aBPORT FOR SURVEY PAR~ 1 : OPERATIONAL REPORT ......i Airborne Magnetometer Surve,y Tanami :Barrow...
Transcript of aBPORT FOR SURVEY PAR~ 1 : OPERATIONAL REPORT ......i Airborne Magnetometer Surve,y Tanami :Barrow...
aBPORT FOR AERO~GNETIC SURVEY
PAR~ 1 OPERATIONAL REPORT p~ 2 INTERPRETATIONAL
REPORT
AMEyenICAN OVERSEAS PETROLEUM
i Airborne Magnetometer Survey
Tanami Barrow Cleek Aeronagnet1c Survey
664624 WISO BASIN NT
being part of Northern TerritorY Permits
to explore nwnbers
118 119 120 123 136 152 f8ld 153
on behalf ot
AMampRICAN OVERSEAS PETROLEUM LTD
119-123 Le1chhardt Stleet
r Brisbane Queensland
Field Work from
17th August 1966 to 2nd May 1967
Part I ONSHORE Operational Report
r Part II
Interpretation Report
Adastra Hunting Geophysics Pty Ltd 41-45 Vickers Avenue MASCOT NSWDEPT OF rES amp ENERGY
DO NOT REMOVE
111111111 ~__~__~ l-_ __~=~-===---~~=~==============--=~==-~O-== -J
Part I
Operational Report
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I INTRODUCTION
I 1 GENERAL INFORMATION
The area surveyed is irregularly shaped with its long axis
lying approximately northwest-southeast It is contained within
the rectangle bounded in the north by latitude 18000 I south in
the south by latitude 230 00 south in the westby longitude
129000 east and in the east by longitude 1370 00 east (See
Plate No1 of this report)
One hundred and seventy seven (177) flight lines were
flown one hundred and fifty seven (157) in a direction
approximately northeast-southwest seventeen (17) northsouth
and three (3) approximately northwest-southeast Seventeen
(17) tie lines were flown at right angles to the flight lines
for purposes of magnetic control (For flight line-tie line
diagram seamp Plate No2 of this report)
The initial plan was to use Lockheed Hudson aircraft VH-AGE
carrying the magnetometer detecting head in a towed bird with
navigation and flight path recovery by means of a Marconi 623
Series Doppler but when this aircraft was lost on September 24th
having flown lines 157 to 173 inclusive the Companys Douglas
DC3 aircraft VH-AGU carrying the magnetometer detecting head as
a fixed boomlbull installation was substituted with ~avigation and
flight path recovery by means of one inch to one mile photo mosaics
prepared by Adastra AirWays Pty Ltd
Several lines flown by VH-AGE were reflown by VH-AGU as
repeatability checks between the two magnetometers
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All flying was
Northern Territory
carried out from Tennant Creek Aerodrome
middot I 2
Hudson flying operations commenced on 17th August 1966 and
came to an untimely finish when the aircraft crashed on 24th September
1966 DC3 flying operations started on 22nd October 1966 but were
suspended on December 20th owing to continued bad weather DC3 o
operations were recommenced on 13th January 1967 and the survey
completed on 8th June 1967
PURPOSE OF THE SURVEY
The purpose of the survey was to ascertain the depth trends
of major structural features such as faults and magnetic intrusions
and the location of 8JJ1 basin boundaries
The information gained from the survey to be used as a guide
in seleating areas of interest for further more detailed geophysical
surveys in the search for oil bearing struotures
II THE FLYING PROGRAMME
II 1 PRE-FLIGHT PREPARATIONS
As the initial plan was to use Doppler for navigation and
flight path recovery east-west photo strips were prepared from
available 9 inch by 9 inch photography spaced at approximately
twenty-five miles and showing the positions of flight lines and tie
lines These were to serve as a check on the Doppler navigator but
after the loss of VH-AGR complete one inch to one mile mosaic
ooverage was prepared These were used throughout the survey for
navigation and subsequent flight path recovery
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II 2 PROGRAMME DETAILS
(a) Survey Altitude
The survey was poundlown at a constant barometric altitude of
2250 feet above mean sea level (amsl) by reference to a
standard aircraft type pressure altimeter with frequent comparisons
against an APN-l radio altimeter
Average terrain clearance was 1250 feet
(b) Flight Lines Flown
One hundred and seventy three (173) poundlight lines were poundlown
the average spacing being two (2) miles Tie lines were flown
perpendicular to the flight lines as sho~ in the flight line shy
tie line diagram (see Plate 2)
Individual Line Mileages
The individual mileage flown for each tie and flight line is
listed below
Line Miles Line Miles Line Miles
2 25 6 51 10 79
3 26 7 52 11 112
4 30 8 79 12 110
5 50 9 79 13 108
14 121 40 104 66 123
15 120 41 103 67 123
16 117 42 103 68 122
17 96 43 59 69 121
18 39 44 103 70 75
19 131 45 10l 71 75
~ lt
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Line Miles Line Miles Line Miles
20 53 46 82 72 74
21 120 47 101 73 73
22 118 48 101 74 73
23 102 49 101 75 72
24 70 50 150 76 72
25 70 51 149 77 71
26 109 52 149 78 71
27 109 53 148 79 70
28 85 54 99 80 69
29 61 55 99 81 69
30 60 56 9~ 82 68
31 107 57 98 83 67
32 107 58 98 84 67
33 48 59 97 85 66
34 60 60 97 86 ~
66
35 106 61 95 87 65
36 156 62 92 88 55
37 155 63 91 89 55
38 110 64 89 90 55
39 23 65 86 91 55
92 55 117 46 l42 63
93 53 118 116 l43 63
94 53 119 116 144 109
95 53 120 116 145 109
96 52 121 116 l46 110
97 52 122 101 147 112
- 5 shy
Line Miles Line Miles Line Miles
98 51 123 101 148 64
99 51 124 101 149 63
100 51 125 101 150 63
101 51 126 44 151 64
102 51 127 48 152 65
middot103 50 128 50 153 18
104 50 129 51 154 17
105 50 130 51 155 15
106 49 131 61 156 12
107 49 132 61 157 35
108 49 133 61 158 35
109 48 134 61 159 35
llO 48 135 62 160 35
III 47 136 63 161 35
ll2 47 137 61 162 35
ll3 47 138 62 163 35
114 47 139 63 164 35
ll5 47 un 63 165 35
ll6 46 l4l 63 166 35
167 35 II 80 E 3g-f
168 35 III 8J F 369
169 35 IV 57 G 206
170 35 V 57 H 356
171 35 I 150
172 35 A 76 J 34
173 35 B 76 K 6
0 44 L 6
~ 1
middot1
-----------~-~--~--~ ---~-~---- --~-~-- --~~~-
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Line Miles Line Miles Line Miles
I 79 D 250 M 6
N 6
0 56
P 56
Q 56
Total Flight Line Miles 12903
1 IITie 2138
Grand Total l50u
(c) Storm Monitor
The storm monitor vas installed at Tennant Creek Aerodrome
NT and was run continuously 24 houra per day throughout the
following periods
17th August to
24th October to
22nd January to
25th May to
24th September 1966
20th December 1966shy
2nd May 1967
8th June 1967
III METHODS AND INSTRUMENTS USED FOR THE SURVEY
III 1 AIRBORNE (TarAL FORCE) MAGNETOMampTER
The instrument used for this survey was a Gulf Mark III
total force saturable core fluxgate magnetometer manufactured by
the Gulf Research and Development Company Pittsburgh PennsylVania
uSA
The airborne magnetometer is intended primarily for measuring
and recording the Earth 1 s total magnetio field intensity and in
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particular looal variations of intensitr such as Dl8yen be oaused by
geologioal inhomogeneities
The equipment oomprises a measuring (deteoting) element an
osoillator to exoite it a vacuum tube oircuit to amplity and deteot
the output variations of the element orientating devices (which keep
the detector element aligned) a potentiometer circuit to compensate
or buckoutlt large changes of field and a recorder
The eqUipment was designed for use in a moving airoraft to
provide a continuous and accurate record of variations of the Earths lt
total magnetic field intensity Beoause an aircratt does not
accurately maintain its orientation in space provision has been made
to hold the measuring or deteoting element in a fixed orientation
(parallel) with respect to the Earths total field
The Earths magnetic field itself is used as a reference the
detector element being aligned with its axis ot sensitivitr parallel
to this field This arrangement places the detector element in the
most favourable position and errors due to improper orientation are
at a minimum Two simUar sets of deteotor elements are used to
sense and seek the position of zero (null) field I
When the axis of sensitivity ot the detector element is aligned
parallel to the Earth t s magnetic field 8DT error in alignment results
in a decrease of the total field reading the magnitude of the error
is proportional to the siDe of the angle ot misorientation and is of
the order ot 05 gamma tor at degree misalignment in a total magnetic
field of 55000 gamma
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The value of the step (automatic reset procedure) when
variations of magnetic field exceed the tull scale deflection
(ie 600 gamma) was 500 gamma
III 2 MAGNETOMETER CALIBRATIONS
The instrument was calibrated prior to the survey using
standard Helmholtz coils whose radius and coil constant provided
a force of 10 gamma per 1 milliampere of applied current
(a) Lag Test
Lag or delay of response considered in relation to the ground
position of the aircraft is due to a combination of the following
i) Delay due to electrical resistance in the circuitry
ii) Delay due to meChanical transference ot received signal
to the recorder Chart
iii) Delay due to difference in position between the 35mm film
recording camera and the detector head
To establish this lag it is necessary that the survey aircraft
fly over an easily identifiable magnetic body on a reciprocal course
eg a ship a large metal pipeline an iron bridge etc which will
give a well-defined sharp anomaly Then by identifying the centre
of the disturbing body on the 35mm tracking tilm and plotting its
position on the Chart record the difference between this point
(average of the reciprocal headings) and the peak of the magnetic
anomaly is the total lag for the installation tested
The lag test for this survey was flown owr the Sydney Harbour
Bridge immediately prior to the aircrafts departure
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The installation used in this vurvey was checked as described
and no readable lag was detected The side fiducial index pen is
always aligned with the main recording pen
(b) Heading Effect Test
When using the H~son aircraft which towed the magnetometer
there was no heading effect
In the case of DC bull3 aircraft VH-AGU the detector head (measuring
fluxgate~and their respective servo-motors were installed in an
shy extension to the tail section of the survey aircraft
With this type of installation proper compensation for the
asypunetrical distribution of the aircraft IS permanently magnetised
material and the induced magnetic field effects caused by the aircraft
cutting magnetic lines of force on the detecting fluxgate must be made
Compensation for the aircraftls permanent magnetic material m~
be achieved in two ways one by the use of permanently magnetised bar
magnets or two by using induction (air core) coils
Induction coils with variable magnetic intensities controlled
directly by the magnetometer operator are preferable to bar magnets
in that they allow a trial and error compensation pr~cedure to be
carried out whilst airborne whereas in the case of bar magnets fmiddot
adjustments have to be made on the ground
The coil system of compensation was used on this survey tor the
DCbull3 compensation
Variations of induced magnetic fields caused by the aircraft
cutting linea 0 magnetic force on different headings was compensated
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for by the use of high permeabUity strips of metal (IIPermelloyR)
attached to the side af the magnetometer detector head housing
A selected magneticallr quiet area near Tennant Creek was used
for final compensation and a table showing residual heading errors
is included (plate 9) with this report
(c) Repeatability TesectN
I t is required to show that a traverse when poundlown in opposite
directions gives rise to identical (but laterallr reversed)
middot profUes
This test was carried out several times during the survey and
particularly in order to establish repeatabUity between the
magnetometers of VH-AGE and VH~U
III 3 32mm POSITIONING CAMERA
The instrument used to record the position of the aircraft in
relation to the ground was a single frame 35mm c~era using 400 foot
capacity film magazines details of which are as follows
Type Vinten ~5mm Geological Survey Camera
Focal Length 28mm (110 inches)
Shutter Speed l250th sec (set)
Diaphragm Range I f2 - pound32
Format 18mm x 25mm
The camera was mounted in the aircraft with its optical axis
vertical for straight and level poundlight
Exposures were made automaticallr using an electronically
controlled intervalometer set at 30 second intervals With the
exposure interval so set each 35mm frame is overlapped by
approximately 25 - 30 thus ensuring complete ground coverage
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The camera exposures are related to the magnetic field record
by the use of a fiducial pen on the recorder which operates
simultaneously with the Camera Veeder Counter at every tenth
eleventh and twelfth exposure These fiducials appear on the
right hand side of the magnetometer reqord
Du Pont type 936 Superior Two 35mm fUm rated at 160 ASA was
used throughout the survey
shyAutomatic Film Laboratories Moore Park Sydney processed
the film
III 4 RADIO ALTIMETER
Apart from the pressure (barometric) altimeter which is
standard equipment in all aircraft a radio altimeter type APN-l
was used continuously to record terrain clearance
The instrument was set on IIhigh rangell (0-4000 ft) throughout
the survey and was checked at intervals over the base aerodrome
An Esterline Angus recording potentiometer was used in
conjunction with this instrument to obtain a continuous profile on
a five inch wide curvilinear chart recording at a chart speed of
It inches per minute
III 5 STORM MONITOR (Ground Magnetometer)
During aeromagnetic surveys it is essential that ~ time
variation of the Earths total magnetic field is monitored and
recorded throughout the survey on a 24 hour d~ basis
The normal diurnal change of the Earth I s field which occurs
daily is normally of low gradient and cyclic and is compensated
for in the standard flight linetie line control pattern
_~_______________L______ ___~_)
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However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
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is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
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V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
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00
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
i Airborne Magnetometer Survey
Tanami Barrow Cleek Aeronagnet1c Survey
664624 WISO BASIN NT
being part of Northern TerritorY Permits
to explore nwnbers
118 119 120 123 136 152 f8ld 153
on behalf ot
AMampRICAN OVERSEAS PETROLEUM LTD
119-123 Le1chhardt Stleet
r Brisbane Queensland
Field Work from
17th August 1966 to 2nd May 1967
Part I ONSHORE Operational Report
r Part II
Interpretation Report
Adastra Hunting Geophysics Pty Ltd 41-45 Vickers Avenue MASCOT NSWDEPT OF rES amp ENERGY
DO NOT REMOVE
111111111 ~__~__~ l-_ __~=~-===---~~=~==============--=~==-~O-== -J
Part I
Operational Report
-1shy
I INTRODUCTION
I 1 GENERAL INFORMATION
The area surveyed is irregularly shaped with its long axis
lying approximately northwest-southeast It is contained within
the rectangle bounded in the north by latitude 18000 I south in
the south by latitude 230 00 south in the westby longitude
129000 east and in the east by longitude 1370 00 east (See
Plate No1 of this report)
One hundred and seventy seven (177) flight lines were
flown one hundred and fifty seven (157) in a direction
approximately northeast-southwest seventeen (17) northsouth
and three (3) approximately northwest-southeast Seventeen
(17) tie lines were flown at right angles to the flight lines
for purposes of magnetic control (For flight line-tie line
diagram seamp Plate No2 of this report)
The initial plan was to use Lockheed Hudson aircraft VH-AGE
carrying the magnetometer detecting head in a towed bird with
navigation and flight path recovery by means of a Marconi 623
Series Doppler but when this aircraft was lost on September 24th
having flown lines 157 to 173 inclusive the Companys Douglas
DC3 aircraft VH-AGU carrying the magnetometer detecting head as
a fixed boomlbull installation was substituted with ~avigation and
flight path recovery by means of one inch to one mile photo mosaics
prepared by Adastra AirWays Pty Ltd
Several lines flown by VH-AGE were reflown by VH-AGU as
repeatability checks between the two magnetometers
-2shy
All flying was
Northern Territory
carried out from Tennant Creek Aerodrome
middot I 2
Hudson flying operations commenced on 17th August 1966 and
came to an untimely finish when the aircraft crashed on 24th September
1966 DC3 flying operations started on 22nd October 1966 but were
suspended on December 20th owing to continued bad weather DC3 o
operations were recommenced on 13th January 1967 and the survey
completed on 8th June 1967
PURPOSE OF THE SURVEY
The purpose of the survey was to ascertain the depth trends
of major structural features such as faults and magnetic intrusions
and the location of 8JJ1 basin boundaries
The information gained from the survey to be used as a guide
in seleating areas of interest for further more detailed geophysical
surveys in the search for oil bearing struotures
II THE FLYING PROGRAMME
II 1 PRE-FLIGHT PREPARATIONS
As the initial plan was to use Doppler for navigation and
flight path recovery east-west photo strips were prepared from
available 9 inch by 9 inch photography spaced at approximately
twenty-five miles and showing the positions of flight lines and tie
lines These were to serve as a check on the Doppler navigator but
after the loss of VH-AGR complete one inch to one mile mosaic
ooverage was prepared These were used throughout the survey for
navigation and subsequent flight path recovery
-3shy
II 2 PROGRAMME DETAILS
(a) Survey Altitude
The survey was poundlown at a constant barometric altitude of
2250 feet above mean sea level (amsl) by reference to a
standard aircraft type pressure altimeter with frequent comparisons
against an APN-l radio altimeter
Average terrain clearance was 1250 feet
(b) Flight Lines Flown
One hundred and seventy three (173) poundlight lines were poundlown
the average spacing being two (2) miles Tie lines were flown
perpendicular to the flight lines as sho~ in the flight line shy
tie line diagram (see Plate 2)
Individual Line Mileages
The individual mileage flown for each tie and flight line is
listed below
Line Miles Line Miles Line Miles
2 25 6 51 10 79
3 26 7 52 11 112
4 30 8 79 12 110
5 50 9 79 13 108
14 121 40 104 66 123
15 120 41 103 67 123
16 117 42 103 68 122
17 96 43 59 69 121
18 39 44 103 70 75
19 131 45 10l 71 75
~ lt
-4shy
Line Miles Line Miles Line Miles
20 53 46 82 72 74
21 120 47 101 73 73
22 118 48 101 74 73
23 102 49 101 75 72
24 70 50 150 76 72
25 70 51 149 77 71
26 109 52 149 78 71
27 109 53 148 79 70
28 85 54 99 80 69
29 61 55 99 81 69
30 60 56 9~ 82 68
31 107 57 98 83 67
32 107 58 98 84 67
33 48 59 97 85 66
34 60 60 97 86 ~
66
35 106 61 95 87 65
36 156 62 92 88 55
37 155 63 91 89 55
38 110 64 89 90 55
39 23 65 86 91 55
92 55 117 46 l42 63
93 53 118 116 l43 63
94 53 119 116 144 109
95 53 120 116 145 109
96 52 121 116 l46 110
97 52 122 101 147 112
- 5 shy
Line Miles Line Miles Line Miles
98 51 123 101 148 64
99 51 124 101 149 63
100 51 125 101 150 63
101 51 126 44 151 64
102 51 127 48 152 65
middot103 50 128 50 153 18
104 50 129 51 154 17
105 50 130 51 155 15
106 49 131 61 156 12
107 49 132 61 157 35
108 49 133 61 158 35
109 48 134 61 159 35
llO 48 135 62 160 35
III 47 136 63 161 35
ll2 47 137 61 162 35
ll3 47 138 62 163 35
114 47 139 63 164 35
ll5 47 un 63 165 35
ll6 46 l4l 63 166 35
167 35 II 80 E 3g-f
168 35 III 8J F 369
169 35 IV 57 G 206
170 35 V 57 H 356
171 35 I 150
172 35 A 76 J 34
173 35 B 76 K 6
0 44 L 6
~ 1
middot1
-----------~-~--~--~ ---~-~---- --~-~-- --~~~-
-6shy
Line Miles Line Miles Line Miles
I 79 D 250 M 6
N 6
0 56
P 56
Q 56
Total Flight Line Miles 12903
1 IITie 2138
Grand Total l50u
(c) Storm Monitor
The storm monitor vas installed at Tennant Creek Aerodrome
NT and was run continuously 24 houra per day throughout the
following periods
17th August to
24th October to
22nd January to
25th May to
24th September 1966
20th December 1966shy
2nd May 1967
8th June 1967
III METHODS AND INSTRUMENTS USED FOR THE SURVEY
III 1 AIRBORNE (TarAL FORCE) MAGNETOMampTER
The instrument used for this survey was a Gulf Mark III
total force saturable core fluxgate magnetometer manufactured by
the Gulf Research and Development Company Pittsburgh PennsylVania
uSA
The airborne magnetometer is intended primarily for measuring
and recording the Earth 1 s total magnetio field intensity and in
-7shy
particular looal variations of intensitr such as Dl8yen be oaused by
geologioal inhomogeneities
The equipment oomprises a measuring (deteoting) element an
osoillator to exoite it a vacuum tube oircuit to amplity and deteot
the output variations of the element orientating devices (which keep
the detector element aligned) a potentiometer circuit to compensate
or buckoutlt large changes of field and a recorder
The eqUipment was designed for use in a moving airoraft to
provide a continuous and accurate record of variations of the Earths lt
total magnetic field intensity Beoause an aircratt does not
accurately maintain its orientation in space provision has been made
to hold the measuring or deteoting element in a fixed orientation
(parallel) with respect to the Earths total field
The Earths magnetic field itself is used as a reference the
detector element being aligned with its axis ot sensitivitr parallel
to this field This arrangement places the detector element in the
most favourable position and errors due to improper orientation are
at a minimum Two simUar sets of deteotor elements are used to
sense and seek the position of zero (null) field I
When the axis of sensitivity ot the detector element is aligned
parallel to the Earth t s magnetic field 8DT error in alignment results
in a decrease of the total field reading the magnitude of the error
is proportional to the siDe of the angle ot misorientation and is of
the order ot 05 gamma tor at degree misalignment in a total magnetic
field of 55000 gamma
-8shy
The value of the step (automatic reset procedure) when
variations of magnetic field exceed the tull scale deflection
(ie 600 gamma) was 500 gamma
III 2 MAGNETOMETER CALIBRATIONS
The instrument was calibrated prior to the survey using
standard Helmholtz coils whose radius and coil constant provided
a force of 10 gamma per 1 milliampere of applied current
(a) Lag Test
Lag or delay of response considered in relation to the ground
position of the aircraft is due to a combination of the following
i) Delay due to electrical resistance in the circuitry
ii) Delay due to meChanical transference ot received signal
to the recorder Chart
iii) Delay due to difference in position between the 35mm film
recording camera and the detector head
To establish this lag it is necessary that the survey aircraft
fly over an easily identifiable magnetic body on a reciprocal course
eg a ship a large metal pipeline an iron bridge etc which will
give a well-defined sharp anomaly Then by identifying the centre
of the disturbing body on the 35mm tracking tilm and plotting its
position on the Chart record the difference between this point
(average of the reciprocal headings) and the peak of the magnetic
anomaly is the total lag for the installation tested
The lag test for this survey was flown owr the Sydney Harbour
Bridge immediately prior to the aircrafts departure
-9shy
The installation used in this vurvey was checked as described
and no readable lag was detected The side fiducial index pen is
always aligned with the main recording pen
(b) Heading Effect Test
When using the H~son aircraft which towed the magnetometer
there was no heading effect
In the case of DC bull3 aircraft VH-AGU the detector head (measuring
fluxgate~and their respective servo-motors were installed in an
shy extension to the tail section of the survey aircraft
With this type of installation proper compensation for the
asypunetrical distribution of the aircraft IS permanently magnetised
material and the induced magnetic field effects caused by the aircraft
cutting magnetic lines of force on the detecting fluxgate must be made
Compensation for the aircraftls permanent magnetic material m~
be achieved in two ways one by the use of permanently magnetised bar
magnets or two by using induction (air core) coils
Induction coils with variable magnetic intensities controlled
directly by the magnetometer operator are preferable to bar magnets
in that they allow a trial and error compensation pr~cedure to be
carried out whilst airborne whereas in the case of bar magnets fmiddot
adjustments have to be made on the ground
The coil system of compensation was used on this survey tor the
DCbull3 compensation
Variations of induced magnetic fields caused by the aircraft
cutting linea 0 magnetic force on different headings was compensated
- 10 shy
for by the use of high permeabUity strips of metal (IIPermelloyR)
attached to the side af the magnetometer detector head housing
A selected magneticallr quiet area near Tennant Creek was used
for final compensation and a table showing residual heading errors
is included (plate 9) with this report
(c) Repeatability TesectN
I t is required to show that a traverse when poundlown in opposite
directions gives rise to identical (but laterallr reversed)
middot profUes
This test was carried out several times during the survey and
particularly in order to establish repeatabUity between the
magnetometers of VH-AGE and VH~U
III 3 32mm POSITIONING CAMERA
The instrument used to record the position of the aircraft in
relation to the ground was a single frame 35mm c~era using 400 foot
capacity film magazines details of which are as follows
Type Vinten ~5mm Geological Survey Camera
Focal Length 28mm (110 inches)
Shutter Speed l250th sec (set)
Diaphragm Range I f2 - pound32
Format 18mm x 25mm
The camera was mounted in the aircraft with its optical axis
vertical for straight and level poundlight
Exposures were made automaticallr using an electronically
controlled intervalometer set at 30 second intervals With the
exposure interval so set each 35mm frame is overlapped by
approximately 25 - 30 thus ensuring complete ground coverage
-11shy
The camera exposures are related to the magnetic field record
by the use of a fiducial pen on the recorder which operates
simultaneously with the Camera Veeder Counter at every tenth
eleventh and twelfth exposure These fiducials appear on the
right hand side of the magnetometer reqord
Du Pont type 936 Superior Two 35mm fUm rated at 160 ASA was
used throughout the survey
shyAutomatic Film Laboratories Moore Park Sydney processed
the film
III 4 RADIO ALTIMETER
Apart from the pressure (barometric) altimeter which is
standard equipment in all aircraft a radio altimeter type APN-l
was used continuously to record terrain clearance
The instrument was set on IIhigh rangell (0-4000 ft) throughout
the survey and was checked at intervals over the base aerodrome
An Esterline Angus recording potentiometer was used in
conjunction with this instrument to obtain a continuous profile on
a five inch wide curvilinear chart recording at a chart speed of
It inches per minute
III 5 STORM MONITOR (Ground Magnetometer)
During aeromagnetic surveys it is essential that ~ time
variation of the Earths total magnetic field is monitored and
recorded throughout the survey on a 24 hour d~ basis
The normal diurnal change of the Earth I s field which occurs
daily is normally of low gradient and cyclic and is compensated
for in the standard flight linetie line control pattern
_~_______________L______ ___~_)
-12shy
However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
- ll
Z
Q)
w
0 ~ --shy
U
W
0 ()
~
-II
D~
06
- I
-
e ~l
ri 0
~ shy
No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
Part I
Operational Report
-1shy
I INTRODUCTION
I 1 GENERAL INFORMATION
The area surveyed is irregularly shaped with its long axis
lying approximately northwest-southeast It is contained within
the rectangle bounded in the north by latitude 18000 I south in
the south by latitude 230 00 south in the westby longitude
129000 east and in the east by longitude 1370 00 east (See
Plate No1 of this report)
One hundred and seventy seven (177) flight lines were
flown one hundred and fifty seven (157) in a direction
approximately northeast-southwest seventeen (17) northsouth
and three (3) approximately northwest-southeast Seventeen
(17) tie lines were flown at right angles to the flight lines
for purposes of magnetic control (For flight line-tie line
diagram seamp Plate No2 of this report)
The initial plan was to use Lockheed Hudson aircraft VH-AGE
carrying the magnetometer detecting head in a towed bird with
navigation and flight path recovery by means of a Marconi 623
Series Doppler but when this aircraft was lost on September 24th
having flown lines 157 to 173 inclusive the Companys Douglas
DC3 aircraft VH-AGU carrying the magnetometer detecting head as
a fixed boomlbull installation was substituted with ~avigation and
flight path recovery by means of one inch to one mile photo mosaics
prepared by Adastra AirWays Pty Ltd
Several lines flown by VH-AGE were reflown by VH-AGU as
repeatability checks between the two magnetometers
-2shy
All flying was
Northern Territory
carried out from Tennant Creek Aerodrome
middot I 2
Hudson flying operations commenced on 17th August 1966 and
came to an untimely finish when the aircraft crashed on 24th September
1966 DC3 flying operations started on 22nd October 1966 but were
suspended on December 20th owing to continued bad weather DC3 o
operations were recommenced on 13th January 1967 and the survey
completed on 8th June 1967
PURPOSE OF THE SURVEY
The purpose of the survey was to ascertain the depth trends
of major structural features such as faults and magnetic intrusions
and the location of 8JJ1 basin boundaries
The information gained from the survey to be used as a guide
in seleating areas of interest for further more detailed geophysical
surveys in the search for oil bearing struotures
II THE FLYING PROGRAMME
II 1 PRE-FLIGHT PREPARATIONS
As the initial plan was to use Doppler for navigation and
flight path recovery east-west photo strips were prepared from
available 9 inch by 9 inch photography spaced at approximately
twenty-five miles and showing the positions of flight lines and tie
lines These were to serve as a check on the Doppler navigator but
after the loss of VH-AGR complete one inch to one mile mosaic
ooverage was prepared These were used throughout the survey for
navigation and subsequent flight path recovery
-3shy
II 2 PROGRAMME DETAILS
(a) Survey Altitude
The survey was poundlown at a constant barometric altitude of
2250 feet above mean sea level (amsl) by reference to a
standard aircraft type pressure altimeter with frequent comparisons
against an APN-l radio altimeter
Average terrain clearance was 1250 feet
(b) Flight Lines Flown
One hundred and seventy three (173) poundlight lines were poundlown
the average spacing being two (2) miles Tie lines were flown
perpendicular to the flight lines as sho~ in the flight line shy
tie line diagram (see Plate 2)
Individual Line Mileages
The individual mileage flown for each tie and flight line is
listed below
Line Miles Line Miles Line Miles
2 25 6 51 10 79
3 26 7 52 11 112
4 30 8 79 12 110
5 50 9 79 13 108
14 121 40 104 66 123
15 120 41 103 67 123
16 117 42 103 68 122
17 96 43 59 69 121
18 39 44 103 70 75
19 131 45 10l 71 75
~ lt
-4shy
Line Miles Line Miles Line Miles
20 53 46 82 72 74
21 120 47 101 73 73
22 118 48 101 74 73
23 102 49 101 75 72
24 70 50 150 76 72
25 70 51 149 77 71
26 109 52 149 78 71
27 109 53 148 79 70
28 85 54 99 80 69
29 61 55 99 81 69
30 60 56 9~ 82 68
31 107 57 98 83 67
32 107 58 98 84 67
33 48 59 97 85 66
34 60 60 97 86 ~
66
35 106 61 95 87 65
36 156 62 92 88 55
37 155 63 91 89 55
38 110 64 89 90 55
39 23 65 86 91 55
92 55 117 46 l42 63
93 53 118 116 l43 63
94 53 119 116 144 109
95 53 120 116 145 109
96 52 121 116 l46 110
97 52 122 101 147 112
- 5 shy
Line Miles Line Miles Line Miles
98 51 123 101 148 64
99 51 124 101 149 63
100 51 125 101 150 63
101 51 126 44 151 64
102 51 127 48 152 65
middot103 50 128 50 153 18
104 50 129 51 154 17
105 50 130 51 155 15
106 49 131 61 156 12
107 49 132 61 157 35
108 49 133 61 158 35
109 48 134 61 159 35
llO 48 135 62 160 35
III 47 136 63 161 35
ll2 47 137 61 162 35
ll3 47 138 62 163 35
114 47 139 63 164 35
ll5 47 un 63 165 35
ll6 46 l4l 63 166 35
167 35 II 80 E 3g-f
168 35 III 8J F 369
169 35 IV 57 G 206
170 35 V 57 H 356
171 35 I 150
172 35 A 76 J 34
173 35 B 76 K 6
0 44 L 6
~ 1
middot1
-----------~-~--~--~ ---~-~---- --~-~-- --~~~-
-6shy
Line Miles Line Miles Line Miles
I 79 D 250 M 6
N 6
0 56
P 56
Q 56
Total Flight Line Miles 12903
1 IITie 2138
Grand Total l50u
(c) Storm Monitor
The storm monitor vas installed at Tennant Creek Aerodrome
NT and was run continuously 24 houra per day throughout the
following periods
17th August to
24th October to
22nd January to
25th May to
24th September 1966
20th December 1966shy
2nd May 1967
8th June 1967
III METHODS AND INSTRUMENTS USED FOR THE SURVEY
III 1 AIRBORNE (TarAL FORCE) MAGNETOMampTER
The instrument used for this survey was a Gulf Mark III
total force saturable core fluxgate magnetometer manufactured by
the Gulf Research and Development Company Pittsburgh PennsylVania
uSA
The airborne magnetometer is intended primarily for measuring
and recording the Earth 1 s total magnetio field intensity and in
-7shy
particular looal variations of intensitr such as Dl8yen be oaused by
geologioal inhomogeneities
The equipment oomprises a measuring (deteoting) element an
osoillator to exoite it a vacuum tube oircuit to amplity and deteot
the output variations of the element orientating devices (which keep
the detector element aligned) a potentiometer circuit to compensate
or buckoutlt large changes of field and a recorder
The eqUipment was designed for use in a moving airoraft to
provide a continuous and accurate record of variations of the Earths lt
total magnetic field intensity Beoause an aircratt does not
accurately maintain its orientation in space provision has been made
to hold the measuring or deteoting element in a fixed orientation
(parallel) with respect to the Earths total field
The Earths magnetic field itself is used as a reference the
detector element being aligned with its axis ot sensitivitr parallel
to this field This arrangement places the detector element in the
most favourable position and errors due to improper orientation are
at a minimum Two simUar sets of deteotor elements are used to
sense and seek the position of zero (null) field I
When the axis of sensitivity ot the detector element is aligned
parallel to the Earth t s magnetic field 8DT error in alignment results
in a decrease of the total field reading the magnitude of the error
is proportional to the siDe of the angle ot misorientation and is of
the order ot 05 gamma tor at degree misalignment in a total magnetic
field of 55000 gamma
-8shy
The value of the step (automatic reset procedure) when
variations of magnetic field exceed the tull scale deflection
(ie 600 gamma) was 500 gamma
III 2 MAGNETOMETER CALIBRATIONS
The instrument was calibrated prior to the survey using
standard Helmholtz coils whose radius and coil constant provided
a force of 10 gamma per 1 milliampere of applied current
(a) Lag Test
Lag or delay of response considered in relation to the ground
position of the aircraft is due to a combination of the following
i) Delay due to electrical resistance in the circuitry
ii) Delay due to meChanical transference ot received signal
to the recorder Chart
iii) Delay due to difference in position between the 35mm film
recording camera and the detector head
To establish this lag it is necessary that the survey aircraft
fly over an easily identifiable magnetic body on a reciprocal course
eg a ship a large metal pipeline an iron bridge etc which will
give a well-defined sharp anomaly Then by identifying the centre
of the disturbing body on the 35mm tracking tilm and plotting its
position on the Chart record the difference between this point
(average of the reciprocal headings) and the peak of the magnetic
anomaly is the total lag for the installation tested
The lag test for this survey was flown owr the Sydney Harbour
Bridge immediately prior to the aircrafts departure
-9shy
The installation used in this vurvey was checked as described
and no readable lag was detected The side fiducial index pen is
always aligned with the main recording pen
(b) Heading Effect Test
When using the H~son aircraft which towed the magnetometer
there was no heading effect
In the case of DC bull3 aircraft VH-AGU the detector head (measuring
fluxgate~and their respective servo-motors were installed in an
shy extension to the tail section of the survey aircraft
With this type of installation proper compensation for the
asypunetrical distribution of the aircraft IS permanently magnetised
material and the induced magnetic field effects caused by the aircraft
cutting magnetic lines of force on the detecting fluxgate must be made
Compensation for the aircraftls permanent magnetic material m~
be achieved in two ways one by the use of permanently magnetised bar
magnets or two by using induction (air core) coils
Induction coils with variable magnetic intensities controlled
directly by the magnetometer operator are preferable to bar magnets
in that they allow a trial and error compensation pr~cedure to be
carried out whilst airborne whereas in the case of bar magnets fmiddot
adjustments have to be made on the ground
The coil system of compensation was used on this survey tor the
DCbull3 compensation
Variations of induced magnetic fields caused by the aircraft
cutting linea 0 magnetic force on different headings was compensated
- 10 shy
for by the use of high permeabUity strips of metal (IIPermelloyR)
attached to the side af the magnetometer detector head housing
A selected magneticallr quiet area near Tennant Creek was used
for final compensation and a table showing residual heading errors
is included (plate 9) with this report
(c) Repeatability TesectN
I t is required to show that a traverse when poundlown in opposite
directions gives rise to identical (but laterallr reversed)
middot profUes
This test was carried out several times during the survey and
particularly in order to establish repeatabUity between the
magnetometers of VH-AGE and VH~U
III 3 32mm POSITIONING CAMERA
The instrument used to record the position of the aircraft in
relation to the ground was a single frame 35mm c~era using 400 foot
capacity film magazines details of which are as follows
Type Vinten ~5mm Geological Survey Camera
Focal Length 28mm (110 inches)
Shutter Speed l250th sec (set)
Diaphragm Range I f2 - pound32
Format 18mm x 25mm
The camera was mounted in the aircraft with its optical axis
vertical for straight and level poundlight
Exposures were made automaticallr using an electronically
controlled intervalometer set at 30 second intervals With the
exposure interval so set each 35mm frame is overlapped by
approximately 25 - 30 thus ensuring complete ground coverage
-11shy
The camera exposures are related to the magnetic field record
by the use of a fiducial pen on the recorder which operates
simultaneously with the Camera Veeder Counter at every tenth
eleventh and twelfth exposure These fiducials appear on the
right hand side of the magnetometer reqord
Du Pont type 936 Superior Two 35mm fUm rated at 160 ASA was
used throughout the survey
shyAutomatic Film Laboratories Moore Park Sydney processed
the film
III 4 RADIO ALTIMETER
Apart from the pressure (barometric) altimeter which is
standard equipment in all aircraft a radio altimeter type APN-l
was used continuously to record terrain clearance
The instrument was set on IIhigh rangell (0-4000 ft) throughout
the survey and was checked at intervals over the base aerodrome
An Esterline Angus recording potentiometer was used in
conjunction with this instrument to obtain a continuous profile on
a five inch wide curvilinear chart recording at a chart speed of
It inches per minute
III 5 STORM MONITOR (Ground Magnetometer)
During aeromagnetic surveys it is essential that ~ time
variation of the Earths total magnetic field is monitored and
recorded throughout the survey on a 24 hour d~ basis
The normal diurnal change of the Earth I s field which occurs
daily is normally of low gradient and cyclic and is compensated
for in the standard flight linetie line control pattern
_~_______________L______ ___~_)
-12shy
However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
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U
W
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06
- I
-
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ri 0
~ shy
No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-1shy
I INTRODUCTION
I 1 GENERAL INFORMATION
The area surveyed is irregularly shaped with its long axis
lying approximately northwest-southeast It is contained within
the rectangle bounded in the north by latitude 18000 I south in
the south by latitude 230 00 south in the westby longitude
129000 east and in the east by longitude 1370 00 east (See
Plate No1 of this report)
One hundred and seventy seven (177) flight lines were
flown one hundred and fifty seven (157) in a direction
approximately northeast-southwest seventeen (17) northsouth
and three (3) approximately northwest-southeast Seventeen
(17) tie lines were flown at right angles to the flight lines
for purposes of magnetic control (For flight line-tie line
diagram seamp Plate No2 of this report)
The initial plan was to use Lockheed Hudson aircraft VH-AGE
carrying the magnetometer detecting head in a towed bird with
navigation and flight path recovery by means of a Marconi 623
Series Doppler but when this aircraft was lost on September 24th
having flown lines 157 to 173 inclusive the Companys Douglas
DC3 aircraft VH-AGU carrying the magnetometer detecting head as
a fixed boomlbull installation was substituted with ~avigation and
flight path recovery by means of one inch to one mile photo mosaics
prepared by Adastra AirWays Pty Ltd
Several lines flown by VH-AGE were reflown by VH-AGU as
repeatability checks between the two magnetometers
-2shy
All flying was
Northern Territory
carried out from Tennant Creek Aerodrome
middot I 2
Hudson flying operations commenced on 17th August 1966 and
came to an untimely finish when the aircraft crashed on 24th September
1966 DC3 flying operations started on 22nd October 1966 but were
suspended on December 20th owing to continued bad weather DC3 o
operations were recommenced on 13th January 1967 and the survey
completed on 8th June 1967
PURPOSE OF THE SURVEY
The purpose of the survey was to ascertain the depth trends
of major structural features such as faults and magnetic intrusions
and the location of 8JJ1 basin boundaries
The information gained from the survey to be used as a guide
in seleating areas of interest for further more detailed geophysical
surveys in the search for oil bearing struotures
II THE FLYING PROGRAMME
II 1 PRE-FLIGHT PREPARATIONS
As the initial plan was to use Doppler for navigation and
flight path recovery east-west photo strips were prepared from
available 9 inch by 9 inch photography spaced at approximately
twenty-five miles and showing the positions of flight lines and tie
lines These were to serve as a check on the Doppler navigator but
after the loss of VH-AGR complete one inch to one mile mosaic
ooverage was prepared These were used throughout the survey for
navigation and subsequent flight path recovery
-3shy
II 2 PROGRAMME DETAILS
(a) Survey Altitude
The survey was poundlown at a constant barometric altitude of
2250 feet above mean sea level (amsl) by reference to a
standard aircraft type pressure altimeter with frequent comparisons
against an APN-l radio altimeter
Average terrain clearance was 1250 feet
(b) Flight Lines Flown
One hundred and seventy three (173) poundlight lines were poundlown
the average spacing being two (2) miles Tie lines were flown
perpendicular to the flight lines as sho~ in the flight line shy
tie line diagram (see Plate 2)
Individual Line Mileages
The individual mileage flown for each tie and flight line is
listed below
Line Miles Line Miles Line Miles
2 25 6 51 10 79
3 26 7 52 11 112
4 30 8 79 12 110
5 50 9 79 13 108
14 121 40 104 66 123
15 120 41 103 67 123
16 117 42 103 68 122
17 96 43 59 69 121
18 39 44 103 70 75
19 131 45 10l 71 75
~ lt
-4shy
Line Miles Line Miles Line Miles
20 53 46 82 72 74
21 120 47 101 73 73
22 118 48 101 74 73
23 102 49 101 75 72
24 70 50 150 76 72
25 70 51 149 77 71
26 109 52 149 78 71
27 109 53 148 79 70
28 85 54 99 80 69
29 61 55 99 81 69
30 60 56 9~ 82 68
31 107 57 98 83 67
32 107 58 98 84 67
33 48 59 97 85 66
34 60 60 97 86 ~
66
35 106 61 95 87 65
36 156 62 92 88 55
37 155 63 91 89 55
38 110 64 89 90 55
39 23 65 86 91 55
92 55 117 46 l42 63
93 53 118 116 l43 63
94 53 119 116 144 109
95 53 120 116 145 109
96 52 121 116 l46 110
97 52 122 101 147 112
- 5 shy
Line Miles Line Miles Line Miles
98 51 123 101 148 64
99 51 124 101 149 63
100 51 125 101 150 63
101 51 126 44 151 64
102 51 127 48 152 65
middot103 50 128 50 153 18
104 50 129 51 154 17
105 50 130 51 155 15
106 49 131 61 156 12
107 49 132 61 157 35
108 49 133 61 158 35
109 48 134 61 159 35
llO 48 135 62 160 35
III 47 136 63 161 35
ll2 47 137 61 162 35
ll3 47 138 62 163 35
114 47 139 63 164 35
ll5 47 un 63 165 35
ll6 46 l4l 63 166 35
167 35 II 80 E 3g-f
168 35 III 8J F 369
169 35 IV 57 G 206
170 35 V 57 H 356
171 35 I 150
172 35 A 76 J 34
173 35 B 76 K 6
0 44 L 6
~ 1
middot1
-----------~-~--~--~ ---~-~---- --~-~-- --~~~-
-6shy
Line Miles Line Miles Line Miles
I 79 D 250 M 6
N 6
0 56
P 56
Q 56
Total Flight Line Miles 12903
1 IITie 2138
Grand Total l50u
(c) Storm Monitor
The storm monitor vas installed at Tennant Creek Aerodrome
NT and was run continuously 24 houra per day throughout the
following periods
17th August to
24th October to
22nd January to
25th May to
24th September 1966
20th December 1966shy
2nd May 1967
8th June 1967
III METHODS AND INSTRUMENTS USED FOR THE SURVEY
III 1 AIRBORNE (TarAL FORCE) MAGNETOMampTER
The instrument used for this survey was a Gulf Mark III
total force saturable core fluxgate magnetometer manufactured by
the Gulf Research and Development Company Pittsburgh PennsylVania
uSA
The airborne magnetometer is intended primarily for measuring
and recording the Earth 1 s total magnetio field intensity and in
-7shy
particular looal variations of intensitr such as Dl8yen be oaused by
geologioal inhomogeneities
The equipment oomprises a measuring (deteoting) element an
osoillator to exoite it a vacuum tube oircuit to amplity and deteot
the output variations of the element orientating devices (which keep
the detector element aligned) a potentiometer circuit to compensate
or buckoutlt large changes of field and a recorder
The eqUipment was designed for use in a moving airoraft to
provide a continuous and accurate record of variations of the Earths lt
total magnetic field intensity Beoause an aircratt does not
accurately maintain its orientation in space provision has been made
to hold the measuring or deteoting element in a fixed orientation
(parallel) with respect to the Earths total field
The Earths magnetic field itself is used as a reference the
detector element being aligned with its axis ot sensitivitr parallel
to this field This arrangement places the detector element in the
most favourable position and errors due to improper orientation are
at a minimum Two simUar sets of deteotor elements are used to
sense and seek the position of zero (null) field I
When the axis of sensitivity ot the detector element is aligned
parallel to the Earth t s magnetic field 8DT error in alignment results
in a decrease of the total field reading the magnitude of the error
is proportional to the siDe of the angle ot misorientation and is of
the order ot 05 gamma tor at degree misalignment in a total magnetic
field of 55000 gamma
-8shy
The value of the step (automatic reset procedure) when
variations of magnetic field exceed the tull scale deflection
(ie 600 gamma) was 500 gamma
III 2 MAGNETOMETER CALIBRATIONS
The instrument was calibrated prior to the survey using
standard Helmholtz coils whose radius and coil constant provided
a force of 10 gamma per 1 milliampere of applied current
(a) Lag Test
Lag or delay of response considered in relation to the ground
position of the aircraft is due to a combination of the following
i) Delay due to electrical resistance in the circuitry
ii) Delay due to meChanical transference ot received signal
to the recorder Chart
iii) Delay due to difference in position between the 35mm film
recording camera and the detector head
To establish this lag it is necessary that the survey aircraft
fly over an easily identifiable magnetic body on a reciprocal course
eg a ship a large metal pipeline an iron bridge etc which will
give a well-defined sharp anomaly Then by identifying the centre
of the disturbing body on the 35mm tracking tilm and plotting its
position on the Chart record the difference between this point
(average of the reciprocal headings) and the peak of the magnetic
anomaly is the total lag for the installation tested
The lag test for this survey was flown owr the Sydney Harbour
Bridge immediately prior to the aircrafts departure
-9shy
The installation used in this vurvey was checked as described
and no readable lag was detected The side fiducial index pen is
always aligned with the main recording pen
(b) Heading Effect Test
When using the H~son aircraft which towed the magnetometer
there was no heading effect
In the case of DC bull3 aircraft VH-AGU the detector head (measuring
fluxgate~and their respective servo-motors were installed in an
shy extension to the tail section of the survey aircraft
With this type of installation proper compensation for the
asypunetrical distribution of the aircraft IS permanently magnetised
material and the induced magnetic field effects caused by the aircraft
cutting magnetic lines of force on the detecting fluxgate must be made
Compensation for the aircraftls permanent magnetic material m~
be achieved in two ways one by the use of permanently magnetised bar
magnets or two by using induction (air core) coils
Induction coils with variable magnetic intensities controlled
directly by the magnetometer operator are preferable to bar magnets
in that they allow a trial and error compensation pr~cedure to be
carried out whilst airborne whereas in the case of bar magnets fmiddot
adjustments have to be made on the ground
The coil system of compensation was used on this survey tor the
DCbull3 compensation
Variations of induced magnetic fields caused by the aircraft
cutting linea 0 magnetic force on different headings was compensated
- 10 shy
for by the use of high permeabUity strips of metal (IIPermelloyR)
attached to the side af the magnetometer detector head housing
A selected magneticallr quiet area near Tennant Creek was used
for final compensation and a table showing residual heading errors
is included (plate 9) with this report
(c) Repeatability TesectN
I t is required to show that a traverse when poundlown in opposite
directions gives rise to identical (but laterallr reversed)
middot profUes
This test was carried out several times during the survey and
particularly in order to establish repeatabUity between the
magnetometers of VH-AGE and VH~U
III 3 32mm POSITIONING CAMERA
The instrument used to record the position of the aircraft in
relation to the ground was a single frame 35mm c~era using 400 foot
capacity film magazines details of which are as follows
Type Vinten ~5mm Geological Survey Camera
Focal Length 28mm (110 inches)
Shutter Speed l250th sec (set)
Diaphragm Range I f2 - pound32
Format 18mm x 25mm
The camera was mounted in the aircraft with its optical axis
vertical for straight and level poundlight
Exposures were made automaticallr using an electronically
controlled intervalometer set at 30 second intervals With the
exposure interval so set each 35mm frame is overlapped by
approximately 25 - 30 thus ensuring complete ground coverage
-11shy
The camera exposures are related to the magnetic field record
by the use of a fiducial pen on the recorder which operates
simultaneously with the Camera Veeder Counter at every tenth
eleventh and twelfth exposure These fiducials appear on the
right hand side of the magnetometer reqord
Du Pont type 936 Superior Two 35mm fUm rated at 160 ASA was
used throughout the survey
shyAutomatic Film Laboratories Moore Park Sydney processed
the film
III 4 RADIO ALTIMETER
Apart from the pressure (barometric) altimeter which is
standard equipment in all aircraft a radio altimeter type APN-l
was used continuously to record terrain clearance
The instrument was set on IIhigh rangell (0-4000 ft) throughout
the survey and was checked at intervals over the base aerodrome
An Esterline Angus recording potentiometer was used in
conjunction with this instrument to obtain a continuous profile on
a five inch wide curvilinear chart recording at a chart speed of
It inches per minute
III 5 STORM MONITOR (Ground Magnetometer)
During aeromagnetic surveys it is essential that ~ time
variation of the Earths total magnetic field is monitored and
recorded throughout the survey on a 24 hour d~ basis
The normal diurnal change of the Earth I s field which occurs
daily is normally of low gradient and cyclic and is compensated
for in the standard flight linetie line control pattern
_~_______________L______ ___~_)
-12shy
However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
- ll
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w
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U
W
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- I
-
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ri 0
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No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-2shy
All flying was
Northern Territory
carried out from Tennant Creek Aerodrome
middot I 2
Hudson flying operations commenced on 17th August 1966 and
came to an untimely finish when the aircraft crashed on 24th September
1966 DC3 flying operations started on 22nd October 1966 but were
suspended on December 20th owing to continued bad weather DC3 o
operations were recommenced on 13th January 1967 and the survey
completed on 8th June 1967
PURPOSE OF THE SURVEY
The purpose of the survey was to ascertain the depth trends
of major structural features such as faults and magnetic intrusions
and the location of 8JJ1 basin boundaries
The information gained from the survey to be used as a guide
in seleating areas of interest for further more detailed geophysical
surveys in the search for oil bearing struotures
II THE FLYING PROGRAMME
II 1 PRE-FLIGHT PREPARATIONS
As the initial plan was to use Doppler for navigation and
flight path recovery east-west photo strips were prepared from
available 9 inch by 9 inch photography spaced at approximately
twenty-five miles and showing the positions of flight lines and tie
lines These were to serve as a check on the Doppler navigator but
after the loss of VH-AGR complete one inch to one mile mosaic
ooverage was prepared These were used throughout the survey for
navigation and subsequent flight path recovery
-3shy
II 2 PROGRAMME DETAILS
(a) Survey Altitude
The survey was poundlown at a constant barometric altitude of
2250 feet above mean sea level (amsl) by reference to a
standard aircraft type pressure altimeter with frequent comparisons
against an APN-l radio altimeter
Average terrain clearance was 1250 feet
(b) Flight Lines Flown
One hundred and seventy three (173) poundlight lines were poundlown
the average spacing being two (2) miles Tie lines were flown
perpendicular to the flight lines as sho~ in the flight line shy
tie line diagram (see Plate 2)
Individual Line Mileages
The individual mileage flown for each tie and flight line is
listed below
Line Miles Line Miles Line Miles
2 25 6 51 10 79
3 26 7 52 11 112
4 30 8 79 12 110
5 50 9 79 13 108
14 121 40 104 66 123
15 120 41 103 67 123
16 117 42 103 68 122
17 96 43 59 69 121
18 39 44 103 70 75
19 131 45 10l 71 75
~ lt
-4shy
Line Miles Line Miles Line Miles
20 53 46 82 72 74
21 120 47 101 73 73
22 118 48 101 74 73
23 102 49 101 75 72
24 70 50 150 76 72
25 70 51 149 77 71
26 109 52 149 78 71
27 109 53 148 79 70
28 85 54 99 80 69
29 61 55 99 81 69
30 60 56 9~ 82 68
31 107 57 98 83 67
32 107 58 98 84 67
33 48 59 97 85 66
34 60 60 97 86 ~
66
35 106 61 95 87 65
36 156 62 92 88 55
37 155 63 91 89 55
38 110 64 89 90 55
39 23 65 86 91 55
92 55 117 46 l42 63
93 53 118 116 l43 63
94 53 119 116 144 109
95 53 120 116 145 109
96 52 121 116 l46 110
97 52 122 101 147 112
- 5 shy
Line Miles Line Miles Line Miles
98 51 123 101 148 64
99 51 124 101 149 63
100 51 125 101 150 63
101 51 126 44 151 64
102 51 127 48 152 65
middot103 50 128 50 153 18
104 50 129 51 154 17
105 50 130 51 155 15
106 49 131 61 156 12
107 49 132 61 157 35
108 49 133 61 158 35
109 48 134 61 159 35
llO 48 135 62 160 35
III 47 136 63 161 35
ll2 47 137 61 162 35
ll3 47 138 62 163 35
114 47 139 63 164 35
ll5 47 un 63 165 35
ll6 46 l4l 63 166 35
167 35 II 80 E 3g-f
168 35 III 8J F 369
169 35 IV 57 G 206
170 35 V 57 H 356
171 35 I 150
172 35 A 76 J 34
173 35 B 76 K 6
0 44 L 6
~ 1
middot1
-----------~-~--~--~ ---~-~---- --~-~-- --~~~-
-6shy
Line Miles Line Miles Line Miles
I 79 D 250 M 6
N 6
0 56
P 56
Q 56
Total Flight Line Miles 12903
1 IITie 2138
Grand Total l50u
(c) Storm Monitor
The storm monitor vas installed at Tennant Creek Aerodrome
NT and was run continuously 24 houra per day throughout the
following periods
17th August to
24th October to
22nd January to
25th May to
24th September 1966
20th December 1966shy
2nd May 1967
8th June 1967
III METHODS AND INSTRUMENTS USED FOR THE SURVEY
III 1 AIRBORNE (TarAL FORCE) MAGNETOMampTER
The instrument used for this survey was a Gulf Mark III
total force saturable core fluxgate magnetometer manufactured by
the Gulf Research and Development Company Pittsburgh PennsylVania
uSA
The airborne magnetometer is intended primarily for measuring
and recording the Earth 1 s total magnetio field intensity and in
-7shy
particular looal variations of intensitr such as Dl8yen be oaused by
geologioal inhomogeneities
The equipment oomprises a measuring (deteoting) element an
osoillator to exoite it a vacuum tube oircuit to amplity and deteot
the output variations of the element orientating devices (which keep
the detector element aligned) a potentiometer circuit to compensate
or buckoutlt large changes of field and a recorder
The eqUipment was designed for use in a moving airoraft to
provide a continuous and accurate record of variations of the Earths lt
total magnetic field intensity Beoause an aircratt does not
accurately maintain its orientation in space provision has been made
to hold the measuring or deteoting element in a fixed orientation
(parallel) with respect to the Earths total field
The Earths magnetic field itself is used as a reference the
detector element being aligned with its axis ot sensitivitr parallel
to this field This arrangement places the detector element in the
most favourable position and errors due to improper orientation are
at a minimum Two simUar sets of deteotor elements are used to
sense and seek the position of zero (null) field I
When the axis of sensitivity ot the detector element is aligned
parallel to the Earth t s magnetic field 8DT error in alignment results
in a decrease of the total field reading the magnitude of the error
is proportional to the siDe of the angle ot misorientation and is of
the order ot 05 gamma tor at degree misalignment in a total magnetic
field of 55000 gamma
-8shy
The value of the step (automatic reset procedure) when
variations of magnetic field exceed the tull scale deflection
(ie 600 gamma) was 500 gamma
III 2 MAGNETOMETER CALIBRATIONS
The instrument was calibrated prior to the survey using
standard Helmholtz coils whose radius and coil constant provided
a force of 10 gamma per 1 milliampere of applied current
(a) Lag Test
Lag or delay of response considered in relation to the ground
position of the aircraft is due to a combination of the following
i) Delay due to electrical resistance in the circuitry
ii) Delay due to meChanical transference ot received signal
to the recorder Chart
iii) Delay due to difference in position between the 35mm film
recording camera and the detector head
To establish this lag it is necessary that the survey aircraft
fly over an easily identifiable magnetic body on a reciprocal course
eg a ship a large metal pipeline an iron bridge etc which will
give a well-defined sharp anomaly Then by identifying the centre
of the disturbing body on the 35mm tracking tilm and plotting its
position on the Chart record the difference between this point
(average of the reciprocal headings) and the peak of the magnetic
anomaly is the total lag for the installation tested
The lag test for this survey was flown owr the Sydney Harbour
Bridge immediately prior to the aircrafts departure
-9shy
The installation used in this vurvey was checked as described
and no readable lag was detected The side fiducial index pen is
always aligned with the main recording pen
(b) Heading Effect Test
When using the H~son aircraft which towed the magnetometer
there was no heading effect
In the case of DC bull3 aircraft VH-AGU the detector head (measuring
fluxgate~and their respective servo-motors were installed in an
shy extension to the tail section of the survey aircraft
With this type of installation proper compensation for the
asypunetrical distribution of the aircraft IS permanently magnetised
material and the induced magnetic field effects caused by the aircraft
cutting magnetic lines of force on the detecting fluxgate must be made
Compensation for the aircraftls permanent magnetic material m~
be achieved in two ways one by the use of permanently magnetised bar
magnets or two by using induction (air core) coils
Induction coils with variable magnetic intensities controlled
directly by the magnetometer operator are preferable to bar magnets
in that they allow a trial and error compensation pr~cedure to be
carried out whilst airborne whereas in the case of bar magnets fmiddot
adjustments have to be made on the ground
The coil system of compensation was used on this survey tor the
DCbull3 compensation
Variations of induced magnetic fields caused by the aircraft
cutting linea 0 magnetic force on different headings was compensated
- 10 shy
for by the use of high permeabUity strips of metal (IIPermelloyR)
attached to the side af the magnetometer detector head housing
A selected magneticallr quiet area near Tennant Creek was used
for final compensation and a table showing residual heading errors
is included (plate 9) with this report
(c) Repeatability TesectN
I t is required to show that a traverse when poundlown in opposite
directions gives rise to identical (but laterallr reversed)
middot profUes
This test was carried out several times during the survey and
particularly in order to establish repeatabUity between the
magnetometers of VH-AGE and VH~U
III 3 32mm POSITIONING CAMERA
The instrument used to record the position of the aircraft in
relation to the ground was a single frame 35mm c~era using 400 foot
capacity film magazines details of which are as follows
Type Vinten ~5mm Geological Survey Camera
Focal Length 28mm (110 inches)
Shutter Speed l250th sec (set)
Diaphragm Range I f2 - pound32
Format 18mm x 25mm
The camera was mounted in the aircraft with its optical axis
vertical for straight and level poundlight
Exposures were made automaticallr using an electronically
controlled intervalometer set at 30 second intervals With the
exposure interval so set each 35mm frame is overlapped by
approximately 25 - 30 thus ensuring complete ground coverage
-11shy
The camera exposures are related to the magnetic field record
by the use of a fiducial pen on the recorder which operates
simultaneously with the Camera Veeder Counter at every tenth
eleventh and twelfth exposure These fiducials appear on the
right hand side of the magnetometer reqord
Du Pont type 936 Superior Two 35mm fUm rated at 160 ASA was
used throughout the survey
shyAutomatic Film Laboratories Moore Park Sydney processed
the film
III 4 RADIO ALTIMETER
Apart from the pressure (barometric) altimeter which is
standard equipment in all aircraft a radio altimeter type APN-l
was used continuously to record terrain clearance
The instrument was set on IIhigh rangell (0-4000 ft) throughout
the survey and was checked at intervals over the base aerodrome
An Esterline Angus recording potentiometer was used in
conjunction with this instrument to obtain a continuous profile on
a five inch wide curvilinear chart recording at a chart speed of
It inches per minute
III 5 STORM MONITOR (Ground Magnetometer)
During aeromagnetic surveys it is essential that ~ time
variation of the Earths total magnetic field is monitored and
recorded throughout the survey on a 24 hour d~ basis
The normal diurnal change of the Earth I s field which occurs
daily is normally of low gradient and cyclic and is compensated
for in the standard flight linetie line control pattern
_~_______________L______ ___~_)
-12shy
However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
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PLATE 3 SPECIMEN MAGNETOMETER RECORD
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PLATE 5 SPECI EN STORM MONITOR RECORD
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-3shy
II 2 PROGRAMME DETAILS
(a) Survey Altitude
The survey was poundlown at a constant barometric altitude of
2250 feet above mean sea level (amsl) by reference to a
standard aircraft type pressure altimeter with frequent comparisons
against an APN-l radio altimeter
Average terrain clearance was 1250 feet
(b) Flight Lines Flown
One hundred and seventy three (173) poundlight lines were poundlown
the average spacing being two (2) miles Tie lines were flown
perpendicular to the flight lines as sho~ in the flight line shy
tie line diagram (see Plate 2)
Individual Line Mileages
The individual mileage flown for each tie and flight line is
listed below
Line Miles Line Miles Line Miles
2 25 6 51 10 79
3 26 7 52 11 112
4 30 8 79 12 110
5 50 9 79 13 108
14 121 40 104 66 123
15 120 41 103 67 123
16 117 42 103 68 122
17 96 43 59 69 121
18 39 44 103 70 75
19 131 45 10l 71 75
~ lt
-4shy
Line Miles Line Miles Line Miles
20 53 46 82 72 74
21 120 47 101 73 73
22 118 48 101 74 73
23 102 49 101 75 72
24 70 50 150 76 72
25 70 51 149 77 71
26 109 52 149 78 71
27 109 53 148 79 70
28 85 54 99 80 69
29 61 55 99 81 69
30 60 56 9~ 82 68
31 107 57 98 83 67
32 107 58 98 84 67
33 48 59 97 85 66
34 60 60 97 86 ~
66
35 106 61 95 87 65
36 156 62 92 88 55
37 155 63 91 89 55
38 110 64 89 90 55
39 23 65 86 91 55
92 55 117 46 l42 63
93 53 118 116 l43 63
94 53 119 116 144 109
95 53 120 116 145 109
96 52 121 116 l46 110
97 52 122 101 147 112
- 5 shy
Line Miles Line Miles Line Miles
98 51 123 101 148 64
99 51 124 101 149 63
100 51 125 101 150 63
101 51 126 44 151 64
102 51 127 48 152 65
middot103 50 128 50 153 18
104 50 129 51 154 17
105 50 130 51 155 15
106 49 131 61 156 12
107 49 132 61 157 35
108 49 133 61 158 35
109 48 134 61 159 35
llO 48 135 62 160 35
III 47 136 63 161 35
ll2 47 137 61 162 35
ll3 47 138 62 163 35
114 47 139 63 164 35
ll5 47 un 63 165 35
ll6 46 l4l 63 166 35
167 35 II 80 E 3g-f
168 35 III 8J F 369
169 35 IV 57 G 206
170 35 V 57 H 356
171 35 I 150
172 35 A 76 J 34
173 35 B 76 K 6
0 44 L 6
~ 1
middot1
-----------~-~--~--~ ---~-~---- --~-~-- --~~~-
-6shy
Line Miles Line Miles Line Miles
I 79 D 250 M 6
N 6
0 56
P 56
Q 56
Total Flight Line Miles 12903
1 IITie 2138
Grand Total l50u
(c) Storm Monitor
The storm monitor vas installed at Tennant Creek Aerodrome
NT and was run continuously 24 houra per day throughout the
following periods
17th August to
24th October to
22nd January to
25th May to
24th September 1966
20th December 1966shy
2nd May 1967
8th June 1967
III METHODS AND INSTRUMENTS USED FOR THE SURVEY
III 1 AIRBORNE (TarAL FORCE) MAGNETOMampTER
The instrument used for this survey was a Gulf Mark III
total force saturable core fluxgate magnetometer manufactured by
the Gulf Research and Development Company Pittsburgh PennsylVania
uSA
The airborne magnetometer is intended primarily for measuring
and recording the Earth 1 s total magnetio field intensity and in
-7shy
particular looal variations of intensitr such as Dl8yen be oaused by
geologioal inhomogeneities
The equipment oomprises a measuring (deteoting) element an
osoillator to exoite it a vacuum tube oircuit to amplity and deteot
the output variations of the element orientating devices (which keep
the detector element aligned) a potentiometer circuit to compensate
or buckoutlt large changes of field and a recorder
The eqUipment was designed for use in a moving airoraft to
provide a continuous and accurate record of variations of the Earths lt
total magnetic field intensity Beoause an aircratt does not
accurately maintain its orientation in space provision has been made
to hold the measuring or deteoting element in a fixed orientation
(parallel) with respect to the Earths total field
The Earths magnetic field itself is used as a reference the
detector element being aligned with its axis ot sensitivitr parallel
to this field This arrangement places the detector element in the
most favourable position and errors due to improper orientation are
at a minimum Two simUar sets of deteotor elements are used to
sense and seek the position of zero (null) field I
When the axis of sensitivity ot the detector element is aligned
parallel to the Earth t s magnetic field 8DT error in alignment results
in a decrease of the total field reading the magnitude of the error
is proportional to the siDe of the angle ot misorientation and is of
the order ot 05 gamma tor at degree misalignment in a total magnetic
field of 55000 gamma
-8shy
The value of the step (automatic reset procedure) when
variations of magnetic field exceed the tull scale deflection
(ie 600 gamma) was 500 gamma
III 2 MAGNETOMETER CALIBRATIONS
The instrument was calibrated prior to the survey using
standard Helmholtz coils whose radius and coil constant provided
a force of 10 gamma per 1 milliampere of applied current
(a) Lag Test
Lag or delay of response considered in relation to the ground
position of the aircraft is due to a combination of the following
i) Delay due to electrical resistance in the circuitry
ii) Delay due to meChanical transference ot received signal
to the recorder Chart
iii) Delay due to difference in position between the 35mm film
recording camera and the detector head
To establish this lag it is necessary that the survey aircraft
fly over an easily identifiable magnetic body on a reciprocal course
eg a ship a large metal pipeline an iron bridge etc which will
give a well-defined sharp anomaly Then by identifying the centre
of the disturbing body on the 35mm tracking tilm and plotting its
position on the Chart record the difference between this point
(average of the reciprocal headings) and the peak of the magnetic
anomaly is the total lag for the installation tested
The lag test for this survey was flown owr the Sydney Harbour
Bridge immediately prior to the aircrafts departure
-9shy
The installation used in this vurvey was checked as described
and no readable lag was detected The side fiducial index pen is
always aligned with the main recording pen
(b) Heading Effect Test
When using the H~son aircraft which towed the magnetometer
there was no heading effect
In the case of DC bull3 aircraft VH-AGU the detector head (measuring
fluxgate~and their respective servo-motors were installed in an
shy extension to the tail section of the survey aircraft
With this type of installation proper compensation for the
asypunetrical distribution of the aircraft IS permanently magnetised
material and the induced magnetic field effects caused by the aircraft
cutting magnetic lines of force on the detecting fluxgate must be made
Compensation for the aircraftls permanent magnetic material m~
be achieved in two ways one by the use of permanently magnetised bar
magnets or two by using induction (air core) coils
Induction coils with variable magnetic intensities controlled
directly by the magnetometer operator are preferable to bar magnets
in that they allow a trial and error compensation pr~cedure to be
carried out whilst airborne whereas in the case of bar magnets fmiddot
adjustments have to be made on the ground
The coil system of compensation was used on this survey tor the
DCbull3 compensation
Variations of induced magnetic fields caused by the aircraft
cutting linea 0 magnetic force on different headings was compensated
- 10 shy
for by the use of high permeabUity strips of metal (IIPermelloyR)
attached to the side af the magnetometer detector head housing
A selected magneticallr quiet area near Tennant Creek was used
for final compensation and a table showing residual heading errors
is included (plate 9) with this report
(c) Repeatability TesectN
I t is required to show that a traverse when poundlown in opposite
directions gives rise to identical (but laterallr reversed)
middot profUes
This test was carried out several times during the survey and
particularly in order to establish repeatabUity between the
magnetometers of VH-AGE and VH~U
III 3 32mm POSITIONING CAMERA
The instrument used to record the position of the aircraft in
relation to the ground was a single frame 35mm c~era using 400 foot
capacity film magazines details of which are as follows
Type Vinten ~5mm Geological Survey Camera
Focal Length 28mm (110 inches)
Shutter Speed l250th sec (set)
Diaphragm Range I f2 - pound32
Format 18mm x 25mm
The camera was mounted in the aircraft with its optical axis
vertical for straight and level poundlight
Exposures were made automaticallr using an electronically
controlled intervalometer set at 30 second intervals With the
exposure interval so set each 35mm frame is overlapped by
approximately 25 - 30 thus ensuring complete ground coverage
-11shy
The camera exposures are related to the magnetic field record
by the use of a fiducial pen on the recorder which operates
simultaneously with the Camera Veeder Counter at every tenth
eleventh and twelfth exposure These fiducials appear on the
right hand side of the magnetometer reqord
Du Pont type 936 Superior Two 35mm fUm rated at 160 ASA was
used throughout the survey
shyAutomatic Film Laboratories Moore Park Sydney processed
the film
III 4 RADIO ALTIMETER
Apart from the pressure (barometric) altimeter which is
standard equipment in all aircraft a radio altimeter type APN-l
was used continuously to record terrain clearance
The instrument was set on IIhigh rangell (0-4000 ft) throughout
the survey and was checked at intervals over the base aerodrome
An Esterline Angus recording potentiometer was used in
conjunction with this instrument to obtain a continuous profile on
a five inch wide curvilinear chart recording at a chart speed of
It inches per minute
III 5 STORM MONITOR (Ground Magnetometer)
During aeromagnetic surveys it is essential that ~ time
variation of the Earths total magnetic field is monitored and
recorded throughout the survey on a 24 hour d~ basis
The normal diurnal change of the Earth I s field which occurs
daily is normally of low gradient and cyclic and is compensated
for in the standard flight linetie line control pattern
_~_______________L______ ___~_)
-12shy
However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
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c D
E
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+-
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ri 0
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No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-4shy
Line Miles Line Miles Line Miles
20 53 46 82 72 74
21 120 47 101 73 73
22 118 48 101 74 73
23 102 49 101 75 72
24 70 50 150 76 72
25 70 51 149 77 71
26 109 52 149 78 71
27 109 53 148 79 70
28 85 54 99 80 69
29 61 55 99 81 69
30 60 56 9~ 82 68
31 107 57 98 83 67
32 107 58 98 84 67
33 48 59 97 85 66
34 60 60 97 86 ~
66
35 106 61 95 87 65
36 156 62 92 88 55
37 155 63 91 89 55
38 110 64 89 90 55
39 23 65 86 91 55
92 55 117 46 l42 63
93 53 118 116 l43 63
94 53 119 116 144 109
95 53 120 116 145 109
96 52 121 116 l46 110
97 52 122 101 147 112
- 5 shy
Line Miles Line Miles Line Miles
98 51 123 101 148 64
99 51 124 101 149 63
100 51 125 101 150 63
101 51 126 44 151 64
102 51 127 48 152 65
middot103 50 128 50 153 18
104 50 129 51 154 17
105 50 130 51 155 15
106 49 131 61 156 12
107 49 132 61 157 35
108 49 133 61 158 35
109 48 134 61 159 35
llO 48 135 62 160 35
III 47 136 63 161 35
ll2 47 137 61 162 35
ll3 47 138 62 163 35
114 47 139 63 164 35
ll5 47 un 63 165 35
ll6 46 l4l 63 166 35
167 35 II 80 E 3g-f
168 35 III 8J F 369
169 35 IV 57 G 206
170 35 V 57 H 356
171 35 I 150
172 35 A 76 J 34
173 35 B 76 K 6
0 44 L 6
~ 1
middot1
-----------~-~--~--~ ---~-~---- --~-~-- --~~~-
-6shy
Line Miles Line Miles Line Miles
I 79 D 250 M 6
N 6
0 56
P 56
Q 56
Total Flight Line Miles 12903
1 IITie 2138
Grand Total l50u
(c) Storm Monitor
The storm monitor vas installed at Tennant Creek Aerodrome
NT and was run continuously 24 houra per day throughout the
following periods
17th August to
24th October to
22nd January to
25th May to
24th September 1966
20th December 1966shy
2nd May 1967
8th June 1967
III METHODS AND INSTRUMENTS USED FOR THE SURVEY
III 1 AIRBORNE (TarAL FORCE) MAGNETOMampTER
The instrument used for this survey was a Gulf Mark III
total force saturable core fluxgate magnetometer manufactured by
the Gulf Research and Development Company Pittsburgh PennsylVania
uSA
The airborne magnetometer is intended primarily for measuring
and recording the Earth 1 s total magnetio field intensity and in
-7shy
particular looal variations of intensitr such as Dl8yen be oaused by
geologioal inhomogeneities
The equipment oomprises a measuring (deteoting) element an
osoillator to exoite it a vacuum tube oircuit to amplity and deteot
the output variations of the element orientating devices (which keep
the detector element aligned) a potentiometer circuit to compensate
or buckoutlt large changes of field and a recorder
The eqUipment was designed for use in a moving airoraft to
provide a continuous and accurate record of variations of the Earths lt
total magnetic field intensity Beoause an aircratt does not
accurately maintain its orientation in space provision has been made
to hold the measuring or deteoting element in a fixed orientation
(parallel) with respect to the Earths total field
The Earths magnetic field itself is used as a reference the
detector element being aligned with its axis ot sensitivitr parallel
to this field This arrangement places the detector element in the
most favourable position and errors due to improper orientation are
at a minimum Two simUar sets of deteotor elements are used to
sense and seek the position of zero (null) field I
When the axis of sensitivity ot the detector element is aligned
parallel to the Earth t s magnetic field 8DT error in alignment results
in a decrease of the total field reading the magnitude of the error
is proportional to the siDe of the angle ot misorientation and is of
the order ot 05 gamma tor at degree misalignment in a total magnetic
field of 55000 gamma
-8shy
The value of the step (automatic reset procedure) when
variations of magnetic field exceed the tull scale deflection
(ie 600 gamma) was 500 gamma
III 2 MAGNETOMETER CALIBRATIONS
The instrument was calibrated prior to the survey using
standard Helmholtz coils whose radius and coil constant provided
a force of 10 gamma per 1 milliampere of applied current
(a) Lag Test
Lag or delay of response considered in relation to the ground
position of the aircraft is due to a combination of the following
i) Delay due to electrical resistance in the circuitry
ii) Delay due to meChanical transference ot received signal
to the recorder Chart
iii) Delay due to difference in position between the 35mm film
recording camera and the detector head
To establish this lag it is necessary that the survey aircraft
fly over an easily identifiable magnetic body on a reciprocal course
eg a ship a large metal pipeline an iron bridge etc which will
give a well-defined sharp anomaly Then by identifying the centre
of the disturbing body on the 35mm tracking tilm and plotting its
position on the Chart record the difference between this point
(average of the reciprocal headings) and the peak of the magnetic
anomaly is the total lag for the installation tested
The lag test for this survey was flown owr the Sydney Harbour
Bridge immediately prior to the aircrafts departure
-9shy
The installation used in this vurvey was checked as described
and no readable lag was detected The side fiducial index pen is
always aligned with the main recording pen
(b) Heading Effect Test
When using the H~son aircraft which towed the magnetometer
there was no heading effect
In the case of DC bull3 aircraft VH-AGU the detector head (measuring
fluxgate~and their respective servo-motors were installed in an
shy extension to the tail section of the survey aircraft
With this type of installation proper compensation for the
asypunetrical distribution of the aircraft IS permanently magnetised
material and the induced magnetic field effects caused by the aircraft
cutting magnetic lines of force on the detecting fluxgate must be made
Compensation for the aircraftls permanent magnetic material m~
be achieved in two ways one by the use of permanently magnetised bar
magnets or two by using induction (air core) coils
Induction coils with variable magnetic intensities controlled
directly by the magnetometer operator are preferable to bar magnets
in that they allow a trial and error compensation pr~cedure to be
carried out whilst airborne whereas in the case of bar magnets fmiddot
adjustments have to be made on the ground
The coil system of compensation was used on this survey tor the
DCbull3 compensation
Variations of induced magnetic fields caused by the aircraft
cutting linea 0 magnetic force on different headings was compensated
- 10 shy
for by the use of high permeabUity strips of metal (IIPermelloyR)
attached to the side af the magnetometer detector head housing
A selected magneticallr quiet area near Tennant Creek was used
for final compensation and a table showing residual heading errors
is included (plate 9) with this report
(c) Repeatability TesectN
I t is required to show that a traverse when poundlown in opposite
directions gives rise to identical (but laterallr reversed)
middot profUes
This test was carried out several times during the survey and
particularly in order to establish repeatabUity between the
magnetometers of VH-AGE and VH~U
III 3 32mm POSITIONING CAMERA
The instrument used to record the position of the aircraft in
relation to the ground was a single frame 35mm c~era using 400 foot
capacity film magazines details of which are as follows
Type Vinten ~5mm Geological Survey Camera
Focal Length 28mm (110 inches)
Shutter Speed l250th sec (set)
Diaphragm Range I f2 - pound32
Format 18mm x 25mm
The camera was mounted in the aircraft with its optical axis
vertical for straight and level poundlight
Exposures were made automaticallr using an electronically
controlled intervalometer set at 30 second intervals With the
exposure interval so set each 35mm frame is overlapped by
approximately 25 - 30 thus ensuring complete ground coverage
-11shy
The camera exposures are related to the magnetic field record
by the use of a fiducial pen on the recorder which operates
simultaneously with the Camera Veeder Counter at every tenth
eleventh and twelfth exposure These fiducials appear on the
right hand side of the magnetometer reqord
Du Pont type 936 Superior Two 35mm fUm rated at 160 ASA was
used throughout the survey
shyAutomatic Film Laboratories Moore Park Sydney processed
the film
III 4 RADIO ALTIMETER
Apart from the pressure (barometric) altimeter which is
standard equipment in all aircraft a radio altimeter type APN-l
was used continuously to record terrain clearance
The instrument was set on IIhigh rangell (0-4000 ft) throughout
the survey and was checked at intervals over the base aerodrome
An Esterline Angus recording potentiometer was used in
conjunction with this instrument to obtain a continuous profile on
a five inch wide curvilinear chart recording at a chart speed of
It inches per minute
III 5 STORM MONITOR (Ground Magnetometer)
During aeromagnetic surveys it is essential that ~ time
variation of the Earths total magnetic field is monitored and
recorded throughout the survey on a 24 hour d~ basis
The normal diurnal change of the Earth I s field which occurs
daily is normally of low gradient and cyclic and is compensated
for in the standard flight linetie line control pattern
_~_______________L______ ___~_)
-12shy
However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
- ll
Z
Q)
w
0 ~ --shy
U
W
0 ()
~
-II
D~
06
- I
-
e ~l
ri 0
~ shy
No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
- 5 shy
Line Miles Line Miles Line Miles
98 51 123 101 148 64
99 51 124 101 149 63
100 51 125 101 150 63
101 51 126 44 151 64
102 51 127 48 152 65
middot103 50 128 50 153 18
104 50 129 51 154 17
105 50 130 51 155 15
106 49 131 61 156 12
107 49 132 61 157 35
108 49 133 61 158 35
109 48 134 61 159 35
llO 48 135 62 160 35
III 47 136 63 161 35
ll2 47 137 61 162 35
ll3 47 138 62 163 35
114 47 139 63 164 35
ll5 47 un 63 165 35
ll6 46 l4l 63 166 35
167 35 II 80 E 3g-f
168 35 III 8J F 369
169 35 IV 57 G 206
170 35 V 57 H 356
171 35 I 150
172 35 A 76 J 34
173 35 B 76 K 6
0 44 L 6
~ 1
middot1
-----------~-~--~--~ ---~-~---- --~-~-- --~~~-
-6shy
Line Miles Line Miles Line Miles
I 79 D 250 M 6
N 6
0 56
P 56
Q 56
Total Flight Line Miles 12903
1 IITie 2138
Grand Total l50u
(c) Storm Monitor
The storm monitor vas installed at Tennant Creek Aerodrome
NT and was run continuously 24 houra per day throughout the
following periods
17th August to
24th October to
22nd January to
25th May to
24th September 1966
20th December 1966shy
2nd May 1967
8th June 1967
III METHODS AND INSTRUMENTS USED FOR THE SURVEY
III 1 AIRBORNE (TarAL FORCE) MAGNETOMampTER
The instrument used for this survey was a Gulf Mark III
total force saturable core fluxgate magnetometer manufactured by
the Gulf Research and Development Company Pittsburgh PennsylVania
uSA
The airborne magnetometer is intended primarily for measuring
and recording the Earth 1 s total magnetio field intensity and in
-7shy
particular looal variations of intensitr such as Dl8yen be oaused by
geologioal inhomogeneities
The equipment oomprises a measuring (deteoting) element an
osoillator to exoite it a vacuum tube oircuit to amplity and deteot
the output variations of the element orientating devices (which keep
the detector element aligned) a potentiometer circuit to compensate
or buckoutlt large changes of field and a recorder
The eqUipment was designed for use in a moving airoraft to
provide a continuous and accurate record of variations of the Earths lt
total magnetic field intensity Beoause an aircratt does not
accurately maintain its orientation in space provision has been made
to hold the measuring or deteoting element in a fixed orientation
(parallel) with respect to the Earths total field
The Earths magnetic field itself is used as a reference the
detector element being aligned with its axis ot sensitivitr parallel
to this field This arrangement places the detector element in the
most favourable position and errors due to improper orientation are
at a minimum Two simUar sets of deteotor elements are used to
sense and seek the position of zero (null) field I
When the axis of sensitivity ot the detector element is aligned
parallel to the Earth t s magnetic field 8DT error in alignment results
in a decrease of the total field reading the magnitude of the error
is proportional to the siDe of the angle ot misorientation and is of
the order ot 05 gamma tor at degree misalignment in a total magnetic
field of 55000 gamma
-8shy
The value of the step (automatic reset procedure) when
variations of magnetic field exceed the tull scale deflection
(ie 600 gamma) was 500 gamma
III 2 MAGNETOMETER CALIBRATIONS
The instrument was calibrated prior to the survey using
standard Helmholtz coils whose radius and coil constant provided
a force of 10 gamma per 1 milliampere of applied current
(a) Lag Test
Lag or delay of response considered in relation to the ground
position of the aircraft is due to a combination of the following
i) Delay due to electrical resistance in the circuitry
ii) Delay due to meChanical transference ot received signal
to the recorder Chart
iii) Delay due to difference in position between the 35mm film
recording camera and the detector head
To establish this lag it is necessary that the survey aircraft
fly over an easily identifiable magnetic body on a reciprocal course
eg a ship a large metal pipeline an iron bridge etc which will
give a well-defined sharp anomaly Then by identifying the centre
of the disturbing body on the 35mm tracking tilm and plotting its
position on the Chart record the difference between this point
(average of the reciprocal headings) and the peak of the magnetic
anomaly is the total lag for the installation tested
The lag test for this survey was flown owr the Sydney Harbour
Bridge immediately prior to the aircrafts departure
-9shy
The installation used in this vurvey was checked as described
and no readable lag was detected The side fiducial index pen is
always aligned with the main recording pen
(b) Heading Effect Test
When using the H~son aircraft which towed the magnetometer
there was no heading effect
In the case of DC bull3 aircraft VH-AGU the detector head (measuring
fluxgate~and their respective servo-motors were installed in an
shy extension to the tail section of the survey aircraft
With this type of installation proper compensation for the
asypunetrical distribution of the aircraft IS permanently magnetised
material and the induced magnetic field effects caused by the aircraft
cutting magnetic lines of force on the detecting fluxgate must be made
Compensation for the aircraftls permanent magnetic material m~
be achieved in two ways one by the use of permanently magnetised bar
magnets or two by using induction (air core) coils
Induction coils with variable magnetic intensities controlled
directly by the magnetometer operator are preferable to bar magnets
in that they allow a trial and error compensation pr~cedure to be
carried out whilst airborne whereas in the case of bar magnets fmiddot
adjustments have to be made on the ground
The coil system of compensation was used on this survey tor the
DCbull3 compensation
Variations of induced magnetic fields caused by the aircraft
cutting linea 0 magnetic force on different headings was compensated
- 10 shy
for by the use of high permeabUity strips of metal (IIPermelloyR)
attached to the side af the magnetometer detector head housing
A selected magneticallr quiet area near Tennant Creek was used
for final compensation and a table showing residual heading errors
is included (plate 9) with this report
(c) Repeatability TesectN
I t is required to show that a traverse when poundlown in opposite
directions gives rise to identical (but laterallr reversed)
middot profUes
This test was carried out several times during the survey and
particularly in order to establish repeatabUity between the
magnetometers of VH-AGE and VH~U
III 3 32mm POSITIONING CAMERA
The instrument used to record the position of the aircraft in
relation to the ground was a single frame 35mm c~era using 400 foot
capacity film magazines details of which are as follows
Type Vinten ~5mm Geological Survey Camera
Focal Length 28mm (110 inches)
Shutter Speed l250th sec (set)
Diaphragm Range I f2 - pound32
Format 18mm x 25mm
The camera was mounted in the aircraft with its optical axis
vertical for straight and level poundlight
Exposures were made automaticallr using an electronically
controlled intervalometer set at 30 second intervals With the
exposure interval so set each 35mm frame is overlapped by
approximately 25 - 30 thus ensuring complete ground coverage
-11shy
The camera exposures are related to the magnetic field record
by the use of a fiducial pen on the recorder which operates
simultaneously with the Camera Veeder Counter at every tenth
eleventh and twelfth exposure These fiducials appear on the
right hand side of the magnetometer reqord
Du Pont type 936 Superior Two 35mm fUm rated at 160 ASA was
used throughout the survey
shyAutomatic Film Laboratories Moore Park Sydney processed
the film
III 4 RADIO ALTIMETER
Apart from the pressure (barometric) altimeter which is
standard equipment in all aircraft a radio altimeter type APN-l
was used continuously to record terrain clearance
The instrument was set on IIhigh rangell (0-4000 ft) throughout
the survey and was checked at intervals over the base aerodrome
An Esterline Angus recording potentiometer was used in
conjunction with this instrument to obtain a continuous profile on
a five inch wide curvilinear chart recording at a chart speed of
It inches per minute
III 5 STORM MONITOR (Ground Magnetometer)
During aeromagnetic surveys it is essential that ~ time
variation of the Earths total magnetic field is monitored and
recorded throughout the survey on a 24 hour d~ basis
The normal diurnal change of the Earth I s field which occurs
daily is normally of low gradient and cyclic and is compensated
for in the standard flight linetie line control pattern
_~_______________L______ ___~_)
-12shy
However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
- ll
Z
Q)
w
0 ~ --shy
U
W
0 ()
~
-II
D~
06
- I
-
e ~l
ri 0
~ shy
No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-6shy
Line Miles Line Miles Line Miles
I 79 D 250 M 6
N 6
0 56
P 56
Q 56
Total Flight Line Miles 12903
1 IITie 2138
Grand Total l50u
(c) Storm Monitor
The storm monitor vas installed at Tennant Creek Aerodrome
NT and was run continuously 24 houra per day throughout the
following periods
17th August to
24th October to
22nd January to
25th May to
24th September 1966
20th December 1966shy
2nd May 1967
8th June 1967
III METHODS AND INSTRUMENTS USED FOR THE SURVEY
III 1 AIRBORNE (TarAL FORCE) MAGNETOMampTER
The instrument used for this survey was a Gulf Mark III
total force saturable core fluxgate magnetometer manufactured by
the Gulf Research and Development Company Pittsburgh PennsylVania
uSA
The airborne magnetometer is intended primarily for measuring
and recording the Earth 1 s total magnetio field intensity and in
-7shy
particular looal variations of intensitr such as Dl8yen be oaused by
geologioal inhomogeneities
The equipment oomprises a measuring (deteoting) element an
osoillator to exoite it a vacuum tube oircuit to amplity and deteot
the output variations of the element orientating devices (which keep
the detector element aligned) a potentiometer circuit to compensate
or buckoutlt large changes of field and a recorder
The eqUipment was designed for use in a moving airoraft to
provide a continuous and accurate record of variations of the Earths lt
total magnetic field intensity Beoause an aircratt does not
accurately maintain its orientation in space provision has been made
to hold the measuring or deteoting element in a fixed orientation
(parallel) with respect to the Earths total field
The Earths magnetic field itself is used as a reference the
detector element being aligned with its axis ot sensitivitr parallel
to this field This arrangement places the detector element in the
most favourable position and errors due to improper orientation are
at a minimum Two simUar sets of deteotor elements are used to
sense and seek the position of zero (null) field I
When the axis of sensitivity ot the detector element is aligned
parallel to the Earth t s magnetic field 8DT error in alignment results
in a decrease of the total field reading the magnitude of the error
is proportional to the siDe of the angle ot misorientation and is of
the order ot 05 gamma tor at degree misalignment in a total magnetic
field of 55000 gamma
-8shy
The value of the step (automatic reset procedure) when
variations of magnetic field exceed the tull scale deflection
(ie 600 gamma) was 500 gamma
III 2 MAGNETOMETER CALIBRATIONS
The instrument was calibrated prior to the survey using
standard Helmholtz coils whose radius and coil constant provided
a force of 10 gamma per 1 milliampere of applied current
(a) Lag Test
Lag or delay of response considered in relation to the ground
position of the aircraft is due to a combination of the following
i) Delay due to electrical resistance in the circuitry
ii) Delay due to meChanical transference ot received signal
to the recorder Chart
iii) Delay due to difference in position between the 35mm film
recording camera and the detector head
To establish this lag it is necessary that the survey aircraft
fly over an easily identifiable magnetic body on a reciprocal course
eg a ship a large metal pipeline an iron bridge etc which will
give a well-defined sharp anomaly Then by identifying the centre
of the disturbing body on the 35mm tracking tilm and plotting its
position on the Chart record the difference between this point
(average of the reciprocal headings) and the peak of the magnetic
anomaly is the total lag for the installation tested
The lag test for this survey was flown owr the Sydney Harbour
Bridge immediately prior to the aircrafts departure
-9shy
The installation used in this vurvey was checked as described
and no readable lag was detected The side fiducial index pen is
always aligned with the main recording pen
(b) Heading Effect Test
When using the H~son aircraft which towed the magnetometer
there was no heading effect
In the case of DC bull3 aircraft VH-AGU the detector head (measuring
fluxgate~and their respective servo-motors were installed in an
shy extension to the tail section of the survey aircraft
With this type of installation proper compensation for the
asypunetrical distribution of the aircraft IS permanently magnetised
material and the induced magnetic field effects caused by the aircraft
cutting magnetic lines of force on the detecting fluxgate must be made
Compensation for the aircraftls permanent magnetic material m~
be achieved in two ways one by the use of permanently magnetised bar
magnets or two by using induction (air core) coils
Induction coils with variable magnetic intensities controlled
directly by the magnetometer operator are preferable to bar magnets
in that they allow a trial and error compensation pr~cedure to be
carried out whilst airborne whereas in the case of bar magnets fmiddot
adjustments have to be made on the ground
The coil system of compensation was used on this survey tor the
DCbull3 compensation
Variations of induced magnetic fields caused by the aircraft
cutting linea 0 magnetic force on different headings was compensated
- 10 shy
for by the use of high permeabUity strips of metal (IIPermelloyR)
attached to the side af the magnetometer detector head housing
A selected magneticallr quiet area near Tennant Creek was used
for final compensation and a table showing residual heading errors
is included (plate 9) with this report
(c) Repeatability TesectN
I t is required to show that a traverse when poundlown in opposite
directions gives rise to identical (but laterallr reversed)
middot profUes
This test was carried out several times during the survey and
particularly in order to establish repeatabUity between the
magnetometers of VH-AGE and VH~U
III 3 32mm POSITIONING CAMERA
The instrument used to record the position of the aircraft in
relation to the ground was a single frame 35mm c~era using 400 foot
capacity film magazines details of which are as follows
Type Vinten ~5mm Geological Survey Camera
Focal Length 28mm (110 inches)
Shutter Speed l250th sec (set)
Diaphragm Range I f2 - pound32
Format 18mm x 25mm
The camera was mounted in the aircraft with its optical axis
vertical for straight and level poundlight
Exposures were made automaticallr using an electronically
controlled intervalometer set at 30 second intervals With the
exposure interval so set each 35mm frame is overlapped by
approximately 25 - 30 thus ensuring complete ground coverage
-11shy
The camera exposures are related to the magnetic field record
by the use of a fiducial pen on the recorder which operates
simultaneously with the Camera Veeder Counter at every tenth
eleventh and twelfth exposure These fiducials appear on the
right hand side of the magnetometer reqord
Du Pont type 936 Superior Two 35mm fUm rated at 160 ASA was
used throughout the survey
shyAutomatic Film Laboratories Moore Park Sydney processed
the film
III 4 RADIO ALTIMETER
Apart from the pressure (barometric) altimeter which is
standard equipment in all aircraft a radio altimeter type APN-l
was used continuously to record terrain clearance
The instrument was set on IIhigh rangell (0-4000 ft) throughout
the survey and was checked at intervals over the base aerodrome
An Esterline Angus recording potentiometer was used in
conjunction with this instrument to obtain a continuous profile on
a five inch wide curvilinear chart recording at a chart speed of
It inches per minute
III 5 STORM MONITOR (Ground Magnetometer)
During aeromagnetic surveys it is essential that ~ time
variation of the Earths total magnetic field is monitored and
recorded throughout the survey on a 24 hour d~ basis
The normal diurnal change of the Earth I s field which occurs
daily is normally of low gradient and cyclic and is compensated
for in the standard flight linetie line control pattern
_~_______________L______ ___~_)
-12shy
However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
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+ + +0middot AldVld
+ I
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I I I I
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I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
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PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
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--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
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-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
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-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-7shy
particular looal variations of intensitr such as Dl8yen be oaused by
geologioal inhomogeneities
The equipment oomprises a measuring (deteoting) element an
osoillator to exoite it a vacuum tube oircuit to amplity and deteot
the output variations of the element orientating devices (which keep
the detector element aligned) a potentiometer circuit to compensate
or buckoutlt large changes of field and a recorder
The eqUipment was designed for use in a moving airoraft to
provide a continuous and accurate record of variations of the Earths lt
total magnetic field intensity Beoause an aircratt does not
accurately maintain its orientation in space provision has been made
to hold the measuring or deteoting element in a fixed orientation
(parallel) with respect to the Earths total field
The Earths magnetic field itself is used as a reference the
detector element being aligned with its axis ot sensitivitr parallel
to this field This arrangement places the detector element in the
most favourable position and errors due to improper orientation are
at a minimum Two simUar sets of deteotor elements are used to
sense and seek the position of zero (null) field I
When the axis of sensitivity ot the detector element is aligned
parallel to the Earth t s magnetic field 8DT error in alignment results
in a decrease of the total field reading the magnitude of the error
is proportional to the siDe of the angle ot misorientation and is of
the order ot 05 gamma tor at degree misalignment in a total magnetic
field of 55000 gamma
-8shy
The value of the step (automatic reset procedure) when
variations of magnetic field exceed the tull scale deflection
(ie 600 gamma) was 500 gamma
III 2 MAGNETOMETER CALIBRATIONS
The instrument was calibrated prior to the survey using
standard Helmholtz coils whose radius and coil constant provided
a force of 10 gamma per 1 milliampere of applied current
(a) Lag Test
Lag or delay of response considered in relation to the ground
position of the aircraft is due to a combination of the following
i) Delay due to electrical resistance in the circuitry
ii) Delay due to meChanical transference ot received signal
to the recorder Chart
iii) Delay due to difference in position between the 35mm film
recording camera and the detector head
To establish this lag it is necessary that the survey aircraft
fly over an easily identifiable magnetic body on a reciprocal course
eg a ship a large metal pipeline an iron bridge etc which will
give a well-defined sharp anomaly Then by identifying the centre
of the disturbing body on the 35mm tracking tilm and plotting its
position on the Chart record the difference between this point
(average of the reciprocal headings) and the peak of the magnetic
anomaly is the total lag for the installation tested
The lag test for this survey was flown owr the Sydney Harbour
Bridge immediately prior to the aircrafts departure
-9shy
The installation used in this vurvey was checked as described
and no readable lag was detected The side fiducial index pen is
always aligned with the main recording pen
(b) Heading Effect Test
When using the H~son aircraft which towed the magnetometer
there was no heading effect
In the case of DC bull3 aircraft VH-AGU the detector head (measuring
fluxgate~and their respective servo-motors were installed in an
shy extension to the tail section of the survey aircraft
With this type of installation proper compensation for the
asypunetrical distribution of the aircraft IS permanently magnetised
material and the induced magnetic field effects caused by the aircraft
cutting magnetic lines of force on the detecting fluxgate must be made
Compensation for the aircraftls permanent magnetic material m~
be achieved in two ways one by the use of permanently magnetised bar
magnets or two by using induction (air core) coils
Induction coils with variable magnetic intensities controlled
directly by the magnetometer operator are preferable to bar magnets
in that they allow a trial and error compensation pr~cedure to be
carried out whilst airborne whereas in the case of bar magnets fmiddot
adjustments have to be made on the ground
The coil system of compensation was used on this survey tor the
DCbull3 compensation
Variations of induced magnetic fields caused by the aircraft
cutting linea 0 magnetic force on different headings was compensated
- 10 shy
for by the use of high permeabUity strips of metal (IIPermelloyR)
attached to the side af the magnetometer detector head housing
A selected magneticallr quiet area near Tennant Creek was used
for final compensation and a table showing residual heading errors
is included (plate 9) with this report
(c) Repeatability TesectN
I t is required to show that a traverse when poundlown in opposite
directions gives rise to identical (but laterallr reversed)
middot profUes
This test was carried out several times during the survey and
particularly in order to establish repeatabUity between the
magnetometers of VH-AGE and VH~U
III 3 32mm POSITIONING CAMERA
The instrument used to record the position of the aircraft in
relation to the ground was a single frame 35mm c~era using 400 foot
capacity film magazines details of which are as follows
Type Vinten ~5mm Geological Survey Camera
Focal Length 28mm (110 inches)
Shutter Speed l250th sec (set)
Diaphragm Range I f2 - pound32
Format 18mm x 25mm
The camera was mounted in the aircraft with its optical axis
vertical for straight and level poundlight
Exposures were made automaticallr using an electronically
controlled intervalometer set at 30 second intervals With the
exposure interval so set each 35mm frame is overlapped by
approximately 25 - 30 thus ensuring complete ground coverage
-11shy
The camera exposures are related to the magnetic field record
by the use of a fiducial pen on the recorder which operates
simultaneously with the Camera Veeder Counter at every tenth
eleventh and twelfth exposure These fiducials appear on the
right hand side of the magnetometer reqord
Du Pont type 936 Superior Two 35mm fUm rated at 160 ASA was
used throughout the survey
shyAutomatic Film Laboratories Moore Park Sydney processed
the film
III 4 RADIO ALTIMETER
Apart from the pressure (barometric) altimeter which is
standard equipment in all aircraft a radio altimeter type APN-l
was used continuously to record terrain clearance
The instrument was set on IIhigh rangell (0-4000 ft) throughout
the survey and was checked at intervals over the base aerodrome
An Esterline Angus recording potentiometer was used in
conjunction with this instrument to obtain a continuous profile on
a five inch wide curvilinear chart recording at a chart speed of
It inches per minute
III 5 STORM MONITOR (Ground Magnetometer)
During aeromagnetic surveys it is essential that ~ time
variation of the Earths total magnetic field is monitored and
recorded throughout the survey on a 24 hour d~ basis
The normal diurnal change of the Earth I s field which occurs
daily is normally of low gradient and cyclic and is compensated
for in the standard flight linetie line control pattern
_~_______________L______ ___~_)
-12shy
However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
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U
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06
- I
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e ~l
ri 0
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No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-8shy
The value of the step (automatic reset procedure) when
variations of magnetic field exceed the tull scale deflection
(ie 600 gamma) was 500 gamma
III 2 MAGNETOMETER CALIBRATIONS
The instrument was calibrated prior to the survey using
standard Helmholtz coils whose radius and coil constant provided
a force of 10 gamma per 1 milliampere of applied current
(a) Lag Test
Lag or delay of response considered in relation to the ground
position of the aircraft is due to a combination of the following
i) Delay due to electrical resistance in the circuitry
ii) Delay due to meChanical transference ot received signal
to the recorder Chart
iii) Delay due to difference in position between the 35mm film
recording camera and the detector head
To establish this lag it is necessary that the survey aircraft
fly over an easily identifiable magnetic body on a reciprocal course
eg a ship a large metal pipeline an iron bridge etc which will
give a well-defined sharp anomaly Then by identifying the centre
of the disturbing body on the 35mm tracking tilm and plotting its
position on the Chart record the difference between this point
(average of the reciprocal headings) and the peak of the magnetic
anomaly is the total lag for the installation tested
The lag test for this survey was flown owr the Sydney Harbour
Bridge immediately prior to the aircrafts departure
-9shy
The installation used in this vurvey was checked as described
and no readable lag was detected The side fiducial index pen is
always aligned with the main recording pen
(b) Heading Effect Test
When using the H~son aircraft which towed the magnetometer
there was no heading effect
In the case of DC bull3 aircraft VH-AGU the detector head (measuring
fluxgate~and their respective servo-motors were installed in an
shy extension to the tail section of the survey aircraft
With this type of installation proper compensation for the
asypunetrical distribution of the aircraft IS permanently magnetised
material and the induced magnetic field effects caused by the aircraft
cutting magnetic lines of force on the detecting fluxgate must be made
Compensation for the aircraftls permanent magnetic material m~
be achieved in two ways one by the use of permanently magnetised bar
magnets or two by using induction (air core) coils
Induction coils with variable magnetic intensities controlled
directly by the magnetometer operator are preferable to bar magnets
in that they allow a trial and error compensation pr~cedure to be
carried out whilst airborne whereas in the case of bar magnets fmiddot
adjustments have to be made on the ground
The coil system of compensation was used on this survey tor the
DCbull3 compensation
Variations of induced magnetic fields caused by the aircraft
cutting linea 0 magnetic force on different headings was compensated
- 10 shy
for by the use of high permeabUity strips of metal (IIPermelloyR)
attached to the side af the magnetometer detector head housing
A selected magneticallr quiet area near Tennant Creek was used
for final compensation and a table showing residual heading errors
is included (plate 9) with this report
(c) Repeatability TesectN
I t is required to show that a traverse when poundlown in opposite
directions gives rise to identical (but laterallr reversed)
middot profUes
This test was carried out several times during the survey and
particularly in order to establish repeatabUity between the
magnetometers of VH-AGE and VH~U
III 3 32mm POSITIONING CAMERA
The instrument used to record the position of the aircraft in
relation to the ground was a single frame 35mm c~era using 400 foot
capacity film magazines details of which are as follows
Type Vinten ~5mm Geological Survey Camera
Focal Length 28mm (110 inches)
Shutter Speed l250th sec (set)
Diaphragm Range I f2 - pound32
Format 18mm x 25mm
The camera was mounted in the aircraft with its optical axis
vertical for straight and level poundlight
Exposures were made automaticallr using an electronically
controlled intervalometer set at 30 second intervals With the
exposure interval so set each 35mm frame is overlapped by
approximately 25 - 30 thus ensuring complete ground coverage
-11shy
The camera exposures are related to the magnetic field record
by the use of a fiducial pen on the recorder which operates
simultaneously with the Camera Veeder Counter at every tenth
eleventh and twelfth exposure These fiducials appear on the
right hand side of the magnetometer reqord
Du Pont type 936 Superior Two 35mm fUm rated at 160 ASA was
used throughout the survey
shyAutomatic Film Laboratories Moore Park Sydney processed
the film
III 4 RADIO ALTIMETER
Apart from the pressure (barometric) altimeter which is
standard equipment in all aircraft a radio altimeter type APN-l
was used continuously to record terrain clearance
The instrument was set on IIhigh rangell (0-4000 ft) throughout
the survey and was checked at intervals over the base aerodrome
An Esterline Angus recording potentiometer was used in
conjunction with this instrument to obtain a continuous profile on
a five inch wide curvilinear chart recording at a chart speed of
It inches per minute
III 5 STORM MONITOR (Ground Magnetometer)
During aeromagnetic surveys it is essential that ~ time
variation of the Earths total magnetic field is monitored and
recorded throughout the survey on a 24 hour d~ basis
The normal diurnal change of the Earth I s field which occurs
daily is normally of low gradient and cyclic and is compensated
for in the standard flight linetie line control pattern
_~_______________L______ ___~_)
-12shy
However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
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00
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-9shy
The installation used in this vurvey was checked as described
and no readable lag was detected The side fiducial index pen is
always aligned with the main recording pen
(b) Heading Effect Test
When using the H~son aircraft which towed the magnetometer
there was no heading effect
In the case of DC bull3 aircraft VH-AGU the detector head (measuring
fluxgate~and their respective servo-motors were installed in an
shy extension to the tail section of the survey aircraft
With this type of installation proper compensation for the
asypunetrical distribution of the aircraft IS permanently magnetised
material and the induced magnetic field effects caused by the aircraft
cutting magnetic lines of force on the detecting fluxgate must be made
Compensation for the aircraftls permanent magnetic material m~
be achieved in two ways one by the use of permanently magnetised bar
magnets or two by using induction (air core) coils
Induction coils with variable magnetic intensities controlled
directly by the magnetometer operator are preferable to bar magnets
in that they allow a trial and error compensation pr~cedure to be
carried out whilst airborne whereas in the case of bar magnets fmiddot
adjustments have to be made on the ground
The coil system of compensation was used on this survey tor the
DCbull3 compensation
Variations of induced magnetic fields caused by the aircraft
cutting linea 0 magnetic force on different headings was compensated
- 10 shy
for by the use of high permeabUity strips of metal (IIPermelloyR)
attached to the side af the magnetometer detector head housing
A selected magneticallr quiet area near Tennant Creek was used
for final compensation and a table showing residual heading errors
is included (plate 9) with this report
(c) Repeatability TesectN
I t is required to show that a traverse when poundlown in opposite
directions gives rise to identical (but laterallr reversed)
middot profUes
This test was carried out several times during the survey and
particularly in order to establish repeatabUity between the
magnetometers of VH-AGE and VH~U
III 3 32mm POSITIONING CAMERA
The instrument used to record the position of the aircraft in
relation to the ground was a single frame 35mm c~era using 400 foot
capacity film magazines details of which are as follows
Type Vinten ~5mm Geological Survey Camera
Focal Length 28mm (110 inches)
Shutter Speed l250th sec (set)
Diaphragm Range I f2 - pound32
Format 18mm x 25mm
The camera was mounted in the aircraft with its optical axis
vertical for straight and level poundlight
Exposures were made automaticallr using an electronically
controlled intervalometer set at 30 second intervals With the
exposure interval so set each 35mm frame is overlapped by
approximately 25 - 30 thus ensuring complete ground coverage
-11shy
The camera exposures are related to the magnetic field record
by the use of a fiducial pen on the recorder which operates
simultaneously with the Camera Veeder Counter at every tenth
eleventh and twelfth exposure These fiducials appear on the
right hand side of the magnetometer reqord
Du Pont type 936 Superior Two 35mm fUm rated at 160 ASA was
used throughout the survey
shyAutomatic Film Laboratories Moore Park Sydney processed
the film
III 4 RADIO ALTIMETER
Apart from the pressure (barometric) altimeter which is
standard equipment in all aircraft a radio altimeter type APN-l
was used continuously to record terrain clearance
The instrument was set on IIhigh rangell (0-4000 ft) throughout
the survey and was checked at intervals over the base aerodrome
An Esterline Angus recording potentiometer was used in
conjunction with this instrument to obtain a continuous profile on
a five inch wide curvilinear chart recording at a chart speed of
It inches per minute
III 5 STORM MONITOR (Ground Magnetometer)
During aeromagnetic surveys it is essential that ~ time
variation of the Earths total magnetic field is monitored and
recorded throughout the survey on a 24 hour d~ basis
The normal diurnal change of the Earth I s field which occurs
daily is normally of low gradient and cyclic and is compensated
for in the standard flight linetie line control pattern
_~_______________L______ ___~_)
-12shy
However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
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c D
E
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E
+-
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u
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ri 0
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
- 10 shy
for by the use of high permeabUity strips of metal (IIPermelloyR)
attached to the side af the magnetometer detector head housing
A selected magneticallr quiet area near Tennant Creek was used
for final compensation and a table showing residual heading errors
is included (plate 9) with this report
(c) Repeatability TesectN
I t is required to show that a traverse when poundlown in opposite
directions gives rise to identical (but laterallr reversed)
middot profUes
This test was carried out several times during the survey and
particularly in order to establish repeatabUity between the
magnetometers of VH-AGE and VH~U
III 3 32mm POSITIONING CAMERA
The instrument used to record the position of the aircraft in
relation to the ground was a single frame 35mm c~era using 400 foot
capacity film magazines details of which are as follows
Type Vinten ~5mm Geological Survey Camera
Focal Length 28mm (110 inches)
Shutter Speed l250th sec (set)
Diaphragm Range I f2 - pound32
Format 18mm x 25mm
The camera was mounted in the aircraft with its optical axis
vertical for straight and level poundlight
Exposures were made automaticallr using an electronically
controlled intervalometer set at 30 second intervals With the
exposure interval so set each 35mm frame is overlapped by
approximately 25 - 30 thus ensuring complete ground coverage
-11shy
The camera exposures are related to the magnetic field record
by the use of a fiducial pen on the recorder which operates
simultaneously with the Camera Veeder Counter at every tenth
eleventh and twelfth exposure These fiducials appear on the
right hand side of the magnetometer reqord
Du Pont type 936 Superior Two 35mm fUm rated at 160 ASA was
used throughout the survey
shyAutomatic Film Laboratories Moore Park Sydney processed
the film
III 4 RADIO ALTIMETER
Apart from the pressure (barometric) altimeter which is
standard equipment in all aircraft a radio altimeter type APN-l
was used continuously to record terrain clearance
The instrument was set on IIhigh rangell (0-4000 ft) throughout
the survey and was checked at intervals over the base aerodrome
An Esterline Angus recording potentiometer was used in
conjunction with this instrument to obtain a continuous profile on
a five inch wide curvilinear chart recording at a chart speed of
It inches per minute
III 5 STORM MONITOR (Ground Magnetometer)
During aeromagnetic surveys it is essential that ~ time
variation of the Earths total magnetic field is monitored and
recorded throughout the survey on a 24 hour d~ basis
The normal diurnal change of the Earth I s field which occurs
daily is normally of low gradient and cyclic and is compensated
for in the standard flight linetie line control pattern
_~_______________L______ ___~_)
-12shy
However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
- ll
Z
Q)
w
0 ~ --shy
U
W
0 ()
~
-II
D~
06
- I
-
e ~l
ri 0
~ shy
No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-11shy
The camera exposures are related to the magnetic field record
by the use of a fiducial pen on the recorder which operates
simultaneously with the Camera Veeder Counter at every tenth
eleventh and twelfth exposure These fiducials appear on the
right hand side of the magnetometer reqord
Du Pont type 936 Superior Two 35mm fUm rated at 160 ASA was
used throughout the survey
shyAutomatic Film Laboratories Moore Park Sydney processed
the film
III 4 RADIO ALTIMETER
Apart from the pressure (barometric) altimeter which is
standard equipment in all aircraft a radio altimeter type APN-l
was used continuously to record terrain clearance
The instrument was set on IIhigh rangell (0-4000 ft) throughout
the survey and was checked at intervals over the base aerodrome
An Esterline Angus recording potentiometer was used in
conjunction with this instrument to obtain a continuous profile on
a five inch wide curvilinear chart recording at a chart speed of
It inches per minute
III 5 STORM MONITOR (Ground Magnetometer)
During aeromagnetic surveys it is essential that ~ time
variation of the Earths total magnetic field is monitored and
recorded throughout the survey on a 24 hour d~ basis
The normal diurnal change of the Earth I s field which occurs
daily is normally of low gradient and cyclic and is compensated
for in the standard flight linetie line control pattern
_~_______________L______ ___~_)
-12shy
However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
- ll
Z
Q)
w
0 ~ --shy
U
W
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~
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06
- I
-
e ~l
ri 0
~ shy
No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-12shy
However ~ abnormal variation (field strength varying
erratically over short periods) will affect the recorded results
of all sorties carried out during these periods Where such
variations occur reflying would be necessary
The storm monitor used for this survey was a saturable-core
single fiuxgate magnetometer manufactured by the Gulf Research
and Development Comp~ Pittsburgh Pennsylvania USA
The complete equipment consists of
i) Fluxgate Element (detector head)
ii) Ceramic Spacing Rod
iii) IIBuckoutII magnet
iv) Tripod
v) Esterline Angus Recorder
vi) Compensator
The fluxgate element operates on the same principle as the
airborne instrument ie it comprises two coils having ferroshy
magnetic cores which are driven cyclically through saturation
A secondary (compensating) coil surrounds both primaries
which are connected in series opposition and so arranged that one
core saturates slightly ahead of the other The resultant output
is in the form of sharp pulses when there is no external magnetic
field the positive and negative pulses are equal in amplitude
Should an external field eg the Earth I S normal field be
applied to these cores their times of saturation would be altered c
causing a change in output thus increasing the etfect
To balance or null this introduced external field a current
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
- ll
Z
Q)
w
0 ~ --shy
U
W
0 ()
~
-II
D~
06
- I
-
e ~l
ri 0
~ shy
No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-13shy
is passed through the secondary coil (surrounding the primaries)
equal and opposite to the disturbing field
It is this current measured amplified and passed through a
potentiometer which is recorded and translated into magnetic units
To obtain maximum sensitivity a permanent magnet (dipole)
supported by a ceramic structure is mounted on the head together
with the detecting element This magnet adjustable in distance
and azimuth from the detector element is used to cancel or IlbuckoutU
the major portion of the Earths normal magnetic field
(a) Record
The recorder chart speed was set at It inches per minute
whilst the aircraft was on survey and It inches per hour at w other times
(b) Calibration
Prior to commencement of the survey the instrument was
calibrated using a standard Helmholtz coil (as used for the airborne
magnetometer)
Full scale deflection over the 4t inch wide recorder chart
was 240 gammas The chart is divided into 50 equal divisions thus
each division has a value of approximately 48 gammas
(c) Storm Monitor
For the period of the survey the sto~ monitor was installed
at Tennant Creek Aerodrome and its operation was continuous
During ~orties the recorder chart speed was It inches per
minute and at othez times It inches per hour
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
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0 -~
00
+-
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E
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+-
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-14shy
IV FLYING OPERATIONS
IV bull 1 AIRCRAFl
Lockheed Hudson aircraft VH-AGE began flying this survey in
the Elkedra area using a towed birdlt installation but after its
loss the survey was completed by the companys DC3 aircraft VH-AGU
using a fixed tail boom installation
IV 2 BASES
The survey aircraft operated from Tennant Creek Aerodrome
N bull T throughout the survey
IV 3middot MAPS AND CHARTS
The following maps charts and photographs were used to plan
fly and subsequen~ plot the aeromagnetic data
(a) World Aeronautical Charts (ICAO)
Scale 11000000 (158 miles to 1 inch)
Halls Creek 3222
Newcastle Waters 32J2
Lake Mack~ 3231
Alice Springs 3232
(b) Planimetric Series Maps
Scale 1250000 (approx 395 miles to 1 inch)
Birrindudu Mount Solitaire
Winnecke Ck Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Barrow Ck
Green Swamp Well Elkedra
Tennant Ck Alcoota
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
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I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
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00
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
- 15 shy
(c) National MaQ~ing Photo-Index ~~s
(approx 4 miles to 1 inch)
Winnecke Ok Lander River
South Lake Woods (Lake Surprise)
Tanami Bonney Well
Tanami East Frew River
Green Swamp Well Barrow Ok
Tennant Ok Elkedra
Mount Solitaire Alcoota
(d) Photo Mosaics
(Scale 1 inch to 1 mile)
Oompiled by Adastra Airwavs Pty Ltd from available
9tl x 9 vertical photography (53 sheets)
IV RECORD OF OPERATIONS
Originally the company I S Lockheed Hudson aircraft VH-AGE
was assigned to this area utilising a Marconi 623 Series Doppler
navigational system in conjunction with selected strip photograpby
However when this aircraft was lost having flown only the Elkedra
block of lines a company DO3 aircraft VH-AGU Was substituted
Because the Doppler unit Was also lost in VH-AGE the navigation
was completed visually by reference to prepar4d one inch to one
mile photomosaics
During the period 17th August to 16th September operations
of VB-AGE Were continuous except for the follOwing dates
18th August Rain anOor cloud
23rd - 28th August inclusive Turbulence (severe)
29th August Doppler equipment unserviceable
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
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c D
E
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+-
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ri 0
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No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
30th August
31st August
1st September
2nd September
3rd-6th II incl
7th September
8th September
9th September
lOth September
11th-15th II incl
-16 shy
Equipment unserviceable (awaiting spares)
II 100 hourly inspection as weather
unsuitable for survey
Turbulent conditions - sortie abandoned bull
Aircraft unserviceable - uls starter motor
Turbulent conditions
Magnetic storms
II II
II Doppler equipment unserviceable
16th September Dust storms - gusting 35 knot winds
The operations of VH-AGU were continuous during the period
24th October to 2nd M~
25th October
26th October
29th October
1st November
4th November
5th November
6th November
7th-lOth II incl
12th November
14th-20th incL
26th November
2nd-4th December
except for the following dates
Bad weather condi tiona
II 1 n
II Magnetometer equipment unserviceable
Awaiting navigation mosaics
II 1 Bad weather conditions
35mm Tracking camera breakdown shyawaiting spare parts
Bad weather - gusting 30-40 knot winds
Magnetometer equipment unserviceable shyawaiting additional spare parts
Crew stand-down in accordance with DCA regulat~ons
Bad weather conditions
_____________---_~______________----~t-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
- ll
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U
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06
- I
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ri 0
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No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-17 shy
6th-11th December incl
13th December
16th December
18th December
2C t- gtecember
14th-21st January incl
25th January
28th January
29th January
30th January
31st January
1st February
2nd February
4th-15th February incl
Aircraft unserviceable - burst tyre shyawaiting spare
Magnetometer equipment unserviceable
Orew stand-down in accordance with DOA regulations
Bad weather conditions
Aircraft withdrawn owing continU4d bad weather
Bad weather conditions
II It II
Aircraft unserviceable - hydraulic leak
II II
II II II Dust storms
Oontinuous rain and low cloud
16th-20th February incl Oontinued bad weather
22nd-26th February incl
27th FebiUary
28th February
2nd-1L1~th March incl
18th March
23rd~26th March incl
28th March
29th March
1st-8th April incl
10th April
II II Equipment unserviceable - magnetometer water-logged
II II Heavy rain trom tropical cyclone - t100ds
No tuel available - used tor emergency services in NT
Gusty winds reaching 4ltgt-45 knots
Bad weather conditions
II
II
II
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
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Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
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I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-18 shy18th April Magnetometer equipment unservioeable
23rd April Crew stand-down in acoordanoe with DCA regulations
25th April Bad weather oonditions
26th April II
28th April 29th April
V AIRBORNE PROCEDURES
V 1 WARMING-UP OF INSTRUMENTS
All eleotronio instruments were switched on and left running for
at least half an hour before recording began to ensure their proper
and stea4y funotion
v bull 2 ATIlli OTATION OF REGORDS
During the survey reoords were annotated for future and plotting-
purposes the following annotations being made
v bull 3 AEROHACh~ETIG REGORD
i Line identifioation and direction
ii Numbering of the first fiducial number and every fiducial mark
equivalent to each lOOth frame
iii Time - synchronised with storm monitor
iv Step numbers
v Reoorder standardise marked RS
vi Measuring cirouit standardise marked MS
vii Instrument drift errors
V 4 RADIO ALTIMETER REGORD
i Start 8nd finish of line showing line numbers and direotions
ii Camera fiduoial numbers
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
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0 -~
00
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-19 -
V DAILY FLIGHT REPORTS
These reports give a detailed description of the sorties from
an operational point of view and show the following information
i Time of start and end of sortie
ii Instruments used and relevant details
iii 35mm photography frame numbers
iv Time of start and end of individual survey lines
v Direction of lines flown
vi Change s of film magazine s and recorder charts
vii Navigators diagram showing the flYing achieved for the
sortie and line directions
VI GEOPHYSICAL TECHNIQUES
VI 1 CONTROL OF OBSERVATIONS
Control of observations (magnetic datum) was achieved by the
use of all tie lines and selected flight lines so distributed as
to form rectangular circuits with approximatelY 20 mile sides
At all these intersections-readings were taken and using a
system based on least squares (successive approximations) each
control line was adjusted to a standard datum
Lines used for control distribution of errors and remaining
errors are shown in Plate 8 of this report
VI 2 rnSTRUMENT DRIFl
The instrument drift was checked at the end of each surveyed
line using the normal standardising procedures Both the recorder
and measuring circuits were adjusted in this w~
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
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0 -~
00
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ri 0
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
- 20 shy
VI 3 REGION AL CORRECTION
A regional correction was applied to the magnetic profiles
middotand is stated on each Total Magnetic Intensity (TMI) contour map
as follows
Minus 95 gammas per statute mile South
Minus 08 gammas per statute mile West
VI 4middot ~YfAGNETIC INFORMATION (Average)
Total Force Field (f)
Inclination of Field (I)
Deviation of Field (D)
Vertical Component (Z)
Horizontal Component (H)
I 50500
_490
40 East
-38000 gamma
33500 gamma
VII REDUCTION OF DArA
VII 1middot PLOTTING OF FLIGHT PATH AND TRANSFER TO OVERLAYS
The position of the aircraft was first plotted on to 1 inch
to 1 mile photomosaics from 35mm traCking film These mosaics
were then reduced photographically to a scale of 1100000 and a
standard 10000 yard Transverse Mercator grid plotted by reference
to 250000 planimetric maps
VII 2
The aircraft track was then traced on to 1100000 overl~s
using the Transverse Mercator grid as reference
RELATING PROFILES TO THE PLOTTED FLIGHT PATH
All points plotted on the base overlay sheets were also plotted
011 the aeromagnetic profiles Ten gamma intercepts all highsll and
lows were then transferred from the profiles to the base overlqs
__-----shy
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-21shy
by scale
VII 3 DATUM LINING AND INTERCEPlING
Aeromagnetic profiles were referred to a common datum based
on the adjusted values of the control intersections
The assigned datum value was sufficien~ high to avoid an1
negative datum anomalies
Intercepts from the aeromagnetic profiles were taken at minima
axima and at intervals of ten gammas Where the anomaly gradients were steep other intervals were selected according to the gradients
The intercepted values were transferred to the base overl~s
using the positions plotted from the IIRqdist co-ordinates
VIII MAPS CHARrS RECORDS ETC bullbull SUPPLIED ON COMPLETION OF THE SURVEY
The following maps charts records etc were supplied on
completion of the survey
i All original annotated magnetometer profiles with positions
of flight linetie line intersections marked with adjusted
datum levels and titled with traverse number date flown
and direction
ii One Xerox copy of all magnetometer profiles for supply
to the Bureau of Mineral Resources under PSSA regulations
iii All original radio altimeter profiles showing height of
aircraft above terrain with annotations similar to ~agnetic
profiles in (i) above
iv All original storm monitor profiles (marked whilst on survey
at 10 minute intervals) These records show the variation
of the magnetic strength throughout the period of the survey
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
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W
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- I
-
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ri 0
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No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-22shy
v All original daily flight reports listing traverses flown
35mm film exposure numbers and fiducial numbers directions
of lines times of start and end of survey lines and all
pertinent operational data for each sortie poundlawn
vi Original 35mm tracking film
vii One (1) copy each of the Total Magnetic Intensity contour
maps on a uCronaflex base at a scale of 1100000
(approximately 158 miles to 1 inch)
viii One (1) copy each of Total Magnetic Intensity contour maps
on a Cronapoundlexll base at a scale of 1250000 (approximately
394 wdles to 1 inch)
ix One (1) copy of Basement Depth Contour map on a Cronapoundlex
base at a scale of 125000P (approximately 394 miles to
1 inch)
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
- ll
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w
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U
W
0 ()
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06
- I
-
e ~l
ri 0
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No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-23shy
IX INDEX OF FLIGHT LINES AND TIE
LINES FLOWN
Line No Direction Section Date Sortie Frame No Remarks
157 S 2081966 3 301-570 I II158 N 571-840 II II159 s 1001-1340 II II160 N 13u-1630 II II161 S 1671-2010
162 N 2011-2290
163 N 2181966 ~ 001-290 II164 s 291-6()() II II165 N 66i-960
166 S II n 16u-2020 II II167 N 1341-1640 II I168 N 101-420
169 N 5121966 22 1110-1460
170 N 2281966 5 131-410 II I171 s 611-1020 II I172 N 1021-1290
173 S 5121966 22 651-1050
TL nOli E 5121966 22 51-650 TLllpll E 1981966 2 281-699
TLIIQ E I 001-540 (Roll 2)
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
- ll
Z
Q)
w
0 ~ --shy
U
W
0 ()
~
-II
D~
06
- I
-
e ~l
ri 0
~ shy
No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
2f
-24shy
Line No Direotion Seotion Date Sortie Frame No Remarks
I NW VL 151967 52 2620-3260 II IIII SE L-V 1700-2619 II IIIII NW V-L 1130-1699
IV NE 111 -16 2041967 46 5200-5809 II ItV S~1 16- I 4670-5199
2 700 E-D 3041967 51 1071-1381 II II3 2500 D-E 791-1070 II II4 700 E-D 461-790 II5 2500 E-F 71-460
5 700 E-D 861967 57 2l4l-2450 6 2500 D-F 2741967 50 1811-2169
II II7 700 F-D 1301-1810 8 700 G-D 3131967 39 1490-2220
II II9 2500 D-G 2221-2800 10 700 G-D 2801-3539 11 2500 C-F 2741967 50 170-950
- II11 700 G-F 951-1300 11A 2500 D-E 861967 57 1870-2llO
II12A 2500 C-D 1550-1869 12 700 F-C 2241967 48 1540-2679 12 700 G-F 27401967 50 951-1300
13 2500 C-F 2241967 48 561-1420 13 2500 F-G 661967 56 332-550
14 2500 C-H 3131967 39 3540-4420
14A 2500 C-E 661967 56 61-331
15 70deg F-C 2141967 47 2940-3579 15 700 G-F 661967 56 551-910
15 70deg H-G 2141967 47 940-2419 16 2500 C-F 2141967 2420-2939
It16 2500 F-H 490-939 17 2300 16-1 2041967 46 3120-3860
II18 500 I-F 11 3861-4669 II II19 500 I-F 2121-3119
19 450 F-16 561967 55 2492-2920
20 2300 G-I 2041967 46 1570-2009
21 2300 D-1 II II 60-999
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
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c D
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+-
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ri 0
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
~ J I
-25shy
Line No bull Direction Section Date Sortie Frame No Remarks
22 500 I-D 1941967 45 2241-3530 23A 2250 F-I 561967 55 1872-2491 23 2300 D-I 1941967 45 61-919 24 NE I-F It 920-1679
II If25 SW F-I 1680-2240 26 S1d D-F 251967 53 2050-2429 26 NE I-F 1741967 44 8E1J-1679 27 NE I-D 3031967 38 3151-4179
128 H-D 1641967 43 1830-2689 29 S~ D-G 561967 55 1321-1871 30 NE G-D 3031967 38 4731-5340
31 st~ D-I II 2290-3150 32 NE G-D 14041967 41 581-ll89 32 NE I-G 3031967 38 1231-2289
n33 stf G-I 831-1230 34 sti D-G 2121967 35 3221-3720
35 SV[ D-I 1441967 41 1381-2230
36 NE I~A -321967 34 1811-3169
37 STt AI If If 680-1810
38 S A-D 2711967 33 671-1590
38 sw D-G 1541967 42 200-620 If If39 sw H-I 621-879
40 NE H-D 2611967 32 2661-3470
40 NE I-H 1541967 42 880-ll89 m[ D-I 2611967 32 1771-26EIJ41
II II42- Npound E-D 1001-1770
42- S1l E-I 16467 43 71-680 043 ST D-G 2611967 32 61-950
44 Sid D-I 2411967 31 EIJ-799
45 Si-l D-G 2121967 35 3E1J-1049
45 NE I-G 1641967 43 681-ll79
46 sw E-I 2311967 30 1690-2330 II II47 NE E-D 780-1689
48 NE G-D 561967 55 712-J320
48 SW G-I 2311967 30 ~60-779
--------~--
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
- ll
Z
Q)
w
0 ~ --shy
U
W
0 ()
~
-II
D~
06
- I
-
e ~l
ri 0
~ shy
No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-26shy
Line No Direction Section Date Sortie Frame No Remarks
49 SW D-I 2211967 29 61-839 50 SW A-D 251967 53 1650-2049 50 NE I-D 1121966 21 981-1890 51 NE D-A 251967 53 1ll0-1649
51 SW D-I 1121966 21 171-980 52 SW A-D 251967 53 721-1109
52 SW D-I 30111966 20 3571-4450
53 NE D-A 251967 53 241-720
53 NE I-D 30111966 20 4451-5320 II II
54 NE I-D 2551-3470
55 SW D-I II 1751-2550 II II56 NE I-D 831-1750
amp ( SW D-G 561967 55 191-711
57 SW D-I 30111966 20 51-830
52 SW D-I 15121966 25 1551-2320
59 Svl D-I 29111966 19 4941-5800 II II60 NE I-D 4151-494Q
CDJ NE F-D 3151967 54 951-1480 shy0 SW D-I 29111966 19 3261-4150
62 NE I-D It II 2361-3110 II IIS~r D-I 1581-23tD
0 NE I-D 15121966 25 671-1550 t shy0 SW D-H 29111966 19 101-840
66 NE H-A 27111966 17 7001-8020 If It67 SW A-H 5981-7000 II II68 NE F-A 4981-5980
68 sw F-H 3151967 54 582-950
9 sw A-B 28111966 18 middot51-471
69 SW B-D 251967 53 fIJ-240
69 SW D-H 27111966 17 4231-4980
70 SW D-F 3151967 54 162-531
70 NE G-F 27111966 17 3491-4230
70 SW G-H II 2691-3390 Ii- NE H-D 3131967 39 6380-7030
72 NE H-D 27111966 17 2020-2690
73 SW D-H II II 1341-1990
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
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06
- I
-
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ri 0
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No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-27shy
Line No Direction Section Date Sortie Frame No Remarks
74 NE H-D Z7ll1966 17 671-1340 75 SW D-H 1 61-670 76 SW D-H 25ll1966 16 2581-3250 77 SW D-H 17 bull 12 1966 26 51-650
middot78 Sw D-H 1212 1966 23 1031-1620
79 SW D-H 14121966 24 101-660 STshy80 D-H 15121966 25 101-670
81 H-D 3ll1966 9 5281-5889 ~
82 D-H 11 9 4601-5179 I II83 - H-D 9 3970-4600 ~
u~v84 D-H II 9 3401-3969
85 NE H-D II 9 2751-3400 86 SM D-H II 9 2181-Z750
187 H-D II 9 1570-2180
BE ) E-H II 9 1061-1569 ~
Ivt89 H-E II 9 520-1060
9U SW E-H 11 9 51-519
91 sw E-H 30101966 6 61-509
92 NE H-E 6 601-1119 II 693 sw E-H ll20-1589
94 H-E II 6 1590-2100
95 E-H n II 6 2101-2579
96 NE H-E II 6 2580-3139
97 SW E-H 31101966 7 l4J-619
98 ~~E H-E II 7 620-1059 II 799 sw E-H 1060-1549
lOC NE H-E II 7 1550-1990 II~
-- SW E-H 7 1991-2449 i102 NE H~ 7 2450-2879 It103 SW E-H 7 2880-3320 II104 NE H~ 7 3321-3779
105 SW E-H II 7 3780-4229
106 NE H-E II 7 4230-4659
107 SW E-H 7 4761-5219
108 NE H~ It 7 5220-5679
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
- ll
Z
Q)
w
0 ~ --shy
U
W
0 ()
~
-II
D~
06
- I
-
e ~l
ri 0
~ shy
No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
f I bull
-28shy
Line No Direction Section Date Sortie Frame No Remarks
109 SW E-H 2111966 8 50-479 110 NE H-E II 480-960 III SW E-H II 961-1399 112 NE H-E 1400-1889
II II113 S-~ E-H 1890-2319 114 l~E H-E II 2320-2799 115 ~~J E-H II 2800-3229 116 ~2 H-E II It 3230-37J0
- II II ~117 E-H 3711-4149
118 s E-M 24111966 15 51-970 119 NE M-E II II 1001-2130
II II120 SW E-M 2131-3140 121 NE M-E 11 3211-4220 122 NE H-K 21111966 12 2651-3610 123 s~ K-H II 1841-2650
h124 4 H-K II II 801-1840
II II125 S~ K-H 51-800 -~126 - J-E 2111966 8 4150-4610
II II127 Svt E-J 4611-4999 I~ II128 N3 J-E 5000-5410
127 SW E-J 13111966 II 51-460 130 1E J-E 1l1l1966 10 651-1070 131 SW E-J II 1071-144D
II II132 NE J-E 1441-1860 133 SV1 E-J II 1861-2300 134 NE J-E 2301-2810 135 SW E-J II 2811-3260 136 NE J-E II II 3261-3790 137 SW E-J II 3791-4260 138 NE J-E II 4261-4810 139 SW E-J II 4811-5280
II II140 NE J-E 5411-6000 141 NE J-E 23111966 14 3831-4410 142 SW E-J II 3311-3830
II t143 NE J-E 2741-3310
144 SW E-N II 51-1000
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
O
()
c D
E
u Q)
E
+-
LD
u
(Y)
shy+
- ll
Z
Q)
w
0 ~ --shy
U
W
0 ()
~
-II
D~
06
- I
-
e ~l
ri 0
~ shy
No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-29shy
Line No Direction Section Date Sortie Frame No Remarks
145 NE J-E 22111966 13 5571-6220
145 NE N-J 23111966 14 1001-1530 146 SW E-J 22111966 13 5011-5571 146 SW J-N 23111966 14 1631-2140
147 NE J-E 22111966 13 4401-5010
147 NE N-J 23111966 14 2241-2740 148 S~(l E-J 22111966 13 3861-4400
149 NE J-E II II 3221-3860 150 SW E-J II II 2581-3120 151 llE J-E II II 1901-2580
152 SW E-J II 1371-1900
153 SW H-J 25111966 16 4681-4890
154 NE J-H 13111966 11 1581-1780
155 SW H-J II II 1381-1580 156 NE J-H II II 1191-1380
TIE LlNES
Ali 3100 36-69 321967 34 70-679 Bil 3100 3amp-69 Zl11967 33 60-670 GU 1800 11-23 861967 57 520-1010 liD 3100 77-2 941967 40 81-1539 DIt 3100 87-78 12121966 23 711-1030 nEil 3100 78-2 2731967 37 60-1870 liE 1000 78-87 12121966 23 431-710 tEn 100deg_1300 87-14l 22111966 13 4l-1110 IIEll 900 141-152 II It llll-1370 FII 1300 5-76 2731967 37 4581-6599 II Fit 1200 78-143 19121966 27 211-1600 II Gil 3100 71-8 3131967 39 51-1489 URU 1300 14-78 II II 4571-6379 RII
II III
1100
2700
3100
78-156 64-IV
17121966 151967
26
52 651-2360 61-1129
IIJII 1650
1200 125-156 13111966 11 461-820 821-1190
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
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LOCATION DIAGRAM
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PLATE 3 SPECIMEN MAGNETOMETER RECORD
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-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
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PLATE 5 SPECI EN STORM MONITOR RECORD
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-30shy
Line No Direction Section Date Sortie Frame No Remarks
KII 1300 122-125 21111966 12 3721-3850
L 2250 I-III 151967 52 3261-3329 II Mil 3100 121-118 24111966 15 3141-3210 liN II 3100
147-144 23111966 14 2J4l-2240
---------~ --~ ------- ---~--------- -
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
t-
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()
c D
E
u Q)
E
+-
LD
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ri 0
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No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-31shy
AHlaJ LIST OF PLATES AND DIAGRAMS
1 Location Diagram
2 Diagram of Flight Pattern
3 Specimen Magnetometer P~cord
4 II Radio Altimeter Record C
5 II Storm Monitor Record
6 II 35mm F~ Record
7 Mosaic Coverage
8 Distribution of Magnetic Closure Error and Residual Error
9 Residual Heading Error
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
- ~ ~ ~ tt~ ~ Ui ~ ~ Ij
T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
J
(lgt
0 gt r 0
0 0
gtshy C 0 gt
+
7 0 -1
~
r - C
-i ~ 0
Z
+
+
+
N
H~rl t~) pound(O elM )4 hur k tf-j
I ~ Lmiddotmiddot~l t _ r _-- t1 I
C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
j~ 1~ 25 ~if - lit1-lt we ll__L_+i U(POSORE Iii j
~ --~ltlt Imiddotmiddot tmiddot~-
~
E V) ~ E uJ ----+--=t--=--+ f ~=L~~~h~~~F=f~r~[Er~r=zr~[~bJU tj ~-g-~-f==-r=-=--+-cshy
==t--t-==i~ III) ~ It g- --o t-_~I~u= __ --c+ -z
o - i=
u-shyt==t==t=-l=03==r-tmiddotmiddot w shyljJ~~4~~2~~$~t~~1j~--~f~~rsp~[ ~g~~~_=-+_lt===-I ii ==1=t=plusmn
~~~~~~~~~[1~e~V4e~f~~~Stta~bliSfh~e~d9~fr~o~mIt~f~~~~~~~]Jr=t~~-~-~-~==----4---C==+-- ~~2~~yen
t 1==
-----shy
PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
- ---1
lL
0 -~
00
+-
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c D
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No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
middot ~~
-32shy
Plate 9
RESIDUU HEADING EFFECT TABLE
Mg Ela12sed CorrsRdg Residu~ Error Direction Gamma Time mins Corm Gamma Gamma
3600 100 0 0 100 0
1800 91 3 -09 90 -10
6900 100 5 -15 99 -1
2700 104 9 -26 101 + 1
0450 1~2 II -32 99 -1
2250 100 13 -38 96 -4
135deg 96 17 -50 91 - 9
3150 06 20 -58 100 0
360deg 107 -70 100 024
Date 10th November 1966
Place Tennant Creek NT
shyAircraft DC3 VH-AGU
Magnetometer Gulf Mark III (Stinger)
Height 2250 feetams1
Time 1609 - 1634
Method Induction Coils (Permanent)
Permalloy-II Strips (Induced)
--
iii
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T ~
+ + +0middot AldVld
+ I
I~~ I I
I
ooy WI
I I I I
I
c
I I
II QLOWA +
I NTII
t----------______ +
_ ________L SA
100
LOCATION DIAGRAM
PLATE 1IH61 Ma$o~
+
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0 0
gtshy C 0 gt
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C NXD ~~~Llmiddot ~-lmiddot~middot--liIT-lmiddot~-B 1 q 1 tomiddotmiddotmiddot 0 r 0
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PLATE 3 SPECIMEN MAGNETOMETER RECORD
------
L_
Frome
-------- ----
~-----_-+------
~------
----middot_---------1-----shy
--~----- ------------- --- __--shy-------- ------~---
-------- ------_ ------ __ _------shy ----~-
-------- ------ -----f------middot----- 1----shy
-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
-----~-~
-----+------ ---i--------- c
deg -------r_------~-------~---~------_+------_4------r_-----+_------_-----~r-----~~------~ ~
--shy~~~~==~~i=~~~~====~~~~=t==~~l=~~~~~~=-~======~~====~~8=-=--=middot--=--8~1----~
-----1-----shy
----~-I--===cHART SPEED 1- 5 inch per minute on ~ I J
=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
~
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
--
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PLATE 5 SPECI EN STORM MONITOR RECORD
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
H~rl t~) pound(O elM )4 hur k tf-j
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PLATE 3 SPECIMEN MAGNETOMETER RECORD
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-------- ------_ ------ __ _------shy ----~-
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-----+___ CHART SPEED-3 if1_chesp~r minute_---=- _____ +------------- ----------t---------t-shy
PLATE 4 SPECIMEN RADIO A METER RECORD (curvilinear)
middot SM_----gt
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PLATE 5 SPECI EN STORM MONITOR RECORD
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
------
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
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=~~n _1-5 in~ff per houiJoff survex)-shy---~
PLATE 5 SPECI EN STORM MONITOR RECORD
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
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-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
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A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
~
-II
D~
06
- I
-
e ~l
ri 0
~ shy
No -
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-33shy
Part II
Interpretation Report
Q OJ oR J
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-34shy
CONTENTS
I INTRODUCTlOO
II METHODS OF STUDYING DATA
III GEOLOGY OF THE AREA
IV OTHER GEOPHYSICAL SURVEYS
V MAGNETIC INTERPRETATlOO
Eastern Area Blocksl B C and D
Main Area
(a) Depth to Basement
(b) Structure
Western Area Blocks E F and G
Main Area Block H
SUMlfARY AND RECOMMBNDATIONS
APPENDU I Methods used in the Interpretation of the Aeromagnetic Data
APPENDIX II Elements of the Earth I s Magnetic Field
Page No
35
36
37
39
JI)
41
44
45
47
49
51
53
54
58
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-35shy
I INTROOOCTlOO
I The area coveredby the survey is approximately 25000
square miles and as far as we know includes considerably
varied geology within these very extensive limits Most of
the area is covered by sand so that our present knowledge of
the geology is based on outcrops which cover only a small
fraction of the total area
The aim of this interpretation is threefold
1 To obtain a general picture of the geology and
especially the distribution of the shallow basement
areas which ~ be used to plan further exploration
within the area
2 ~o find the depth of magnetic basement in areas in
which a considerable thickness of potentially oil ~
bearing sediments ~ exist
3 To recognise certain structures mainly major faults
in the basement which ~ have affected overlying
sedimentary rocks
The very size of the area presents special problems for the
interpreter With so little known about the geology and the
problems not yet clearly defined it is difficult to know which
particular aspects of the interpretation should be emphasised
~le feel therefore that a re-interpretation of the airborne
results at some later date when more is known about the geology
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-36shy
of the area will be well worth while
A special problem in this area is the abundance of shallow
magnetic bodies Over almost the entire area there are magnetic
bodies close to the surface Some of these are probably due to
lavas or basic igneous intrustions within the younger sediments
These minor anomalies make itmiddot very difficult
(a) To distinguish between the areas in which a weakly
magnetic basement lies close to the surface and
areas in which non-magnetic sediments with igneous
intrusions are near the surface
(b) To calculate the dept of the magnetic basement
accurately because they distort the shape of the
anomalies associated with the deeper bodies
There is a second interpretation problem namely that in
places the tlbasement ll for oil exploration may not be magnetic
some areas of apparently deep basement may in fact be areas of
very weakly magnetic basement
II METHODS OF STUDyrnG DATA
The major part of the interpretation was carried out by
measuring various parameters of the anomalies using the original
magnetometer records These anomalies were selected from the
magnetic contour map as the ones best suited for depth estimation
using the methods described in Appendix 1 Corrections were
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-37shy
made for anomalies which cross the flight lines obliquely
In the course of the interpretation the calculated depths
were related to the pattern of magnetic anomalies seen on the
contour maps and a comparison made with structures and outcrops
shown on the geological maps of the adjacent areas
The numerous small anomalies from near surface bodies
distortthe shape of the anomalies from rocks in the magnetic
basement thus reducing the accuracy of the estimated depths of
basement
The trend of a few of the shallow magnetic bodies can be
followed from line 92 to line 112 If this information is useful
for the appreciation of the geology further interpretation of the
shallow anomalies might be attempted especially in areas where
it is known that conformable lavas or sill occur within the sediments
Until more geological control is available we do not know to which
areas this may be applied
If lavas ar~ found in any part of the area the aeromagnetic
records should be re-examined first to relate the small anomalies
to the lavas and then as suggested above to work out the structure
from them
III GEOLOGY OF THE AREA
The most detailed information about the geology of the area
came from two maps provided by American Overseas Petroleum Ltd
Exploration Department One map - D-172-G at 1500000 scale
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-38shy
shows all known rock outcrops in the north western part ot the
survey area
Map C118G at 11000000 scale shows the geology as it is
known over the whole area and over a considerable part ot the
surrounding country This map also shows the gravity contours
which are based on surveys carried out by the Bureau ot Mineral
Resources
Information about the surrounding area comes trom the
12534000 scale Tectonic Map ot Australia published by the
Bureau ot Mineral Resources Department ot National Development
1960
Depth contours based on information provided by an aeroshy
magnetic survey tlown by Aero Service Ltd in 1964 are available
in the north western part ot the area
We can summarise the geology as tollowsshy
The oldest rocks in the area are Archean rocks ot the
Arunta Complex These rocks where they outcrop are
strongly magnetic and provide a well defined magnetic
basement they torm much ot the southern limit ot the
sediments in the survey area
Lower and Upper Proterozoic rocks which include
sediments lavas and acid and basic intrusions are tound
on both the southern and northern siQes of the south eastern
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
part of the area (that is on sheets 5 6 7 and 8 on the
1250000 scale maps) These rocks are 3trongly magnetic
where they outcrop
Most of the outcrops of non-metamorphic rocks seen
within the survey area are of Cambrian age We knoW little
about the structures which affect them
Devonian rocks in OP 123 south east of BarroW Creek
form a syncline Devonian rocks are also found in the
southern part of OP 118 in an areavhere there is a large
negative gravity anomaly
To the north of the area a thin cover of Mesozoic and
Tertiary sediments lie on top of rocks of unknown age
A narrow belt of Tertiary sediments occurs about 20
miles south of Barrow Creek Another thin belt of Tertiary
sediments occurs about 20 miles south east of Devils Marbles
Both narroW belts of Tertiary sedi~nts look as though they
might folloW some eroded major fault line
IV OTHER GEOPHYSICAL SURVEYS
An airborne magnetic survey was flown over oil prospect 118
by Aero Service Ltd and deptnto basement has been calculated
This was a reconnaissance survey flown for Exoil Company Pty Ltd
with flight lines 8 and 12 miles apart In places we have more
data and can get a little more detail from our interpretation
However the picture of basement configuration is not appreciably
changed
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-40shy
A gravity survey was carried out in the BJea by the Bureau
of Mineral Resources which supports this interpretation of the
present aeromagnetic data As we do not know the density of the
various rock groups the interpretation of the gravity results in
quantitative not qualitative The gravity lowsoccur in areas
where the basement according to the magnetic results is deep
Various gravity trends stop abruptly wheregtmagnetic trends indicate
a change in geology
V MAGNETIC INTERPRETATION
The methods used for the interpretation of aeromagnetic data
is described in Appendix I
Examination of the records shows that over much of the area
there are magnetic bodies of at least two depths Some of the
magnetic anomalies are obviously due to weakly magnetic or small
bodies which lie close to the surface These bodies produce a
geologic noise ll through which we can recognise anomalies from
a much deeper source which we consider constitutes the magnetic
basement in this area Several thin linear magnetic bodies near
the surface can be followed for twenty miles from line to line
they are probably lava flows or sill but may be dykes
The survey consists of one large area described in two parts
for which the basement geology is different There are also eight
blocks of four lines which were flown over better exposed areas
on the periphery of the main sand covered area and three blocks
of lines flown over Oambrian rocks at the eastern end of oP 123
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-Jshy
These blocks have been described separately
In the eastern part of the main area the anomalies are
strong and trend west-northwest In the western area the
anomalies are weaker and the trends more variable The
boundary between the two areas which runs across from sheet
12 to sheet 13 and across sheet 15 is in the Pre-Cambrian
rocks and not necessarily an important one in the Palaeozoic
rocks bull
Eastern Area
The blocks of lines are marked A B C and D on the
interpretation map These areas are described first and will
then be referred to occasionally in the description of the main
area These areas differ from the main area in the amount of
geological information available They provide a calibration
for the interpretation by showing the type of magnetic response
which may be expected from a number of the rock groups
Block A
The geology in this area consists mainly of Middle Protershy
ozoic rocks (Hatches Creek Group) and some Cambrian rocks The
rocks are folded along axes which strike north-west or northshy
northwest and are cut by faults which strike north-south and
north-west
The anomalies in the area have a high amplitude ard obviously
lie very close to the surface The southwest~rn part of the
block on sheet 17 is very strongly magnetic the part of the
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-42shy
block on sheet 18 is less magnetic The boundary between them
which is shown on the interpretation map appears to strike
northwest and is possibly a fault There appears to be a
second boundary between Block A and the main area this boundary
also appears to strike northwest The Middle Proterozoic rocks
in this area would constitute a magnetic basement if they occur
beneath less magnetic sedimentary rocks
At the northeast end of the lines where the Cambrian rocks
outcrop the basement is about 2500 feet below sea levelbull
Block B
Block B consists of three bands of lines Bl B2 and B3
Huch of the area is covered by sand Cambrian rocks outcrop
along the southeastern edge
The magnetic anomalies in this area strike east-west and
southwest and have a high amplitude In the northwestern part
of the block the magnetic basement is less than 1000 feet bel~w
below sea level it also appears to be shallow in the southeastern
corner Between these two areas there lies a basin I in which
the magnetic basement is about 4000 feet deep This basin
extends southwest outside the limit of Block B and it also
extends to the north east across a shallower part There are
two major faults in the basement rocks One crosses B3 and
strikes east-northeast This fault appears to be a continuation of
a large fault seen near the southern end of the main area (sheet 20)
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-43shy
The northern limit of the sedimentary basin in Block B
might also be related to an extension of a large east-northeast
fault in the main area (sheet 25) but if it is it is not evident
on the magnetic map The boundary between the basin and the
shallow magnetic area in the southeast corner of Block B also
appears to be a fault which strikes northeast This suggests
that the deeper sediments in this area occur in a trough-like
fault
Block C
Upper Proterozoic rocks outcrop in the northern part of
Block C and Archaean rocks of the Arunta Complex outcrop in the
south
I The anomalies over the Upper Proterozoic rocks are large
and tta magnetic bodies are obviously very shallow or actually
reach the surface The anomalies are presumably due to basic
rocks In the southern part the rocks are less magnetic but
are still shallow This is quite consistent with what is known
about the geology
The strike of the magnetic anomalies in the southern part
of the area is east-west in contrast to the northwest strike
which is seen in the northern part This indicates adifference
in the structural pattern of the two parts of the block The
boundary between the main block and Block C strikes northwest and
from the sharp change we think that it may be a large fault
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-44shy
Block D
The rocks exposed in Block D consist of Upper Proterozoic
in the north and Archaean rocks in the ArunJa Complex in the
south A narrow band of Tertiary sediments which strikes
northwest runs across the area and coincides with the steep
gravity gradient The geological map shows shear zones in the
north An inlier of Cambrian rocks occurs near this Block
The magnetic basement is shallow on sheet 20 where the
upper Proterozoic rocks outcrop The well developed gravity
minimum coincides with areas in which the magnetic basement
reaches a depth of 3000 feet This suggests that there is a
basin II within the Upper Proterozoic rocks and faulted with
either weakly magnetic Proterozoic or younger rocks possibly
bounded on its northern side by a fault~
A basement depth of 1700 feet southeast~f Barrow Creek
seems to occur over Proterozoic rocks and not over the Cambrian
outlier This apparent depth found over weakly magnetic rocks
may ~ccount for a number of conflicting depths observed elsewhere
in the area
Main Area
The rocks which outcrop in this area include one which range
in age from Archaean to Devonian Except for the major basin
implied by the outcrops of Pre-Cambrian to the north and south
of the Lower Palaeozoic rocks in the middle of the area there
are no major structures to be seen The Devonian rocks occur
with the fold which strikes northwest There are few outcrops
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-45shy
within the area which is almost entirely covered by sand The
southern boundary of the area lies close to the most northerly
outcrops of the Arunta Complex
A gravitY survey was carried out in this area by the BMR
and two extensive negative anomalies indicate areas in which there
m~ be greater thicknesses of light sediments
Both gravitY and magnetic maps indicate three areas in which
sediments ~ be thicker than elsewhere One area lies 50 miles
east of Barrow Creek the second lies along the southern ~dge of
OP 118 and OP 119 the third area lies in the southeast corner
of OP 120
(a) Depth of Basement
In the northern part of the area the depth determinations
made on the magnetic anomalies indicate that the basement is shallow
most of it being less than 2000 feet below sealevel and some of it
being less than 1000 feet On sheet 20 (south west corner of
sheet 6) the limit of this area of shallow magnetic basement seems
to be a fault which strikes northwest or west-northwest and
separates the shallow basement area from the deeper basin 111
which lies in the south west corner of 0P123 This basin is
4000 feet and 5000 feet deep Magnetic bodies at shallower
depths in the centre of the basin may be due either to anuplifted
block to an intrusion or to a mass of lavas within the basin
A small basin IVIt which lies 15 miles southwest of the end
of Block A is possib~ due to a small basin of Cambrian or nonshy
magnetic Middle Proterozoic rocks
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-46shy
A large magnetic structure which strikes west-northwest
is associated with the northern margin of the basin V which
lies on the southern edge of 0Pll9and OPllS and for the most
part coincides with the large negative gravity anomaly This
basin is about 4000 feet deep at its eastern end and is about
10000 feet deep in the middle
The shallower depths on sheet 15 correspond to relatively
higher gravity values within the gravity low already mentioned
To the west of this a greater depth m~ be associated with a
transverse fault which runs north-northeast this structure
could affect the distribution of oil or gas in this area
At the western end of the basin a west north west trending
anomaly within the basin gives unusually shallow depths flanked
by much deeper values This magnetic boQy presents a puzzle~
It is very narrow for a horst block but on the other hand it is
unusual to have a wide Qyke in this position~ It is possible
that the deep values to the north of this block come from a
weakly magnetic basement We can only draw attention to this
boQy and remark that there is some peculiarity in the geology
It ~ justify fUrther ground investigation This shallow
structure and the main basin gtoth end against a large northshy
northeast fault
The southern limit of this basin V is the outcrop of the
Arunta Complex which is distinguishable on the magnetic records
by the abundance of shallow anomalies The boundary is abrupt
and may be a fault There are a number of III1ch shallower values
found within the area and it is difficult to interpret them
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-47shy
They m~ be due either to local shallowing of the basin floor
or to magnetic bodies within the sediments
In the northern part of the area (on sheet 17 northwest
corner of sheet 6) several shallow anomalies can be followed from
line 92 to line 112 This suggests that the shallow anomalies
here are due to a bedded magnetic horizon or to qykes Because
volcanic rocks are found within the Cambrian rocks in 0P152
these magnetic anomalies maT be due to lava flows or volcanic
ash Similar shallow magnetic bodies occur in profusion over
most of the area
Area VI to the south of those bedded magnetic rocks
referred to above (close to the western edge of sheets 17 and 20)
there are a number of shallow depth determinations in the middle
of deeper values We think these shallower values are due to
intrusions or volcanio rocks which lie olose to the surface
Between this area and the basin VII in OP 123 there aPpears to
be an area in which magnetic rocks are abundant at shallow depths
This coincides wi~h an increase in the gravity field and thus
likely to be an area in which the sediments are thin
Elsewher~ in the eastern area the cover of weakly magnetic
rooks is not thick
(b) Structure
The magnetic anomalies within this area run mainly northwest
and southeast with an occasional swing in the strike to east-westbullbull
Some information about the main structural divisions of the
Jl
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-48shy
basement rocks can be obtained from the pattern of the magnetic
anomalies which indicate a number of major changes within the
Pre-Cambrian rocks There appear to be major fault zones striking
east-northeast near basin III judging from the depth estimation
made from the magnetic data some of these faults have moved since
the Palaeozoic sediments were deposited in this area Most of
these faults have a very considerable strike length One fault
which cuts across the area near point 136 E and 210 45 S m~ extend
to Block B as described earlierand may form the northern edge of
one of the areas of deeper sedimentation 111 bull
The southern boundary of the main outcrop of Lower Proterozoic
rocks in OP 123 also follows a well-defined magnet~c feature in
the basement rocks
There are some north-south trends VIII on the magnetic map
which we think ~ be associated with faultsbut it is difficult
to make out the direction of movement of these faults or whether
they have been moved since the sediments were deposited (In other
areas there is an obvious change in the thickness ot sedimentson
either side of the big faults)
The Basin IlIon sheet 20 is cut by one of these north-south
faults and there are several anomalies superimposed upon the larger
ones which couldcome from igneous rocks whichmiddothavebeen intrudedmiddot
along the fault plane
On the eastern side of sheet 16 a break in the magnetic
pattern suggests that a large north-northeast fault IX cuts across
this area The shallow anomalies which were picked out on lines
92 and 112 stop on the line ot this fault This suggests that the
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
---~
-49shy
fault affects both the basement and the sediments
On the southern side of the area on sheet 20 the change
from basement (Archaean rocks) to non-magnetic sediments is
abrupt Anomalie s here are superimposed on one very large
anomaly whose source appears to lie 12000 feet below sea level
and is very well seen on flight line llJ It is possible that
this deep feature controls the boundary of the Archaean rocks
both here and to the west where the rapid change in depths as
determined from the magnetic map suggests a large fault bull
The northern edge of the basin is complex We think that
it is bounded bY a fault which is marked both by its strong
magnetic trends and by the change in depths indicated by the
magnetic anomalies
The western end of basin V is a large north-northeast fault
which m~ extend to basin XII The actual division between the
eastern and western part of the area is not a clear cut line
The boundary includes a zone in which much shallower but more
erratic basement depths are observed
Western Area
The western area has a completely different magnetic pattern
from that of the east Unlike the pronounced linear pattern in
the east the anomalies in this area have no well-defined trend
direction
Most of it is covered by sand through which occasional outshy
crops of Cambrian rock are seen At the extreme western end of
the area Upper Proterozoic rocks outcrop The only structures
lt
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-50shy
indicated in this area are faults which strike northwest there
is no indication of folding in the Palaeozoic rocks
In the western part of the area the flight lines were
flown in bands so that the info~mation about part of this area
is less complete than it is in the east and the results should
treated as reconnaissance survey findings only
For convenience we could describe the blocks separate~
Blocks E F and G lie to the north of the area Block H lies at
the western end of the area
Block E
This block or l~nes covers outcrops of Lower Proterozoic
rocks in the north to Cambrian rocks in the south Magnetic
rocks are shallow in the north and are presumably Proterozoic
The boundary of the shallow rocks is abrupt and is possibly a -
fault Depths of 5000 feet are found to the south and mark
the beginning of a basin X which extends to the west We do
not know how far this basin may extend tothe east To the
southeast the basement is less than 1000 feet below sea level
Block F
Shallow magnetic rocks are found on the nor~hern part of
Block F and are separated by a well defined boundary from
deeper magnetic basement in the south This boundary m~ be a
continuation of the one seen on Block E The basin of weakly
magnetic rocks is 5000 feet deep
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-51shy
Block G
The rocks which outcrop are of Cambrian age In the
northern part of this strip the depth estimates vary considerably
and it is difficult to know whether we are dealing with a
sedimentary seotion containing magnetic rocks or a weakly magnetshy
ised basement The anomalies which run along the direction of
the flight lines could indicate a shallow basement In the
south the more consistent values indicate depths ot 4000 teet
and ~ mark the continuation ot the basin X seen in Blocks E
and F
Main Area
Within the main area east of block F (on sheet 13) we
show a small basement high on the interpretation map which is
based on three value s only As there are a number ot shallow
anomalies in the area probably due to magneticJqers within
sediments we cannot be quite sure that these three values are
in fact trom the basement If they are due to other larger
magnetic masses within the sediments then the shallow basement
which divides basin X disappears and we can take the basin through
trom Strip E to Strip F This basin deepens to about 7000 teet
and widens towards the west and ends at a basement ridge XI which
runs north-northeast
There is probably one basin XII about 7000 teet deep on
the western side ot this ridge but as it lies in that part of the
area where the aeromagnetic cover is not complete we cannot be
sure about the probable north-south strike or continuity ot the
~ t i -~
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-52shy
of the basin The eastern edge of the basin lies on the line
of the large north-northeast fault postulated at the western
end of Basin V There is no evidence for a continuation of
this fault on the aeromagnetic lines flown but this may be due
to the combination of flight path direction line spacing and
unfavourable basement geology
In the north western part of the area there is another
basin XIII which is separated from XII by a ridge This basin
is about 10000 feet deep The survey has not been extended
far enough to indicate the northern limits of this basin
Block H
Block H lies on Sheets 5 and 10 and has been flown almost
entirely over Upper Proterozoic rocks in which northwest faults
are seen The southeastern part ofthe stripis magnetically
flat and the contours run parallel to the flight lines so
that we obtain satisfactory depth values from the records bull
On the margin of Sheets 10 and 6 the magnetic basement is
obviously very shallow At the northwestern end of the blOck
shallow values indicate the outcrop ping of magnetic basement
Between these two groups of shallow anomalies the smooth contours
indicate a considerably deeper magnetic basement it is not
possible to make a proper depth estimate because of the numerous
small shallow magnetic bodies which distort the curve The
shallow values determined from the magnetic survey correspond to
areas in which the gravity values are high and the broad anomaly
which would indicate a basin corresponds to a gravity low
bull 5 bull
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-53shy
SUHMARY AND RECOMr-tENDATIONS
The results o~ the interpretation are most clearly
~arised on the 11250000 interpretation maps All sheet
numbers quoted in the text refer to the 100000 sheet layout
The major basins correspond closely to those indicated
by previous aeromagnetic surveys gravity surveys and the geology
~~ar basement faults striking east-northeast and north-northeast
are shown on the interpretation maps these faults may produce
minor folds or faults in the overlying sediments which could
affect the distribution of hydrocarbons in the area
Most of the ground surveys should be carried out in the
area where the basins occur We also suggest that particular
attention be paid to the origin of the shallow anomalies in all
areas If these anomalies are due to magnetic sediments the
magnetic map could yield an immense amount of structural informshy
ation the magnetic properties of the ~ediments pound-rom drill core~
should be studied especially if they-are found in an area in
which there are no igneous rocks to account for the anomalies
If igneous rocks occur the magnetic susceptibility of samples
should be measured so that the anomlies can be fully accounted
for the pattern of dykes and sills in an area might throw some
light on the structures If volcanic ashes are a potential
reservoir rock the changes in the magnetic anomalies may take
on a new significance
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-54shy
APPENDIX I
METHODS USED IN THE INTERPRETATlm OF THE
AEROMAGNETIC DATA
Harf-Slope Method
The measurements required for the Half-5lope method were
taken directly from the magnetometer profUe charts
The distance between the tangential points of contact of the
anomaly curve and the line of half maxinum slope have been empiricallJr
related by Peters to the depth of a dyke-like body so orientated
with respect to the Earths magnetic field that it produces a
symmetrical anom~ the distance between the inflection points or
points of maxinum slope is related to the maximum horizontal width
of the body This ratio of width to depth varies from anomaly to
anomaly and partially controls the choice of empirical factors used shy
in depth determinations the application of an appropriate factor
converts the scale distance between Half-Slope contact points into
a depth below the aircraft Where the anomaly is not quite symmetrical
the two flanks of an anomaly are processed independently and the results
averaged
This method could be in severe error if applied to anomalies
which are too disturbed too asymmetrical too comp1lex or not dyke-like
therefore care has to be taken in the selection of suitable anomalies
The method presents a number of advantages however mainly speed and
ease of use but especially its applicability to slightly disturbed
or complex anomalies which do Dot JustifY more elaborate analytical
techniques
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-55-
The depth obtained by this method is plotted midway between
the two inflection points of the anomalJr one or both flanks of
some very wide anomalies are treated separatel1 1n these cases
the depths are plotted at the 1ntleotion points on the assumption
that they represent the depth of the ~vidual contacts
A modification of this method which is more frequently used
permits the ~dth of the anomalous body to be calculated and makes
allowance for anomalies which are not symmetrical More accurate
depths may be calculated in this way than can be achieved by the use
of the simple half-slope method
Dipping Dyke Method
The Dipping Dyke method was developed by personnel of Huntec
Limited of Toronto Canada Although a paper is in preparation whichdescribes its application it is at present unpublished
Utilising the position of the inflection points the gradient
at these points and the maximum magnetic field value on a profile
at right-angles to the strike of the body information on depth width
dip location and magnetic susceptibility contrast is obtained with
the aid of prepared charts and tables
Under certain conditions this method may give reasonable
answers from anomalies which are not caused by dyke-like bodies
However specific checks such as the shape of the anomalJr are provided
by the method and help in detecting these cases If this method does
not function on a given anomalT it is an indication that the anomaly
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-56shy
is not derived from a Qyke-like boQy or that it is distorted by
anomalies from adjacent bodies An undetected or misinterpreted
regional gradient could also prevent its 8pp+ication
Depths obtained using this method are plotted onto Total
Magnetic Intensity contour maps at the calculated centre of the
dyke-like boQy
Characteristic Curve Method
This method resolves basically into matching the field anomaly
to a suitable theoretical anomaly derived from the mathematical
expression for the induced external magnetic field of the chosen model
by the use of co~ted characteristic curves
A comprehensive series of such curves has been prepared for
use with total magnetic -intensity measurements by Computer Applications
and Systems Engineering of Toronto Canada the curves have been
derived for various models which have geol~gical eqUivalants ie
tabular bodies dipping dykes vertical prisms stepcontacts horizontal
plates and rods
Several parameters of the theoretical anomaly are measureq and
combined to produce dimensionless quantities the most diagnostic
combinations are then chosen as estimators and plotted against the
parameters in families of characteristic curves
The interpretation involves measuring the most diagnostic
parameters on the magnetometer field record or contour sheet from
the estimators so produced it is possible with the characteristic
curves to interpolate the characteristics of the causative boQy
The results of the analyses are converted into map scale units bT the
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
-57shy
comparison of suitable linear parameters the choice ot parameter
being dependant on the chosen model A poundUrther set at curves enables the magnetic susceptibility contrast to be determined
Half-Width Method
Using this method a number at parameters can be measured on a
wide range of theoretical dyke curves and the results presented as a
series at curves which relate these parameters to the depth at the
causative body The distances measured include bull
(a) The distance separating the maxillllDl and minimum
gamma values of the anomaly
(b) The horizontal extent of the anomaly at half the
maxillllm amplitude
(c) The distanc-e separating tpe points of quarter and
three quarters at the maxiJJlllll amplitude on each
flank at the anomalybull
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-
APPENDIX II
ELEMENTS OF THE EARTHS MAGNETIC FIELD
The elements of the Earth s magnetic field vary appreciably
over the very large area covered by this ~ey At the northwest
end of this area the field is as follows I
Declination 4015 east of north
Inclination _490
Magnetic Intensity 50500 gammas
At the south end of the area the field iSI
Declination 4o30 east of north
Inclination _510
Magnetic Intensity
- Part 1 - Operational Report
- Introduction
- The Flying Programme
- Methods and Instruments Used for the Survey
- Flying Operations
- Airborne Procedures
- Geophysical Techniques
- Reduction of Data
- Maps Charts Records Etc Supplied on Completion of the Survey
- Index of Flight Lines and Tie Lines Flown
- Part 2 - Interpretation Report
- Introduction
- Methods of Studying Data
- Geology of the Area
- Other Geophysical Surveys
- Magnetic Interpretation
- Summary and Recommendations
- Appendix 1
- Appendix 2
-