Exploration Geophysics - Volume 23, Issue 1-2, 1992

Fresnel-zone considerations are the essence of horizontal resolution on seismic sections. The first Fresnel-zone is described as the subsurface area over which reflected energy adds constructively to cause a seismic reflection event. Contributions from higher order Fresnel-zones are assumed to cancel each other out. Therefore only the first Fresnel-zone defines the actual reflector response. The size or extent of this zone determines the lateral resolving power of the seismic method. This establishes the spatial resolution with which important lithological changes along with seismic profile direction may be observed.

Seismic wave velocity is a function of direction in an elastically anisotropic medium and hence the wavefront shape is non-spherical. It can assume an elliptical or non-elliptical shape. The Fresnel-zone dimensions for P and SH waves are calculated using numerical modelling techniques. Results obtained at varying reflector dips using transversely anisotropic velocity functions are compared with the corresponding values for the isotropic case. This comparison is carried out for both P and SH waves. The size of the Fresnel-zone is found to be predominantly dependent on the shapes or curvatures and wavelength of the wavefront as well as the elastic constants δ* and γ. The dip or attitude of the reflector is also found to have a remarkable influence on the dimensions of the Fresnzel-zone.

Results from numerical studies show considerable variations between the Fresnel-zones for anisotropic and isotropic velocity functions at various reflector dips. Consequently the spatial resolution in an anisotropic medium would be significantly different from that determined for the same medium if it is assumed to be isotropic. This observation indicates that the lateral resolution of reflection events from the base of thick shale sequences is most likely to be affected.

One aspect of the inversion of refraction data is the determination of the velocity stratification of the layers above the target refractor. This aspect is notoriously unreliable with the standard approach in which the travel-time graphs are used to determine the relevant model and compute its parameters, because of the common existence of undetected layers, velocity inversions, variable velocity media, and vertical seismic anisotropy. While forward modelling with ray tracing can ensure that any result is compatible with the travel-time data, it does not ensure that the model selected is appropriate or that its parameters are well determined. Furthermore, there can be a considerable range in the depths computed with the various models which honour the travel-time data. Therefore, forward modelling is of little value in resolving most ambiguities in the velocity stratification above the target refractor.

The other aspect of refraction inversion is the resolution of structure and seismic velocity within the refractor. This can be achieved with the generalized reciprocal method (GRM) and the minimum variance criterion, while still honouring the traveltimes to the target refractor. The use of specific ray tracing routines is not essential. In addition, the optimum XY value is a function of the thicknesses and seismic velocities of all layers above the refractor. Depths computed with the GRM approach have smaller errors than with the standard approach, they are usually related to the accuracy in determining the optimum XY value, and they are largely independent of the model selected. Furthermore, the GRM is able to make reasonable depth estimates for models which include the velocity inversion and the vertical anisotropy problems, for which the standard approach has no recognised solutions.

It is proposed that forward modelling with ray tracing is neither a necessary nor a sufficient condition in refraction inversion.

Over the past two decades guided waves — propagating within low-velocity/low-density coal seams — have regularly been exploited by the coal industry to detect geologic disturbances ahead of longwall mining. In these standard inseam transmission surveys, channel waves are generated by firing shots within the coal seam; the seam waves are detected by placing array of geophones within the coal.

In this paper we investigate a novel low-cost 2D method for locating faults and dykes in coal seams. The method entails running a simple one-pass walk-away source VSP survey over a suspected lateral discontinuity in the coal panel and imaging the scatterers by multicomponent recording of the channel waves. The channel waves are excited at the discontinuity (fault) as a result of scattering of seismic waves travelling downward from the surface source. The borehole geophone, at the seam level, and laterally displaced from the fault, detects some of the forward scattered channel wave energy captured by the waveguide.

Simple numerical and laboratory-scale seismic models studies were undertaken to simulate typical shallow coal seam field situations found in Australia. Different source types were to run 2D profiles over the coal/host-rock geological sequence. In these studies, both offset VSP and walkaway VSP recoding geometries were employed. Substantial forward scattering from incident S-wave to channel wave propagation was observed on the borehole receiver. Our results suggest that the waveguide walk-away survey can be an effective method to reveal small faults (less than one-tenth of a wavelength) and other seam disruptions. It is therefore possible to map small seam faults in the field — with a resolution that is otherwise unattainable — at a long horizontal distance from a drill hole.

Survey technology is entering a new age of maturity which seems destined to make airborne geophysics an even more important solution to many exploration problems. In terms of quantity, more than 40 years of aeromagnetic survey gives an emerging completeness of reconnaissance coverage world-wide. In terms of quality, superlative resolution of geological detail is evident in surveys (magnetic and radiometric) carried out to the latest specifications.

The steady accumulation of aeromagnetic survey data is approaching first-pass completeness, even in areas of the world where this has not been deliberately planned. While the technical quality of older surveys often leaves something to be desired, even the poorest early data can give valuable new structural information when digitized and compiled together into maps covering large areas at small scales. Such a compilation of North America was published in 1987. Compilations of Europe, Africa and Australia will be completed in time to display at this meeting. Some new technological solutions for digitizing map data, linking and levelling separate surveys and the incorporation of long wavelength (near-DC) information will be described.

The quest for high resolution in magnetic and radiometric surveys is surprisingly new and distinctly Australian. The need for low ground clearance and close line spacing is predicted by simple theory, and new technology can cope with the consequent demands of accurate position fixing, more frequent sampling of geophysical parameters with low noise envelopes, high-resolution magnetic recording and adequate crystal volumes for spectrometry. Current techniques of imaging the resulting data — both magnetic and spectrometric — test the data quality to its limits, and suggest new areas where improved data acquisition and processing technology could stretch the exploration power of airborne surveys even further in the immediate future. Meanwhile, the full digital integration of geophysical information with geological and other geo-data is still in its infancy.

Certain materials respond to a seismic stress by becoming electrically polarized. There are several different mechanisms for creating the polarization including piezoelectricity, the electrokinetic or E effect, and the triboelectric effect. We have carried out a number of laboratory and field experiments to investigate geophysical exploration techniques based on such phenomena.

This paper focuses on an experiment at Humboldt, Australia, at which we made our first credible observations of piezoelectricity under field conditions. The experiment used a compressed-air jackhammer as a source and four 50-m ungrounded long-wire antennas for detectors. Despite the fact that the signals are significantly contaminated with noise, principally from a nearby radio transmitter, they demonstrate conclusively the presence of electrical responses. Seismic energy equivalent to a single hammer blow interacted with the quartz body to produce electrical fields with strengths from microvolts to tens of microvolts at the antennas. These results are in accord with earlier, unpublished results of Alan Boyle (CRA Exploration Pty Ltd). Our interpretation is limited to two dimensions, but the results suggest a quartz structure dipping at a shallow angle beneath the line of shot points. Later arrivals show a source close to the centre of the antenna array, for which there may be explanations other than piezoelectricity.

Thus, our experiments support the many Soviet claims that such phenomena can form the basis of valid prospecting tools.

A new correlation and picking algorithm has been developed which uses multi-component seismic data from two stations. A conventional single-station covariance matrix analysis is well known to provide polarization estimates of the particle motion at a given point in space. This single-station technique is successful if any noise present on the data is purely random. However, in the presence of coherent noise, the single-station analysis breaks down. A two-station (binocular) covariance matrix provides similar information at two points in space, as well as indicating the correlation between the two stations. The major advantage of the procedure is that it can successfully identify overlapping arrivals (in the time domain) where a single-station analysis fails. The binocular analysis may be sequentially applied to a multi-station seismic gather using the same reference signal to identify a coherent event. The technique was applied to synthetic data and then successfully used to pick overlapping events on data acquired in a physical scale-model experiment.

A spectral matrix technique described by Mari and Gavin has been developed and extended for analysing interfering seismic events. It may be used with both single and multi-component data. Spectral matrix filtering removes random noise and may be used to enhance picked coherent events. This leads to a possible wave-field decomposition procedure for use in separating interfering coherent events.

Conventional event-separation procedures require large geophone arrays to avoid spatial aliasing. Spectral matrix filtering does not require such dense coverage and so may be used in areas where conventional surveys may not be possible. The technique is illustrated using synthetic and physical scale-model data.

The HYC deposit was discovered in the McArthur River area of the Northern Territory in 1955. Although surface expression is very limited, it was essentially an outcrop discovery resulting from drilling under a small, zinc-rich, siliceous dolomite outcrop. The deposit is described as a sediment-hosted stratiform sulphide. It lies at the base of the Barney Creek Formation in Proterozoic sedimentary rocks of the McArthur Group. The resource is estimated to contain 227 Mt, grading 9.2% Zn, 4.1% Pb, 4.1 g/t Ag and 0.2% Cu.

The body lies beneath a black-soil plain in the McArthur River valley so, while geophysics played no part in the initial discovery, it became an important element in the detailed exploration of the deposit and of the surrounding areas. Induced polarization/resistivity was successful in outlining the HYC deposit and it led to the discovery of new mineralization in the Ridge and Cooley areas. The gravity signature of the deposit is complicated by structure and carbonate rocks, but the method was used successfully to locate a new zone of massive sulphide — unfortunately pyrite only. Early EM methods tested in the area were unsuccessful, but the more recently developed TEM systems have given excellent results, both in profiling and sounding modes. Both the Geotem and Questem airborne systems produce clear anomalies over the deposit. The HYC mineralization has no magnetic signature, but regional airborne magnetics delineates structures, including the Emu Fault, which may be important in the localization of the deposit. A trial seismic reflection line showed some structural elements of the deposit.

A practical definition of DMO (Dip Move-Out) is that it is a mapping to zero offset. In order to avoid the arbitrary attenuation of dipping events in this mapping process, Amplitude-Preserving or True-Amplitude DMO is needed. Such a DMO algorithm will be an invaluable tool in the study of AVO (Amplitude-Versus-Offset) effects.

A method is described whereby both anti-aliasing and Amplitude-Preserving DMO can be implemented using the integral approach (also known as Kirchhoff DMO, Summation DMO, and (x-t) or space-time DMO).

Anti-aliasing is accomplished by using band-limited sinc-function interpolators based upon local dip of the impulse response of the DMO operator. This enables the appropriate amplitude weighting along the DMO impulse response to be applied. That gives rise to the so-called True-Amplitude and Amplitude-Preserving DMO algorithms. The F-K domain offers a useful platform for the comparison and implementation of various weighting factors for the DMO impulse response. F-K analyses of impulse responses can be used to illustrate the effects of anti-aliasing and amplitude properties of integral DMO. In 2D, where in general there is regular sampling in the common offset plane, the anti-aliasing and Amplitude-Preserving DMO algorithm improves the image quality in the mapping of pre-stack data to zero offset.

In the 3D case the DMO ellipse is exactly the same as the 2D DMO ellipse, apart from the fact that for 3D DMO we must rotate the ellipse to lie in the direction given by the source-receiver axis with respect to the processing grid. This grid is usually organised in terms of 3D binline and 3D CMP (Common Mid-Point). In marine data, streamer feathering has the effect of spreading midpoints (3D CMPs), for a given sail line, over several 3D binlines. The result of this is to vary the distribution of source-receiver azimuths (feather angles) within a given bin. This leads to irregular sampling in the common offset-azimuth plane where 3D DMO is performed.

Whilst Kirchhoff methods have the flexibility of dealing with irregularly sampled data, the desirability of regular sampling in this common offset-azimuth plane in order to preserve amplitudes is illustrated. Methods for data regularisation in offset and azimuth are not discussed here. Instead we question the need for 3D DMO in certain marine applications. A simple criterion for determining the condition under which cable feathering becomes a significant factor in determining the need for 3D DMO is presented in tabular form and its implications are discussed.

A new compilation of magnetic and gravity data for Victoria has enabled the production of images of these data bases. The magnetic image of Victoria has been produced by combining regional and detailed airborne surveys. All the 1:250 000 standard map sheets in Victoria (21 in total) have been flown by the Bureau of Mineral Resources (BMR) and the Geological Survey of Victoria (GSV), and the data have been gridded using a 150 m x 150 m cell size and merged together by linear adjustment to adjacent data sets. Detailed company and government airborne surveys, and marine surveys have been superimposed onto the regional data to produce a composite image of the total magnetic intensity of Victoria.

The GSV has edited existing onshore gravity data, added new data, and converted the data base to the IGSN71 (Isogal 84) datum. A summary is given of the five methods used to convert the gravity data from the Potsdam datum (Isogal 65) to IGSN71 datum (Isogal 84).

The resultant gravity data base has been gridded using a 300 m x 300 m cell size and imaged. The images of the magnetic and gravity data sets highlight a number of major geological features not previously apparent. The improved ability and increased computing power of modern image-processing software to handle and merge images of large, multiple data sets has substantially enhanced the contribution that regional geophysical surveys have made to the geological knowledge of Victoria.

The occurrence of localised high velocity zones due to cementation within the Tertiary section is a well documented phenomenon in the Timor Sea. The presence of such anomalies over the Skua Oil field in Licence area AC/L4 has complicated the depth conversion of the top porosity time structure. These velocity anomalies are small in relation to the spread length of a common depth point (CDP) gather, and in the vicinity of an anomaly only a fraction of the traces in a CDP gather are affected. In order to compensate for the effect of the anomaly it is necessary to know exactly which traces are affected by velocity pull-up and by how much. This can be calculated only if the anomaly can be accurately delineated.

The areal extent of the anomalies can be estimated from areas of velocity ‘pull-up’ on the Base Eocene time structure map. However, they cannot be delineated precisely enough from this map, as the relationship between the shape of the velocity pull-up and the anomalies is very complex.

In order to delineate an anomaly in the vicinity of the Skua-7a well, a radial pattern of walkaway checkshot surveys was acquired. Some 28 equally spaced profiles, with offset ranges varying from 2000 to 3000 m, were acquired between azimuths 90° and 225° east of north. Single shots were fired at approximately 35-m intervals along each line, and the first-break P-wave seismic energy travel times to a three-component geophone at a depth of 2516 m subsea (ss) was measured for every location.

The velocity pull-up effect of the anomaly was clearly seen in the variation of first-break time with offset along each profile. A two-dimensional image of the anomaly along each profile was generated through inverse modelling of the travel times, and a three-dimensional model of the anomaly was created by combining the 28 two-dimensional images.

The resultant anomaly model was significantly smaller in areal extent than indicated by the areas of pull-up on the Base Eocene time structure map, although the measured reduction in travel time, the velocity pull-up, through the anomaly was much larger than was indicated from the same map.

This depth model is a more accurate representation of the velocity anomaly in the vicinity of the Skua-7a well and highlights the shortcomings of delineating velocity anomalies from their expression on surface seismic data. This model represents a starting point for more sophisticated inversion techniques and more accurate depth conversions.

Exploration beneath a near-surface white magnetic noise source, such as maghemitic and lateritic soils or thin basalt flows, can best be achieved at ground level. The objective of such exploration is to discriminate between the noise and signal from underlying sources. The greater the ratio of signal-source depth to noise-source depth, the easier it is to distinguish between the two by filtering.

Spectral analysis confirmed that low-pass linear filtering by either upward continuation (as implemented in airborne surveying) or digital processing (e.g. by running average or Butterworth filtering) is not appropriate because it does not distinguish between the low-frequency component of the noise originating near the surface and the low-frequency signal arising from a deep source. A filter was required which would cut all frequencies arising from random dipolar magnetic sources occurring between the surface and a selected depth.

A running median filter with a window width greater than twice the expected noise-source depth (but less than twice the expected signal-source depth) has been confirmed by spectral analysis to attenuate the full spectrum of magnetic noise arising from dipolar sources within the surface noise-source layer. Application of the filter to fully sampled, ground-level data, recorded where surface maghemite overlies the Elura base-metal orebody near Cobar (NSW), demonstrated its effectiveness.

The median filter may be further refined by defining a “range” value, judiciously selected such that the magnetic texture due to near-surface geological structure is preserved while still adequately discriminating against the low-frequency component of intense, random dipole noise. A typical range value selection is half the RMS noise amplitude.

Signal-to-noise ratios were determined from the areas under the signal and filtered noise spectra. Graphing the signal-to-noise ratio against sensor elevation above the noise source demonstrated that the ratio achievable 0.5 m above ground level was five times better than that at the optimum airborne elevation of 75 m. Moreover, a survey elevation of 10 m (as may be conducted from a helicopter) would result in the very worst signal-to-noise ratio.

A detailed comparison is presented of results for Vertical Seismic Profiling using a surface vibrator source and a buried explosive charge.

The geological setting was the southern region of the Sydney basin near Appin where the Bulli coal seam is at a burial depth of approximately 500 m, and where overlying material is sandstone with interbedded shale.

Borehole geophones were spaced at 8 m, commencing at 28 m below the surface and continuing down nearly to the Bulli coal seam. The vibrator operated at a location offset 60 m from the borehole, where 50 linear vibrator sweeps of 40 Hz to 360 Hz and 3 s duration were used. The vibrator performance is compared to results of using 0.5 kg of explosive at the same offset location, and buried beneath the weathering layer generally of depth 13 m. The source bandwidth of the buried explosive charge was 40 Hz to beyond 350 Hz, determined by spectral analysis of the unfiltered record from the downhole geophone closest to the surface.

Results of VSP sections showing downgoing waves, and an upgoing wave (Bulli coal-seam reflection) are presented, together with processing details. An examination of vibrator and explosive signal bandwidth attenuation versus depth is presented. These results were determined from the unfiltered downhole geophone records of the downgoing waves.

Digitally recorded vibrator ground force was obtained using the accepted practice of accelerometers mounted on the vibrator ground contact plate and reaction mass, and was also obtained using an accelerometer buried one metre beneath the ground contact plate. These signals were analysed to establish the quality of the ground excitation; results are presented graphically.

VSP sections, both filtered band pass 60-250 Hz, clearly show downgoing waves of good continuity and comparable wavelet width. However, the explosive downgoing wave showed evident broadening of wavelet width at geophone depths greater than 276 m. This observation was supported by spectral analysis of the unfiltered explosive downgoing wave, which showed a marked reduction of the explosive energy bandwidth with depth, compared with the bandwidth of the wavelet energy produced by the vibrator. The vibrator output however, suffered substantial bandwidth attenuation in passing through the weathered layer, which effectively filtered out vibrator frequencies above 250 Hz. The lower energy output of the vibrator compared with the explosive showed up in the Bulli coal-seam reflection, which in the case of the vibrator became buried in noise at downhole geophone locations shallower than 188 m. On the other hand, the explosive reflection was much stronger, and still evident in the record from the geophone located at a depth of 116 m.

Following clearing of native vegetation for agriculture, large parts of agricultural areas in Western Australia have become affected by soil salinity. The decrease in water discharge via transpiration of vegetation has caused water tables to rise and mobilise salts stored in the weathered profile. Investigations of the surface have failed to explain the causes of the patterns of saline outbreaks. Street and Engel (1990) showed that ground geophysical methods have application in detecting hydrological controls within the landscape. This paper reports on the research carried out by World Geoscience Corporation in conjunction with the WA Department of Agriculture into the application of airborne magnetic and electromagnetic surveys for soil salinity.

The East Yornaning Catchment in Western Australia has been studied in considerable detail using airborne geophysics followed by ground investigations. The geophysics shows that on a broad scale salt storage within the landscape is controlled by topography and lateral changes in hydraulic conductivity due to weathered dolerite dykes.

The knowledge of and controls on the salt storage distribution is essential in good land management planning to achieve sustainable agricultural practices.

The Bureau of Mineral Resources has been routinely acquiring airborne magnetic surveys over the land area of Australia since 1951 to record and map anomalies in the earth’s magnetic field attributable to geological structures and lithologies. In forty years, over four million line kilometres of survey data have been flown, while the technology of survey practice has passed through various stages of development.

About 83 per cent of the land area has now been covered with so-called reconnaissance surveys flown 150 m above terrain at line spacing between 1.5 and 3.2 km. Located profile data for these surveys have been gridded using a minimum curvature technique, to 15 second of arc (approximately 400 m) and, where necessary, micro-levelled. Data for much of the remaining areas — particularly the inland sedimentary basins covered by surveys of lower specifications — were obtained from digital data on an approximately 2-km grid (72 seconds of arc) published in 1976; these have also been interpolated to 15 second of arc.

The data were first assembled for each of over five hundred 1:250 000 map sheets. The 1:250 000 sheets were linked by minimizing the discrepancies along their common boundaries (which were often also survey boundaries) and reducing remaining mis-ties through Laplacian smoothing to minimize the visibility of boundaries between surveys acquired separately.

While the data quality varies with instrumentation and survey parameters, it is almost everywhere good enough to provide a useful synoptic view of magnetic anomaly patterns, which can be expected to give important new insights into geology and tectonics at a continental scale, and to provide a regional framework within which to interpret more local magnetic anomalies.

The purpose of this short paper is to report the latest progress on compilation of the Magnetic Anomaly Map of Australia, which is scheduled for publication late in 1992.

The In Situ Minerals Analysis Group in the CSIRO Division of Geomechanics has developed quantitative borehole logging techniques applicable to iron-ore and coal deposits. They are used currently to determine the formation density, either the iron-ore grades or the raw coal-ash contents, as appropriate, and the borehole diameter. The in-situ analyses depend on probe-calibration equations which were formulated by linear regression analysis that related the probe’s spectral outputs with the required geological variables.

Calibration equations consisting of a linear combination of first-order terms gave excellent assaying accuracy. The group achieved further improvements in assaying accuracy by developing a more generalised calibration model based on second-order terms and cross-product terms of the probe’s spectral parameters. The logging data used for the statistical analysis were recorded in mine development boreholes at three Pilbara iron-ore mines and at a Queensland coal mine.

Application of the generalised model, in place of the first-order model, resulted in a reduction of the root mean square (RMS) deviation between assays obtained in the laboratory and by logging, of about 15% relative for iron-ore grades and of about 8% relative for raw coal-ash content.

The study also shows that the accuracy obtained using the conventional, non-spectrometric calibration model is inferior to that obtained by using either of the two spectrometric models, where the comparisons made are based on the same set of logging data.

A significantly improved geological and structural map of the Ravenswood Batholith is being developed by the integration of high-resolution magnetic and radiometric data with detailed geological information acquired from 1986 to 1991. Some new geological models being proposed are a direct result of geophysical input.

The Ravenswood Batholith evolved during three main periods of intrusion spanning the Middle Ordovician to Early Permian. Most of the plutons of the first two intrusive periods are relatively shallow, sheet-like bodies, probably emplaced through a buoyancy-controlled mechanism. However, some plutons in the southwest of the batholith tend to be deep-rooted diapiric intrusives. The third main period of plutonism (Late Carboniferous-Early Devonian) comprises felsic stocks and ring complexes, and mafic to felsic dyke-like complexes which were intruded into the cooled Ordovician - Devonian granitoids.

Almost all of the granitoids are magnetite-bearing, and aeromagnetic anomalies associated with the plutons vary considerably owing to the effects of remanent magnetisation as well as changes in magnetic susceptibilities. A general relationship between pluton ages and their associated aeromagnetic anomalies is established within the Ravenswood 1:100 000 sheet. Potassium, thorium and uranium variations within and between individual plutons are clearly illustrated by high-resolution radiometric data.

Predominant trend directions of geophysical lineaments in the Ravenswood Batholith are northeast, northwest and less commonly north-south. Some of the major aeromagnetic lineaments appear to have controlled the emplacement of plutons within the batholith. A set of parallel northeasterly trending magnetic lineaments extends for up to 200 km and reflects major, probably pre-existing structures.

Mineralisation within the Ravenswood Batholith is almost completely represented by gold — the Charters Towers and Ravenswood Gold Fields, between them, having produced in excess of 200 t of gold — with seven mines recently active. The source of the gold mineralisation in the eastern portion of the batholith is unknown but some of the major deposits can be spatially and temporally related to major structural features and to rocks emplaced during the final Late Carboniferous-Permian intrusive phase of the Ravenswood Batholith.

The Century Zn-Pb-Ag deposit, located about 250 km north-northwest of Mount Isa, Queensland, was discovered by CRA Exploration in 1990 following several years of regional area selection studies and initial reconnaissance field work. The field work focussed attention on a zinc soil geochemical anomaly, which was delineated on one of two regional gravity, ground magnetic and soil sample traverses. No anomalous response was detected on the gravity or magnetic data.

Follow-up grid-soil sampling delineated an area about 1.5 km square of enhanced zinc anomalism, located adjacent to the Termite Range Fault, a major northwest trending structure. However, concurrent magnetic and SIROTEM surveys failed to reveal any targets to support the immediate drill testing of the geochemical anomaly.

Subsequent siting of the discovery drillhole was based largely on the observed geochemical anomaly, and recognition of near-surface leaching of zinc elsewhere in the Century area. Post-discovery drilling has outlined a resource containing 116 Mt of 10.3% zinc, 1.5% lead, and 35 g/t silver.

Geophysical techniques have been applied at Century since the discovery to help define the limits of mineralisation, map lithologic boundaries and structure, provide data for ore reserve estimates and mine planning, and to aid ongoing exploration in the area. Methods used have been gravity, ground and airborne magnetics, ground and airborne electromagnetics, IP/resistivity, downhole geophysical logging, and reflection seismic. The most useful technique to date has been IP/resistivity, which has assisted in mapping the extent of mineralisation beneath limestone and younger cover to depths of at least 150 m.

The Radio Imaging Method (RIM-MET) is a geophysical technique which uses medium frequency (50-520 kHz) radio waves to evaluate subsurface geology. The method is well established in the Australian coal mining industry and has recently been applied to base metal deposits in the Mount Isa area.

RIM relies on propagating radio waves in a transmission mode through rock, either from borehole to borehole, borehole to mine, or drive to drive. The resulting EM wave signal will attenuate as a direct response to variation in the conductivity of the intervening rock between transmitter and receiver. Multiple ray paths are measured through the area of interest by moving the transmitter relative to the receiver. This is repeated for a number of receiver locations until sufficient data are collected for tomographic image processing.

Tomographic imaging procedures, developed by the CSIRO Division of Radiophysics, are employed to produce an image of the distribution of ore within the survey section. The production of a tomographic image is an iterative process, incorporating inverse modelling and hypothesis testing.

This paper describes the results of in-mine and borehole surveys through Pb/Zn orebodies, carried out for M.I.M. Exploration Pty Ltd in and around Hilton Mine. The surveys at Mount Isa have demonstrated the role the method can play in late-stage ore definition and mine exploration. Determination of ore ‘shape’ in section and correlation of ore zones between boreholes is possible using the Radio Imaging Method. At this stage the technique does not appear to be applicable to green-field exploration but has the most potential as a mine-planning tool. The advantage of RIM over other EM geophysical techniques is its ability to look between boreholes.

The method has responded to low-grade Zn-dominated ore intersections in addition to high-grade Pb ore. Radio-wave penetration varies according to the conductivity of the host, but useful tomographic images using 50-kHz antennas were possible at 64-m borehole separation in the country rocks encountered at Mount Isa.

The Radio Imaging Method has demonstrated that it can be used to improve geological interpretation between boreholes and may reduce the amount of drilling required for mine planning.