Technical Report WRD87050 · 2019-08-14 · Technical Report WRD87050 Viewed at 15:07:09 on...

Technical Report WRD87050

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I POWER AND WATER AUTHORITY

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HYDROGEOPHYSICAL REPORT 87/4

I RESISTIVITY PROFILES - KAKADU NATIONAL PARK

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Peter Furness

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CONTENTS

1. INTRODUCTION

2. INSTRUMENTATION AND PROCEDURES

3. RESULTS

APPENDIX A:

APPENDIX B:

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GEOLOGICAL HAPS SHOWING LOCATION OF RESISTIVITY TR~VERSES

POLE DIPOLE APPARENT RESISTIVITY PROFILES


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1. INTRODUCTION

During the period December 1 to 12, 1986 a number of resistivity profiles were completed within the Kal@du National Park by geophysical staff of the Water Resources Division of the NT Department of Mines and Energy.

In total this work comprised approximately 20.5 line km of single spacing apparent resistivity profiling distributed over 14 individual traverses in six district locations. The work was completed by a full crew comprising one operator and two assistants in a total of five working days.

The aim of the present survey was generally to elucidate the proterozoic geology in the selected areas with particular interest in possible structural elements within those lithologies.

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2. INSTRUMENTATION AND PROCEDURES

Most traverses on the present survey were completed with a standard pole dipole resistivity array hawing the following parameters:

1. A potential dipole of 50 metres,

2. A current pole, potential dipole spacing of 100 metres and

3. A minimum remote current electrode separation of 100 metres.

The traverse at Boggy Plains, however employed an array of considerably larger dimensions in order to achieve a greater depth of investigations and also to facilitate easier comparisons with previous work in the general area. For these reasons (and in this location only) a current pole, potential dipole spacing of 250 metres was used.

In all cases a station spacing of 50 metres was employed.

Details of the dc resistivity instrumentation used on the profiles are as follows:-

Transmitter:

Receiver:

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A moderate powered dc transmitter capable of outputting up to 300 watts at current levels to 2 amps and voltages to 1200 volts.

Fluke DMM model 8062A with an input impedance of 10 megohm shunted by a 10 pF, a dc voltage resolution of 10 microvolts and provision for the automatic offset of noise voltages.


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3. RESULTS

The locations of all traverses are indicated on ~the maps of Appendix A. These have been reproduced at a magnification of xl.S from the East Alligator and Cahill 1:100 000 Australian Geological Series Sheets.

All represent resistivity profiles are shown in Appendix B, plotted at a logarithmic ordinate scale of 10 em to one decade. Array orientation and details are included in each illustration.

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RANGER HEADQUARTERS

These lines traverse archaen schists and gneiss of the Nanambu Complex largely observed by a thin cover- of unconsolidated sands and laterite (see Map 1 for details).

The profile data show basically two types of apparent resistivity behaviour ie:

1.

2.

broad zones of relatively apparent resistivities ohm-metres, and

conductive response with of less than 200

broad zones of relatively showing apparent resistivities of 200 ohm-metres.

resistive response generally in excess

Since only single separation profiles were completed the problem in evaluating the present data is that of deciding whether the causation of the above responses is due to resistivity changes within the basement rocks (ie lithology and structure) or due to resistivity and/or thickness changes in the typically more conductive overburden (ie surficial cover and weathered zone) or indeed due to a combina tion 0 f all these effects.

Parallel lines 1 and 2 show similar behaviour with a resistive western section and predominantly conductive eastern section. The transition separating these two zones is abrupt on each line and defines an azimuth that is in general agreement with the regional strike of the Nanambu Complex in this location. This argues in favour of a basement lithology causation of the above two response types in the present instances from schists? in the east to more resistive gneissic? rocks in the Vlest of each traverse. The rather irregular distributions of the tvlO apparent resistivity responses on line 3 would then possibly result from shallow excursions of this traverse into the two lithologies.

Again it should be cautioned that the above interpretation is only one of several possibilities. Indeed the possibility of the responses being due to changes in the resistivity and/or thickness of the surficial layers is particularly significant.

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The location of producing bores drilled in schist in the vicinity of SOOw on line 2 suggests that this aquifer may be available throughout the broad conductive zone to the west of 1500w (and best targeted at 1250w).

On line 1 the eastern conductive zone is generally more resistive than on line 2 but with a relatively intense conductive indication lying between 8S0w and 1050w. This feature appears to be the most significant indication on line 1 and is possibly continuous with the producing conductive zone on line 2.

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BOGGY PLAIN

This profile was completed on the southern side of the Arnhem Highway at a location some eight kID east of~the South Alligator River (see Map 2 for details). The basement rocks comprise archean granite, gneiss and schist of the Nanambu Complex as well as schists with carbonate and gneiss of the lower proterozoic Cahill Formation.

Apparent resistivities observed on the western section of the profile are generally high averaging approximately 500 ohm-metres. These clearly reflect the resistive lithologies of the Nanambu Complex. Nanambu Complex rocks can be expected to be more resistant to weathering processes and the consequent lack of any significant depth of weathering will also contribute to the high apparent resistivities.

East of 200E significantly 100.;er apparent resistivities are observed over the more conductive and deeply weathered Ii thologies of the Cahill Formation. The probable location of the boundary separating the above two lithologies is in the vicinity of 2125E.

"Jithin the generally more conductive lithologies of the Cahill Formation there is evidence of:

1.

2.

An extremely narrow conductor extending to the near surface and centred at approximately 2525E.

A narrow resistive body with little depth of burial centred at approximately 2900E.

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OBIRI ROCK

Two small resistivity lines were completed in the vicinity of Obiri Rock to explore for any faults. or joint developments that could possibly contribute' to the development of secondary porosity within the sandstones of the Kombolgie Formation. Available access was severely limited by the frequent occurrence of residual outcrops of sandstone at this location (see Map 3 for details) .

Both resistivity profiles show the large resistive responses typical of the extremely resistive Kombolgie Formation sandstones.

The decreasing apparent resistivities noted to the east of 200W on line 1 and north of 400N on line 2 probably result from the onset of shallow conductive floodplain deposits at these locations and are not considered meaningful in terms of the resistivity of the sandstone.

The e:<tremely small amplitude conductive indications centred at 700W on line 1 and 225N on line 2 define a zone trending approximately north east. This may reflect a local increase in the depth to unweathered sandstone and as such is a possible source of a limited groundwater su?ply.

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JIM JIM ROAD

Lines 1, 2 3

These three short lines were completed in an attempt to locate the southern extremity of the Jim Jim Fault which at this location penetrates quartz mica schists of the lower proterozoic Nourlangie Schist (see Map 4) .

The resistivity data is extremely noisy - particularly on the northern lines. This behaviour correlates with the occurrence of extreme surface resistivities as evidenced by low transmitted currents and received signals.

Apparent resistivities on the northern two lines are anomalously low for the present lithologies and electrode array. It appears that the intensity and/or depth of weathering in the schists increase from south to north in this location.

No sensible indication of any fault like structure can be made from the present data.

Lines 4, 5

These two short lines were completed some five km to the south east of lines 1, 2 and 3. They were located to position an inferred fault trending north by north west in the Nourlangie Schist.

The apparent resistivity data again appear quite noisy. However, both transmitted currents and received signals are quite strong implying that the origin of this noise is geological and not related to instrumentation or procedure.

The change in character of the data on line 4 east of 500E to a smooth, broad apparent resistivity low probably reflects the dolerite bedrock mapped at this location. The same effect appears to be present west of 250E on line 5. Simalarly, the high apparent resistivities noted on this latter line east of 550E probably derive from a residual block of resistive Kombolgie Formation mapped in the vicinity.

Again, no sensible indication of the inferred fault can be made.

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COOINDA AIRSTRIP

This line was completed along the northern boundary of the Cooinda Airstrip (see Map 5 for details).

The objective was to locate the Jim Jim Fault which in this location disrupts lower proterozoic Cahill Formation (schists, gneisses and quartzites) .

The very low apparent resistivities observed on the western portion of the profile are probably the result of very conductive and surficial floodplain deposits. This data is not considered significant in terms of the resistivity of the proterozoic basement rocks.

It is suggested that this profile does not adequately cover the probable location of the Jim Jim Fault (see Map 5 for inferred position). It is recommended that the profile be continued westwards or south westwards to the banks of the Cooinda Waterhole.

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NOURLANGIE CAMP

Two lines were completed in this location (see Map 6 for details) in order to define the faulted? bOLlnd.ary between the Nanambu Complex rocks to the west and 'the generally more conductive Cahill Formation lithologies to the east.

The northern sections of both lines show apparent resistivities that are anomalously low for the Cahill Formation and the present electrode array. Presumably they derive in large part from the conductive quarternary cover. This is almost certainly the case for the very low responses seen sOLlth of 12008 on line 2 and indeed probably influences the entire northern half of line 1 to some extent.

The onset of higher apparent resistivities south of 700S on line 1 and south of 9008 on line 2 appear to reflect the onset of the more resitive and presumably less weathered Nanambu lithologies to the west. Very approximate locations for this boundary are 6508 on line 1 and 900S on line 2.

The cause of the low apparent resistivities between 5508 and 7508 on line 1 and 4505 and 900S on line 2 is uncertain. These may both be due to a zone of the conductive cover mentioned above.

Alternatively they may reflect resistivity changes within the lower proterozoic basement rocks in which case the likely causation would be a very narrow conductor centred at 6505 on line 1 and a broad conductive zone extending from 5508 to 9008 on line 2. It should be mentioned however that the amplitude of the above indications argues to some extent in favour of a surficial causation.

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Technical Report WRD87050 · 2019-08-14 · Technical Report WRD87050 Viewed at 15:07:09 on...

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