Iof some 82 measurements of apparent conductivity. Technical Report WRD83095 Viewed at 14:07:31 on...

Technical Report WRD83095

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HYDROGEOPHYSTCAL REPORT 83/7

• NUMBULWAR GEOPHYSICAL SURVEY

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I Prepared by: P Furness

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CONTENTS

1. INTRODlJCTION

2. GEOLOGY OF THE AREA

3. SURVLY OBJECTIVES

4. INSTRUMENTATION A~u TECHNIQUES

4.1 Resistivity

4.2 Electromagnetic

5. RESULTS

5.1 Resistivity Soundings

5.2 Resistivity Profiles

5.3 Electromagnetic Profiles

APPENDIX 1: Schlumberger vertical electric soundings and interpreted earth models.


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1. INTRODUCTION

During the period 28 September to 5 October, 1983 a geophysical survey was completed in the vicinity of Numbulwar Aboriginal Settlement by staff of the ,1ater Division, N. T. Department of Transport and Works. The survey was undertaken in the initial phases of a groundwater investigation aimed at extending the settlement's existing supply~

During a total field period of cOTflPrislng one geophysicist and two completed the following operations:

6.5 days a crew field assistants

1. 12 line-km of pole-dipole resistivity proIlllng involving approximately 240 observations of apparent resisti vi ty,

2. 11 Schlumberger vertical electric soundings involving approximately 220 apparent resistivity measurements, and

3. 2 line-~~ each of horizontal coplanar and vertical coplanar coil electromagnetic profiles involving a total of some 82 measurements of apparent conductivity.


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2. GEOLOGY OF THE AREA

The geology of the area is presently only poorly known.

The most obvious geological feature in the area is an exposure of resistant, fine to rnea~um grained sandstone outcropping over a substantial al-ea to the north-west of the :c,ettlement (see map 1). A S}U( sheet based on 1959 mapping Describes this particular exposure as undifferentiated quartz sandstone of upper (?) proterozoic age. The same document indicat.es southerly dips of less than 15° at the eastern margi~ of the sandstone.

Photo trend lines paralleling the to the north-east suggest the presence conformable beds underlying this unit. these however, is unknown~

sandstone particularly of less resistant, but

The lateral extent of

Further to the north the geology is totally obscured by an area of no outcrop. However, here air photos show the location of several lagoon like features suggestive of sink holes and other collapse structures normally associated ~lith carbonate sediments. To the south and east of the sandstone any outcrop is obscured by an extensive development of coastal sand dunes (see map 1.)

Rotary drilling completed during the course of the present survey provided some further geological information. Two holes drilled through the sandstone near its northeastern margin indicated a dip of somewhat less than 3° for this location. These holes also identified the underlying conformable lithology as dark grey shales.

Other holes drilled off the sandstone have all encountered an extensive cover of white, medium grained, clayey sandstone of probable lower cretaceous age. This unit has been found to be up to 30 metres thick and is underlain in the southern portion of traverse 2 by the shales mentioned above. Several holes drilled at the north of this traverse were terminated prematurely and failed to enter the underlying lithology.


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3. SURVEY OBJECTIveS

The present survey was conducted as an initial phase of a groundwater investigation aimed at providing an additional ~ater supply for the Nuwbulwar Settlement. Numbulwar cLlrrently derives its i,vater from a number of sand spear systerns located in a surficial aquifer associated with the coastal sand dunes surrounding the settlement.

the paucity of detai led geological information the present survey was aimed primarily at elucidating the geology in an area to the north of the townshi:?, remote [yom the influence of saline 1daters.. Additionally J' some effoyt

Due to

was expended in monitoring the influence of saline groundwatel-s in areas more adjacent to the coast and the Rose River estuary.

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4. INSTRUMENTATTON AND TECHNIQUES

4.1 Resisti~ity

The resistivity instrumentation employed documented In detail in Eydrogeophysical brief specification follo",s:

at NUr;IDul"War Report 80/2 .

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transmitter

Teceiver

de to de converter c~pable of outputting up to 300 -watts at selectable volt2ge levels to 1500 volts and output currents to 2 amps.

digital millivoltmeter with a resolution of 10 microvolts and provision for automatically offsetting up to 200 millivolts of de noise voltages.

Due to the extremely dry and resistive surface cover at Numbulwar all resistivity profiling operations had to be preceeded by the emplacement of wetted metal foil electrodes to ensure adequate transmitted currents. Great operational difficulties "Were experienced during sounding operations where the use of metal foil electrodes "Was not practical.

4.2 Electromagnetic

The electromagnetic instrumentation used at Numbul"War was a commercially available terrain conductivity meter (GEONICS EM34-3). This instrument involves a dual coil arrangement in either horizontal or vertical coplanar mode with three coil separation/frequency combinations. These latter parameters have been chosen to ensure the system's operation at 10"W induction numbers over a wide range of earth resistivities. For such operation the apparent ground conductivity is definable from the instrument's observed quadrature response, and is read directly from the receiver in millimhos/meter.


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S. RESULTS

S.l Resistivity soundings

During the course of the survey a total of 11 vertical ele,tric soundings were completed with the Schlumberger array to a maximum AB/2 of 200 metres. These soundings were located at the sites indicated in map 1 and al.""e presented along with their interpretations in Appendix 1.

The quality of the sounding caLa is ralr" - occasionally poor. This unfortunate circumstance is largely the result of a dry and extremely resistive sandy surface cover existing over much of the area. These conditions caused!

1. low transmitted. currents and consequently low received signals,

2. high impedance potential circuits, and

3. unstable S.P. voltages.

The above problems were for the most part overcome bv the normal proc~dures of electrode construction, use of high vol tage transmitters and high impedance potential measuring circuitry etc.

However, the sounding data is to some extent degraded by these effects, and indeed in several instances soundings were aborted due to the operational difficulties.

In spite of the quality of the data all soundings were treated quantitatively by means of an unconstrained, least squares inversion technique. The resulting solution earth models are listed in Appendix 1 along with their computed sounding curves - the unprocessed field data is also included to facilitate a better evaluation of the interpretation.

Also produced by the present computerised interpretation scheme (although not included in this report) is a-singular value analysis of the matrix system involved in the inversion process, as well as a documentation of the extreme parameter sets consistent with the observed data and an estimated relative observation error of 5%.

It is stressed that the interpretations presented in Appendix 1 represent minimum layered, best ·fit (in a least squares sense) solutions to the field data. While the true resistivity layering is ambiguous to a greater or less extent due to the observation errors in the field data, no effort has been made to resolve this ambiguity by constraining the solutions with geological information. However, the range of the ambiguity (consistent with an estimated 5% relative error in the 'data and a 68% confidence level for the estimated solution parameters) is available in the extreme parameter sets referred to above.


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50undinqs 5/1, 5/4, 5/8, 5/9.

These soundings were all completed in the area of sandstone outcrop. For all the reasons outlined above the aualitv of these' soundings is generally poor indeed, - Si9 "was terminated prematurely due to difficulty in achieving 3.dequate transmissions. Otherwise, 'Cne data at larger ~lectrode separations can be ex~ected to be contaminated bv - . " ] arger than normal observation errors.. This clearly will result in poorer estimates of layer parameters - particularly those deeper in the geoelectric section.

Sounding 5/1 was completed over what is expected to be tIle thickest section of sandstone. In addition to showing a total thickness of some 11 metres comprising various surface layers, 5/1 clearly identi fies the sandstone as a 65 meter thick layer of high resistivity (l134 ohm-metres).

Sounding 5/8 was completed approximately 500 metres up dip of the previous location and shows an identical trend in resistivities with depth as 5/1. There are some significant differences however. In particular, the resistivity of layer 4 (sandstone) is somewhat higher here at 1364 ohm-metres. While this could reflect an increase in salinity of the formation water towards the Rose River, it is probably not significant (bearing in mind the data quality etc). The total depth to the underlying ultimate layer is substantially the same as indicated by 5/1.

5/9 was completed a further 500 metres up dip from the site of 5/8 and shows the presence of a thin second layer of extreme resistivity ( 60 000 ohm-metres). ~~ile the estimated thickness and resistivity of the sandstone at this site (layer 3) vary from what is expected on the basis of 5/1 and 5/8, it must be mentioned that these parameters derived from 5/9 are highly questionable. This obtains from the suppression of layer 3 due to its position between the extremely resistive layer 2 and relatively conductive layer 4. As a result the resistivity and thickness of layer 3 can vary over wide ranges with little significant effect on the sounding curve. The problem is further exacerbated by the poor accuracy of the sounding data at larger electrode spacings.

Not-with-standing, in common with 5/1 and 5/8, 5/9 shows the effect of a moderately conductive ultimate layer (100-300 ohm-metres) .

The interpretation of 5/4 shows (beside the usual surface layers) some 15.5 metres of a layer exhibiting a resistivity of 1282 ohm-metres - a typical figure for the sandstone. This thickness of sandstone is also to be expected on the basis of geological information and the sounding· location (See Map 1).


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Sounding S/4 however, shows the effect of an unexpected ultimate layer of resistivity 1837 ohm-metres underlying ~he normal relatively conductive layer 4. It is suggested that the terminal section of S/4 responsible for this interpretation is distorted by causes unknown. An alternative explanation-that the rising segment is the result of an interbedded resistive sandstone bed wi thin the shale unit (whose possible manifestation on resistivity profile occurs between 1450 Wand 1700 W) -is unlikely fo·r reasons indicated in section 5.2.

Soundinas S/2, 5/3, 5/10, 5/11. -----.~

ThesE' sOltnaings were completed along the nort.hern section of trav8rse 2 over lithologies not encountered elsewhere on the profiles. All show the presence of various, typically resistive surface layers to.a maximum depth of 10 metres. With the exception of 5/2 all indicate a significant layer of approximately 190 ohm-metres resi~tivity underlying these surface layers. Note that while S/2 estimates a somewhat lower resistivity (107 ohm-metres) for this layer, the credibility of the latter results is discounted to some extent by the indicated presence at this site of significant lateral resistivity changes (see resistivity traverse 2). It is probable that the top of the 190 o~m-metre layer coincides with the water table in the area.

An observation of perhaps more regional geological significance is that both S/2 and Sill indicate the presence of an ultimate relatively resistive layer under the 190 ohm-metre layer. This unit is understandably not detected by SilO at the array dimensions used due to the presence of a shallower, quite conductive layer 3 at this site. However, some distance to the south, S/3 indicates a conductive ultimate layer (17 ohm-metres) underlying the 190 ohm-metre bed. This last result should be qualified by noting that it is effectively based on the last two data points of the sounding curve whose accuracy is highly questionable. Whatever its nature, there is evidence to suggest that the depth to this ultimate layer increases substantially from the north to the south.

A comparison of SilO with the remainder of the soundings in this suite indicates the origin of the significant conductive anomaly observed on line 2 between 2800 Nand 3500 N (see sheet 1). It derives· from a conductive layer 3 ¢eveloped on top of the 190 ohm-metre bed. While its expected thickness is 35 metres, an analysis indicates that this figure could range from 25· to 46 metres with corresponding changes in its resistivity. The quite low expected resistivity of this layer (19 ohm-metres) suggests a significant component of matrix conduction due to clays etc. (The alternative explanation of saline groundwater at this site is argued against by its location.)


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Soundinq S/5.

S/5 was completed north of the settlement in the general vicinity of the existing sand spear systems. This sounding was amongst those most seriously effected by the surface conditions and l~S interpretation should be accordingly assessed~ Besides a thin resistive surface layer S/5 ir,dicates approximately 5 metres of layer 2 (105 oh:n-metre) overlying a relatively conductive ultimate layer (34 ohrn-metYes) . While the resistivity of layer 2 is quite ('()nY;Jatible Vlith reasonably clean sands saturated with fresh wa ter, the interpretation of layer 3 is ambiguous. Its low rAsistivity could either derive from a significant clay fraction or else the presence of saline water.

Sounding S/6

S/6was completed in an area until recently used for agriculture. It inclcat;es a surface layer some 6 metres thick with a resistivity of 80 ohm-metres a reasonable figure for dirty sands saturated by fresh water. Underlying this is a substantial thickness (33 metres) of very conductive material (6.8 ohms-metres). The same options are available here concerning tnis layer's significance as were discussed in conjunction with S/5 .

Sounding S/7

S/7 was completed J..n a southern extension of that area of salt-flats shown to the east of the airstrip in Map 1. The extremely low interpreted resistivities indicate that salt water contamination (·obviously from a surface source) penetrates the section to significant depths.

5.2 Resistivity profiles

As part of the orientation phase of the present survey four Schlumberger vertical electric soundings (S/l, S/2, 8/3, S/4) were' completed at sample sites throughout the area of investigation. Based on 'these soundings a profiling array was designed to best accomplish the stated survey alIDS. This array' comprised a pole-dipole electrode configuration with the follm.;ing parameters,

(1) A potential dipole of 50 metres,

(2). a current electrode separation of 100 metres, and·

(3) a station spacing of 50 metres.

In a sounding context such an arrangement loosely corresponds to. a Schlumberger configuration with an AB/2 of somewhat less than 125 metres. An inspection of appendix 1 suggests' that this point on the sounding curves effectively monitors the basement resistivity (geology) while also being sensitive to shallower effects .


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All resistivity profiles are shown in Sheet 1 plotted at a logarithmic ordinate scale and a horizontal scale of 1:100 000. Also shown are the array details and the plotting ,:onvention employed. The location of the profiles is indicated in Map 1. Line 1 ---- The western half 0-: this profile shows the extremely .(t2sistive response of the sandstone uDit~ The eastern half exhibits apparent resistivities of up to two orders of maani tude lower which clearly derive from the significantly If,ore conductive shale unit .. - The boundary between these tw'o litholooics is well defined at between 1800 Wand 1850 Wand (paradoxically) appears to dip fairly steeply.

The isolated resistive anomaly located bet\veen 1450 W and 1700 H deserves some CO,[ullent. This could possibly reflect a thin, resistive (sandstone ?) interbed within the shale unit. In this case however, the shallow dip of such a unit to ~ne west would produce a much smaller gradient bounding the anomaly in this direction. Moreover, the same feature could sensibly be expected at the same stratigraphic position on other profiles.

1-1hile the above explanation is to SOme extent substantiated by S/4 a more favoured explanation for both the form and magnitude of the anomaly is that it represents the effect of a small remnant of sandstone isolated by the westward retreat of the scarp bordering this unit.

The broad conductive indication superimposed on the sandstone response between 2500 Wand 3500 W is somewhat amD~guous. It could represent an increase in thickness and/or conductivity of the surface layers (weathered zones etc) at this location. Conversely it may indicate the presence of less resistive interbeds wi thin the sandstone uni t in which case the former explanation will also be relevant due to the likelihood of differential weathering of such features.

Line 2. (south)

This line crosses the same units (although at a much more oblique angle) as line 1. The responses of the various lithologies are quite similar to those of the previous line with a notable exception the apparent, resistivities observed over the shale unit are . substantially lower in magni tude than those observed on line 1. Likewise, - the response pattern over this unit is somewhat smoother than on the previous line - although this effect may be explained to some 'extent by the more oblique orientation of traverse 2 with respect to the strike of the unit.

The Im"er apparent resistivities observed over the shale unit app-ear to reflect an increase in the thickness and/or the conductivity of the surficial layers. (It is unlikely it reflects an ~ncrease in bedrock conductivity). Rough calculations indicate that it would require an increase in total longitudinal conductance (5) of the suprabasement layers of approximately 2.5-times to account for the observed effect. Likely causes of such an increase in S at this near shore location are:


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1. Deep accumulations of saturated sands,

2. the occurrence of conductive clay layers, or

3. possible salt water contamination of the groundwater.

\~Jhatever its cause, this effect becomes most pronounced around 3000 S.

The sandstone response appears strongly in the far south of the profile and indicates the nor~nern boundary of this unit at approximately 3400 S. Again, this boundary appears to be near vertical in orientation.

A symetric, isolated resistive anomaly occurs between 600 Sand 900 S. For the same reasons as indicated previously in a discussion of a similar anomaly on line I, this present anomaly probably derives from an isolated remnant of sandstone at this location.

Line 2 (north)

This line traverses lithologies not otherwise encountered on the resistivity profiles: In general it exhibits intermediate apparent resistivities (measured with the present electrode array) substantially higher than those observed over the shale unit.

Several closed conductive significance appear on this line.

indications of They include:

1. A broad zone lying between 500 Sand 500 N,

possibly

2. A small amplitude anomaly located between 1000 Nand 1400 N,

3. A narrow anomaly centred at 2800 N, and

4. A broad, intense indication between 2900 Nand 3600 N.

The last two of these anomalies were considered of particular sign~I~cance and were further investigated with electromagnetic profiling and resistivity sounding_ The significance attached to these anomalies derived in part from their nature and magnitudes, as well as from their proximity to lagoons and depressions suggesting sink holes (see map 1)

5.3 Electromagnetic Profiles

Electromagnetic profiles were 2000 N to 4000 N. The results the resistivity profiles of the

completed along line 2 from are plotted in Sheet 1, below same line.


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In order to obtain some depth control on the previously observed re sistivi ty anomalies two coil configurations were employed at a constant intercoil spacing of 40 metres. The horizontal coplanar coil configura'tion senses significantly 6eeper than the vertical coplanar arrangement.

Both configuratiorls show an almost identical response over the northern portion of resistivity anomaly 4. Significant differences exist over the southern portion of +:h i s arlomaly however.. In particular the vertica 1 coplanar response indicates a continuity of anomalous conductivity at ~;ha i lOvl d.epth bet~ween resistivity anomal ies 3 and 4.. The deeper sensing horizontal co?lanar measureillents on the other hand indicate that the southern extremlty of Lne deeper conductivity distribution lies at approximately 2950 N.


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APPENDIX 1:

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Schl~uberger vertical electric soundings and interpreted earth models.

Technical R

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SCHLUMBERGER ELECTRICRL S~UNDING

RREA: NUMBULWAR

DESIGNATIljN; S/l

MljDEL PARAMETERS-> LAiER RESISTIVlTY THICKNESS

1 10942.09 0.67

2 1567.63 5.6

3 3395.53 4.69

4 1134.64 64.89

5 256.69

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DES I GNAT I ~N: S/2

M~DEL PRRRMETERS-> LAYER RESISTIVITY THICKNESS

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1765.85

416.67

107.98

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RRER: NUMBULWRR

DE51GNRTI~N; 5/3

M~DEL PRRRMETER5-> LAYER RES 1 STl V lTY THICKNESS

1 887.03 0.99

2 264.51 4.08

3 1159.97 3.2

4 193.78 169.51

5 17.38

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RRER; NUMBULWRR

DESIGNRTI~N; S/4


1 1242.63 1.2

2 10224.02 1.8

3 1281.97 15.47

4 368.38 14.27

5 1837.89

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SCHLUMBERGER ELECTRICAL S~UNDING

AREA; NUMBULWRR

DESIGNATlelN: S/6

MeJDEL PRRRMETERS-> LAYER RESISTIVITY THICKNESS

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2

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RRER; NUMBULWRR

DES I GNRT I (jN; S/7

M(jDEL PRRRMETERS-> LAjER RESISTIVITY THlCKNESS

1 4.44 1. 92

2 0.27 1. 33

3 14.67

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RRER: NUMBULWRR

DES 1 GNRT I ClN: S/8

MClDEL PRRRMETERS-> LAIER RES I ST I V ITI 1H 1 CKNESS

1 E780.47 D.9E

2 648.19 1.92

3 7697.78 3.36

4 1364.37 75.16

5 116.35

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ed at 14:07:31 on 29/07/2010P

age 24 of 26.

I

I

II II I II I I I I I I I I I I I I

SCHLUMBERGER ELECTRICAL S~UNDING

AREA; NUMBULWAR

DE5IGNATI~N; 5/9

M~DEL PARAMETERS-> LAYER RES I 5T 1 V ITY THICKNESS

1 11140.55 2.33

2 61814.23 3.33

3 4588.18 29. 11

4 259.87

hfjur'e A. ~ .. ~~-----

1\ I

~

tr)

4J

C)

C)

C)

o C)

...-.


MClDEL CURVE. •.• -

~ ~.

~o ~ I 0 ::EO :c Cl rD .-.

>t-.-.. > ...... tIf) ..., If) LiJ 0:::0

C)

r-- 0 z ...... w n::: a: 0.... 0.... a:

o

.&

1000 ~ L ______ -:+::-______ -;;,;;--_ __.

1 10 100 AB/2 (METRES)->

-~ ......... ~----~---------.--

Technical R

eport WR

D83095

View

ed at 14:07:31 on 29/07/2010P

age 25 of 26.

~ I I I I I I I I I I


RRER; NUMBULWRR

DESIGNRTl~N: SilO

M~DEL PRRRMETERS-> LA'l'ER RESISTIVlT'l' THICKNESS

1 1578.37 2. 16

2 641. 16 7.81

3 18.67 34.72

4 184.56

1\ I

r-"

t.r)

4-l n::: tw ::c

o o o o .-.

I a ::co :1:0 to ..... ~

~ t...... > >-;

t(/j ...... t.r)

w n:::

a J- Cl Z .-. tl-l n::: a: a... a... a:

-~

/1/ -~

fIELD DATA ..•• ~ ~

MClDEL CURVE •••• -

~L-------~----~~~-----l~ 1 10 100 1000

hgtJf7C A. 10. AB/2 (METRES)->

---.-.--~-~---------------------~----

Technical R

eport WR

D83095

View

ed at 14:07:31 on 29/07/2010P

age 26 of 26.


AREA: NUMBULWRR

DESIGNATI~N: S/ll

MLJDEL PRRAMETERS-> LAYER RESISTIVITY THICKNESS

1 5573.38 0.76

2 938.82 2.4

3 319.69 9.45

4 190.21 76.12

5 801.48

FigcJT'e A .11.

f\ I

~

IF)

4.1 tl:: t-

C) C) C) C)

C) .....

W :::£:0 10

:::£:0 :co to .....

>-1-t-;

>->-1

tIF) t-t

IF)

4-1 tl::a

Cl t- = Z .-. 4J tl:: ee a_ (L ee

Cl Cl


MeJDEL CURVE .... -

"-"4"'/ .--. • --------4

1 10 100 1000 AB/2 (METRES)->

---~------ .-~ --------------------- ----------_._----------------------------

Iof some 82 measurements of apparent conductivity. Technical Report WRD83095 Viewed at 14:07:31 on...

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Transcript of Iof some 82 measurements of apparent conductivity. Technical Report WRD83095 Viewed at 14:07:31 on...