We derive an unambiguous apparent conductivity from total magnetic field TEM data for fixed-loop geometry. For single-component fixed-loop TEM measurements, apparent conductivity is either dual-valued or undefined. The ambiguity or non-existence is particularly evident for readings taken outside the transmitter loop, both for step and impulse response data. Therefore, computing apparent conductivity from single-component fixed-loop TEM data can be problematic, especially at intermediate delay times. However, if multi-component fixed-loop magnetic field data is available, an unambiguous apparent conductivity can be derived from |B (t )| at all times, except in the inductive limit. Impulse response measurements can be time-weighted and summed to yield “quasi-|B |” data. Apparent conductivity derived from quasi-|B | amplitudes is dual-valued, but usually only one of the alternatives is geologically plausible. Computing apparent conductivity from |B | or quasi-|B | amplitudes expedites generation of conductivity-depth sections from fixed-loop TEM. A field data example with multi-component SQUID data shows significant improvement in the conductivity-depth section when |B (t )| is transformed rather than the vertical component, Bz , alone; the lateral extent of a conductive target is under-estimated in the Bz CDI

Many techniques exist for computer-aided interpretation of geological, geotechnical, and resource quality parameters from downhole geophysical data. Usually the measurements at any one depth are interpreted without regard to the data at other depths. However, in some circumstances, it is desirable to generate interpretations that conform to a known stratigraphic succession. Accordingly, this paper describes progress towards the development of an a posteriori stratigraphic correction procedure. The correction procedure operates on a preexisting interpretation of stratigraphic units based on their wireline signatures alone. The inputs are the stratigraphic order itself, and minimum, maximum, and standard (average) thicknesses defined for each of the stratigraphic units. Correction is effected in three stages, targeting successively (a) layers that exceed the minimum thickness, (b) thin layers, and (c) excessively thick layers. During each stage, a directed trial-and-error process resolves stratigraphic conflicts between adjacent layers. The process is expedited if a petrophysically distinct “marker bed” occurs in the sequence. The correction algorithm has been illustrated in the context of geotechnical modelling at the German Creek coal mine, Queensland. Results achieved to date have been sufficiently encouraging to warrant a trial implementation of computer-aided geotechnical interpretation at the mine.

Cross-hole frequency domain electromagnetic (EM) methods are most commonly applied at high frequencies (> 1 kHz) for definition of petroleum and coal deposits. Interpretation is often based on tomographic reconstruction of the data, assuming far-field, ray-like behaviour. In some cases these assumptions are untenable. Conventional tomography is also questionable if data are recorded at depths which are small in comparison to a wavelength, i.e. if surface reflections are appreciable. In order to address these limitations of tomography, a program has been written to invert cross-hole data from a vertical magnetic dipole transmitter in a layered earth. 1D inversion and conventional tomography are applied to the same radio frequency data set to illustrate the advantages of the more rigorous approach at shallow depths and short ranges.

Conductivity-depth imaging is a convenient form of presentation for preliminary interpretation of ground and airborne EM data. This paper describes the airborne EM adaptation of the Emax conductivity-depth transformation, originally developed for ground TEM. The transformation proceeds in two stages: first the apparent conductivity is determined at a given delay time; then the depth of the current maximum in a half-space with conductivity equal to the apparent conductivity is adopted as the apparent depth at that time.The advantage of the Emax transformation is that it is readily adaptable to a wide variety of TEM data. The disadvantage is that apparent conductivity is not unique, nor always defined. In practice this does not usually pose difficulties for transformation of airborne EM.The utility of the Emax transformation to airborne data is illustrated via application to GEOTEM_DEEP total field data. The total field provides a degree of immunity to receiver mis-orientation.

Velocity and absorption tomograms are the two most common forms of presentation of radar tomographic data. In this paper a simple transformation is developed, to convert velocity and attenuation coefficient to conductivity and dielectric constant. The approach is illustrated via application to data from the Hellyer zinc-lead-silver mine, Tasmania. The conversion to conductivity and dielectric constant was especially advantageous because mining personnel, geophysicists included, are often unfamiliar with radar velocity and absorption. Encouraging results were obtained in mapping the distribution of electrical properties between boreholes at Hellyer, illustrating the potential of radar tomography to delineate sulphide ore zones.

Just as geophysics, in the form of 3D seismic and wireline logging, plays a key role in petroleum reservoir management, so too will geophysics play an important role in mining of coal, metals, and minerals in the 21(st) century. The commercial and social imperatives driving greater use of geophysics are reduced costs, increased revenues, and enhanced safety. In short, superior utilisation of capital and management of risks.In relatively undeformed sedimentary environments it is possible to adapt petroleum-style 3D reflection seismic to image mines. This has been demonstrated most impressively in the Witwatersrand. Anglo-American, for example, completed a $1 million 3D seismic survey at Western Deeps in order to site a new $300 million shaft with confidence. Likewise, 3D seismic has gained rapid acceptance at Australian coal mines in recent years. Encouraged by these successes, metalliferous mining companies are supporting research in Canada and Australia, as well as South Africa, to adapt 3D seismic for more highly structured, metamorphic terranes.Because the mining industry is far more diverse than the petroleum industry in terms of commodities and geological environments, 3D seismic is not always cost-effective. A plethora of other geophysical techniques can be applied, many of which are employed in mineral exploration. Mine applications of geophysics differ from traditional exploration applications in two main ways: time scales and length scales are shorter, and boreholes are more plentiful. Borehole geophysical techniques therefore play a more significant role, both for logging and imaging. Logging systems detail the variations of in situ physical properties down the borehole at scales measured in centimetres, while geophysical imaging techniques can map features located tens or even hundreds of metres from the sensors. While geologists tend to think of boreholes in terms of chips and core, geophysicists perceive them first and foremost as access paths for instruments.Borehole logging has been employed at iron and coal mines for decades, mainly to accurately define ore boundaries in delineation holes. In base metal mines, Outokumpu implemented logging widely for orebody delineation in percussion holes in the 1980s. The benefit was a direct cost-saving, arising from substitution of core drilling with percussion drilling plus borehole logging. The practice has since spread to other companies. INCO, for example, defines nickel boundaries in blast holes at Sudbury using conductivity logs. The benefits are in the form of reduced dilution and enhanced ore recovery, flowing from a more accurate mine model. Density, natural gamma radiation, magnetic susceptibility, and conductivity are the principal metalliferous mine logging parameters because they can be recorded in both dry and water-filled holes.Sonic velocity is the premier geotechnical logging parameter, given its sensitivity to rock strength, stress, porosity, and degree of fracturing. Sonic is recorded routinely in exploration and geotechnical holes at coal mines. With the advent of slimline dipmeter, full waveform sonic, and optical and acoustic scanner tools, the role of borehole logging in geotechnical evaluations is expanding. monitoring of strain and micro-seismic activity enhances safety during mine production.While qualitative interpretation of logs is adequate for stratigraphic identification or definition of boundaries, a wealth of quantitative information, eg density, can be derived from properly calibrated, repeatable geophysical logs. Moreover, geophysical logs can sometimes serve as surrogates for geochemical assays, and not only for magnetite and uranium. At Outokumpu's Kemi chromite mine, for example, gamma-gamma logging provides the basis for grade control. By reducing reliance on assaying, three benefits can be realised: reduced reliance on core drilling; lower core handling and assaying costs; and shorter turn-around times.Conventional downhole EM and borehole magnetics are used for near-mine exploration, and for ground sterilisation. Inco has enjoyed considerable success with borehole UTEM in the Sudbury Basin, for example, and Geopeko has applied three-component borehole magnetics successfully in the Tennant Creek area. Electrical techniques such as applied potential and magnetometric resistivity (MMR) are also finding application at mines. For higher resolution, borehole seismic, radio imaging, and radar have been invoked, with varying degrees of success, to delineate orebodies and map structures, or to geotechnically characterise the rock mass and identify hazards. Success mapping nickel sulphide shoots with borehole radar has been reported by WMC at Kambalda.There are no universal geophysical panaceas, and each mine imposes different geological, logistical, and economic constraints. Geophysics will not always be cost-effective. However, the greatest single impediment to expanded use of geophysics at mines has been the low level of awareness of geophysics on the part of most mine geologists, engineers, and managers and, equally, the limited understanding of mine geology and engineering exhibited by the majority of geophysicists. This myopia will be remembered as a 20(th) century affliction!Blue Sky areas for mine geophysics in the next ten years include integration of geophysical data acquisition with drilling, enhanced grade estimation and rock mass characterisation, and the incorporation of geophysical information into mine models using geostatistical techniques. All these advances will be predicated on an expansion of petrophysical knowledge.

In an effort to reduce costs and increase revenues at mines, there is a strong incentive to develop high‐resolution techniques both for near‐mine exploration and for delineation of known orebodies. To investigate the potential of high‐frequency EM techniques for exploration and delineation of massive sulfide orebodies, radio frequency electromagnetic (RFEM) and ground‐penetrating radar (GPR) surveys were conducted in boreholes through the McConnell massive nickel‐copper sulfide body near Sudbury, Ontario, from 1993–1996. Crosshole RFEM data were acquired with a JW-4 electric dipole system between two boreholes on section 2720W. Ten frequencies between 0.5 and 5.0 MHz were recorded. Radio signals propagated through the Sudbury Breccia over ranges of at least 150 m at all frequencies. The resulting radio absorption tomogram clearly imaged the McConnell deposit over 110 m downdip. Signal was extinguished when either antenna entered the sulfide body. However, the expected radio shadow did not eventuate when transmitter and receiver were on opposite sides of the deposit. Two‐dimensional modeling suggested that diffraction around the edges of the sulfide body could not account for the observed field amplitudes. It was concluded at the time that the sulfide body is discontinuous; according to modeling, a gap as small as 5 m could have explained the observations. Subsequent investigations by INCO established that pick‐up in the metal‐cored downhole cables was actually responsible for the elevated signal levels. Both single‐hole reflection profiles and crosshole measurements were acquired using RAMAC borehole radar systems, operating at 60 MHz. Detection of radar reflections from the sulfide contact was problematic. One coherent reflection was observed from the hanging‐wall contact in single‐hole reflection mode. This reflection could be traced about 25 m uphole from the contact. In addition to unfavorable survey geometry, factors which may have suppressed reflections included host rock heterogeneity, disseminated sulfides, and contact irregularity. Velocity and absorption tomograms were generated in the Sudbury Breccia host rock from the crosshole radar. Radar velocity was variable, averaging 125 m/μs, while absorption was typically 0.8 dB/m at 60 MHz. Kirchhoff‐style 2-D migration of later arrivals in the crosshole radargrams defined reflective zones that roughly parallel the inferred edge of the sulfide body. The McConnell high‐frequency EM surveys established that radio tomography and simple radio shadowing are potentially valuable for near‐ and in‐mine exploration and orebody delineation in the Sudbury Breccia. The effectiveness of borehole radar in this particular environment is less certain.

Radio-frequency tomography (RFT) normally is assumed to sense the geological structure in a plane between the transmitters and receivers, loosely termed the image plane. In practice, out-of-plane objects also affect the tomograms. We illustrate these effects on synthetic crosshole tomograms generated for a conductive sphere in a more resistive host. The sources and receivers are vertical magnetic dipoles. The tomographic image of a highly conductive sphere located wholly or partially between the sources and the receivers is a conductive feature the shape of which is governed by the conductivity of the host. As the sphere moves around one of the boreholes at a fixed distance, its expression on the tomogram is transformed from a conductive feature to a resistive feature. This reversal also occurs as the conductive sphere moves radially away from the transmitter. The period of these reversals can be related to the change in path length from source to sphere to receiver. Thus, if only amplitudes are recorded, an out-of-plane conductor could be misinterpreted as a resistive object in the plane; the two cases could probably be differentiated if phase data were also recorded. The influence of the sphere on the tomogram is also negligible at certain azimuths and radial separations determined by the host conductivity and source frequency. A good conductor close to a borehole could be missed by an RFT survey if it were located in such a null. The risk of this occurrence is greatly reduced if multifrequency data are collected. Also, the shape of the tomographic expression of the sphere is insensitive to azimuth and radial separation.

A cross-hole RFEM (Radio Frequency Electromagnetic) tomographic survey was conducted at the Mt. Isa Copper Mine in 1995 as part of a CMTE/AMIRA project investigating the application of geophysics in metalliferous mines. The primary objective of the survey was to evaluate the capability of RFEM for orebody delineation, in a section of the mine where a correlation had previously been established between conductivity and copper grade.An absorption tomogram constructed from the limited 52.5 kHz data set demonstrated that RFEM has potential in this environment for resolving orebody boundaries and establishing ore continuity between drill holes. The calculated absorption coefficients on the tomogram lie between 0.94 and 5.165 dB/m, consistent with laboratory absorption measurements on rock samples from the survey site.The continuity of the footwall orebody, paralleling the Paroo Fault, was not well represented in the tomogram, due to low ray coverage in the comer of the image. However, a simple amplitude mask, depicting only the less attenuated ray paths, provided evidence for continuous ore between the holes. This provides encouragement for efforts to combine amplitude masking with tomography.

The radio imaging method is utilised in mines and oil fields to obtain detailed geological information between drill holes or mine roadways. When radio waves are transmitted through the ground at a fixed frequency, variations in absorption as the transmitter-receiver geometry changes are indicative of variations in conductivity of the geological section. With operating frequencies typically between 103 and 106 Hz, radio frequency electromagnetics (RFEM) is intermediate in range and resolution between low frequency electromagnetics used in exploration and high frequency ground penetrating radar (GPR).At many metalliferous mines, the ore is characterised by high conductivity contrasts and well-defined boundaries. Radio tomography between holes or mine roadways has a role to play in orebody delineation, but standard SIRT reconstruction produces unrealistic smooth images. In order to generate tomographic images with sharp boundaries, a weighted SIRT algorithm has been developed. A ‘clamping weight’ has been designed to fix the absorption coefficient at the low (host) value in regions that do not attenuate the radio signals, thereby localising high absorption into discrete zones. A ‘central weighting’ has also been introduced to concentrate high absorption towards the centre of the image in an attempt to compensate for the sensitivity of the acquisition system to variations in conductivity close to the receiver or transmitter.The weighted tomography has been tested on simulated cross-hole radio frequency data, as well as data collected from the Levack mine, Canada. Generally, the inclusion of weights in tomographic reconstructions has produced more realistic images of the geology. For the synthetic data sets, the resulting tomograms resembled the true model more closely than the standard SIRT images. For the Levack data set, the weighted tomography improved the definition of a mineralised lens.

Approximate conductivity-depth transformation has become popular for presentation of transient electromagnetic (TEM) profile data. The majority of published schemes apply only to data acquired using the coincident-loop and in-loop configurations. TEM surveys for mineral exploration or groundwater studies, however, sometimes employ the ‘slingram’ configuration, in which the receiver is located at a fixed distance outside the transmitter loop. Accordingly, a conductivity-depth transformation, based on existing in-loop and fixed-loop schemes, has been developed for slingram impulse-response TEM data.Apparent conductivity for the slingram configuration is almost always dual-valued. Nonetheless, it is possible to select a unique apparent conductivity for the purpose of depth transformation, since only one apparent conductivity at each delay time is compatible with downward diffusion of current. The unique apparent conductivities selected in this manner may not, however, be suitable for time-depth transformation, as they do not always define a smooth conductivity versus time curve. In particular, apparent conductivity is least reliable as an indicator of ground conductivity for measured voltages close to the maximum (or minimum) possible half-space response for the given system geometry and delay time. Such values are automatically edited from the apparent conductivity versus time curve. Finally, the depth assigned at each remaining delay time is the depth to the physical current maximum in a half-space with conductivity equal to the apparent conductivity at that time.The conductivity-depth transformation yields a valid qualitative portrayal of the actual variation of conductivity with depth when applied to theoretical slingram data for layered-earth models. Conductivity-depth pseudo-sections generated from field data show good correspondence to known geology, and may be generated in a fraction of the time required for inversion.

The role of geophysics in mineral exploration has expanded rapidly in recent decades, but in mining its importance is only just being recognised. The barriers to greater acceptance of geophysics in mining are more 'cultural' than technical, insofar as mining companies have historically (albeit unconsciously) effectively quarantined geophysicists from mine development and production personnel. The principal motivation of this paper is therefore to raise the geophysical awareness of mining geologists, engineers, and managers.In resource definition and mine development, major capital expenditures are committed on the basis of very sparse information. Local inaccuracies in mine models based on incomplete datasets are not infrequently the root cause of unexpected and sometimes costly production shortfalls, through lost ore or bad ground. When suitable physical contrasts exist, geophysics has the potential to reduce the risks in mine development decision-making via timely and cost-effective mapping of the orebody and its environment. Geophysics, appropriately applied, can underpin mine performance improvements in a number of spheres, including cost per tonne, safety, and environmental impact.Geophysical methods can be classified into two broad categories: borehole logging, for determination of in situ physical properties of the borehole wall rocks; and geophysical imaging, for mapping features at tens or even hundreds of metres from the sensors. Imaging methods can be applied from the pit floor, or underground from individual holes, between holes, or from hole to roadway. Petrophysical borehole logs can expedite ore boundary delineation, rock mass characterisation, and (sometimes) grade estimation. Benefits during production include substitution of core drilling with cheaper percussion drilling, and blasting pattern optimisation. Less well recognised is the value of petrophysical measurements during feasibility studies and at the onset of mine development. Sonic velocity logging of delineation holes can yield a continuous in situ record of rock strength, for example. The benefit of petrophysical data at an early stage in a project is enhanced three-dimensional understanding of the geological and geomechanical environment in the appropriate time frame to influence major mine design decisions. Geophysical imaging techniques can be applied in mines for a variety of applications, including orebody delineation, hazard detection, and exploration. Radio-frequency tomography, for example, is used to map conductive orebodies between drill-holes. Ground-penetrating radar can generate detailed images of potential hazards such as cavities or aquifers, while passive seismic monitoring reduces safety risks by delineating zones of high rock stress. Downhole electromagnetics and borehole magnetics are employed both for detecting off-hole mineralisation and for ground sterilisation.Geophysics is not a panacea at mining operations, but should be viewed as an additional source of tools to deploy in the continuous struggle to maximise overall performance. The benefits flowing from application of geophysics may be direct, as immediate cost reductions, or indirect in the form of an enhanced ore recovery, optimised blasting pattern, or early warning of a safety hazard.

In resource definition and mine development, major capital expenditures are committed on the basis of sparse information and even in production surprises such as bad ground are not uncommon. Drill holes constitute the principal source of information about the orebody and its environment and optimisation of drilling implies maximising the yield of useful information per dollar expended. Geophysical borehole logging provides new flexibility for optimising the drilling budget.Economic benefits can flow from geophysical logging at all stages of the mining cycle. The most commonly cited benefit is substitution of diamond delineation drilling with cheaper percussion or reverse circulation drilling in cases where geophysical logs can substitute for core. This approach can deliver an attractive direct saving in drilling costs (and time) if drill meterage is unchanged, or a potentially greater indirect benefit from better ore control if more holes are drilled within the original drilling budget. More ore contact intercepts per dollar can translate into lower dilution and enhanced ore recovery, thereby increasing revenue as well as reducing mining costs.Substitution of diamond drilling with percussion drilling is not always feasible. However, geophysical logging of diamond holes is often highly advantageous in its own right, both for geotechnical characterisation as well as ore delineation. Operational advantages of logging include data objectivity, speed of interpretation and reduced core handling and analysis costs. Geophysical logging assists mining engineers by providing a continuous measure of in situ rock strength, even over intervals with poor core recovery.The additional expense of geophysical logging in blast holes can be justified in terms of more accurate ore boundary delineation and rock strength evaluation. Precise ore boundary delineation permits refinement of charge placement, to minimise dilution and maximise recovery. Detailed knowledge of rock strength can enable optimisation of blast design. Rock strength can be evaluated in real-time via analysis of drill performance data recorded with measurement-while-drilling (MWD) technology.To successfully apply geophysical logging in mines, its technical, operational and economic viability must all be established. This entails verification of the existence of appropriate physical property contrasts, investigation of the best way to integrate the geophysical results with the mining method and an objective assessment of the value of the resulting mine performance benefit.Geophysical logging is already well established in many mining operations. The greatest impediments to its wider utilisation at mines are: lack of appreciation of geophysics by most mine geologists, engineers and managers; limited interpretational aids; and the necessity to re-enter completed drillholes. In this paper we endeavour to address these issues by informing mine geologists about potential applications of geophysics, by illustrating automated interpretation of geophysical logs and by highlighting the importance of emerging MWD and logging-while-drilling (LWD) technology.

The McConnell deposit, a highly conductive tabular nickel sulphide body, was imaged successfully between two holes about 100m apart. The mineralisation was defined as a continuous high absorption zone on the 5MHz tomogram. Numerical modelling of the signal amplitudes suggested that diffraction around the ends and bottom of the deposit was inadequate to explain the strength of the observed fields when transmitter and receiver were on opposite sides of the deposit. Consequently, it was concluded that the sulphide body is discontinuous. The existence of a gap is supported by an intermediate drill hole which passed through two massive sulphide intervals, separated by about 6m; this barren interval had been originally interpreted as evidence for a re-entrant on the sulphide contact. The potential of radio frequency surveys to refine mine plans was thus clearly demonstrated.

A computationally efficient transformation is being developed to convert fixed loop TEM profiles to apparent conductivity versus depth pseudo-sections. Such pseudo-sections facilitate first-pass interpretation for geologists and geophysicists alike. Following Fullagar (1989), the apparent depth at each delay time is equated to the depth of the current maximum in a half-space with conductivity corresponding to the apparent conductivity at that time. The non-uniqueness in the apparent conductivity at any (x,t) can be overcome by identifying the unique apparent conductivity associated with downward current diffusion. Initial applications of the transformation to theoretical TEM decays and to field data from the Canning Basin demonstrate its capability to provide a reliable qualitative portrayal of conductivity structure.

A computationally-efficient transformation has been developed to convert coincident loop or in-loop TEM profiles to apparent conductivity versus depth pseudo-sections. The transformation is based on the depth to the maximum physical current induced in a half-space, not on attributes of abstract current filaments. Tests on both synthetic and field data indicate that these pseudo-sections provide a useful qualitative portrayal of conductivity structure, thus facilitating first-pass interpretation of TEM in mapping applications.

An appreciation of errors is a prerequisite for scientific interpretation of any experimental result. In seismic deconvolution three main sources of error can be identified: ambient random noise, coherent noise, and uncertainty in the wavelet. In this paper a deconvolution method is described which (1) takes wavelet uncertainty into account during inverse filter design, and (2) determines the error in the deconvolved output. Wavelet uncertainty is characterised by the time domain covariance matrix for wavelet deviations and the vector of frequency domain wavelet variances. Thus a prerequisite for implementation of the method is a representative suite of source wavelets. This requirement is most readily satisfied for offshore data, where far-field signatures can be measured or calculated. Thus the method is introduced in the context of marine seismic, and illustrated with a synthetic example. Deconvolution with respect to a time-invariant far-field signature provides an estimate of the impulse response function. The wavelet-related errors in the impulse response model are model dependent and hence time dependent. Their distribution provides an immediate indication of the likelihood and size of errors in event amplitude, time, and shape.

In spike recovery deconvolution (SRD) (Fullagar, 1985) it is assumed a priori that the reflectivity consists of event spikes separated by zeros, and a spiky reflectivity is constructed which, when convolved with the wavelet, reproduces the seismogram to an accuracy consistent with the perceived level of noise. The principal attraction of SRD is improved resolution for which the price is increased computation relative to conventional linear deconvolution.

A circular current filament in free space constitutes a useful idealisation of late-time secondary current flow in a geological conductor. The mathematical simplicity of the current filament model permits efficient inversion of down-hole TEM (DHEM) logs via iterative adjustment of a starting guess. The application of current filament modelling in exploration is illustrated using data from a shallow hole intersecting a mineralised quartz vein. The DHEM log at 1.2 msec is closely replicated by combining the effects of two filaments centred up-dip, one encircling the hole and one away from the hole; these are interpreted to represent a single distributed current system pierced by the hole. Inversion of the 5.8 msec DHEM log suggests that this current system contracts and migrates up-dip. Inductive coupling between the vein and the regolith zone is hypothesised as the cause of the upward migration. The current system dominating the DHEM response is small in diameter and located near the hole; the down-hole survey therefore provides no indication of the better mineralisation later intersected down-dip.

A procedure has been developed to perform automatic inversion of HLEM frequency soundings by the method of Backus and Gilbert (BG). Using a linearized iterative scheme, a layered conductivity is constructed satisfying the available soundings to an accuracy consistent with the observational uncertainties. Subsequently, in order to investigate the resolving power of the data and hence (hopefully) to determine the broad scale features which are common to all conductivity models which satisfy the data, BG average conductivities are computed for the constructed model at a number of depths. In examples with both synthetic data and field data these averages proved to be identical (except for random error) to the corresponding average conductivities computed for a number of different models satisfying the same data. The additional models for this analysis were generated by maximizing or minimizing the conductivities of individual layers in the original constructed model using linear programming techniques. On the basis of these empirical results, in conjunction with the uniqueness theorem for HLEM soundings, it is concluded that BG average conductivities computed for a particular constructed model will often provide a valid characterization of the true ground conductivity over a depth range governed by survey parameters.