Frequency-domain Electromagnetic Research Articles

Determination of the 3D structure of an earthflow by geophysical methods: The case of Super Sauze, in the French southern Alps

In order to evaluate the risk associated by an earthflow to abruptly evolve into a torrential flow, the knowledge of its internal structure is necessary. Geotechnical methods are important to reach this goal. However, because of the rough topography associated with earthflows, their surface heterogeneities, and the spatial variations of the thickness of the potentially moving mass, non-intrusive geophysical methods offer a very useful tool that complements traditional geotechnical methods. We report the results of a comprehensive study covering a 150 m by 200 m area of the Super Sauze earthflow. This earthflow developed in black marls in the southern French Alps. Shallow electrical conductivity investigations, derived using low frequency domain electromagnetics, maps hidden gullies and crests and lateral variations of the clay and the water content within the first 5 m below the ground surface. Electrical resistivity tomography allows to extrapolate this information down to 10 m below the ground surface along selected transects. The vertical structure of the earthflow, down to the substratum, is defined precisely thanks to joint inversion of DC and TDEM vertical soundings along one profile: the flowing upper layer and the position of the substratum are clearly evidenced. Combining this geophysical datasets with geotechnical tests and drill holes, we provide an estimate of both the location and the volume of the potentially most dangerous areas of the earthflow.

On Optimal Convergence Rate of the Rational Krylov Subspace Reduction for Electromagnetic Problems in Unbounded Domains

We solve an electromagnetic frequency domain induction problem in $\mathbf{R}^3$ for a frequency interval using rational Krylov subspace (RKS) approximation. The RKS is constructed by spanning on the solutions for a certain a priori chosen set of frequencies. We reduce the problem of the optimal choice of these frequencies to the third Zolotaryov problem in the complex plane, having an approximate closed form solution, and determine the best Cauchy–Hadamard convergence rate. The theory is illustrated with numerical examples for Maxwell's equations arising in 3D magnetotelluric geophysical exploration.

Portable Magnetic/Frequency Domain Electromagnetic Induction Sensor System Development

An unexploded ordnance (UXO) survey instrument that simultaneously collects total field magnetic data and frequency domain electromagnetic (FDEM) induction data was developed and tested for the detection and characterization of buried UXO. The system is comprised of an FDEM sensor operating at 9.8 kHz and a cesium vapor magnetometer. The system was initially tested in dynamic survey (detection) and static cued survey (characterization) modes at the Naval Research Laboratory Blossom Point UXO test facility in Maryland. During these tests, electromagnetic (EM) induced bias in the magnetic data was mitigated by physically offsetting the magnetometer from the EM transmitter coils. In the dynamic survey, the aggregate performance exceeded the detection rates for the individual component sensor technologies. The cued analysis tests showed that target features can be determined by using model-based analyses, and the location estimate errors provided by these analyses were consistently better than the dynamic survey results.

Current and Charge Integral Equation Formulations and Picard's Extended Maxwell System

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Important connection between computational and mathematical electromagnetics is presented. The newly developed well-conditioned electromagnetic frequency domain surface integral equation formulations, the current and charge integral equations, are shown to be related to Picard's extended Maxwell system, an extended partial differential equation system that has the correct static behavior. Electromagnetic surface integral representations are derived in this paper for traditional surface integral equation formulations and for the Picard system using the fundamental solution approach, i.e., from the definition of Dirac's delta function. The surface integral representations are constructed with proper solid angle coefficients starting from the scalar Helmholtz equation. The traditional surface integral equation formulations are shown to be derived from Maxwell's curl equations and are thus lacking the contribution of the divergence equations at zero frequency. It is shown that the new current and charge formulations can be derived from the surface integral representation of the Picard system. </para>

Frequency domain electromagnetic and ground penetrating radar investigation of ephemeral streams: case study near the Southern High Plains, Texas

A frequency domain electromagnetic (FDEM) and ground penetrating radar (GPR) study was conducted on an ephemeral stream in north-central Texas to determine if FDEM and GPR measurements can be combined to determine the electrical characteristics of current and ancient stream channels. GPR data were collected at several frequencies to image sedimentary structures of different scale lengths, and to determine the formation porosity and water content of stream sediments. FDEM measurements were collected using Geonics EM31 and EM34 loop–loop instruments on a profile along the current stream channel and five profiles perpendicular to the channel. The results indicate that the greater spatial resolution of the EM31 mapped the current and possible ancient channels better than the EM34, however, the EM34 provided depth information on the formations underlying the channel sediments that the EM31 could not image. GPR measurements taken along a point bar deposit with 200, 100 and 25 MHz antennae indicated that the higher frequency antenna better resolved channel structures including laminar bedding, trough scours and cross-bedding, however, lower frequency antenna (25 MHz) imaged sedimentary structures within the underlying channel sediments. Common midpoint GPR measurements collected along the point bar deposit were used to estimate the sediment formation porosity (26%) and the water content, during a dry period, of the unsaturated (12%) and saturated (26 or 100% of the pore space) sediments. The combined results indicated that the FDEM data should be collected first because of the speed and ease of measurements. The FDEM data (especially the EM31) pointed to the locations of possible sedimentary structures, which can then be resolved by using different frequency GPR measurements.

3D animated visualization of EM diffusion for a frequency-domain helicopter EM system

The electromagnetic fields inside the earth are calculated by continuation downward of the electromagnetic (EM) solutions at the location of a frequency-domain helicopter electromagnetic (HEM) sensor. Models examined using the continuation approach include a layered isotropic and anisotropic earth. The finite-element approach is used to model 2D structures of a dipping contact or a dipping dike. By incorporating a time factor, we display the EM diffusion in the earth (change in direction and amplitude of the EM field through time) as 3D animated vectors or contours. The propagation of the EM smoke ring, influenced by the resistivity and structure of the earth, is apparent from the dynamic presentations. The current propagates downward and outward with time, becoming wider and more diffuse, and the phase varies with time, depth, and outward distance. The downward propagation of EM fields is slower in more conductive geology. In a layered isotropic earth, the current ring is symmetric with no vertical current flow for both vertical and horizontal dipole transmitters. In anisotropic or 2D structures; however, the current flow is significantly distorted resulting in vertical current flow and nonsymmetric smoke rings.

Mapping Groundwater in Regolith and Fractured Bedrock using Ground and Airborne Geophysics: Case studies from Malawi and Brazil

Water supply exploration in rural regions of eastern Malawi (Africa) and northeastern Brazil (South America) is complicated by the highly irregular nature of aquifers in weathered regolith and crystalline bedrock. Drilling success can be as low as 30%. Improvements in the success rate can be achieved by using simple geophysical investigations to target the thickest zones of weathering. Sophisticated geophysical surveys that image the full range in aquifer types can further improve the success rate to 70%. In Malawi, a total of 250 hand pumps were sited using a combination of borehole geophysics (natural gamma, conductivity and magnetic susceptibility), ground-based frequency-domain electromagnetics (Geonics EM34 and Apex Max-Min), and two-dimensional electrical resistivity imaging. In a similar terrain in northeastern Brazil, resistivity profiling and Very Low Frequency electromagnetic surveys are the mainstay of water exploration. A recent project introduced ground-based and airborne frequency-domain electromagnetics (Geonics EM34 and Aerodat helicopter electromagnetics) to improve the imaging of complex fracture zones and the detection of saline aquifers. Helicopter-borne surveys are a valuable tool in groundwater exploration because they cover large areas at high resolution. In both case studies, inversion of geophysical data integrated with remote sensing images and hydrogeology has increased the chances of drilling successful wells and finding potable water for rural communities.

Post-Processing Calibration of Frequency-Domain Electromagnetic Data for Sea-Ice Thickness Measurements

Sea-ice thickness measurements using electromagnetic (EM) instruments require accurate data. Calibration of sea-ice thickness data acquired using a low induction number (LIN) EM sensor can be performed by conducting a geometric sounding at a range of heights over level sea ice of known thickness, and by comparing the observed data with the expected layered-earth response. Calibration corrections for scaling, phase-mixing, and zero-offset errors can be derived using least-squares inversion to minimise the misfit between the observed data and the theoretical response, and can be incorporated in modelling algorithms used to determine sea-ice thickness.This paper presents a case history illustrating identification and correction of calibration errors in frequency-domain EM data for Antarctic sea-ice thickness measurements. Comparison of coincident EM measurements made using three EM31 instruments showed that measured apparent conductivities disagreed by up to around 100 mS/m, resulting in errors in the estimated sea-ice thickness of up to 60%. Separate calibration corrections were determined for each instrument by analysis of geometrical sounding data acquired over level sea ice. Sea-ice thickness at the calibration site was determined by making a large number of drilled thickness measurements over the footprint of the EM instrument, and seawater and sea-ice conductivities were determined using independent measurements. Best-fit scaling, phase-mixing, and zero-offset errors were determined via inversion of the geometrical sounding data, with the sea-ice thickness and seawater conductivity fixed at their known values. After application of the calibration corrections, sea-ice thicknesses derived from the three instruments agreed closely with each other and with drilling results. Calibration corrections derived in this manner have been shown to be valid over a period of at least several weeks.

Helicopter Frequency Domain(FDEM) Surveys for Characterizing Environmental Problems Related to Mining: Observations and Conclusions from 30 Surveys

Since 1999, the National Energy Technology Laboratory has conducted helicopter frequency domain electromagnetic (FDEM) surveys of 30 sites to identify hydrologic problems that have resulted from mining activity. The FDEM data from these surveys was processed to generate conductivity maps and conductivity/depth images (CDI), which together show the lateral and vertical distribution of ground conductivity. This information has been used to determine: 1) the location, depth, and thickness of regional water tables and perched water tables, 2) the location of infiltration zones, 3) the location of pyritic wastes either on the surface or at depth, 4) the location of flooded mine workings (if filled with conductive water and located at depths less than 50 m), and 5) the likely locations for springs or mine discharges. Helicopter FDEM results have been validated using down-hole geophysical measurements, traditional hydrologic measurements from dense networks of groundwater monitoring wells, ground-based electromagnetic surveys, airborne thermal infrared imagery, and field

Geophysical Evaluation Of Potential Refuse Disposal Sites A Case Study from George Town, Northern Tasmania

Geophysical techniques were applied at the George Town municipal refuse disposal site in northern Tasmania to delineate areas of thick Jurassic dolerite-derived clays and colluvial material for future waste disposal trenches. A variety of techniques were applied including seismic refraction, magnetics and frequency domain electromagnetics (EM31). Seismic refraction surveys effectively discriminate zones of thick clay and colluvial sediments from near-surface dolerite bedrock and residual core stones. Travel-time curves are typically complex due to the irregular dolerite surface and data was interpreted by tomographic inversion. Correlation of refraction tomograms with engineering data, auger drilling and point load measurements suggest that the boundary between ``easy' and ``difficult' excavation corresponds to a seismic velocity of ~2000m/s. Areas of near-surface dolerite are characterised by high total magnetic intensity and low apparent conductivity while zones of thick colluvial material and clay are conductive and have lower total magnetic intensity. The results of magnetic and EM surveys are complementary and correlate closely with the seismic interpretation. In this geological environment the combination of magnetic and frequency-domain EM surveys provide the most cost-effective geophysical exploration strategy for rapid delineation of suitable sites for waste disposal.

An Introduction to the Use of Airborne Technologies for Watershed Characterization in Mined Areas

Airborne geophysical and imagery technologies can be used to rapidly characterize watersheds. Frequency domain electromagnetics (FDEM) and thermal infrared (TIR) imagery are being used to map surface and subsurface hydrologic features. FDEM is being used to map water-filled fractures and voids that serve as groundwater conduits; TIR is being used to identify locations where groundwater discharges to the surface. The benefits of airborne reconnaissance include: significant cost savings through large-scale data collection, reduced landowner-access issues, improved watershed management practices, efficient targeting of costly, high resolution ground investigations, and superior techniques for locating drilling sites. The papers that follow this overview provide detailed descriptions of the application of these technologies on a case study basis.

A Geophysical Investigation of Dryland Salinity at Cressy, Tasmania

Geophysical methods have been used at Panshanger Estate near Cressy, Tasmania, to map the distribution of salt-affected areas, and the underlying catchment structure. Frequency-domain electromagnetic (FEM) techniques were used to detect nearsurface salt-affected zones based on their high conductivity. Areas visibly affected by salt were found to coincide with areas displaying the highest conductivities. Other areas displaying moderate to high conductivity did not appear to be salt-affected but may represent areas of future salt risk. Magnetic, gravity and timedomain electromagnetic (TEM) techniques, in conjunction with drill logs from holes near the field area, indicate that the western half of the field area is underlain by shallow Jurassic dolerite, while to the east and northeast, the dolerite is covered by a thicker layer of sediment. In the western part of the survey area, high conductivities occur close to the edges of bedrock highs, indicating that bedrock topography may be controlling groundwater movement. These bedrock highs generally correlate with surface topographic highs, and the elevated conductivities, indicative of high salt storage, are measured over depressions and creeks. A sinuous conductivity anomaly in the northeast of the field area occurs on slightly elevated ground, in an area where Tertiary cover is thick. There was no visible evidence of salinity at the surface over this feature. It was interpreted as a shallow palaeochannel based on EM measurements. An investigation borehole intercepted an upwardly fining sandy clay sequence, representative of a meandering fluvial system. EC 1:5 analysis of the sediment core and water analysis indicates that the palaeochannel may act as a mechanism of salt transport and storage.

Three-dimensional inversion of multitransmitter electromagnetic data based on the localized quasi-linear approximation

Three-dimensional (3-D) electromagnetic (EM) inversion is increasingly important for the correct interpretation of EM data sets in complex environments. To this end, several approximate solutions have been developed that allow the construction of relatively fast inversion schemes. We have developed a localized quasi-linear (LQL) approximation that is source-independent and, therefore, appropriate for multisource array-type surveys, typical in many geophysical applications, such as airborne EM, cross-well tomography, and well logging. This method is based on the assumption that the anomalous electric field within an inhomogeneous domain is linearly proportional to the background electric field through an electrical reflectivity tensor This tensor is determined by solving a source-independent minimization problem based on the integral equation for the scattering currents. We have also developed a new, fast 3-D EM inversion method, based on this new approximation, and applied it to synthetic and real helicopter-borne EM data. The results demonstrate the stability and efficiency of the method and show that the LQL approximation can be a practical solution to the problem of 3-D inversion of multitransmitter frequency-domain EM data.

Full 3-D inversion of electromagnetic data on PC

Three-dimensional (3-D) electromagnetic (EM) inversion might be believed to require high-performance computers. However, with the rapid progress of recent computer technology, running 3-D inversions on personal computers (PC) is becoming a rational choice. This paper describes an attempt to carry out full 3-D inversions of synthetic frequency-domain EM data on a PC. In the inversion, a staggered-grid finite difference scheme is used to solve for the secondary electric field. The system of equations is solved using the incomplete Cholesky biconjugate gradient (ICBCG) method. By applying the static divergence correction proposed by Smith [Geophysics 61 (1996) 1319–1324], the rates of convergence are dramatically improved, and the forward calculation per source location on a medium-size grid takes only a few minutes on a PC. The sensitivities of the EM responses to subsurface resistivity changes are calculated from forward solutions using the reciprocity relation. An inverse problem is formulated so that a model is found that has a smooth structure and at the same time is close to an initial model, and is solved with an iterative least-squares method. A synthetic example for an airborne EM survey shows that the lateral extents of 3-D bodies are well resolved, and the vertical coil coaxial system gives a better lateral resolution than the horizontal coil coplanar system. An example for a ground EM survey shows that expanding the survey coverage or aperture is needed to improve the resolution at depth.

Recent base metal discovery at the Hill 800 Prospect

Hill 800 Prospect is located northwest of Licola, near the Jamieson River in the eastern highlands of Victoria. It was discovered in 1994 and is currently held under EL 3592 by Mount Wellington Gold N.L. (Mount Wellington Gold). The exploration model is for volcanic-hosted massive sulphides in the Cambrian Jamieson Volcanics. Mount Wellington Gold's interest in the area was sparked by new regional magnetic and radiometric data that became available in 1993 under the Victorian Initiative for Minerals and Petroleum. Investigation of potassium anomalies resulted in the discovery of gossans at Hill 800. Soil geochemical data show high gold values over Hill 800 and high copper values surrounding this, to the south and west. The best assay values in drillholes are 37 m @ 3.16 g/t gold from 0.5 m, and 6 m @ 1.1% copper from 253 m in HED1. The high gold values occur in an oxidised and altered cap near the top of Hill 800. The high copper values occur separately from the gold values, in altered volcanics. The geophysical surveys carried out by Mount Wellington Gold include: airborne radar (AIRSAR), airborne frequency-domain electromagnetics (DIGHEM), ground magnetics, time-domain electromagnetics (SIROTEM), induced polarization, downhole electromagnetics, downhole magnetometric resistivity (DHMMR), spontaneous potential and controlled source audio magnetotellurics (CSAMT). The gold and copper mineralisation currently detected at Hill 800 does not have a significant geophysical response. Responses recorded to date are generally from geological rather than mineralised sources. The latest induced polarization results accurately predicted pyrite enrichment (+- gold). A drilling target was defined from a 3-D model of the inverted 100 m line-spaced data. The induced polarization chargeability anomaly was associated with high resistivity, and when drilled, was found to be caused by disseminated sulphides (pyrite). The DHMMR survey on diamond drillhole HED4A indicates a conductive source, possibly massive sulphides, above the hole at about 460 m depth. The SIROTEM data also show subtle high conductivity at late times in this area. These data have many anomalies, which have not yet been effectively tested by the drilling program. Magnetic susceptibility measurements on rock chips from the drilling program indicate that the weathered and altered volcanics have lower susceptibility than the fresh volcanics - perhaps due to magnetite destruction associated with the alteration/weathering. The regional magnetics show a highly magnetic source beneath Hill 800, probably an intrusion. Modelling shows that this intrusion may come to within a hundred metres of the surface, so it is a likely source of the alteration fluids.

Depth of penetration of frequency-domain airborne electromagnetics in resistive terrains

The depth of investigation of a fixed-wing frequency-domain airborne electromagnetic system is studied. Starting from the definitions for plane-wave EM and a homogeneous half-space, the analysis is extended to dipole fields and a layered earth, and finally to more realistic two-and three-dimensional conductive sources. The main factors affecting the depth of penetration are coil configuration, operating frequency and signal-to-noise ratio of the AEM system. The analysis is based on forward modeling of AEM responses in the frequency domain using existing software for conductive ID, 2D, and 3D bodies. The AEM system analysed as an example is a vertical coaxial, frequency-domain system flown in a DC-3 aircraft. Summary diagrams for different model geometries are constructed from the modeling results. Based on the analysis of the modeling results at the inductive limit, the depth of penetration in resistive terrain for the AEM system studied is between 85 m and 210 m depending on the geometry and the size of the conductor. Host rock and overburden of low but finite conductivity do not cause a significant change in penetration values. The screening and the attenuation effects from conductive overburden and conductive host rock will, however, decrease the penetration depth.

Electromagnetic mapping of saline contamination at an active brine pit

Frequency-domain and time-domain electromagnetic methods were used to investigate groundwater contamination at an active brine pit in southwestern Manitoba, Canada. The objectives of the survey were to delineate contamination suspected to be occuring at the site and to compare frequency-domain electromagnetic (FDEM) and time-domain electromagnetic (TDEM) measurements in a survey area containing pipelines, fences, and power lines. The survey successfully delineated a region of high conductivity around brine pit, confirming that leakage is occurring from the pit. Modelling of the FDEM results suggests the contamination is spreading within a series of shallow sand units. Comparison of FDEM and TDEM survey results indicate that small-separation FDEM systems are much more useful for mapping in a developed area containing sources of cultural noise. The FDEM systems permit rapid mapping of spatial variations in conductivity, are affected to only a limited degree by cultural features, and provide some resolution of the depth variation of conductivity at shallow depth. It was not possible to obtain useful TDEM measurements anywhere near the active brine pit because of the signal distortion in the late-time response. Key words: geophysics, electromagnetic, brine pit, saline contamination.

The Application of Electromagnetic and Electrical Methods to Groundwater Problems in Urban Environments

The use of electromagnetic (EM) methods for the mapping and monitoring of groundwater contamination in urban environments presents difficulties because of the presence of cultural noise and impediments to surveys from manmade structures. This paper shows the successful application of EM and dc methods for the detection and monitoring of groundwater conductivity changes caused by leachate contamination of groundwater, and for monitoring saline intrusions in these difficult environments. This paper discusses results obtained at four sites in the Canning Vale, Morley, Mindarie and Quinns Rocks suburbs of Perth, where any contamination of the groundwater poses a major threat because the city and suburbs rely on groundwater resources for up to 60% of their total water supply. Geophysical measurements were made by the transient electromagnetic (TEM) method using SIROTEM, dc sounding using a Schlumberger array and the down-hole frequency-domain electromagnetic (FEM) method using a Geonics EM-39 instrument. At Morley, where waste disposal began about 12 years ago, and TEM and dc measurements commenced 5 years later, inversion of the data assuming a layered earth showed that a region of low resistivity was developing in 1987 in the saturated zone to the southwest of the waste pit. Data obtained since then have shown a continuing development of this resistivity low further southwest. At the Tamala Park site in Mindarie, waste disposal commenced at the beginning of 1991 and, by November 1991, borehole sampling near the waste pit showed no significant change in groundwater conductivity. From background data collected at Tamala Park, it is possible to quantify seasonal variations in the apparent resistivity derived from the TEM and dc response and to predict that changes caused by leachate contamination should readily be detected above the seasonal variations. The data obtained from one TEM and one dc survey at the Quinns Rocks-Mindarie site appear to indicate that sea water could be affecting the conductivity of the aquifer at a distance up to 180 m inland. This interpretation of the surface TEM data has been difficult to verify with down-hole FEM measurements in the area because the boreholes do not penetrate to the base of the aquifer and measurements have not been made at sufficient depth to detect a significant increase in conductivity below the watertable.

Depth Profiles of Electrical Conductivity from Linear Combinations of Electromagnetic Induction Measurements

AbstractWithin the past 10 yr, frequency‐domain electromagnetic (FEM) induction techniques have become more and more widely used in soil science and hydrology. Each measurement of apparent electrical conductivity represents a depth‐weighted average of soil electrical conductivity. However, the depth weighting corresponding to each measurement may be very different from that required by the user. The simultaneous use of multiple measurements (with different depth weightings) should allow some aspects of the depth distribution of electrical conductivity to be inferred. We illustrate a method for obtaining linear combinations of FEM measurements to estimate the soil electrical conductivity within the depth interval of interest. The method relies on the simple fact that the measurement‐system response is linear, so that a linear combination of apparent‐conductivity readings corresponds to a linear combination of response functions. One can seek a linear combination of response functions that has desirable characteristics. This sets up an optimization problem that can be solved by standard methods, which avoids difficulties encountered with layered inversions to resolve nonlayered systems. The approach was applied to GEONICS EM31, EM34, and EM38 instruments in three examples: (i) single frequency measurements at vertical and horizontal dipole configurations, (ii) frequency measurements using the EM34 at all six possible configurations, and (iii) frequency measurements using the EM38 held at varying heights above the ground. The derived linear combinations were applied to field data from southern Australia. Soil conductivity profiles predicted using linear combinations showed good agreement with profiles measured with a conductivity probe.

Time‐domain electromagnetic soundings at the Nevada Test Site, Nevada

Structural discontinuities and variations in the resistivity of near‐surface rocks often seriously distort dc resistivity and frequency‐domain electromagnetic (FDEM) depth sounding curves. Reliable interpretation of such curves using one‐dimensional (1-D) models is difficult or impossible. Short‐offset time‐domain electromagnetic (TDEM) sounding methods offer a number of advantages over other common geoelectrical sounding methods when working in laterally heterogeneous areas. In order to test the TDEM method in a geologically complex region, measurements were made on the east flank of Yucca Mountain at the Nevada Test Site (NTS). Coincident, offset coincident, single, and central loop configurations with square transmitting loops, either 305 or 152 m on a side, were used. Measured transient voltages were transformed into apparent resistivity values and then inverted in terms of 1-D models, Good fits to all of the offset coincident and single loop data were obtained using three‐layer models. In most of the area, two well‐defined interfaces were mapped, one which corresponds closely to a contact between stratigraphic units at a depth of about 400 m and another which corresponds to a transition from relatively unaltered to altered volcanic rocks at a depth of about 1000 m. In comparison with the results of a dipole‐dipole resistivity survey, the results of the TDEM survey emphasize changes in the geoelectrical section with depth. Nonetheless, discontinuities in the layering mapped with the TDEM method delineated major faults or fault zones along the survey traverse. Schlumberger resistivity soundings expanded to an AB/2 of 1220 m detect only the first interface mapped by the TDEM method. Schlumberger and large‐offset, frequency‐domain, sounding curves appear to be more distorted by lateral variations than the TDEM sounding curves.