bluuebunny/crossref_metadata_2025 huggingface dataset:
Recognition and reconstruction of coherent energy with application to deep seismic reflection data
Authors: van der Baan Mirko , Paul Anne
Published: October 2002

Reflections in deep seismic reflection data tend to be visible on only a limited number of traces in a common midpoint gather. To prevent stack degeneration, any noncoherent reflection energy has to be removed. In this paper, a standard classification technique in remote sensing is presented to enhance data quality. It consists of a recognition technique to detect and extract coherent energy in both common shot gathers and final stacks. This technique uses the statistics of a picked seismic phase to obtain the likelihood distribution of its presence. Multiplication of this likelihood distribution with the original data results in a “cleaned up” section. Application of the technique to data from a deep seismic reflection experiment enhanced the visibility of all reflectors considerably. Because the recognition technique cannot produce an estimate of “missing” data, it is extended with a reconstruction method. Two methods are proposed: application of semblance weighted local slant stacks after recognition, and direct recognition in the linear τ-p domain. In both cases, the power of the stacking process to increase the signal‐to‐noise ratio is combined with the direct selection of only specific seismic phases. The joint application of recognition and reconstruction resulted in data images which showed reflectors more clearly than application of a single technique.
Mapping of elastic properties of gas hydrates in the Carolina trough by waveform inversion
Authors: Xia Ganyuan , Sen Mrinal K. , Stoffa Paul L.
Published: October 2002

Gas hydrates are frozen methane gas that forms at appropriate pressure and temperature conditions. They are found in the marine sediments along continental margins worldwide. They have the economic potential of being tapped as a fuel source and also have the potential as a “greenhouse” agent after being freed into the atmosphere. In seismic sections, the occurrence of the base of gas hydrates, in some areas is often marked by a bright amplitude reflection. Such reflections follow the sea floor topography and are called bottom‐simulating reflectors (BSR). The BSRs have negative polarity with respect to the sea‐floor reflection and, in a common shot or a CDP gather, the amplitude increases with offset. The negative impedance contrast causing BSRs may be due to negative velocity contrast between hydrated sediments and normal sediment below or due to the presence of free gas at the base of the hydrates. In this paper, we carry out a prestack seismic waveform inversion of multichannel seismic data collected in the offshore of South Carolina to investigate the origin of the BSRs. We apply a multistage seismic waveform inversion for this purpose. A nonlinear optimization method is applied to estimate the low‐frequency component of the velocity, whereas an amplitude‐variation‐with‐offset inversion is applied to determine high‐frequency components of the velocity field. Our detailed seismic waveform inversion along the seismic line results in at least three low‐velocity zones where the velocity is well below the velocity of the normal sediments. Such low‐velocity zones correlate very well with negative fluid factors indicating the presence of free gas. Thus we conclude that the BSR is caused by free gas at the base of the hydrates in this region. We identify at least two other layers of free gas beneath the hydrates. The thickness of each of these layers is below the resolution of the source wavelet. Our results confirm similar findings reported from Ocean Drilling Program drilling and vertical‐seismic‐profiling analysis in the same general area.
P- andS-wave attenuation logs from monopole sonic data
Authors: Sun Xinhua , Tang Xiaoming , Cheng C. H. (Arthur) , Frazer L. Neil
Published: October 2002

In this paper, a modification of an existing method for estimating relative P-wave attenuation is proposed. By generating synthetic waveforms without attenuation, the variation of geometrical spreading related to changes in formation properties with depth can be accounted for. With the modified method, reliable P- and S-wave attenuation logs can be extracted from monopole array acoustic waveform log data. Synthetic tests show that the P- and S-wave attenuation values estimated from synthetic waveforms agree well with their respective model values. In‐situ P- and S-wave attenuation profiles provide valuable information about reservoir rock properties. Field data processing results show that this method gives robust estimates of intrinsic attenuation. The attenuation profiles calculated independently from each waveform of an eight‐receiver array are consistent with one another. In fast formations where S-wave velocity exceeds the borehole fluid velocity, both P-wave attenuation ([Formula: see text]) and S-wave attenuation ([Formula: see text]) profiles can be obtained. P- and S-wave attenuation profiles and their comparisons are presented for three reservoirs. Their correlations with formation lithology, permeability, and fractures are also presented.
Global nonlinear inversion of transient EM data from conducting surroundings using a free‐space plate model
Authors: Sharma Shashi P. , Kaikkonen Pertti
Published: October 2002

A platelike conducting body in free space is used as a model to invert transient electromagnetic data using the very fast simulated annealing procedure as a global optimization tool. When the host rock conductivity is non‐zero, acceptable fits between the observed and computed responses are difficult to obtain. In general, the conducting body is assigned a lower conductance, larger dimensions (strike length and depth extent) and a smaller depth than the true values. We approximate the response of a conducting host to yield reliable estimates of model parameters as well as a good fit between the observed and computed responses. Our procedure is based on the assumption that the observed electromagnetic response is the sum of the response due to the conductive target and the response due to conducting surroundings (host and overburden). It is also assumed that the host response is laterally invariant, implying a layered earth and fixed source‐receiver geometry. The validity of the superposition assumption is tested against the full solution for a conductive plate in a finite conducting host. The efficacy of our approach is demonstrated using noise‐free and noisy synthetic data and two field examples measured in different geological conditions.
An efficient data‐subspace inversion method for 2-D magnetotelluric data
Authors: Siripunvaraporn Weerachai , Egbert Gary
Published: October 2002

There are currently three types of algorithms in use for regularized 2-D inversion of magnetotelluric (MT) data. All seek to minimize some functional which penalizes data misfit and model structure. With the most straight‐forward approach (exemplified by OCCAM), the minimization is accomplished using some variant on a linearized Gauss‐Newton approach. A second approach is to use a descent method [e.g., nonlinear conjugate gradients (NLCG)] to avoid the expense of constructing large matrices (e.g., the sensitivity matrix). Finally, approximate methods [e.g., rapid relaxation inversion (RRI)] have been developed which use cheaply computed approximations to the sensitivity matrix to search for a minimum of the penalty functional. Approximate approaches can be very fast, but in practice often fail to converge without significant expert user intervention. On the other hand, the more straightforward methods can be prohibitively expensive to use for even moderate‐size data sets. Here, we present a new and much more efficient variant on the OCCAM scheme. By expressing the solution as a linear combination of rows of the sensitivity matrix smoothed by the model covariance (the “representers”), we transform the linearized inverse problem from the M-dimensional model space to the N-dimensional data space. This method is referred to as DASOCC, the data space OCCAM’s inversion. Since generally N ≪ M, this transformation by itself can result in significant computational saving. More importantly the data space formulation suggests a simple approximate method for constructing the inverse solution. Since MT data are smooth and “redundant,” a subset of the representers is typically sufficient to form the model without significant loss of detail. Computations required for constructing sensitivities and the size of matrices to be inverted can be significantly reduced by this approximation. We refer to this inversion as REBOCC, the reduced basis OCCAM’s inversion. Numerical experiments on synthetic and real data sets with REBOCC, DASOCC, NLCG, RRI, and OCCAM show that REBOCC is faster than both DASOCC and NLCG, which are comparable in speed. All of these methods are significantly faster than OCCAM, but are not competitive with RRI. However, even with a simple synthetic data set, we could not always get RRI to converge to a reasonable solution. The basic idea behind REBOCC should be more broadly applicable, in particular to 3-D MT inversion.
3-D inversions of magnetic gradiometer data in archeological prospecting: Possibilities and limitations
Authors: Herwanger Jörg , Maurer Hansruedi , Green Alan G. , Leckebusch Jürg
Published: October 2002

A vertical‐gradient magnetic system based on optically pumped Cesium sensors has been used to map subtle magnetic anomalies across infilled pit houses and ditches at a medieval archeological site in northern Switzerland. For estimating the locations and dimensions of these features from the recorded data, we have designed and implemented an appropriate inversion scheme. Tests of this scheme on realistic synthetic data sets suggested that suitable minimum magnetic susceptibility contrasts and smoothing parameters for the inversion may be directly extracted from the data. Inversions with minimum magnetic susceptibility contrasts generated causative bodies with maximum plausible sizes. By using higher magnetic susceptibility contrasts, a complete suite of models that matched the data equally well was produced. To constrain better the magnetic susceptibility constrast within a selected area of the archeological site, shallow samples of topsoil and sediment were analyzed in the laboratory. An inversion based on the measured magnetic susceptibility contrast yielded reliable estimates of the locations, 3-D geometries, and sizes of two small pit houses. The depth extent of one pit house was subsequently verified by shallow drilling. We concluded that inversions of vertical‐gradient magnetic data constrained by magnetic susceptibility or shallow borehole information are rapid and inexpensive means of providing key knowledge on the depth distribution of inductively magnetized bodies.
Capillarity‐induced resonance of oil blobs in capillary tubes and porous media
Authors: Hilpert Markus , Jirka Gerhard H. , Plate Erich J.
Published: October 2002

We investigate the excitation by sound waves of capillary trapped oil blobs. The three‐phase contact lines either remain pinned to the heterogeneities of the solid surface or slide if sound waves are applied. We derive approximate, analytical expressions for the resonance of oil blobs in capillary tubes for both types of contact line behavior. Based upon these simple model systems, we conclude that resonance of oil blobs is significant for coarse‐grained but not fine‐grained media.
Improved frequency‐domain elastic wave modeling using weighted‐averaging difference operators
Authors: Min Dong‐Joo , Shin Changsoo , Kwon Byung‐Doo , Chung Seunghwan
Published: October 2002

We develop a new finite‐difference scheme that reduces the number of grid points per wavelength required in frequency‐domain elastic modeling. Our approach computes weighted averages of the spatial second‐order derivative and the mass acceleration terms using a 25-point computational stencil. By determining the weighting coefficients to minimize numerical dispersion and numerical anisotropy, we reduce the number of grid points to 3.3 per shear wavelength, with a resulting error in velocities smaller than 1%. Our choice of grid points reduces the computer memory needed to store the complex impedance matrix to 4% of that for a conventional second‐order scheme and to 54% of that for a combined second‐order scheme. The 25-point weighted averaging scheme of this paper makes it possible to accurately simulate realistic models. Numerical examples show that this technique can achieve the same accurate solutions with fewer grid points than those from previous frequency‐domain second‐order schemes. Our technique can be extended directly to 3-D elastic modeling; the computational efficiency will be even greater than that realized for 2-D models.
Efficient 2.5-D true‐amplitude migration
Authors: Dellinger Joe A. , Gray Samuel H. , Murphy Gary E. , Etgen John T.
Published: October 2002

Kirchhoff depth migration is a widely used algorithm for imaging seismic data in both two and three dimensions. To perform the summation at the heart of the algorithm, standard Kirchhoff migration requires a traveltime map for each source and receiver. True‐amplitude Kirchhoff migration in 2.5-D υ(x, z) media additionally requires maps of amplitudes, out‐of‐plane spreading factors, and takeoff angles; these quantities are necessary for calculating the true‐amplitude weight term in the summation. The increased input/output (I/O) and computational expense of including the true‐amplitude weight term is often not justified by significant improvement in the final muted and stacked image. For this reason, some authors advocate neglecting the weight term in the Kirchhoff summation entirely for most everyday imaging purposes. We demonstrate that for nearly the same expense as ignoring the weight term, a much better solution is possible. We first approximate the true‐amplitude weight term by the weight term for constant‐velocity media; this eliminates the need for additional source and receiver maps. With one small additional approximation, the weight term can then be moved entirely outside the innermost loop of the summation. The resulting Kirchhoff method produces images that are almost as good as for exact true‐amplitude Kirchhoff migration and at almost the same cost as standard methods that do not attempt to preserve amplitudes.
Using borehole electroseismic measurements to detect and characterize fractured (permeable) zones
Authors: Mikhailov Oleg V. , Queen John , Toksöz M. Nafi
Published: October 2002

In 1996, we measured Stoneley‐wave‐induced electrical fields in an uncased water well drilled in fractured granite and diorite near Hamilton, Massachusetts. Stoneley waves generated by sledgehammer blows to the surface casing produced a flow of pore fluid in permeable zones intersected by the borehole. In turn, this flow induced a streaming electrical field. Even though these electrical signals were very small (tens of microvolts), we were able to detect them using electrodes placed in the borehole, after power line and telluric signals were canceled by remote referencing and notch‐filtering. Amplitude analysis of the electrical fields confirmed that they were induced by fluid flow in the fractured formation. The normalized amplitudes of these electrical fields correlate with the fracture density log and agree with the theoretical model for this electroseismic phenomenon. Our Biot‐theory‐based model predicts that borehole electroseismic measurements can be used to characterize permeable zones. According to this model, the normalized amplitude of the Stoneley‐wave‐induced electrical field is proportional to the porosity, and the amplitude‐versus‐frequency behavior of this electrical field depends on the permeability of a formation around a borehole.
2-D inversion of gravity data using sources laterally bounded by continuous surfaces and depth‐dependent density
Authors: García‐Abdeslem Juan
Published: October 2002

A description is given of numerical methods for 2-D gravity modeling and nonlinear inversion. The forward model solution is suitable for calculating the gravity anomaly caused by a 2-D source body with depth‐dependent density that is laterally bounded by continuous surfaces and can easily accommodate different kinds of geologic structures. The weighted and damped discrete nonlinear inverse method addressed here can invert both density and geometry of the source body. Both modeling and inversion methods are illustrated with several examples using synthetic and two field gravity data sets—one over a sulfide ore body and other across a sedimentary basin. A sensitivity analysis is carried out for the resulting solutions by means of the resolution, covariance, and correlation matrices, providing insight into the capabilities and limitations of the inversion method. The inversion of synthetic data provides meaningful results, showing that the method is robust in the presence of noise. Its sensitivity analysis indicates an almost perfect resolution and small covariance, but high correlation between some parameters. Differences in the asperity aspect of the inverted‐field data sets turned out to be important for the inversion capabilities of the algorithm, making a significant difference in the resolution achieved, its covariance, and the degree of correlation among parameters.
Explicit analytic expression for normal moveout from horizontal and dipping reflectors in weakly anisotropic media of arbitrary symmetry type
Authors: Rasolofosaon P. N. J.
Published: October 2002

When processing and inverting seismic reflection data, the NMO velocity must be correctly described, taking into account realistic situations such as the presence of anisotropy and dipping reflectors. Some dip‐moveout (DMO) algorithms have been developed, such as Tsvankin’s analytic formula. It describes the anisotropy‐induced distortions in the classical isotropic cosine of dip dependence of the NMO velocity. However, it is restricted to the vertical symmetry planes of anisotropic media, so the technique is unsuitable for the azimuthal inspection of sedimentary rocks, either with horizontal bedding and vertical fractures or with dipping bedding but no fractures. However, under the weak anisotropy approximation the deviations of the rays from a vertical plane can be neglected for the traveltimes computation, and the equation can still be applicable. Based on this approach, an explicit analytic expression for the P-wave NMO velocity in the presence of horizontal or dipping reflectors in media exhibiting the most general symmetry type (triclinic) is obtained in this work. If the medium exhibits a horizontal symmetry plane, the concise DMO equations are formally identical to Tsvankin’s except that the parameters δ and ε are not constant but depend on the azimuth ψ Physically, δ(ψ) is the deviation from the vertical P-wave velocity of the P-wave NMO velocity for a horizontal reflector normalized by the vertical P-wave velocity for the azimuth ψ. The function ε(ψ) has the same definition as δ(ψ) except that the P-wave NMO velocity is replaced by the horizontal P-wave velocity. Both depend linearly on (1) new dimensionless anisotropy parameters and (2) generalizing to arbitrary symmetry the transversely isotropic parameters δ and ε. In the most general symmetry case (triclinic), an additional term to the DMO formula is necessary. The numerical examples, based on experimental data in rocks, show two things. First, the magnitude of the DMO errors induced by anisotropy depends primarily on the absolute value of ε(ψ) − δ(ψ) and not on the individual values of ε(ψ) and δ(ψ), which is a direct consequence of the similarity between Tsvankin’s equation and the equation presented here. Second, the anisotropy‐induced DMO correction can be significant even in the presence of moderate anisotropy and can exhibit complex azimuthal dependence.
Analytic study of the geometrical spreading of P-waves in a layered transversely isotropic medium with a vertical symmetry axis
Authors: Zhou Hongbo , McMechan George A.
Published: October 2002

An analytical formula for geometrical spreading is derived for a horizontally layered transversely isotropic medium with a vertical symmetry axis (VTI). With this expression, geometrical spreading can be determined using only the anisotropy parameters in the first layer, the traveltime derivatives, and the source‐receiver offset. Explicit, numerically feasible expressions for geometrical spreading are obtained for special cases of transverse isotropy (weak anisotropy and elliptic anisotropy). Geometrical spreading can be calculated for transversly isotropic (TI) media by using picked traveltimes of primary nonhyperbolic P-wave reflections without having to know the actual parameters in the deeper subsurface; no ray tracing is needed. Synthetic examples verify the algorithm and show that it is numerically feasible for calculation of geometrical spreading. For media with a few (4–5) layers, relative errors in the computed geometrical spreading remain less than 0.5% for offset/depth ratios less than 1.0. Errors that change with offset are attributed to inaccuracy in the expression used for nonhyberbolic moveout. Geometrical spreading is most sensitive to errors in NMO velocity, followed by errors in zero‐offset reflection time, followed by errors in anisotropy of the surface layer. New relations between group and phase velocities and between group and phase angles are shown in appendices.
3-D ground‐penetrating radar simulation and plane‐wave theory in anisotropic media
Authors: Carcione José M. , Schoenberg Michael A.
Published: October 2002

Modeling ground‐penetrating radar (GPR) waves requires simulation of the 3-D full wavefield and the correct description of the electromagnetic (EM) properties. Magnetic and dielectric relaxations are described by relaxation functions associated with each principal component of the respective tensorial property. Anisotropy is modeled up to orthorhombic symmetry, i.e., the principal coordinate systems of the three EM material tensors coincide, and each property is described by three different principal components. The algorithm uses the pseudospectral method for computing the spatial derivatives and a second‐order finite difference in time. A complete plane‐wave analysis, including energy balance, gives the expressions of measurable quantities such as the EM-wave velocity and the quality factor as a function of frequency and propagation direction. The algorithm reproduces the wavefront shape and attenuation predicted by the plane‐wave analysis. In addition, the results are in excellent agreement with an analytical 3-D transient solution. The modeling is applied to evaluation of the EM response of two pipes buried in an anisotropic sand overlying a clay layer. The results demonstrate that anisotropy and loss mechanisms cause significant amplitude and phase distortions.
The measurement of weak anisotropy with the generalized reciprocal method
Authors: Palmer Derecke
Published: October 2002

Anisotropy parameters can be determined from seismic refraction data using the generalized reciprocal method (GRM) for a layer in which the velocity can be described with the Crampin approximation for transverse isotropy. The parameters are the standard anisotropy factor, which is the horizontal velocity divided by the vertical velocity, and a second poorly determined parameter which, for weak anisotropy, is approximated by a linear relationship with the anisotropy factor. Although only one anisotropy parameter is effectively determined, the second parameter is essential to ensure that the anisotropy does not degenerate to the elliptical condition which is indeterminate using the approach described in this paper. The anisotropy factor is taken as the value for which the phase velocity at the critical angle given by the Crampin equation is equal to the average velocity computed with the optimum XY value obtained from a GRM analysis of the refraction data. The anisotropy parameters can be used to improve the estimate of the refractor velocity, which can exhibit marked dip effects when the overlying layer is anisotropic. In a model study, depths computed with the phase velocity at the critical angle are within 3% of the true values, whereas those calculated with the horizontal phase velocity (which assumes isotropy) are greater than the true depths by about 25%. Anisotropy illustrates the pitfalls of model‐based inversion strategies, which seek agreement between the travetime data and the computed response of the model. With anisotropic layers, the traveltime data provide the seismic velocity in the overlying layer in the horizontal direction, whereas the seismic velocity near the critical angle is required for depth computations. If anisotropy is applicable, then the GRM using the methods described in this paper is able to provide a good starting model for other approaches, such as refraction tomography.
Directional filtering for linear feature enhancement in geophysical maps
Authors: Sykes Michael P. , Das Umesh C.
Published: October 2002

Geophysical maps of data acquired in ground and airborne surveys are extensively used for mineral, groundwater, and petroleum exploration. Lineaments in these maps are often indicative of contacts, basement faulting, and other tectonic features of interest. To aid the interpretation of these maps, a versatile processing technique of directional filtering, based on the 2-D “normal” Radon transform, is used to enhance or suppress specific lineaments. Synthetic data and field examples using electromagnetic and radiometric data are used to demonstrate the superiority of the Radon transform method over conventional Fourier transform filtering. The Radon transform technique is shown to be more versatile and less susceptible to processing artefacts than the Fourier transform methods.
Gas generation and overpressure: Effects on seismic attributes
Authors: Carcione José M. , Gangi Anthony F.
Published: October 2002

Drilling of deep gas resources is hampered by high risk associated with unexpected overpressure zones. Knowledge of pore pressure using seismic data, as for instance from seismic‐while‐drilling techniques, will help producers plan the drilling process in real time to control potentially dangerous abnormal pressures. We assume a simple basin‐evolution model with a constant sedimentation rate and a constant geothermal gradient. Oil/gas conversion starts at a given depth in a reservoir volume sealed with faults whose permeability is sufficiently low so that the increase in pressure caused by gas generation greatly exceeds the dissipation of pressure by flow. Assuming a first‐order kinetic reaction, with a reaction rate satisfying the Arrhenius equation, the oil/gas conversion fraction is calculated. Balancing mass and volume fractions in the pore space yields the excess pore pressure and the fluid saturations. This excess pore pressure determines the effective pressure, which in turn determines the skeleton bulk moduli. If the generated gas goes into solution in the oil, this effect does not greatly change the depth and oil/gas conversion fraction for which the hydrostatic pressure approaches the lithostatic pressure. The seismic velocities versus pore pressure and differential pressure are computed by using a model for wave propagation in a porous medium saturated with oil and gas. Moreover, the velocities and attenuation factors versus frequency are obtained by including rock‐frame/fluid viscoelastic effects to match ultrasonic experimental velocities. For the basin‐evolution model used here, pore pressure is seismically visible when the effective pressure is less than about 15 MPa and the oil/gas conversion is about 2.5% percent.
Physical properties and seismic imaging of massive sulfides
Authors: Salisbury Matthew H. , Milkereit Bernd , Ascough Graham , Adair Robin , Matthews Larry , Schmitt Douglas R. , Mwenifumbo Jonathan , Eaton David W. , Wu Jianjun
Published: October 2002

Laboratory studies show that the acoustic impedances of massive sulfides can be predicted from the physical properties ([Formula: see text], density) and modal abundances of common sulfide minerals using simple mixing relations. Most sulfides have significantly higher impedances than silicate rocks, implying that seismic reflection techniques can be used directly for base metals exploration, provided the deposits meet the geometric constraints required for detection. To test this concept, a series of 1-, 2-, and 3-D seismic experiments were conducted to image known ore bodies in central and eastern Canada. In one recent test, conducted at the Halfmile Lake copper‐nickel deposit in the Bathurst camp, laboratory measurements on representative samples of ore and country rock demonstrated that the ores should make strong reflectors at the site, while velocity and density logging confirmed that these reflectors should persist at formation scales. These predictions have been confirmed by the detection of strong reflections from the deposit using vertical seismic profiling and 2-D multichannel seismic imaging techniques.
Multilayer MIM inversion of AEM data: Theory and field example
Authors: Bergeron Clyde J. , Ioup Juliette W. , Wu Yan , Ioup George E. , Holladay Kenneth W.
Published: October 2002

This paper presents a multilayer generalization of an algebraic method of inverting frequency‐doma in airborne active electromagnetic (AEM) data in terms of 1-D layered earth models. The processing of the AEM data, which includes a recalibration procedure, is also outlined. The inversion is applied to synthetic fields generated from a multilayer model which is intended to approximate a measured conductivity profile of the water column in the Gulf of Mexico and to measured AEM data from a survey of the Barataria Bay estuary region of the Louisiana Gulf of Mexico coast. The inversion results from the synthetic data are in good agreement with the forward model. The conductivities calculated from the inversions of measured AEM data are compared to ground‐and water‐based measurements. The depth variations of the calculated electrical conductivities in the nearshore Gulf waters are in good agreement with measurements of conductivity versus depth by conductivity‐temperature‐depth (CTD) casts at several points on the over‐the‐water portion of two flight lines.
Geophysics in the new millennium
Authors: Lines Larry , Castagna John P. , Treitel Sven
Published: October 2002

Upon entering the twenty‐first century, we see wide‐ranging changes in geophysics. As of this writing, quality and utility of geophysical data continues a trend of inexorable improvement punctuated by individual quantum steps (such as the 3-D seismic revolution). To a large extent, this improvement has been accomplished on the coattails of advances in computing and related disciplines. These advances have allowed cost‐effective implementation of methods that exploit our steadily increasing understanding of geophysical theory in ever increasingly realistic earth models. As a result, geophysical methods can now provide clearer images at greater distances with bette resolution and signal‐to‐noise ratio than ever before.
Random scattering in magnetotellurics
Authors: White Benjamin S. , Kohler Werner E , Srnka Leonard J
Published: October 2002

Typical well logs show substantial variations of formation electrical resistivity over small spatial scales, down to the resolution of the logging tool. Using a plane stratified earth model, we examine the effects of this fine‐scale microstructure on scattering of the naturally occurring electromagnetic (EM) waves used in magnetotellurics. We show how 1-D magnetotelluric (MT) data may be viewed as arising statistically from a smoothed effective medium version of the resistivity‐depth profile. The difference between the data produced by the true medium and the effective medium is attributable to random scattering noise. This noise is fundamental to magnetotellurics and other diffusive‐wave EM exploration methods since it arises from the very small spatial scales that are usually ignored. The noise has unique statistical properties, which we characterize. We show that if scattering is the dominant noise source, a thin layer of increased resistivity at depth can be reliably detected only if the noise statistics are incorporated properly into the detection algorithm. This sets a new fundamental limit on the vertical detection capability of MT data. The theory agrees well with Monte Carlo simulations of MT responses from random resistivity microlayers.
Seismic signal processing—A new millennium perspective
Authors: Cary Peter
Published: October 2002

In the introduction to his comprehensive SEG textbook, Seismic Data Processing, Oz Yilmaz selects deconvolution, common‐midpoint stacking and migration as being the three principal processes that are applied during routine seismic processing. Since Yilmaz’s tome was first published in 1987, a vast number of papers have been published and conference presentations have been given on virtually every aspect of seismic processing. However, I think it is still accurate to say that the same three processes dominate the processing flow of the vast majority of seismic data that is processed now, at the beginning of the twenty‐first century. This is not to say that important progress has not been made in many aspects of seismic processing and that much more sophisticated processing flows are now applied to some datasets. But it is a great tribute to the real pioneers of our profession—the people who advanced our ideas of seismic processing from examining raw analog records in the field to creating crisp computer‐generated images of the subsurface with processes such as deconvolution, stack and migration—that the very same, or similar, algorithms that they invented still form the backbone of everyday processing that is done around the world today. In fact, there are times when it seems that the last great geophysicist was Carl Friedrich Gauss, because the method that he published back in 1823 of minimizing the sum of the squared errors seems to be used almost everywhere one looks in seismic processing, from deconvolution to migration.
3-D traveltimes and amplitudes by gridded rays
Authors: Bernasconi Giancarlo , Drufuca Giuseppe
Published: October 2002

Seismic imaging in three dimensions requires the calculation of traveltimes and amplitudes of a wave propagating through an elastic medium. They can be computed efficiently and accurately by integrating the eikonal equation on an elemental grid using finite‐difference methods. Unfortunately, this approach to solving the eikonal equation is potentially unstable unless the grid sampling steps satisfy stability conditions or wavefront tracking algorithms are used. We propose a new method for computing traveltimes and amplitudes in 3-D media that is simple, fast, unconditionally stable, and robust. Defining the slowness vector as [Formula: see text] and assuming an isotropic medium, the ray velocity v is related to the slowness vector by the relation [Formula: see text]. Rays emerging from gridpoints on a horizontal plane are propagated downward a single vertical grid step to a new horizontal plane. The components of the slowness vector are then interpolated to gridpoints on this next horizontal plane. This is termed regridding; the process of downward propagation of rays, one vertical grid step at a time, is continued until some prescribed depth is reached. Computation of amplitudes is achieved using a method similar to that for obtaining the zero‐order approximation in asymptotic ray theory. We show comparisons with a full‐wave method on readily accessible 3-D velocity models.
Gravity and magnetic methods at the turn of the millennium
Authors: Hansen R. O.
Published: October 2002

Innovation in the 1980s and 1990s has been dominated by new instrumentation. Only recently has processing and interpretation technology appeared which will extract the maximum value from the new instruments. In the following sections, the instrumentation advances are outlined. Processing and interpretation advances are sketched in the last section.
The use of geophysical prospecting for imaging active faults in the Roer Graben, Belgium
Authors: Demanet Donat , Renardy François , Vanneste Kris , Jongmans Denis , Camelbeeck Thierry , Meghraoui Mustapha
Published: October 2002

As part of a paleoseismological investigation along the Bree fault scarp (western border of the Roer Graben), various geophysical methods [electrical profiling, electromagnetic (EM) profiling, refraction seismic tests, electrical tomography, ground‐penetrating radar (GPR), and high‐resolution reflection seismic profiles] were used to locate and image an active fault zone in a depth range between a few decimeters to a few tens of meters. These geophysical investigations, in parallel with geomorphological and geological analyses, helped in the decision to locate trench excavations exposing the fault surfaces. The results could then be checked with the observations in four trenches excavated across the scarp. Geophysical methods pointed out anomalies at all sites of the fault position. The contrast of physical properties (electrical resistivity and permittivity, seismic velocity) observed between the two fault blocks is a result of a differences in the lithology of the juxtaposed soil layers and of a change in the water table depth across the fault. Extremely fast techniques like electrical and EM profiling or seismic refraction profiles localized the fault position within an accuracy of a few meters. In a second step, more detailed methods (electrical tomography and GPR) more precisely imaged the fault zone and revealed some structures that were observed in the trenches. Finally, one high‐resolution reflection seismic profile imaged the displacement of the fault at depths as large as 120 m and filled the gap between classical seismic reflection profiles and the shallow geophysical techniques. Like all geophysical surveys, the quality of the data is strongly dependent on the geologic environment and on the contrast of the physical properties between the juxtaposed formations. The combined use of various geophysical techniques is thus recommended for fault mapping, particularly for a preliminary investigation when the geological context is poorly defined.
Design and application of a towed land‐streamer system for cost‐effective 2-D and pseudo-3-D shallow seismic data acquisition
Authors: van der Veen M. , Spitzer R. , Green A. G. , Wild P.
Published: October 2002

To reduce the field effort required for 2-D and 3-D shallow seismic surveying, we have developed a towed land‐streamer system. It was constructed with self‐orienting gimbal‐mounted geophones housed in heavy (1 kg) cylindrical casings, sturdy seismic cables with reinforced kevlar sheathing, robust waterproof connectors, and a reinforced rubber sheet that helped prevent cable snagging, maintained geophone alignment, and provided additional hold‐down weight for the geophones. Each cable had takeouts for 12 geophones at 1 m or 2 m intervals. By eliminating the need for manual geophone planting and cable laying, acquisition of 2-D profiles with this system proved to be 50–100% faster with 30–40% fewer personnel than conventional procedures. Costs of the land‐streamer system and total weight to be pulled could be minimized by employing nonuniform receiver configurations. Short receiver intervals (e.g., 1 m) at near offsets were necessary for identifying and mapping shallow (<50 m) reflections, whereas larger receiver intervals (e.g., 2 m) at far offsets were sufficient for imaging deeper (>50 m) reflections and estimating velocity‐depth functions. Our land‐streamer system has been tested successfully on a variety of recording surfaces (e.g., meadow, asphalt road, and compact gravel track). The heavy weight of the geophone casings and rubber sheet ensured good geophone‐to‐ground coupling. On the asphalt surface, a greater proportion of high‐frequency (above 300–350 Hz) energy was recorded by the land streamer than by standard baseplate‐mounted geophones. The land‐streamer system is a practical and efficient means for surveying in urbanized areas. Acquisition and processing of 3-D shallow seismic data with the land‐streamer system was simulated by appropriately decimating and reprocessing an existing 3-D shallow seismic data set. Average subsurface coverage of the original data was ∼50 fold, whereas that of the simulated data was ∼5 fold. The effort required to collect the simulated pseudo-3-D data set would have been approximately 7% of that needed for the original field campaign. Application of important data‐dependent processing procedures (e.g., refraction static corrections and velocity analyses) to the simulated data set produced surprisingly good results. Because receiver spacing along simulated cross‐lines (6 m) was double that along in‐lines (3 m), a pattern of high and low amplitudes was observed on cross‐sections and time slices at early traveltimes (⩽50 ms). At greater traveltimes, all major reflections could be identified and mapped on the land‐streamer data set. With this cost‐effective approach to pseudo-3-D seismic data acquisition, it is expected that shallow 3-D seismic reflection surveying will become attractive for a broader range of engineering and environmental applications.
New method for fast computation of gravity and magnetic anomalies from arbitrary polyhedra
Authors: Singh Bijendra , Guptasarma D.
Published: October 2002

We show that at any point the gravity field from a solid body bounded by plane surfaces and having uniform density can be computed as a field from a fictitious distribution of surface mass‐density on the same body. The surface mass density at every surface element is equal to the product of the volume density of the body and the scalar product of (1) the unit outward vector normal to that surface element and (2) the position vector of the surface element with respect to the point of observation. Accordingly, the contribution to the gravity field from any plane surface of the body vanishes if the observation point lies in the plane of that surface. As a result, we can compute the gravity field everywhere, including points inside, on the surface, on an edge, or at a corner of the body where more than two surfaces meet. This new result lets us compute the gravity field using exactly the same simple procedure as for the magnetic field of a uniformly magnetized object, computed from an equivalent surface distribution of magnetic pole density. To get the gravity field while computing the magnetic field, one simply uses the product of this surface mass density and the universal gravitational constant instead of the surface magnetic pole density. Therefore, the same computer program can be used to compute the gravity, the magnetic field, or both simultaneously. This simple and novel approach makes the numerical computations much faster than all other previously published schemes.
The torsion balance as a tool for geophysical prospecting
Authors: Speake Clive C. , Hammond G. D. , Trenkel C.
Published: October 2002

We discuss whether the torsion balance can again become a key tool for geophysical prospecting. We outline the acknowledged disadvantages of the Eötvös torsion balance and seek designs of a torsion balance beam that would enable the torsion balance to be used on moving platforms. A key result is that torsion balance beams designed to be insensitive to the curvature and horizontal gradients of the gravity field are insensitive to the angular motion of the platform about horizontal axes. We suggest that a double torsion balance using these balance beam designs could be used on moving platforms. We point out that second gradients of the gravitational field (third derivatives of the potential) can be determined with reasonable sensitivity with current technology. We describe double torsion balance schemes where the mass, range, and azimuth of localized mass anomalies could be estimated or where local anomalies could be rejected using information from the second gravity gradient.
On: “Resolution limits of migrated images” (J. Chen and G. T. Schuster, GEOPHYSICS, 64, 1046–1053).
Authors: Tabti Hocine , Gelius Leiv‐J. , Lecomte Isabelle
Published: October 2002

The paper by Chen and Schuster is an attempt to provide simplified analytical formulas that can be used for a quick estimate of the horizontal resolution that might be obtained by the “diffraction stack” migration process. To this end the authors used the impulse response of a point diffractor buried in a homogeneous half‐space with sources and receivers at the surface. The paper also provides to some extent an analysis and a discussion of the resolution “limits” achieved by 2-D and 3-D poststack and prestack migration. Some questions arise from the results and conclusions given in the paper which, from our point of view, deserve to be clarified.
On: “Preferential continuation for potential‐field anomaly enhancement” (R. Pawlowski, GEOPHYSICS, 60, 390–398).
Authors: Zeng Hualin , Xu Deshu
Published: October 2002

Pawlowski (1995) presents an excellent method for preferential continuation for potential‐field anomaly enhancement, and it is an appreciated attempt to solve a very common problem in gravity separation. Upward continuation of a gravity anomaly is a very common method for regional‐residual separation in China. One of the main problems in the conventional upward continuation is that it overattenuates regional‐field or useful long‐wavelength information due to deep sources. Sometimes attenuated‐upward continuation of an observed anomaly to a height has to be regarded as the original regional field at the observation plane in order to use gravity inversion to map deep interfaces such as the Moho. Application of the preferential continuation to gravity anomaly in some areas in China has shown very good effectiveness in solving the above problem (Xu and Zeng, 2000).
Wave propagation in rocks with elastic‐plastic deformations
Authors: Sinha Bikash K. , Plona Thomas J.
Published: October 2002

Wave propagation in rocks subject to elastic‐plastic deformations can be described by the equations of motion for small dynamic fields superposed on a static bias. The static bias refers to the statically deformed state of the material with large deformations. Referred to this statically deformed state, the Piola‐Kirchhoff stress equations of motion describe wave propagation in terms of second- and third‐order elastic constants, static stresses, and finite deformation gradients. Plane wave propagation along the principal stress directions can be described in terms of principal stretches, total static strains, and plastic strains. The elastic strains are differences between the total and plastic strains. Calculations are performed for the plane wave speeds in the absence and presence of plastic strains. Theoretical predictions agree very well with the laboratory measurements made on dry Castlegate sandstone samples subject to multiple uniaxial load cycles up to 70% of the unconfined compressive strength of 16 MPa. The difference between the plane wave speeds in these two cases can be calibrated as function of a certain measure of plastic strain. This analysis also provides a framework for the estimation of plastic strain as a measure of mechanical damage in the material from the acoustic wave velocity measurements. Estimation of near‐wellbore damage influences perforation strategy that would avoid sanding during hydrocarbon production.
Gravity anomalies of 2-D bodies with variable density contrast
Authors: Zhang Jianzhong , Zhong Benshan , Zhou Xixiang , Dai Yun
Published: October 2002

A new method is presented to compute gravity anomalies that result from 2-D bodies with variable density contrast. The cross‐section of a body is approximated by a polygon. Density is assumed to vary as any order of polynomial function with depth and lateral position. Results calculated by the proposed method for models with variable density contrast compare well with other methods. Liaohe basin, northeast China, is modeled from field gravity anomalies using the formulas given, showing the method is valid and effective.