Showing papers in "Geophysics in 1992"

Seismic properties of pore fluids

Abstract: Pore fluids strongly influence the seismic properties of rocks. The densities, bulk moduli, velocities, and viscosities of common pore fluids are usually oversimplified in geophysics. We use a combination of thermodynamic relationships, empirical trends, and new and published data to examine the effects of pressure, temperature, and composition on these important seismic properties of hydrocarbon gases and oils and of brines. Estimates of in-situ conditions and pore fluid composition yield more accurate values of these fluid properties than are typically assumed. Simplified expressions are developed to facilitate the use of realistic fluid properties in rock models. Pore fluids have properties that vary substantially, but systematically, with composition, pressure, and temperature. Gas and oil density and modulus, as well as oil viscosity, increase with molecular weight and pressure, and decrease with temperature. Gas viscosity has a similar behavior, except at higher temperatures and lower pressures, where the viscosity will increase slightly with increasing temperature. Large amounts of gas can go into solution in lighter oils and substantially lower the modulus and viscosity. Brine modulus, density, and viscosities increase with increasing salt content and pressure. Brine is peculiar because the modulus reaches a maximum at a temperature from 40 to 80°C. Far less gas can be absorbed by brines than by light oils. As a result, gas in solution in oils can drive their modulus so far below that of brines that seismic reflection bright spots may develop from the interface between oil saturated and brine saturated rocks.

Magnetic interpretation using the 3-D analytic signal

Abstract: A new method for magnetic interpretation has been developed based on the generalization of the analytic signal concept to three dimensions. The absolute value of the analytic signal is defined as the square root of the squared sum of the vertical and the two horizontal derivatives of the magnetic field. This signal exhibits maxima over magnetization contrasts, independent of the ambient magnetic field and source magnetization directions. Locations of these maxima thus determine the outlines of magnetic sources. Under the assumption that the anomalies are caused by vertical contacts, the analytic signal is used to estimate depth using a simple amplitude half-width rule. Two examples are shown of the application of the method. In the first example, the analytic signal highlights a circular feature beneath Lake Huron that has been identified as a possible impact crater. The second example illustrates the continuation of terranes across the Cabot Strait between Cape Breton and Newfoundland in eastern Canada.

Adaptive surface-related multiple elimination

Abstract: The major amount of multiple energy in seismic data is related to the large reflectivity of the surface. A method is proposed for the elimination of all surface-related multiples by means of a process that removes the influence of the surface reflectivity from the data. An important property of the proposed multiple elimination process is that no knowledge of the subsurface is required. On the other hand, the source signature and the surface reflectivity do need to be provided. As a consequence, the proposed process has been implemented adaptively, meaning that multiple elimination is designed as an inversion process where the source and surface reflectivity properties are estimated and where the multiple-free data equals the inversion residue. Results on simulated data and field data show that the proposed multiple elimination process should be considered as one of the key inversion steps in stepwise seismic inversion.

Ultrasonic velocity and anisotropy of hydrocarbon source rocks

Abstract: An experimental study of the physical properties of black, kerogen-rich shales, also including maturation analysis, scanning electron microscope (SEM) observations, and physical modeling, revealed fairly peculiar petrophysical parameters. Specifically, these rocks have very low porosity and density, but most importantly, both P and S ultrasonic velocities normal to bedding are extremely low, whereas they are much higher parallel to bedding, giving rise to a strong anisotropy even at high confining pressures. We found that these parameters primarily reflect kerogen content, microstructure, and maturation level of these rocks. We found also that microcracks inferred from ultrasonic velocity measurements occur only in mature shales. These microcracks are parallel to the bedding plane and further enhance strong intrinsic anisotropy, notably at low effective pressure. Our results show, that on a small scale, kerogen-rich shales are transversely isotropic rocks and can be effectively modeled using the thin-layer composite concept modified to account for the specific distribution of organic matter in the rock fabric.

Wave-equation tomography

Abstract: The relation between ray-trace and diffraction tomography is usually obscured by formulation of the two methods in different domains: the former in space, the latter in wavenumber. Here diffraction tomography is reformulated in the space domain, under the title of wave-equation tomography. With this transformation, wave-equation tomography projects monochromatic, scattered wavefields back over source-receiver wavepaths, just as ray-trace tomography projects traveltime delays back over source-receiver raypaths. Derived under the Born approximation, these wavepaths are wave-theoretic backprojection patterns for reflected energy; derived under the Rytov approximation, they are wave-theoretic back-projection patterns for transmitted energy.Differences between ray-trace and wave-equation tomography are examined through comparison of wavepaths and raypaths, followed by their application to a transmission-geometry, synthetic data set. Rytov wave-equation tomography proves superior to ray-trace tomography in dealing with geometrical frequency dispersion and finite-aperture data, but inferior in robustness. Where ray-trace tomography assumes linear phase delay and inverts the arrival time of one well-understood event, wave-equation tomography accommodates scattering and inverts all of the signal and noise on an infinite trace simultaneously. Interpreted through the uncertainty relation, these differences lead to a redefinition of Rytov wavepaths as monochromatic raypaths, and of raypaths as infinite-bandwidth wavepaths (Rytov wavepaths averaged over an infinite bandwidth).The infinite-bandwidth and infinite-time assumptions of ray-trace and Rytov, wave-equation tomography are reconciled through the introduction of bandlimited raypaths (Rytov wavepaths averaged over a finite bandwidth). A compromise between rays and waves, bandlimited raypaths are broad backprojection patterns that account for the uncertainty inherent in picking traveltimes from bandlimited data.

Compressional velocity and porosity in sand-clay mixtures

Abstract: Laboratory measurements of porosity and compressional velocity were conducted on unconsolidated brine saturated clean Ottawa sand, pure kaolinite, and their mixtures at various confining pressures. A peak in P velocity versus clay content in unconsolidated sand-clay mixtures at 40 percent clay by weight was found. The peak in velocity is 20-30 percent higher than for either pure clay or clean sand. A minimum in porosity versus clay content at 20-40 percent clay by weight is also observed. Such behavior is explained using a micro-geometrical model for mixtures of sand and clay in which two classes of sediments are considered: (1) sands and shaley sands, in which clay is dispersed in the pore space of load bearing sand and thus reduces porosity and increases the elastic moduli of the pore-filling material and (2) shales and sandy shales, in which sand grains are dispersed in a clay matrix. For these sediments, the model reproduces the extrema in velocity and porosity and accounts for much of the scatter in the velocity-porosity relationship.

Reflection tomography in the postmigrated domain

Abstract: Reflection tomography is an inversion method that adjusts a velocity and reflector depth model to be consistent with the prestack time data. This tomography approach minimizes the misfit of the data and model in the premigrated domain. Generally, the data are represented by the traveltimes of reflection events, which has made the technique problematic and unpopular. Techniques generally known as “migration velocity analysis” have a similar objective but use the postmigrated domain. For a variety of practical reasons, this postmigrated domain has advantages over the premigrated domain. With slight modifications, the reflection tomography approach can be implemented in the postmigrated domain. In this domain, a model is determined by optimizing the consistency of imaged reflection events on what has been called a common reflection point (CRP) gather. Extending reflection tomography to the postmigrated domain allows much of the knowledge developed for migration velocity analysis to be coupled with that of re...

Acquisition and processing of wide-aperture ground-penetrating radar data

Abstract: A 40-channel wide‐aperture ground penetrating radar (GPR) data set was recorded in a complicated fluvial/aeolian environment in eastern Canada. The data were collected in the multichannel format usually associated with seismic reflection surveys and were input directly into a standard seismic processing sequence (filtering, static corrections, common‐midpoint gathering, velocity analysis, normal‐ and dip‐moveout corrections, stacking and depth migration). The results show significant improvements, over single‐channel recordings, in noise reduction and depth of penetration (by stacking), and in spatial positioning and reduction of diffraction artifacts (by migration). These characteristics increase the potential for reliable interpretation of structural and stratigraphic details. Thus, without having to develop any new software, GPR data processing technology is brought to the same level of capability, flexibility, and accessibility that is current in seismic exploration.

Finite‐difference solution of the eikonal equation along expanding wavefronts

Abstract: We show that a scheme to solve the 2-D eikonal equation by a finite‐difference method can violate causality for moderate to large velocity contrasts (V2/V1>2). As an alternative, we present a finite‐difference scheme in which the solution region progresses outward from an “expanding wavefront” rather than an “expanding square,” and therefore honors causality. Our method appears to be stable and reasonably accurate for a variety of velocity models with moderate to large velocity contrasts. The penalty is a large increase in computational cost and programming effort.

Suppression of multiple reflections using the Radon transform

Abstract: Multiple suppression using a variant of the Radon transform is discussed. This transform differs from the classical Radon transform in that the integration surfaces are hyperbolic rather than planar. This specific hyperbolic surface is equivalent to parabolae in terms of computational expense but more accurately distinguishes multiples from primary reflections. The forward transform separates seismic arrivals by their differences in traveltime moveout. Multiples can be suppressed by an inverse transform of only part of the data. Examples show that multiples are effectively attenuated in prestack and stacked seismograms.

Fresnel volume ray tracing

Abstract: The concept of “Fresnel volume ray tracing” consists of standard ray tracing, supplemented by a computation of parameters defining the first Fresnel zones at each point of the ray. The Fresnel volume represents a 3-D spatial equivalent of the Fresnel zone that can also be called a physical ray. The shape of the Fresnel volume depends on the position of the source and the receiver, the structure between them, and the type of body wave under consideration. In addition, the shape also depends on frequency: it is narrow for a high frequency and thick for a low frequency. An efficient algorithm for Fresnel volume ray tracing, based on the paraxial ray method, is proposed. The evaluation of the parameters defining the first Fresnel zone merely consists of a simple algebraic manipulation of the elements of the ray propagator matrix. The proposed algorithm may be applied to any high‐frequency seismic body wave propagating in a laterally varying 2-D or 3-D layered structure (P, S, converted, multiply reflected, et...

Influence of temperature on electrical conductivity on shaly sands

Abstract: In boreholes, temperatures vary and to extract hydrocarbon saturation from conductivity measurements, the influence of temperature on water and rock conductivities must be accounted for. The mobility (μDL) of the counter‐ions due to clays and the electrical conductivity of pore‐filling brine show large changes with variation in temperature, whereas the microgeometry of the pore space exhibits negligible change. Using this idea, the temperature dependence of μDL is extracted using data on dc electrical conductivity of shaly sands (σ) containing varying amounts of clay. The mobility of Na+ counter‐ions is found to vary approximately linearly with temperature. This explicit relationship is tested by comparing the predicted temperature dependence against the measured temperature dependence of conductivity of a set of rocks with high and low clay content. While the rock conductivity shows a large temperature dependence, the resistivity index is less sensitive to temperature. An approximate formula, which is su...

Tomostatics: Turning‐ray tomography + static corrections

Abstract: Applications of tomographic velocities to static corrections have drawn much attention recently. In Computing field statics with the help of seismic tomography (Geophysical Prospecting 1987), W.N. de Amorin et al. showed that the waves refracted at the base of a low‐velocity layer can be used successfully for computing and refining field static corrections. In Tomographic inversion for velocity plus statics (SEG Expanded Abstracts 1991), Livia Squires et al. pointed out that velocity variations and static shifts caused by near‐surface effects and positional uncertainties can be solved simultaneously using constraint tomography.

Iterative asymptotic inversion in the acoustic approximation

Abstract: We propose an iterative method for the linearized prestack inversion of seismic profiles based on the asymptotic theory of wave propagation. For this purpose, we designed a very efficient technique for the downward continuation of an acoustic wavefield by ray methods. The different ray quantities required for the computation of the asymptotic inverse operator are estimated at each diffracting point where we want to recover the earth image. In the linearized inversion, we use the background velocity model obtained by velocity analysis. We determine the short wavelength components of the impedance distribution by linearized inversion of the seismograms observed at the surface of the model. Because the inverse operator is not exact, and because the source and station distribution is limited, the first iteration of our asymptotic inversion technique is not exact. We improve the images by an iterative procedure. Since the background velocity does not change between iterations. There is no need to retrace rays,...

Principles of spatial surface electric field filtering in magnetotellurics; electromagnetic array profiling (EMAP)

Abstract: Electromagnetic Array Profiling (EMAP) is an adaptation of magnetotellurics to overcome spatial aliasing effects associated with the sampling of the surface electric field. Undersampling lateral electric field variations can result in misleading geoelectric interpretations of the subsurface, particularly under the common presence of static distortion. In the EMAP field procedure, electric dipoles are positioned end‐to‐end along a continuous survey path; this configuration, in addition to reducing aliasing effects, lends itself to low‐pass filtering of the lateral electric field variations. We show that lengthening an electric dipole can reduce the static effect due to confined resistivity anomalies smaller than a dipole length. This modification of the sensor characteristics involves a spatial filtering process in which the cutoff wavenumber is inversely proportional to the length of the dipole. However, excessively long dipoles may not prove appropriate at high frequencies where the objective is to sense...

Examples of reverse-time migration of single-channel, ground-penetrating radar profiles

Abstract: A single‐channel, ground‐penetrating radar (GPR) profile portrays a distorted, unfocused image of subsurface structure due to apparent position shifts associated with dipping reflectors and to diffractions from corners and edges. A focused image may be produced from such data by using any of the migration algorithms previously developed for seismic data; we use reverse‐time migration based on the scalar wave equation. Field work was performed over a simple stratigraphic soil sequence and a complicated fluvial environment. In the migrated images, reflector continuity is enhanced and the level of detail available for high‐resolution interpretation is significantly increased.

A dynamic programming approach to first arrival traveltime computation in media with arbitrarily distributed velocities

Abstract: Curved‐ray tomographic traveltime inversion, reverse‐time migration and various other seismic modeling applications require the calculation of traveltime and raypath information throughout a two‐ or three‐dimensional medium. When arbitrary velocity distributions and curved rays are involved, traditional ray shooting or bending procedures can be time consuming and error prone. A two‐dimensional dynamic programming traveltime computation technique, based upon Fermat’s principle, uses simple calculus techniques and a systematic mapping scheme to determine first‐arrival times on a uniform grid, given an arbitrary, discrete velocity distribution. It accurately handles large contrast, discontinuous velocity distributions, including those that generate caustics. First arrival seismic energy can travel either as transmitted waves, diffracted waves, or headwaves, and this technique models all types. The traveltime computations begin with starting values computed near the source location. Then, the mapping systemat...

A generalized Maxwell-Wagner theory for membrane polarization in shaly sands

Abstract: The effects of charged clay platelets on the frequency dependent electrical properties of shaly materials are analyzed using simplified models for the membrane polarization around charged spheres immersed in electrolytic solutions, under a thin double layer approximation. The polarization is defined through two possible mechanisms: (1) a surface conductivity related with a modified Stern double layer model (S-model) according to Schurr-Schwarz theory; (2) a coupled electro-diffusional mechanism occurring in a Guoy-Chapman double layer using Fixman's approach (D-model). By comparing the electric potential in such microscopic models with the external potentials derived for the equivalent homogeneous sphere using a Maxwell-Wagner approach, we obtain the total current conductivity functions for these two models. The theory, therefore, provides explicit expressions relating the total conductivity functions to the model parameters.The behavior of the S-model is described by a complex conductivity exhibiting a simple Debye characteristic. In the D-model, both the conductivity and the dielectric permittivity are given as complex properties, showing similar but much wider dispersion than that of a Debye substance. Our representation of the grains and their associated ionic double layers by an equivalent sphere with effective properties allows us to extend our results to simulate rocks containing clays. This is accomplished using generalized mixture equations written in terms of the total conductivity functions. The frequency behavior of both models are compared and their fit to experimental data on clay-water systems and shaly materials suggest that the D-model is more appropriate for representing the dielectric behavior of clay bearing rocks. The theory can be adapted to estimate the clay parameters of a shaly sandstone using electromagnetic borehole measurements.

Electromagnetic modeling of 3-D structures by the method of system iteration using integral equations

Abstract: A new approach for electromagnetic modeling of three‐dimensional (3-D) earth conductivity structures using integral equations is introduced. A conductivity structure is divided into many substructures and the integral equation governing the scattering currents within a substructure is solved by a direct matrix inversion. The influence of all other substructures are treated as external excitations and the solution for the whole structure is then found iteratively. This is mathematically equivalent to partitioning the scattering matrix into many block submatrices and solving the whole system by a block iterative method. This method reduces computer memory requirements since only one submatrix at a time needs to be stored. The diagonal submatrices that require direct inversion are defined by local scatterers only and thus are generally better conditioned than the matrix for the whole structure. The block iterative solution requires much less computation time than direct matrix inversion or conventional point...

The accuracy of estimating Q from seismic data

Abstract: A major aim of seismic interpretation is the inference of petrophysical properties of reservoir rocks. Because the inversion from seismic to petrophysical characteristics is far from unique, this task requires a range of seismic parameters, prominent among which are seismic velocity, impedance, and Poisson’s ratio. The inclusion of seismic absorption in this list could add desirable complementary information. For example, absorption may be more sensitive to clay content than seismic velocity (Klimento and McCann, 1990). However seismic absorption is difficult to measure, particularly over depth intervals as short as most reservoir intervals.

Location of subsurface targets in geophysical data using neural networks

Abstract: Neural networks were used to estimate the offset, depth, and conductivity‐area product of a conductive target given an electromagnetic ellipticity image of the target. Five different neural network paradigms and five different representations of the ellipticity image were compared. The networks were trained with synthetic images of the target and tested on field data and more synthetic data. The extrapolation capabilities of the networks were also tested with synthetic data lying outside the spatial limits of the training set. The data representations consisted of the whole image, the subsampled image, the peak and adjacent troughs, the peak, and components from a two‐dimensional (2-D) fast Fourier transform. The paradigms tested were standard back propagation, directed random search, functional link, extended delta bar delta, and the hybrid combination of self‐organizing map and back propagation. For input patterns with less than 100 elements, the directed random search and functional link networks gave ...

3-D tomographic imaging of near‐surface seismic velocity and attenuation

Abstract: As a result of the similarity between velocity and attenuation imaging, we have implemented both using the same 3-D tomography software, with simple variable changes. The resulting sets of linear equations are solved by the Simultaneous Iterative Reconstruction Technique (SIRT). The algorithm is applied to 3-D estimation of near‐surface velocity and attenuation distributions from 3-D surface‐survey field data from the Ouachita frontal thrust zone in southeastern Oklahoma; the images obtained correlate well with the known surface geology. Resolution analysis by computation of point spread functions indicates highest resolution in the direction parallel to the densest distribution of survey points (the receiver lines).

Imaging and velocity estimation with depth-focusing analysis

Abstract: Prestack depth migration uses two imaging conditions, zero time and zero offset, during downward continuation to form a migrated depth section. When the migration velocities are exact, the two imaging conditions act in a complementary fashion to yield a focused image. When the migration velocities are in error, reflected energy collapses to zero offset at depths that are inconsistent with the zero‐time imaging condition. The result is a deteriorated seismic image. However, by interpreting the nonzero times at which focusing actually occurs, the migration velocities can be updated iteratively in a process called depth‐focusing analysis. To produce a well‐focused seismic image, the goal of depth‐focusing analysis must be the elimination of focusing errors; however, practical considerations can prevent this goal from being achieved. Therefore, to relax the sensitivity of the migrated image to focusing errors, we introduce a nonzero‐time imaging condition by extracting the data along the interpreted surface o...

Detecting organic chemical contaminants by spectral‐induced polarization method in glacial till environment

Abstract: We present results that considerably extend the range of applications of spectral induced polarization (IP) measurements in surveying soil contaminated by organic chemicals. Soil/organic mixtures are polarizable even with very low contents of clay minerals, even though the use of spectral IP has so far been restricted to soils consisting predominantly of clay minerals. Laboratory measurements made on contaminated (toluene, heptane, ethylene glycol) and uncontaminated glacial till samples show that organic chemicals have a distinct effect on the phase spectra. Theoretical modeling further indicates that the phenomena measured in the laboratory are also relevant and detectable in field investigations.

Modeling elastic fields across irregular boundaries

Abstract: Geologists often see the earth as homogeneous blocks separated by smoothly curving boundaries. In contrast, computer modeling algorithms based on finite‐difference schemes require elastic constants to be specified on the vertices of a regular rectangular grid. How can we convert a continuous geological model into a form suitable for a finite‐difference grid? One common way is to lay the finite‐difference grid down on the continuous geological model and use whatever elastic constants happen to lie beneath each of the grid points.

Attenuation mechanisms in sands : laboratory versus theoretical (biot) data

Abstract: The velocity and attenuation of compressional(Q-1P, VP respectively) and shear waves (Q-1S, VS, respectively), determined with the Pulse Transmission technique at a frequency of about 100 kHz, are compared with the grain size, shape, porosity, density, and static frame compressibility of dry and water‐saturated sands. Except for VS, all the quantities VP, Q-1P and Q-1S are dependent on grain size and are higher in coarser grains than in finer grains. Q-1S decreases significantly with increasing differential pressure in coarse‐grained sediments, but the same sediments show an anomalous increase with differential pressure in Q-1P at low pressures. We have also modeled the VP, VS, Q-1P and Q-1S of these samples to understand the mechanisms governing the observed changes. The Contact Radius model with surface force effects predicts both VP and VS to be dependent on grain size. Frictional losses in unconsolidated coarse‐grained sands must also be considered at small strains (10-7). Velocity and losses measured...

Imaging salt with turning seismic waves

Abstract: Turning seismic waves, which first travel downward and then upward before (and after) reflection, have been recorded in a 3-D seismic survey conducted over an overhanging salt dome. Careful processing of these turning waves enables the imaging of the underside of the salt dome and of intrusions of salt into vertical faults radiating from the dome.When seismic wave velocity increases with depth, waves that initially travel downward are reflected and may turn so as to travel upward before reflection. A simple geometrical argument suggests that these turning waves are likely to exhibit abnormal moveout in com-mon-midpoint (CMP) gathers, in that reflection time decreases with increasing source-receiver offset. This abnormal moveout and the attenuation of turning waves by most migration methods suggest that conventional seismic processing does not properly image turning waves.The most important step in imaging turning waves, assuming that they have been recorded, is the migration process. Simple and inexpensive modifications to the conventional phase-shift migration method enable turning waves to be imaged for little additional computational cost. The examples provided in this paper suggest that these and other such modifications to conventional processing should be used routinely when imaging salt domes.

A scattered equivalent-source method for interpolation and gridding of potential-field data in three dimensions

Abstract: Potential‐field geophysical data observed at scattered discrete points in three dimensions can be interpolated (gridded, for example, onto a level surface) by relating the point data to a continuous function of equivalent discrete point sources. The function used here is the inverse‐distance Newtonian potential. The sources, located beneath some of the data points at a depth proportional to distance to the nearest neighboring data point, are determined iteratively. Areas of no data are filled by minimum curvature. For two‐dimensional (2-D) data (all data points at the same elevation), grids calculated by minimum curvature and by equivalent sources are similar, but the equivalent‐source method can be tuned to reduce aliasing. Gravity data in an area of high topographic relief in southwest U.S.A. were gridded by minimum curvature (a 2-D algorithm) and also by equivalent sources (3-D). The minimum‐curvature grid shows strong correlation with topography, as expected, because variation in gravity effect due to...

Plane‐wave reflection and transmission coefficients for a transversely isotropic solid

Abstract: Numerous investigators have studied the P-SV reflection and transmission coefficients of an isotropic solid (Zoeppritz, 1919; Nafe, 1957; Frasier, 1970; Young and Braile, 1976; Kind, 1976; Aki and Richards, 1980).

2-D and 3-D resistivity image reconstruction using crosshole data

Abstract: Theoretical changes in the distribution of electrical potential near subsurface resistivity anomalies have been studied using two resistivity models. The results suggest that the greatest response from such anomalies can be observed with buried electrodes, and that the resistivity model of a volume between boreholes can be accurately reconstructed by using crosshole data. The distributive properties of crosshole electrical potential data obtained by the pole-pole array method have also been examined using the calculated partial derivative of the observed apparent resistivity with respect to a small cell within a given volume. The results show that for optimum two-dimensional (2-D) and three-dimensional (3-D) target imaging, in-line data and crossline data should be combined, and an area outside the zone of exploration should be included in the analysis.In this paper, the 2-D and 3-D resistivity images presented are reconstructed from crosshole data by the combination of two inversion algorithms. The first algorithm uses the alpha center method for forward modeling and reconstructs a resistivity model by a nonlinear least-squares inversion. Alpha centers ex-press a continuously varying resistivity model, and the distribution of the electrical potential from the model can be calculated quickly. An initial general model is determined by the resistivity backprojection technique (RBPT) prior to the first inversion step. The second process uses finite elements and a linear inversion algorithm to improve the resolution of the resistivity model created by the first step.Simple 2-D and 3-D numerical models are discussed to illustrate the inversion method used in processing. Data from several field studies are also presented to demonstrate the capabilities of using crosshole resistivity exploration techniques. The numerical experiments show that by using the combined reconstruction algorithm, thin conductive layers can be imaged with good resolution for 2-D and 3-D cases. The integration of finite-element computations is shown to improve the image obtained by the alpha center inversion process for 3-D applications. The first field test uses horizontal galleries to evaluate complex 2-D features of a zinc mine. The second field test illustrates the use of three boreholes at a dam site to investigate base rock features and define the distribution of an altered zone in three dimensions.