Geophysical Ground Surveys Over Lightning Ridge Opal Prospects

GEOLOGICAL SURVEY OF NEW SOUTH WALES
DEPARTMENT OF MINERAL RESOURCES
Geophysical Ground Surveys
Over Lightning Ridge Opal Prospects
By
M. Moore
(Geophysicist)
Geological Survey Report Nº: GS2002/442
Date: Sept. 2002
Map Reference:
1:250 000:
ANGLEDOOL - SH/55-7
1:100 000:
CUMBORAH - 8438;
LIGHTNING RIDGE - 8439
© Department of Mineral Resources
The information contained in this report has been obtained by the Department of Mineral Resources as part of the policy of
the State Government to assist in the development of mineral resources and furtherance of geological knowledge. It may
not be published in any form or used in a company prospectus, document or statement without the permission in writing of
the Director General, Department of Minerals and Energy, Sydney.
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TABLE OF CONTENTS
1. INTRODUCTION
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2. BACKGROUND
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3. OPAL – THE LIGHTNING RIDGE MODEL
3.1 Geology
3.2 Geophysics
3.2.1 Prior Studies
3.2.2 Regional Surveys
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4. SURVEY CONSIDERATIONS
4.1 Precursor
4.2 A Geophysical Electrical Model
4.3 Field Acquisition
4.3.1 The Resistivity Survey Method
4.3.2 The Electromagnetic Survey Method
4.3.2 The Magnetic Survey Method
4.4 Data Processing
4.4.1 Resistivity Data Processing
4.4.2 Electromagnetic Data Processing
4.4.3 Magnetic Data Processing
4.5 Trial Areas
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5. RESULTS
5.1 “Crusty’s” Study Area
5.1.1 Resistivity Survey
5.1.2 EM Survey
5.1.3 Ground Magnetic Survey
5.1.4 Drill Results
5.2 Grawin – Mulga Rush Study Area
5.2.1 Resistivity Survey
5.2.2 EM Survey
5.2.3 Ground Magnetic Survey
5.2.4 Drill Results
5.3 Old Goodooga Road Traverse
5.3.1 EM Survey
5.3.2 Ground Magnetic Survey
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6. DISCUSSION
6.1 Resistivity
6.2 EM
6.3 Ground Magnetics
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7. CONCLUSIONS AND RECOMMEDATIONS
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8. REFERENCES
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LIST OF FIGURES
Figure 1
Figure 2a
Figure 2b
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15
Figure 16
Figure 17
Figure 18
Figure 19
Figure 20
Figure 21
Figure 22
Figure 23
Figure 24
Figure 25
Lightning Ridge location diagram
Landsat-7 false colour composite image
Landsat-7 clay ratio pseudocolour image
Calweld 3-foot drill rig
Apparent resistivity SIROTEM section
Pseudocolour image of digital topographic
data over part of the Angledool map sheet
Ternary colour image of potassium,
thorium and uranium radioelement data
Pseudocolour gravity data image
Pseudocolour total magnetic intensity
image
ABEM resistivity equipment
EM-31 instrument in operation
GEM GSM-19 magnetometer in operation
Geo-electrical model of the opal-hosting
environment and forward calculated
apparent resistivity response
Lund imaging system resistivity equipment
Resistivity profile cable and electrode
positions
Wenner-alpha array
Inverted model of the proposed 2-D model
presented in Figure 12
Location of "Crusty’s" prospect and the
Old Goodooga Road traverse study areas
Location of Grawin-Mulga Rush trial site
"Crusty’s" study area showing Wenner
array line locations and drill holes, and
EM-31 and ground magnetic data
responses
Apparent resistivity pseudosections over
"Crusty’s" trial area
Inverted electrical resistivity results over
"Crusty's" area
Mulga Rush study area showing Wenner
array location, drill hole locations,
observed and modelled resistivity sections,
and EM-31 and ground magnetic data
responses
Drill logs for the Grawin trial site
Old Goodooga Road geophysical profile
data, showing ground magnetic, EM-31 and
SIROTEM traverse results
3-D inversion results showing depth slices
of the modelled data
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1. INTRODUCTION
During December 2000 the NSW Department of Mineral Resources (NSWDMR)
conducted a pilot geophysical study over prospective opal areas near Lightning Ridge in
north central New South Wales (Figure 1). The emphasis of the study was to trial an
electrical resistivity profiling technique with computer modelling software to delineate
prospective opal-bearing clay-horizons. Ground magnetic and electromagnetic (EM-311)
methods were also trialed. The Lightning Ridge Miners Association provided logistical
assistance, guidance in selection of appropriate trial areas and supply of drill data over the
study areas.
The applicability of each geophysical method as a prospecting tool for general use by the
miners was assessed. This included the suitability and ease of use of the equipment and the
interpretability of the data for each trialed method.
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Use of proprietary or brand names throughout this document does not indicate endorsement of the product
by the New South Wales Department of Mineral Resources.
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Kilometres
Figure 1. Lightning Ridge location diagram (CMA Tourist Map – Lightning Ridge Opal Fields)
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2. BACKGROUND
Lightning Ridge is one of two major opal fields located in New South Wales. In 1998 1999 opal production in NSW was valued at approximately $44 million (NSWDMR,
2000). This was about half of Australia’s estimated annual opal output for that year.
Lightning Ridge is best known for its production of Black Opal and is the world’s major
producer of this variety of precious gemstone.
Since 1989 about 5000 square kilometres of land surrounding Lightning Ridge has been
released for opal exploration. Title holdings in the area are governed by the NSW Mining
Act (1992) and include the choice of either an exploration license, an opal prospecting
license (OPL), a mining lease, or a mineral claim. Prospecting and exploration at and
around the Ridge is predominantly carried out under mineral-claim titles. Claims are
restricted to a one-quarter hectare block (50 x 50 m), with two claims allowed per
individual. Opal prospecting, exploration, mining and processing activities also compete
with shared land uses such as livestock grazing, agricultural cropping and conservation
reserves.
While some miners still use traditional prospecting indicators and methods, over the last
decade there has been an increased interest in the use of modern exploration techniques.
This includes the use of air photos, satellite imagery (Figures 2a and b), geological ground
traversing and exploratory drilling.
The introduction of the nine-inch auger and the three-foot “Calweld” drill rigs (Figure 3),
have provided a relatively cheap and quick way of testing the prospectivity of mineral
claims. The contract rate for a nine-inch auger hole in 2000 was about $10 per metre, while
that of the three-foot hole was about $45 per metre. Increasing moisture content with depth
generally restricts drilling to around 30 m, although the prospective horizon is usually
encountered within 15 m of the surface. Up to four auger or two Calweld holes can be
achieved per day, where ground conditions allow. The Calweld rig is often used as a
prospecting tool and has the added advantage of providing an immediate access shaft. The
relative low cost and the ability to immediately evaluate the opal potential of the intersected
clay horizon makes drilling an ideal prospecting tool.
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Figure 2a. Landsat 7 – “false colour composite”
image. The colour image has been draped over an
east-illuminated topographical model. (Bands 3, 2
& 1 have been assigned to the red, green and blue
colour planes respectively).
Figure 2b. Landsat 7 – “clay ratio” pseudocolour
image. The image has been intensified with an
east-illuminated topographical model. Red
represents areas of high surface clay content; blue
represents low surface clay content. (The clay
ratio has been calculated using ERMapper’s
standard image processing algorithms).
Figure 3. Calweld 3-foot drill rig.
Despite the increased use of modern techniques, the miners at Lightning Ridge are finding
it increasingly difficult to locate further occurrences of opal within or adjacent to the
existing fields. As a consequence the Lightning Ridge Miners Association approached the
NSWDMR to provide assistance in the continued search for opal. The December 2000
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trials followed on from previous geophysical work carried out by the Department in the
Lightning Ridge area.
The completion in 1984 of a geological study by John Watkins (GS Report 1985/119)
closely examined the geological setting and formation of opal within the Lightning Ridge
province. This report highlighted the potential for geophysical methods to locate the
prospective, uppermost, opal-bearing claystone layer. It was also suggested that successful
use of geophysics could significantly reduce the cost of prospecting by reducing the
number of drill holes required to adequately test a mineral claim. Watkins proposed that a
geophysical success would lead the way to regional evaluation of the prospectivity of the
outer lying areas of the opal field. The report contains a comprehensive discussion of the
geological setting, opal genesis, mining techniques, estimated reserves and the
prospectivity of the region.
To follow-up the recommendations made by Watkins the Department has undertaken a
number of pilot geophysical studies. Electrical resistivity and Time Domain
Electromagnetic (TEM) surveys showed the most promise and are discussed further in
section 3.2.
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3. OPAL – THE LIGHTNING RIDGE MODEL
3.1 Geology
Opal at Lightning Ridge occurs within highly weathered, near-surface, Cretaceous
sediments. Watkins (1985) reported that these are “strongly argillized with feldspar
completely altered to kaolinite”. Locally the sedimentary sequence has been divided into
two distinct members, these being the Coocoran Claystone Member and the Wallangulla
Sandstone Member.
The Coocoran Claystone Member generally occurs as a 2 to 4 m thick fine-grained
claystone unit, although up to 11 m thicknesses having been reported. This unit when
silicified is commonly known as “shincracker”. It is the uppermost member in the
sequence. In contrast, the underlying Wallangulla Member is a fine to medium-grained,
clayey sandstone. It is generally observed to be about 4 to 6 m thick, although up to 20 m
thicknesses have been encountered. Opal is found at the top of the Finch clay facies within
the Wallangulla Member in the immediate vicinity of the Lightning Ridge township. This
facies is a soft, almost grit-free claystone composed of kaolinite, smectite and illite clay
minerals. A thin (up to 30 cm) silicified layer, known locally as “steel band”, is observed to
discontinuously overly the uppermost Finch claystone lenses, and, where it exists, is mostly
opaline. Iron oxides are commonly observed to coat joint surfaces within this Cretaceous
sequence. In the outlying opal fields and prospective ridge country Watkins (1985) reported
that the hosting Cretaceous sediments are “similar to, but are not correlatable with, the
sediments which host the opal at Lightning Ridge”.
Opaline material has also been observed to be associated with local jointing, structures and
breccia zones, and adjacent to the intersection of regional linear features (Watkins, 1985).
Lineaments have been interpreted from aerial photographs and satellite imagery, and are
postulated to be the conduits for fluid silica movement during the weathering process,
which is attributed to opal formation (Watkins, 1985).
Generally, opal occurs beneath a relatively thick, fine-grained, sandstone layer at the
interface of the first impermeable clay bed (Watkins, 1985). The gemstone is found in
either sheet or vein form, or as small (approximately 200 to 400mm in size) roughly egg-
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shaped nodules (“nobbies”) in brecciated zones (“blows”), or along local fault or joint
structures (“slides”) which disrupt the stratigraphic sequence.
3.2 Geophysics
3.2.1 Prior Studies
The thin (1-10 centimetres) nature of opal seam, or sporadic occurrence of nobbies,
and the thickness of the covering sedimentary sequence (10-15 metres) prohibits
direct detection of opal with surface based, geophysical techniques. In prior
NSWDMR geophysical trials the objective was to detect the hosting clay lenses, or
layers, within relatively thick occurrences of weathered sandstone.
Between 1986 and 1990, the Department trialed electrical resistivity (Schlumberger
soundings), electromagnetic (TEM), (Leys, 1987 and 1990), seismic reflection, and
ground penetrating radar (Leys and Palmer, 1987) techniques in the Lightning
Ridge area.
The result of these trials showed that a small (25 metre), coincident loop transient
EM (TEM) survey resolved the electrical structure of the opal bearing environment,
(Leys, 1990 and figure 4). Seismic methods were not pursued further due to the
high cost of processing, while instrumental problems plagued the evaluation of the
GPR unit (Leys and Palmer, 1987).
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Ch 1
Delay
Channel
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Ch 17
Figure 4. Apparent resistivity SIROTEM section – Blue zones depict areas of electrically conductive earth
while red areas are zones of greater apparent resistivity. (Old Goodooga road 25 m, coincident loop,
SIROTEM traverse - Leys, 1987a & b).
The electrical and electromagnetic methods defined a physical contrast between the
opal hosting claystone and the overlying sediments. Generally, the electrical
resistivity soundings resolved the geo-electrical section into three layers (Leys,
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1987a & b) while the TEM sections also showed Watkins' (1985) linear features as
zones of higher apparent resistivity (Figure 4).
3.2.2 Regional Surveys
Regional geophysical data was acquired over the area as part of the NSWDMR's
1995 Discovery 2000 coverage of the Surat Basin. Airborne magnetic, radioelement
and digital elevation data were acquired on east-west, 400 m spaced, parallel flightlines, flown at 80m above the terrain. Ground based gravity data was acquired on a
4 km spaced grid. Images of these geophysical parameters over the Lightning Ridge
district are displayed in Figures 5, 6, 7 and 8.
Figure 5. Pseudocolour image of digital
topographic data extracted from the 1:250 000
Angledool map sheet. A north-east sun
illumination intensity filter has been applied. Red
is synonymous with higher topography. (Area as
in Figure 2.)
Figure 6. Ternary colour image of potassium (K),
thorium (Th) and uranium (U) radioelement data.
K-Th-U have been assigned to red-green-blue
colour planes respectively. The image has been
intensified with a north-east-illuminated
topographical model (Figure 5).
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Figure 7. Pseudocolour gravity data image. A
north-east sun illumination intensity filter has
been applied. Red represents higher gravity
values.
Figure 8. Pseudocolour total magnetic intensity
image. A north-east sun illumination intensity
filter has been applied. Red represents higher
magnetic values.
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4. SURVEY CONSIDERATIONS
4.1 Precursor
In June 1998 Leys and Robson presented a paper at the First Lightning Ridge Opal
Symposium in which modern geophysical field and interpretive techniques were discussed
with respect to their use in the search for opal. The authors proposed to test modern
geophysical techniques applicable to the detection of the opal-hosting environment. Ease of
use, data accessibility and data interpretability were identified as key criteria in evaluating
the potential of a geophysical technique as a prospecting tool.
In December 2000 the NSWDMR conducted a field program to test three geophysical
techniques capable of meeting these objectives. This study included the use of a “semiautomatic” resistivity profiling system (an ABEM-LUND instrument combination - Figure
9); a simple, shallow sensing, EM profiling instrument (a Geonics EM-31 system - Figure
10); and a rapid sampling magnetometer (a GEM GSM-19 system - Figure 11).
(a)
(d)
(b)
(c)
Figure 9. (a) ABEM SAS4000 resistivity instrument; (b) ABEM (LUND) electrode selector ES464; (c) the
two instruments in combination; and (d) the complete resistivity set-up.
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Figure 10. EM-31 and data logger in operation in the opal fields.
Figure 11. GEM GSM-19 rapid sampling magnetometer in use.
4.2 A Geophysical Electrical Model
A simple, two-dimensional (2-D), geo-electrical model of the opal-hosting environment
found at Lightning Ridge had been identified from previous work. The model consists of a
thin (less than 1-2 m), resistive (100 - 500 ohm m) surface layer (gravels and or silcrete “shincraker”); a thicker (approximately 10 to 20 m), electrically intermediate (50 - 100 ohm
m) layer (sandstone); and electrically conductive (1 - 5 ohm m) layers or lenses (claystone),
hosted within the sandstone unit. Figure 8 depicts the 2-D model and forward-calculated
apparent resistivity pseudosection response.
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0
16
32
48
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30
64
80
34561
N-layer
2
10
5
500
50
24681012-
5
Apparent Resistivity (Wenner Alpha) in Ohm.m
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32
75
50
Unit Electrode Spacing = 4 m
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64
80
Depth (M)
0
Figure 12. Geo-electrical model of the opal-hosting environment and forward calculated apparent resistivity
response. (Clay lenses are depicted in green, sandstone in pale blue and surface-materials in dark blue. Red
colours in the pseudosection signify higher apparent resistivity values, blue represents lower apparent
resistivity – values as per colour key). The model was generated using program RES2DMOD (Loke, 1999a).
4.3 Field Acquisition
4.3.1 The Resistivity Survey Method
An ABEM SAS4000 Terrameter, LUND automatic switching box, and four 50 m
cables with electrode take-out positions every 2 m, were used for the survey
(Figures 9 and 13). Mild steel (8 and 10 mm diameter) tent pegs were used as
electrodes. These were positioned at every 4 m along the four adjoining cables to
provide a total coverage length of 160 m. Additional electrodes were placed along
the inner two cables to provide a minimum electrode spacing of 2 m within the
centre of the spread (Figure 14). An expanding Wenner-alpha electrode array
(Figure 15) was used because of its greater sensitivity to a horizontally stratified
earth.
The acquisition system was programmed to automatically acquire each complete
section. Each line was comprised of eight electrode a-spacings (4, 8, 12, 16, 24, 32,
40 and 48 m) for the 4 m electrode separations, and three in-fill a-spacings (2, 4 and
6 m) for the 2 m electrode separations. This produced an effective coverage length
of 100 m for a 24 m electrode separation (70 m for a 32 m electrode separation; 45
m for a 40 m electrode separation; and 20 m for a 48 m electrode separation). The
tighter spaced 2 m central electrode configuration (wenner_s - short - mode on the
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ABEM program configuration menu) was used to characterise the electrical
properties of the near surface.
Watering of the electrodes was employed to enhance electrical contact and improve
data quality. However, despite these efforts some rogue data values were
encountered.
Figure 13. Resistivity equipment (Lund imaging system).
Figure 14. Resistivity profile cable and electrode positions. (ABEM Instruction Manual).
C1
n-a
P1
n-a
P2
n-a
C2
Figure 15. Wenner alpha array - current and potential electrode positions, and a-spacing.
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4.3.2 The Electromagnetic Survey Method
A Geonics EM-31 instrument (Figure 10) was used to survey the study areas. The
EM-31 instrument has an operating frequency of 9.8 kHz, a fixed inter-coil spacing
of 3.66 m , and can be used by a single operator.
EM-31 is sufficiently sensitive to detect small changes in conductivity over small
(in the order of metres) spatial distances. The instrument has a penetration depth of
up to 6 m, with a lateral sensing volume approximately equivalent to the penetration
depth (based on either a homogenous or a horizontally stratified earth model McNeill, 1980). EM-31 is operated in either the vertical or horizontal dipole mode.
Both modes provide monitoring of the quadrature and in-phase earth response,
although penetration is reduced to about 3 m in the horizontal dipole position.
Under ideal conditions the quadrature response is linearly proportional to
conductivity.
For the study, the instrument was operated in the vertical dipole mode (ie horizontal
coplanar coil orientation) to allow the maximum achievable depth penetration. At
each site, measurements were made at 5 m increments along selected traverse lines.
The instrument, and hence the coils, were aligned parallel to the profile direction. A
data logger was used to record both the quadrature and in-phase components of the
inductive field response. The acquired quadrature data was treated as conductivity
values, while the in-phase data produced a measure of the ratio of the received
secondary field to that of the primary transmitting field.
4.3.3 The Magnetic Survey Method
A GEM-GSM 19 Offenhauser magnetometer, capable of rapid sampling
acquisition, was used to survey each study area (Figure 11). Recording of the
magnetic field value was set at a pre-determined sample interval and triggered by
cotton topo-fill. Data was acquired along traverse lines over each of the study areas
using station spacings of either 1 or 5 m.
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4.4 Data Processing
4.4.1 Resistivity Data Processing
At the end of each field day the acquired data was downloaded from the ABEM
instrument onto a field computer for further processing. The data was then
reformatted using ABEM software and input into program RES2DINV - a rapid 2D resistivity and IP modelling package based on a non-linear, least-squares, quasiNewton inversion method (Loke and Barker, 1996). The RES2DINV program uses
a finite element or finite difference technique (user selectable) to calculate a
forward apparent resistivity model (based on the acquired data values). The forward
model response is then used as a seed to the inversion process. Prior to inversion the
profile visualisation tool within RES2DINV was used to edit significant noise
spikes from the data.
Figure 16. Inverted model of the proposed 2-D model presented in Figure 12. The top section shows
the original apparent resistivity pseudosection; the bottom section shows the inverse model
resistivity section and approximate position of the clay lenses; and the middle section shows the
apparent resistivity section calculated from the inverse model for comparison with the original
section. Apparent resistivity values are as per the colour key. Depth values are given for the inverted
model section.
A demonstration version of RES2DINV and user documentation was downloaded
from the ABEM web site. In demonstration mode the program allows three inverse
iterations. This can be sufficient for good quality data. As an example the inverted
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result of the forward resistivity model (displayed in Figure 12) is presented in
Figure 16.
A 3-D inversion program was used to model across-line electrical characteristics
between adjacent arrays. The program RES3DINV (Loke, 1999b), developed as an
extension to RES2DINV, employs the same inversion techniques to those described
for the 2-D program.
4.4.2 EM-31 Data Processing
The acquired EM-31 quadrature and in-phase field data were dumped from the data
logger onto the PC and reformatted for use in GEOSOFT’s OASIS MONTAJ
geophysical processing software.
Noise spikes were manually edited from the data and the conductivity values (the
quadrature component) converted to resistivity values - Ohm metres (Ωm). The
OASIS package was used to grid the resistivity and in-phase values.
4.4.3 Magnetic Data Processing
Software supplied with the instrument was used to dump and reformat the acquired
data on the field PC. The data was then imported into OASIS MONTAJ. Noise
spikes were manually edited from the data. Near surface high frequency magnetic
noise was suppressed using low pass and or upward continuation filters. The data
was then either grided or profiled for display using the MONTAJ package.
4.5 Trial Areas
Three test areas for the trail study were chosen in consultation with the Lightning Ridge
Miners Association.
The Crusty's test site was located about 23 km north-west of Lightning Ridge, between the
Castlereagh Hwy and the Old Goodooga Road (Figure 17). This first test area was
comprised of two adjacent 50 m x 50 m mineral claim blocks (46416 and 46422 - Figure
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19). The approximate bounding corner coordinates2 (AGD66, AMG zone 55)of the site
were: 580245 m E, 6761960 m N; 580345 m E, 6761955 m N; 580340 m E, 6761905 m N;
and 580245 m E, 6761915 m N.
Figure 17. Location of “Crusty’s” prospect and the Old Goodooga Road traverse study areas (CMA
Orthophoto Map – 1: 100 00 Lightning Ridge).
The second test site was a single 50 m x 50 m claim (44508) in the Grawin-Glengarry
district at Mulga Rush, about 45 km south-west of Lightning Ridge (Figure 18). The
approximate corner coordinates (AGD66, AMG zone 55) of the mineral claim were:
563900 m E, 6719065 m N; 563940 m E, 6719035 m N; 563910 m E, 6719000 m N; and
563870 m E, 6719030 m N.
The third site was a road traverse about 4 km west of town, along the Old Goodooga Road
(Figure 17 and 18). This site had been used in previous geophysical studies by the
2
Coordinates were obtained using a hand held 12-channel GPS unit (Garmin GPS12XL). Under ideal
conditions this instrument provides horizontal position accuracy to within 15 m.
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Department (Leys, 1987). The road traverse extended from approximately 590560 m E,
6742810 m N to 591425 m E, 6743305 m N (AGD66, AMG zone 55).
Local grids were established and used at each of the three sites.
Figure 18. Location of Grawin - Mulga Rush trial site (CMA Orthophoto Maps – 1: 100 00 Lightning Ridge
and Cumborah).
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5. RESULTS
5.1 Crusty’s Study Area
5.1.1 Resistivity Survey
A series of six 160 m east-west lines were read over the two adjoining claims in the
trial area (Figure 19). The lines were positioned 10m apart, (labelled 0N, 10N, 20N,
30N, 40N and 50N), with each successive array centrally located over the site. The
field results and inverted sections are presented in Figures 20 and 21 respectively.
The sections reveal consistent trends across the six lines. The near surface response
shows a highly variable resistive layer reflecting changes between sand, silcrete and
gravels. With larger a-spacings and increasing depth (consistent with wider
electrode separation) the eastern half of the sections reveal a broad, low resistivity
zone. This is most probably due to the occurrence of clay lenses, or layers, from
about 10 m below the surface. The western half of the sections reveal the influence
of higher electrically resistive material toward the bottom of the sections. As a
result, the more conductive units appear constrained between a resistive surface
cover and a deeper resistive layer.
5.1.2 EM Survey
Eleven, 5 m spaced, east-west lines of EM-31 horizontal coplanar data were
acquired using a 5 m station spacing over the study area.
Sporadic spikes evident in both the in-phase and quadrature results were manually
edited prior to further processing. It is likely that these data inconsistencies were the
result of either localised high induction earth responses, or poor orientation of the
instrument due to obstruction from vegetation. Figure 19 displays a colour image of
the filtered and grided data.
The EM-31 survey clearly defines the electrical response of the near surface.
Compared to the response obtained from the resistivity survey data, the magnitude
of the values reflects the reduced penetration depth, smaller sampling volume and
inductive response of the EM-31 method.
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The southern, central, resistivity low is most likely the result of locally thin surface
covering, indicating the existence of conductive material within a few metres of the
surface. A drill hole adjacent to this feature revealed a clay intersection at about 3
metres.
5.1.3 Ground Magnetic Survey
Ground magnetic data was acquired at 1 m intervals along the EM-31 surveyed
lines. Noise spikes attributed to the metal claim pegs were manually edited from the
data and a low pass filter applied to suppress the effects of surface maghemite
across the area. A grided image of the resultant magnetic response is presented in
Figure 19.
It is suspected that the magnetic low in the south-west of the area may in part be
attributed to the presence of a large truck positioned 5 m to the south of the area and
central to the low response. (The low magnetic response is most likely due to a
combination of effects that include the height of the truck above the magnetic
sensor, a large generator and air compressor on the truck tray and induction coils
within the truck and generator motors).
Despite attempted filtering efforts, the high frequency surface response dominates
the grid image and restricts inference of possible local structures or vertical jointing
within the strata, which are considered favourable for opal occurrence.
5.1.4 Drill Results
Two, three-foot, Calweld holes were drilled on the site during the geophysical test
surveys (Figure 19). Approximate grid coordinates (AGD66, AMG zone 55) for the
two holes were: 580295 m E, 6761910 m N and 580285 m E, 6711920 m N. Both
holes were drilled to about 20 m (3 Kelly bar - drill rod - lengths) and stopped in
moist claystone. The two holes produced similar results and revealed a
predominantly clay rich sequence.
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A further hole located at about 580270 m E, 6761920 m N was drilled
approximately 18 months later. This latter hole intersected a moderately steeply
dipping fault at about 18 m below the surface. The structure strikes at about 325o
(magnetic), in line with a shaft which intersected a fault on the mineral claim to the
south, dips to the east and has a normal displacement of about 2 - 3 m. The location
and strike direction of the intersected structure is consistent with the south western
edge of the resistivity low identified in the grided EM-31 data (Figure 19).
The two former drill holes produced similar results and revealed a predominantly
clay rich sequence. A brief description of these two holes follows:
Depth (m)
Description
0 - 0.9
Soil/Gravel and Clays;
0.9 - 2.4
Shin cracker and “biscuit band” (banded iron stained sediments);
2.4 - 13.8
Claystone with interbedded sandstone.
(Clays: buff-brown, white, waxy, dry and moist);
13.8 - 14.6
Hard band – silicified clays and sandstone;
14.6 - 18.8
Sandstone (iron stained) with traces of claystone;
18.8 - 20.0
Claystone – buff/yellow and moist.
The resistivity sections coincident with these two holes clearly show the response of
the near surface electrically resistive material and the underlying more conductive
claystone. The EM-31 data also reflects the more dominant effects of a conductive
claystone within 2.5 m of the surface, in the vicinity of the two holes.
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Figure 19. “Crusty’s” study area showing Wenner array line locations and drill holes, and EM-31 and ground
magnetic data responses.
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Figure 20. Apparent resistivity pseudosections over “Crusty’s” trial area. (Blue through red colours indicates
low through high electrically resistive areas – values as indicated).
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Figure 21. Inverted electrical resistivity results over “Crusty’s” area. (Blue through red colours represents low
through high resistive areas – values as indicated).
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5.2 Grawin - Mulga Rush Study Area
5.2.1 Resistivity Survey
A single line of resistivity data was obtained and modelled using the ABEM Lund
system over this area. The line bisected the trial site approximately coincident with
a line of drill holes running grid east-west across the claim block (Figure 22). The
observed apparent resistivity pseudosection and modelled apparent resistivity depth
section are presented in Figure 22.
The results indicate a general decrease in apparent resistivity with depth. It appears
that the resistive surface layer and the underlying sandstone extend to about 10 12 m. Below this, resistivity values are lower – particularly on the eastern half of
the line – indicating the likely presence of a claystone. A deeper, low apparent
resistivity zone on the western portion of the line indicates a thicker sequence of
more resistive sediments and the possibility of a fault through the area. These
observations are consistent with the drill logs across the site (Figure 23).
The observed and modelled apparent resistivity values are slightly higher than those
obtained over the Crusty's study area.
5.2.2 EM Survey
EM-31 data was acquired over the test site at 5 m intervals on 5 m separated lines,
parallel to the resistivity survey. The grided quadrature (displayed as resistivity
values) and in-phase components are presented in Figure 22.
The inductive resistivity high in the south western quadrant of the grid is consistent
with a thicker near surface section of electrically resistive sediments in this portion
of the trial area. This result is consistent with the galvanic resistivity and drill
section observations (Figures 22 and 23). The thickness of cover in all of the drill
holes was in excess of the 6 m theoretical sensing depth of the instrument.
The resistivity values over the area are higher than that observed at the Crusty's site.
This is consistent with the galvanic apparent resistivity observations.
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5.2.3 Ground Magnetic Survey
Ultra detailed coverage of the prospect is displayed in Figure 22. The data was
acquired on 5 m separated lines coincident with the EM-31 data, using a sampling
interval of 1 m. The resultant grid shows a broad, weakly magnetic anomaly
trending across the centre of the claim.
The data was relatively noise free indicating a lack of surficial magnetic material.
The magnetic low in the south-western corner of the grid has been attributed to a
large truck, which was parked about 5 m to the south, on the adjacent claim.
5.2.4 Drill Results
Figure 23 displays drill logs obtained over the site prior to the geophysical study.
Potch was reported in all six holes and “colour” was observed in holes 3 to 6. Holes
5 and 6 exhibited the best opal trace across the area. Sand bands were also logged
throughout the claystone unit.
The difference in depth to the claystone “level” between the three western holes (1,
2 and 3 at about 13 m) and the three eastern holes (6, 4 and 5 at about 10 m) most
likely indicates local faulting across the section. The resistivity and EM survey
results support this observation.
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Figure 22. Mulga Rush study area showing Wenner array location, drill hole locations, observed and
modelled resistivity sections, and EM-31 and ground magnetic data responses. (Blue through red colours
represents low through high data values – values as indicated).
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depth
_0
1
2
6
3
4
5
metres
_5
_ 10
_ 15
_ 20
Topsoil/Gravel
Silcrete
Shin cracker
Sandstone
Claystone/Level
Figure 23. Drill logs for the Grawin trial site. Horizontal scale is approximately 1/3 that of the vertical scale.
(Note holes 4 and 5 have been projected onto the section – refer Figure 14 for approximate hole locations).
5.3 The Old Goodooga Road Traverse
5.3.1 EM Survey
A single traverse of EM-31 data was acquired along the Old Goodooga Road. The
line was positioned roughly coincident with previous geophysical trials along the
road (Leys, 1987). The instrument was used in the vertical dipole mode aligned in
the traverse direction to provide maximum penetration of the inducing current.
Quadrature and in-phase responses were recorded at approximately 5 m intervals
along the line.
The resistivity profile derived form the quadrature response is presented in Figure
24. A profile of apparent resistivity from the earliest time channel of a prior
SIROTEM survey (Leys, 1987) located approximately coincident with the EM-31
profile is also presented in Figure 24. The overall trends observed between these
two data sets yield consistent qualitative results (Figure 24). The higher frequency
content of the EM-31 data reflects the effects of a smaller sampling interval.
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ROSSO'S PROJECT AREA
OLD GOODOOGA ROAD
LIGHTNING RIDGE
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
0
0
- 10
- 10
- 20
- 20
- 30
- 30
- 40
- 40
- 50
- 50
- 60
- 60
- 70
- 70
- 80
- 80
- 90
- 90
- 100
- 100
100
0
100
200
metres
Figure 24. Old Goodooga Road geophysical profile data, showing ground magnetic (measured field and lowpass filtered profiles), EM-31 and SIROTEM traverse results. (Note, red through blue colours on the section
indicate areas of high through low resistivity).
It is evident from comparison with the SIROTEM sections that the EM-31 data
reflects variations in the thickness of the electrically resistive surface unit. The
dominant resistive feature located at coordinate 500 along the profiles most likely
represents thicker surface material overl;ying a steeply dipping structure. Increasing
EM-31 resistivity values toward the eastern end of the line are clearly coincident
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with a thickening of resistive surface strata as displayed in the SIROTEM
resistivity-depth section.
5.3.2 Ground Magnetic Survey
A ground magnetic traverse was read coincident with the EM-31 profile. The
magnetic sampling interval was maintained at approximately 5 m along the traverse.
The resultant observed profile and low pass filtered data (50 m wavelength cut-off)
are displayed in Figure 24.
In terms of geological features there is no obvious correlation between the magnetic
and electromagnetic data sets. The negative magnetic response occurring at about
570 m is due to a buried telephone cable. This feature was also evident in the inphase EM-31 data (not shown). The negative magnetic response at about 960 m is
coincident with an overhead power line.
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6. DISCUSSION
6.1 Resistivity
The Wenner-alpha electrode array was selected because of its greater sensitivity to
horizontally stratified geology. This configuration resolves vertical resistivity changes far
better than other array types, with greatest resolution directly under the centre of the spread
(Loke, 1999c). Compared to other array configurations the Wenner-alpha array also
achieves greater signal strength and moderately better depth penetration3. However, the
Wenner set-up is poor at resolving lateral changes in resistivity and provides rapidly
decreasing horizontal coverage with depth, as the electrode separation is increased along
the spread. To provide adequate depth coverage it was necessary to extend each electrode
spread at least 48 m (12 electrode separations) beyond the boundaries of the test areas. If
the technique was to be adopted as a prospecting tool the need to extend survey lines
beyond claim boundaries could create conflict between adjacent claim holders.
The electrode layout, set up and operation although time consuming was straightforward.
Upon start-up, the ABEM system tests and evaluates the contact of each electrode pair with
the earth. If during the course of acquisition poor or no electrical contact is detected, the
instrument prompts the operator to recheck the electrodes prior to proceeding. In full
automatic mode, poor contacting electrodes are skipped with acquisition continuing to the
next transmit-receive pair. This latter mode relinquishes the operator from paying constant
attention to the instrument, but also limits in-field quality control during the acquisition
process.
Throughout the trial every effort was made to ensure that each electrode was in good
electrical contact with the ground. This was achieved through repeated watering, use of
additional electrodes in parallel and ensuring sufficient depth of burial of the metal stakes
to minimise contact resistance. Despite these efforts a number of electrode pairs failed to
provide sufficient contact. Resistivity techniques generally utilise non-polarisable,
receiving electrodes (for example saturated, copper sulphate solution in a porous pot) to
minimise noise in the measured signal. To facilitate the automated acquisition of resistivity
3
Signal strength is generally inversely proportional to the geometric factor of the array, (which in this
instance is 2πa, where 'a' is the electrode separation), while penetration depth is approximately equivalent to
about half of the greatest a-spacing (in this case half of 48 m).
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data along the electrode spread it was logistically easier to use metal electrodes at each
array position. While ordinary steel electrodes may provide satisfactory results under
favourable conditions, stainless steel is more preferable and results with cleaner data.
Consequently, the use of mild steel electrodes, while cheaper and more expedient, may
have introduced some noise into the acquired data.
At larger a-spacings a number of electrode pairs revealed a negative resistivity response.
Initially, when a negative value was encountered, the particular electrodes were checked
and re-watered to improve electrical contact and signal characteristics. However, as the
survey progressed, it became evident that these data errors were occurring at the same
electrode positions on adjacent lines. Field tests to isolate the cause of these data
inconsistencies suggested that the source of the errors was most likely instrumental.
Similarly, one electrode pair produced a spuriously high value on each line. All such
erroneous data points were manually edited from the files prior to processing, display (in
pseudosection form) and modelling. The LUND electrode selector was brought into
Australia specifically for the trial at Lightning Ridge and was immediately returned to
Sweden upon completion of the fieldwork. As a result there was no opportunity to test the
fidelity of the acquisition apparatus under more favourable and controlled conditions.
The model sections, derived from inversion of the edited field data, show a general
correlation with the EM-31 and drilling data over the test areas. Inversion and interpretation
of the results assume a somewhat distinguishable and uniform geological scenario. Drill
logs of the Crusty's trial site holes, however, reveal a complex heterogeneous geology
comprised of thin inter-bedded clay and sandstone units. It is evident from the drilling data
that the assumption of uniform, distinct, horizontally stratified geological and electrical
units oversimplifies a more complex environment.
The two-dimensional inversion results also fail to account for the three-dimensionality of
the geoelectric structure. To investigate this further the Crusty’s resistivity data were
modelled using 3-D inversion software (RES3DINV). The results are displayed as a series
of resistivity depth slices in Figure 25. The linear nature evident in the near surface layer
may in part be attributed to the rectangular discretization (4 m x 10 m) of the data during
the 3-D inversion process. Comparison of the 6 m depth slice and the EM-31 results
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(Figure19) show a general similarity between the two techniques. The model depth slices
reveal the development of a low resistive zone on the eastern side of the grid that is evident
from about 10 m and extends to depth. The 3-D model is consistent with the 2-D results.
Figure 25. 3-D inversion results showing depth slices of the modelled data. Approximate depths are from top
to bottom: 1.5 m, 6 m, 10 m, 15 m, 22 m, and 27 m. Pink represents high resistive areas. Blue represents more
conductive features. The vertical axis gives an idea of the amplitude of the data on each level, however, the
scales are inconsistent from box to box.
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6.2 EM
The EM-31instrument can be used in both horizontal and vertical dipole modes and
oriented parallel and or orthogonal to the survey direction. Similarly, measurements at two
elevations in any one of these configurations can be performed to provide some limited
depth information.
Initially attempts were made to measure both the horizontal and vertical dipole field
contributions oriented along the survey lines at normal survey height (approximately 1 m).
However, unfamiliarity with the instrument and an apparent failure by the data logger to
recognise the horizontal dipole response restricted use of the instrument to its simplest
configuration.
The 3.6 m long instrument was unwieldy and difficult to negotiate around the scrubby bush
found over many of the opal prospects. Along the majority of survey lines the recorded
values included a number of erratic single point data spikes. It has been assumed that these
were the result of near surface geological effects that were outside the 1 – 1000 Ωm range
of the instrument (McNeill, 1980). These single point data spikes were considered
erroneous and were edited from the data.
Due to the shallow penetration capabilities of the instrument it is difficult to assess the
value of the results with respect to delineating the opal environment. The technique would,
however, be useful for delineating near surface changes in conductivity associated with
variations in the type and thickness of surface units.
6.3 Ground Magnetics
Ultra detailed grid survey coverage requires excellent positional accuracy to enable
correlation of identified anomalies across survey lines. This is particularly important when
the objective of the survey is to delineate subtle features of minimal magnetic contrast, as
was the case in this trial exercise.
Two problems were encountered in the use of this technique over the trial areas. Firstly, the
cotton, topo-fill, distance-measuring system failed to maintain sufficient accuracy along the
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lines to enable a high level of confidence in the final grided result. This was due primarily
to the obstacle presented by low scrub and bush (Figure 11, for example) creating stretch
and entanglement of the cotton system. The second problem was the general level of
magnetic noise encountered either from near surface lateritic material or from the influence
of surrounding mine culture (Figures 19 and 22).
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CONCLUSION AND RECOMMENDATIONS
The continuous, electrical resistivity, profiling technique employed during these trials
requires attentive field efforts to ensure that good quality, meaningful data is acquired. The
means to display and interpret the results in a meaningful fashion is also paramount to
obtaining an effective and useful outcome. The occasional instrumental problems and poor
electrical contact encountered during the test surveys hampered definitive conclusions to be
drawn.
Direct current electrical resistivity techniques will only detect and resolve with modelling,
the occurrence and likely depth, of contrasting conductive or resistive earth materials.
Theoretically, the technique will map, and resolve, the contact between the sandstone and
the underlying clay horizon. However, to achieve complete coverage in profile form across
a claim requires surface coverage to extend beyond the lease boundaries. This would
require agreement between adjacent claim holders. While knowledge of the extent and
depth to the opal bearing horizon would assist effective drill siting, it is considered unlikely
that ground based direct current, resistivity profiling techniques, however automated (such
as the ABEM system employed in these trials), would be adopted as a general purpose,
prospecting tool.
The EM-31 technique provides no accurate depth information and is restricted to a
maximum theoretical penetration depth of six metres. Consequently this technique is
considered ineffective for general exploration of the opal level. However, if the results
were calibrated to geology, it is possible that the technique could be used to rapidly detect
changes within near surface lithology and to map near surface silcrete ("shin-cracker")
occurrences. This could assist the siting of drill holes during the general course of
prospecting. While the instrument is simple to operate and the data lends itself to ready
display it is extremely cumbersome to use amongst the low scrub found in the Lightning
Ridge opal fields.
The ground magnetic technique was inconclusive in its ability to define any direct or
indirect opal-hosting indicators such as faulting.
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The geology of the opal-bearing environment around Lightning Ridge possesses a
discernible electrical character. Any means of rapidly mapping the occurrence and depth of
the clay horizon across the ridges would be of great benefit to opal exploration. The likely
flow-on of more prudent drill siting would also benefit other shared land uses.
To achieve such a result would entail the rapid acquisition of detailed data capable of
characterising the electrical properties of the opal environment in both plan and section
format. It is therefore recommended that a multi-frequency, detailed, airborne
electromagnetic survey be trialed over a suitable test site in the Lightning Ridge area.
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7. REFERENCES
ABEM Instrument AB “Resistivity/induced polarization imaging: Odarslov”, ABEM
Printed Matter No 96500 http://www.abem.se/casestories/caselist.htm [online, 21
November 2000].
ABEM Instrument AB, 1998 “Instruction manual - Terrameter SAS 4000, ABEM product
number 33 0020 26”, ABEM Printed Matter No 93067, ABEM Instrument AB.
Central Mapping Authority of New South Wales, 1981, “Cumborah 8438 – orthophotomap
1:100 000 first edition”, Central Mapping Authority of New South Wales.
Central Mapping Authority of New South Wales, 1981, “Lightning Ridge 8439 –
orthophotomap 1:100 000 first edition”, Central Mapping Authority of New South Wales.
Central Mapping Authority of New South Wales, 1983, “Tourist map - Lightning Ridge
opal fields and district”, Central Mapping Authority of New South Wales.
Leys M. and Palmer D., 1987, “Lightning Ridge experimental geophysical surveys – field
data report”, Geological Survey of New South Wales Department of Mineral Resources,
Geological Survey Report No GS 1987/342, Jul. 1987.
Leys M., 1987, “Interpretation of experimental electrical and electromagnetic geophysical
data from Lightning Ridge New South Wales”, Geological Survey of New South Wales
Department of Minerals and Energy, Geological Survey Report No GS 1987/343, Nov.
1987.
Leys M. T. C., 1990, “An application of the early time SROTEM - Lightning Ridge opal
fields”, Geological Survey of New South Wales Department of Minerals and Energy,
Geological Survey Report No GS 1990/338, Nov. 1990.
Leys M. and Robson D., 1998, "Application of geophysical methods to assist in the opal
search at Lightning Ridge", First Lightning Ridge Opal Symposium
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Loke M. H. and Barker R. D., 1996, "Rapid least-squares inversion of apparent resistivity
pseudosections by a quasi-Newton method", Geophysical Prospecting, 44, pp131-152.
Loke M. H., 1999a, “RES2DMOD ver. 2.2 - Rapid 2D resistivity forward modelling using
the finite-difference and finite-element methods”,
http://www.abem.com/Software/demosoft.htm [online, 21 November 2000].
Loke M. H., 1999b, “RES3DINV ver. 2.04 for Windows 3.1, 95 & NT - Rapid 3D
resistivity & IP inversion using the least-squares method”,
http://www.abem.com/Software/demosoft.htm [online, 21 November 2000].
Loke M. H., 1999c, “Electrical imaging surveys for environmental and engineering studies
- a practical guide to 2-D and 3-D surveys”, http://www.abem.com/Software/demosoft.htm
[online, 21 November 2000].
“Lund imaging system” http://www.abem.com/resistivity/dia.htm [online, 21 November
2000].
McNeill J. D., 1980, “Elecromagnetic terrain conductivity measurement at low induction
numbers”, Geonics Limited Technical Note TN-6, Oct. 1980.
New South Wales Department of Mineral Resources, 1993, “Opals in New South Wales”,
Department of Mineral Resources, Oct. 1993.
New South Wales Department of Mineral Resources, 2000, “Mining at Lightning Ridge”,
Minfact 95, NSW Department of Mineral Resources, Dec. 2000.
“RES2DINV ver. 3.44 for Windows 95/98 and NT - Rapid 2-D resistivity & IP inversion
using the least-squares method”, http://www.abem.com/Software/demosoft.htm [online, 21
November 2000].
“RES3DINV ver. 2.0 – 3D resistivity & IP inversion software for Windows 3.1, 95 & NT”,
http://www.abem.se/ftp/Loke/r3dinvnote.htm [online, 21 November 2000].
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Watkins J. J., 1984, “Future prospects for opal mining in the Lightning Ridge region”, New
South Wales Geological Survey Report GS 1985/119, Department of Mineral Resources,
1985.
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