bluuebunny/crossref_metadata_2025 huggingface dataset:
Shear‐wave logging to enhance seismic modeling
Authors: Payne M. A.
Published: October 2002

In an effort to understand better the amplitude variation with offset for reflections from an oil sand and the sensitivity of the AVO response to shear‐wave velocity variations, I studied synthetic and field gathers collected from an onshore field in the Gulf of Mexico basin. A wave‐equation‐based modeling program generated the synthetic seismic gathers using both measured and estimated shear‐wave velocities. The measured shear‐wave velocities came from a quadrupole sonic tool. The estimated shear‐wave velocities were obtained by applying published empirical and theoretical equations which relate shear‐wave velocities to measured compressional‐wave velocities. I carefully processed the recorded seismic data with a controlled‐amplitude processing stream. Comparison of the synthetic gathers with the processed field data leads to the conclusion that the model containing the measured shear‐wave velocities matches the field data much better than the model containing the estimated shear‐wave velocities. Therefore, existing equations which relate shear‐wave velocities to compressional‐wave velocities yield estimates which are not sufficiently accurate for making quantitative comparisons of synthetic and field gathers. Even small errors in the shear‐wave velocities can have a large impact on the output. Such errors can lead to an incomplete and perhaps inaccurate understanding of the amplitude‐versus‐offset response. This situation can be remedied by collecting shear‐wave data for use in amplitude‐versus‐offset modeling, and for building databases to generate better shear‐wave velocity estimator equations.
A solution to the blind‐zone problem by modeling of postcritical reflections
Authors: Sarkar D. , Tewari H. C. , Dixit M. M. , Kaila K. L.
Published: October 2002

An approach for solving the blind‐zone problem in seismic refraction prospecting is investigated. Hidden layer(s), if present in blind zones, can be identified with the help of postcritical reflections. These arrivals carry high energy and are easily identifiable on seismograms. Forward modeling techniques may be employed to determine the hidden‐layer parameters even in complex situations.
On: “Variable‐depth magnetization mapping: Application to the Athabasca basin, northern Alberta and Saskatchewan, Canada” by M. Pilkington (GEOPHYSICS, 54, 1164–1173, September 1989)
Authors: Steenland Nelson C.
Published: October 2002

Apparent magnetizations calculated from magnetic intensity depend on Z, and Pilkington attempts to remove this variableness by annihilating the topographic effect with the convolution of a topographic set from a seismic source (One would have to consult his references for a description of the topographic set.) having a crude contour interval of 300 m where maximal values are 1500 m. Then there are no data given to show that this surface is the surface of a magnetic basement. The largest element, an outcropping dome in the west with no corresponding anomaly on the intensity map, is not magnetic basement.
Wavefront sets analysis of limited aperture migration sections
Authors: Carrion P. M. , Sato H. K. , Buono A. V. D.
Published: October 2002

Although geophysical literature is quite rich in papers on migration, there are only a few papers that treat LAM (Limited Aperture Migration). Deterministic and stochastic criteria for the reconstruction of reflection interfaces in prestack migration were derived and demonstrated on synthetic and real data examples (a real data set was acquired offshore the Brazilian coast, courtesy of Petrobras). The reconstruction of reflection interfaces in LAM depends on the relative position of “normal bundles,” as defined here, and gradients to the reflection interfaces. The derived stochastic criterion relates the variance of “normal bundles” to the variance of slowness in the medium. These criteria will help the geophysicist to identify directly distorted regions caused by LAM and those regions that are correctly migrated.
The effectiveness of journals in exploration geophysics
Authors: Spies Brian R.
Published: October 2002

A detailed citation analysis was conducted for fourteen major journals dealing with exploration geophysics, to judge their cost‐effectiveness and impact. The analysis was for papers published in 1984, so that papers had approximately five years of visibility at the time the citation analysis was conducted. In addition, a study was performed for Geophysics for the years 1980 to 1988, to assess the influence of the length of time a paper was in the literature. The leading journal, in terms of number of citations, was the Journal of Geophysical Research, which received an average of 17.4 citations per paper, followed by the Geophysical Journal of the Royal Astronomical Society (8.6) and Geophysics (5.4). Several journals average less than 1 citation per paper. For Geophysics, the average paper receives an extra 1.2 citations per year over the nine years studied. The percentage of nil citations decreases from 35 percent after one year, to 8 percent after 9 years. Four percent of papers receive 20 percent of all citations; these are the classic papers of exploration geophysics. Short notes, on average, receive half the number of citations as full papers. Self‐citations, which account for approximately one in five citations, do not appear to significantly affect the importance or relevance of a paper. When examined in terms of cost‐effectiveness, SEG publications rate very well. Geophysics and SEG Expanded Abstracts have the lowest cost per 1000 characters of all the journals studied. In terms of the number of citations per unit cost, Geophysics is more than twice as cost‐effective than its nearest neighbor, the Journal of Geophysical Research. The results also confirm those of earlier studies, that commercial journals are not as cost‐effective as those published by not‐for‐profit professional societies.
An electromagnetic study of Maui’s last active volcano
Authors: Lienert B. R.
Published: October 2002

I have used both d.c. resistivity and time‐varying (0.2–5 Hz) magnetic field soundings to study the Ulupalakua‐Makena area on the island of Maui, the site of the last historic eruption in 1970. Using generalized inversions of the combined data, I have estimated the electrical resistivity structure beneath eight magnetometer sites. The resistivity values I obtained for electrical basement (saltwater‐saturated basalt) are reasonably consistent with a single value of 6 ± 1 Ω ⋅ m at all eight sites, although there is a suggestion of an increase in resistivity away from the coastline. The resolution matrices for the magnetic data indicated that, in addition to resolving the resistivity of electrical basement, the data were significantly affected by a combination of the resistivity of the soil layer and the depth to basement. Use of the Schlumberger data in conjunction with the magnetic data removed this correlation and resulted in good resolution for the resistivity/thickness combinations of the four upper model layers as well as for the resistivity of electrical basement. Assuming an average porosity of 20 percent for basalt and using a formation factor obtained for drillhole samples on the island of Hawaii, I estimate the basement temperature required to explain the interpreted resistivities is 59 ± 13°C. Since the penetration depth at the lowest frequency used is about 3 km, it is unlikely that these low resistivities can be explained by single flows having anomalously high porosities. I therefore conclude that the area may be of value as a low‐ to medium‐temperature geothermal resource.
Processing of seismic data in the presence of anisotropy
Authors: MacBeth Colin , Crampin Stuart
Published: October 2002

Processing techniques for extracting the polarization directions of the fast, and slower split shear waves, together with their traveltime splitting from vector wavefield data, form the basis of prospective procedures for imaging details of the internal structure of subsurface rocks. A recently developed technique involving the independent rotation of the source polarization direction, and geophone axes, provides an effective processing tool for extracting the effects of anisotropy from shear‐wave data. It is theoretically capable of handling nonorthogonal polarization directions of split shear waves. It is further developed, and its performance examined using vertical seismic profiling (VSP) shear‐wave datasets for different anisotropic earth models, and computed using full‐wave modeling. The procedure works well for shear waves propagating through an earth model with a uniform crack strike with depth, but produces inaccurate estimates of polarization and time‐delay when the crack strike varies abruptly with depth. It is unlikely that other techniques could do better under these circumstances, as the problems arise due to the complexities introduced by multiple shear‐wave splitting.
Dual tomography for imaging complex structures
Authors: Carrion Philip
Published: October 2002

The geophysical community admits that geotomography is a workable way to obtain accurate estimates of seismic velocity in complex structures. So‐called dual tomography improves the resolution of computed tomograms and thus can be applied to different geophysical and nongeophysical problems. Dual tomography emerges as a generalized approach to linearized constrained inversion. Dual inversion transforms a generalized constrained optimization problem being formulated in the physical space of seismic velocities to a dual unconstrained problem posed in the vector space of Lagrangian multipliers. It is a parametric optimization problem, with unknown solutions that are always perpendicular to the null‐space of a tomographic matrix. Imposed constraints improve the resolution and act as if the angular aperture coverage was extended. Thus dual tomography is able to quash image blurring associated with incomplete angular recording. Dual tomography does not require accurate knowledge of initial model. One starts with an arbitrary homogeneous medium and updates the medium in the course of iterations. Dual tomography does not require direct inversion of matrices and therefore it is relatively fast. Preliminary results indicate that dual tomography typically yields better images compared with algebraic reconstruction technique (ART), simultaneous reconstruction technique (SIRT), and other conventional techniques especially for limited angular aperture experiments typically used in seismic exploration.
Magnetic interpretation using the 3-D analytic signal
Authors: Roest Walter R. , Verhoef Jacob , Pilkington Mark
Published: October 2002

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.
The calculation of instantaneous frequency and instantaneous bandwidth
Authors: Barnes Arthur E.
Published: October 2002

A number of ways have been offered to calculate instantaneous frequency, an important complex seismic trace attribute. The standard calculation follows directly from its definition and requires two differentiations (Taner et al., 1979). By avoiding these differentiations, three formulas that approximate instantaneous frequency are faster to compute. The first employs a two‐point finite‐impulse response (FIR) differentiator in place of the derivative filter (Scheuer and Oldenburg, 1988). The second is nearly the same as the first, except that it employs a three‐point FIR differentiator (Boashash et al., 1991). The third takes a different approach and involves two approximations (Claerbout, 1976, p. 20; Yilmaz, 1987, p. 521). How do these formulas compare, and which is best?
Adaptive minimum prediction‐error deconvolution and source wavelet estimation using Hopfield neural networks
Authors: Wang Li‐Xin , Mendel Jerry M.
Published: October 2002

The massively parallel processing advantage of artificial neural networks makes them suitable for hardware implementations; therefore, using artificial neural networks for seismic signal processing problems has the potential of greatly speeding up seismic signal processing. A commonly used artificial neural network—Hopfield neural network—is used to implement a new adaptive minimum prediction‐error deconvolution (AMPED) procedure which decomposes deconvolution and wavelet estimation into three subprocesses: reflectivity location detection, reflectivity magnitude estimation, and source wavelet extraction. A random reflectivity model is not required. The basic idea of the approach is to relate the cost functions of the deconvolution and wavelet estimation problem with the energy functions of these Hopfield neural networks so that when these neural networks reach their stable states, for which the energy functions are locally minimized, the outputs of the networks give the solution to the deconvolution and wavelet estimation problem. Three Hopfield neural networks are constructed to implement the three subprocesses, respectively, and they are then connected in an iterative way to implement the entire deconvolution and wavelet estimation procedure. This approach is applied to synthetic and real seismic traces, and the results show that: (1) the Hopfield neural networks converge to their stable states in only one to four iterations; hence, this approach gives a solution to the deconvolution and wavelet estimation problem very quickly; (2) this approach works impressively well in the cases of low signal‐to‐noise ratio and nonminimum phase wavelets; and (3) this approach can treat backscatter either as noise or as useful signal.
Transient electromagnetic surveys in the Okiep district
Authors: Maher M. J.
Published: October 2002

In the Okiep District early miners produced massive sulfide ore from some five deposits. Some of these deposits later contributed to the reserves of disseminated ore mined during modern times. It is unreasonable to assume that all of the massive sulfide bodies present within the area are intersected by the erosion surface and thus were discovered by the early miners. Consequently, blind massive sulfide ore bodies could be present and may have large quantities of disseminated ore associated with them. The transient electromagnetic method is ideally suited to exploring for massive sulfide bodies, and six test surveys were carried out at various sites in the district. Four of these surveys were unsuccessful whereas, at the remaining two sites, excellent anomalies were recorded. At Ezelsfontein East Extension an anomaly was recorded indicative of a massive sulfide body at shallow depth and of generally flat attitude. This anomaly has a time constant of 15 ms and the interpreted body was confirmed by a limited diamond drilling program. A deep, flat‐lying conductor was interpreted from the TEM results at Fonteintjie West Prospect. This anomaly, with a time constant of 0.6 ms, has limited size. Diamond drilling confirmed the presence of submassive to massive sulfide mineralization at this locale. Neither of these two drilled prospects had economic mineralization.
Comparisons of downhole geophones and hydrophones
Authors: Krohn Christine E. , Chen S. T.
Published: October 2002

Receiver tests were conducted to compare the responses of downhole geophones and hydrophones. Commercial receiver tools use a maximum of eight geophone levels; however, we use hydrophones because we can record 48 levels simultaneously. For frequencies above 300 Hz, signal‐to‐background‐noise ratios for hydrophones and geophones in a prototype tool were comparable. (This prototype tool is a lightweight, large‐clamping‐force device that can record higher frequencies than commercial geophone tools.) For frequencies below 300 Hz, signal‐to‐noise ratios were greater for the geophones than for the hydrophones. A commercial geophone tool had lower low‐frequency signal‐to‐background‐noise ratios than the prototype tool, but greater than those of the hydrophones. Further analysis was performed to determine why the signal‐to‐background‐noise ratios for geophones were greater than those for hydrophones at low frequencies. The measured signal level for a hydrophone was 2.4 times that for a geophone, compared with a theoretical prediction of 1.8. Thus, the signal levels do not explain the difference in signal‐to‐background‐noise ratios. The low‐frequency background noise was attributed to coherent noise in the form of tube waves, a noise type to which hydrophones are much more susceptible than are geophones. Thus, the low signal‐to‐background‐noise ratios at frequencies below 300 Hz for hydrophones resulted from ambient noise propagating as tube waves in the borehole. The high‐frequency background noise was attributed to random seismic noise in the environment and not to instrument noise. These results show that hydrophones, which do not need to be clamped to the borehole wall, are preferable to geophones for high‐frequency borehole seismic applications using first arrivals. Geophones are preferable to hydrophones for borehole seismic applications using reflector arrivals, because these later‐arriving events are obscured by source‐generated tube waves in hydrophone data. Development of a method to reduce both the source‐generated and ambient tube‐wave noise detected by hydrophones would result in high‐quality borehole seismic data at a greatly reduced cost.
3-D prestack migration in anisotropic media
Authors: Dong Zhengxin , McMechan George A.
Published: October 2002

A three‐dimensional (3-D) prestack reverse‐time migration algorithm for common‐source P‐wave data from anisotropic media is developed and illustrated by application to synthetic data. Both extrapolation of the data and computation of the excitation‐time imaging condition are implemented using a second‐order finite‐ difference solution of the 3-D anisotropic scalar‐wave equation. Poorly focused, distorted images are obtained if data from anisotropic media are migrated using isotropic extrapolation; well focused, clear images are obtained using anisotropic extrapolation. A priori estimation of the 3-D anisotropic velocity distribution is required. Zones of anomalous, directionally dependent reflectivity associated with anisotropic fracture zones are detectable in both the 3-D common‐ source data and the corresponding migrated images.
Wave velocities in sandstones from elastic network simulations
Authors: Gist G. A. , Thompson A. H. , Katz A. J. , Berry M. J.
Published: October 2002

An elastic network model is used to calculate compressional and shear‐wave velocities in a model sandstone. The model consists of mineral components and pores distributed at random on a regular cubic lattice. Springs are used to represent compressional, shear, and rotational forces between neighboring grains. We have used the network model to calculate the dependence of wave velocities on porosity and clay content in sandstones. The calculated velocities are a linear function of both porosity and total clay content. The coefficients in this bilinear relationship are in quantitative agreement with published experimental correlations for sandstone velocities. This result provides theoretical support for the linear velocity‐porosity‐clay content relationship commonly assumed in empirical correlations. The rotational forces between grains are necessary to obtain quantitative agreement with experimental data. Wave velocities calculated using the continuum self‐consistent imbedding approximation agree with the elastic network for quartz/water mixtures, but not for quartz/clay mixtures. This disagreement demonstrates the importance of a granular elastic model with rotational forces for calculating velocities in rocks with multiple mineral components.
Audio‐frequency electromagnetic tomography in 2-D
Authors: Zhou Qiang , Becker Alex , Morrison H. F.
Published: October 2002

The mathematical formulation of acoustic diffraction tomography is applied to the problem of low frequency, diffusive electromagnetic (EM) fields. EM tomographic inversion, in two‐dimensional (2-D) Cartesian geometry, is illustrated for a crosshole source‐receiver configuration. The object function of the conductivity distribution is related to the transformed and filtered data by an inverse Fourier transform in the vertical direction and an inverse Laplace transform in the lateral direction. The reconstructed conductivity image is found to be a band‐limited version of the actual conductivity distribution. To stabilize the inversion, a regularized least‐squares method is used for image reconstruction. As in the seismic case, the inversion quality can be understood by inspecting the wavenumber domain coverage of the object function. Numerical experiments show that the resolution is better in the vertical direction than in the horizontal and it is also a function of source operating frequency. The position and attitude of the target are recovered well.
Ground roll: A potential tool for constraining shallow shear‐wave structure
Authors: Al‐Eqabi Ghassan I. , Herrmann Robert B.
Published: October 2002

The objective of this study is to demonstrate that a laterally varying shallow S‐wave structure, derived from the dispersion of the ground roll, can explain observed lateral variations in the direct S‐wave arrival. The data set consists of multichannel seismic refraction data from a USGS-GSC survey in the state of Maine and the province of Quebec. These data exhibit significant lateral changes in the moveout of the ground‐roll as well as the S‐wave first arrivals. A sequence of surface‐wave processing steps are used to obtain a final laterally varying S‐wave velocity model. These steps include visual examination of the data, stacking, waveform inversion of selected traces, phase velocity adjustment by crosscorrelation, and phase velocity inversion. These models are used to predict the S‐wave first arrivals by using two‐dimensional (2D) ray tracing techniques. Observed and calculated S‐wave arrivals match well over 30 km long data paths, where lateral variations in the S‐wave velocity in the upper 1–2 km are as much as ±8 percent. The modeled correlation between the lateral variations in the ground‐roll and S‐wave arrival demonstrates that a laterally varying structure can be constrained by using surface‐wave data. The application of this technique to data from shorter spreads and shallower depths is discussed.
On: “A waveform inversion technique for measuring elastic wave attenuation in cylindrical bars” by X. M. Tang (June 1992 GEOPHYSICS, p. 854–859)
Authors: Blair Dane P.
Published: October 2002

The author has recently presented a waveform inversion technique for measuring elastic wave attenuation in cylindrical bars. However, it is worthwhile pointing out some previous, relevant studies which may have been overlooked. The Pochhammer technique employed to derive the pulse attenuation characteristics is not new. In fact it was first used by Hsieh and Kolsky (1958); Blair (1985, 1990) also implemented the method using a Fast Fourier Transform.
A stable truncated series approximation of the reduction‐to‐the‐pole operator
Authors: Mendonça Carlos Alberto , Silva João B. C.
Published: October 2002

We combine a stabilized reduction‐to‐the‐pole and an upward continuation filter to produce meaningful reduced‐to‐the‐pole fields at low magnetic latitudes. The stabilizing procedure is based on the development, in Taylor’s series, of the theoretical expression for the reduction‐to‐the‐pole filter in the wavenumber domain. The filter instability is caused by the huge filter amplitudes along the magnetization azimuth, which are expressed by the infinite sum of terms close to unity. The stabilizing procedure reduces to simply truncating the infinite series. The upward continuation filter attenuates the high wavenumber component of the noise and allows us to design a stabilized filter closer to the theoretical one. Besides, quantitative interpretations of source depth based on the filtered field are more reliable when using upward continuation as compared with arbitrary low‐pass filters. The proposed filter was applied to synthetic data of a single prism uniformly magnetized along a supposedly known direction, and it produced a reduced‐to‐the‐pole field very close to the theoretical field at pole. We also applied the filter to magnetic data from Dixon Seamount assuming induced magnetization only. We obtained, within the central part of the anomaly, roughly circular contours of the reduced‐to‐the‐pole anomaly due to the nearly circular shape of the Seamount (evidenced by topographic data).
Poststack migration of P-SV seismic data
Authors: Harrison Mark P. , Stewart Robert R.
Published: October 2002

The exploding‐reflector model is only satisfied for zero‐offset P-SV data when [Formula: see text] is depth‐invariant. P-SV diffractions in a vertically‐inhomogeneous medium are approximately hyperbolic, and an expression for their migration velocity is derived. The resulting migration velocities are 6–11 percent less than the corresponding P-SV rms velocities. Migration of DMO‐corrected synthetic P-SV stacked data, using a conventional phase‐shift algorithm and the derived migration velocities, is found to adequately collapse diffractions, whereas migration using the rms velocity function gives significant overmigration.
A simple approach to calculating refraction statics corrections
Authors: Hatherly Peter J. , Urosevic Milovan , Lambourne Andy , Evans Brian J.
Published: October 2002

The higher the resolution demanded from the interpretation of seismic data, the more care required in the acquisition and processing of that data. Of the processing steps that directly influence resolution, the one that is having the most impact on land data is the statics correction. Geophysical conferences have devoted entire technical sessions to statics. Seismic crews frequently undertake specific refraction surveys just to obtain good statics data. Here we outline a very simple solution to the statics problem that produces remarkable results.
Multichannel Wiener deconvolution of vertical seismic profiles
Authors: Haldorsen Jakob B. U. , Miller Douglas E. , Walsh John J.
Published: October 2002

We describe a technique for performing optimal, least‐squares deconvolution of vertical seismic profile (VSP) data. The method is a two‐step process that involves (1) estimating the source signature and (2) applying a least‐squares optimum deconvolution operator that minimizes the noise not coherent with the source signature estimate. The optimum inverse problem, formulated in the frequency domain, gives as a solution an operator that can be interpreted as a simple inverse to the estimated aligned signature multiplied by semblance across the array. An application to a zero‐offset VSP acquired with a dynamite source shows the effectiveness of the operator in attaining the two conflicting goals of adaptively spiking the effective source signature and minimizing the noise. Signature design for seismic surveys could benefit from observing that the optimum deconvolution operator gives a flat signal spectrum if and only if the seismic source has the same amplitude spectrum as the noise.
Investigating the velocity‐depth ambiguity of reflection traveltimes
Authors: Tieman Hans J.
Published: October 2002

Reflection seismic data contain a long wavelength ambiguity making it difficult to separate traveltime information into velocity and reflector depth components. The existence of this velocity‐depth ambiguity is a feature of the geometry of the subsurface and is not caused by the particular inversion algorithm being used. Factors that control the occurrence of velocity‐depth ambiguities include the effective width of a potential velocity anomaly; i.e., its spatial wavelength, its height above a reflector, and its thickness. Factors that do not affect velocity‐depth ambiguities are the magnitude of the anomaly (the difference in velocity between it and the background) and the cable length with which data were recorded. A thin velocity anomaly induces an ambiguity at a wavelength approximately equal to 4.44 times the height of the anomaly above the reflector. A thick anomaly that spans the entire space from surface to reflector induces an ambiguity at a wavelength approximately equal to 2.57 the depth to the reflector. These are wavelengths that are significant in size, and therefore are of exploration interest. Through Fourier analysis, any subsurface velocity field can be decomposed into spatial frequency components. Thus the wavelength dependent velocity‐depth ambiguity adversely affects all velocity distributions.
The lattice filter in ground‐roll suppression
Authors: Saatçilar Ruhi , Canitez Nezihi
Published: October 2002

Seismic reflections are sometimes masked by Ray‐leigh‐type surface waves that are termed ground roll in seismic literature. An adaptive lattice filter is used to recover reflected signals contaminated by ground roll. Experiments on synthetic and field data showed that the adaptive lattice filter technique is very effective in ground‐roll elimination. In addition, the filter works as a whitening operator, compresses the signal, and increases the signal‐to‐noise ratio.
Streaming potential from multiphase flow
Authors: Sprunt Eve S. , Mercer Tony B. , Djabbarah Nizar F.
Published: October 2002

In trying to understand the affect of electrokinetics on the spontaneous potential (SP) log, the focus has generally been on the solid‐brine streaming potential. Within the accuracy of the measurements, the streaming‐potential coupling coefficient is shown to be independent of the permeability of the rock. The solid‐brine streaming potential is of much smaller magnitude than the electrostatic potentials from gas‐liquid and liquid‐liquid flow. Air bubbles were found to increase the streaming potential coupling coefficient by more than two orders of magnitude over the value for single‐phase brine flow. Thus, two‐phase gas‐liquid flow is more likely to have a significant impact on the SP log than is single phase liquid flow. Two‐phase oil‐brine flow may also produce a larger electrokinetic potential than single‐phase flow. The magnitude of the electrokinetic potential caused by oil‐brine flow will depend on the composition of the oil and the brine. Trace materials can have a major impact on the electrokinetic potential of hydrocarbons. In a system with multiphase flow, the solid‐liquid interaction is probably the smallest component of the electrokinetic potential.
The velocity‐depth ambiguity in seismic traveltime data
Authors: Ross Warren S.
Published: October 2002

An observed disturbance in seismic traveltimes to a reflector can be caused either by an anomalous velocity zone between the surface and the reflector or by a structural variation in the reflector itself. This velocity‐depth ambiguity is formulated in terms of linear estimation theory. Such a formulation allows integration of various published results on velocity‐depth ambiguity and suggests improved methods of stabilizing the solution of a depth‐conversion problem. By solving a relatively simple problem that is amenable to analysis—a single reflector beneath an overburden with a variable velocity—the following conclusions can be drawn: 1) The velocity‐depth ambiguity is caused by traveltime errors and can be quantitatively related to those errors by closed‐form expressions if the velocities do not vary laterally (or vary very slowly). Among other things, those expressions show that for small spread lengths (shorter than haft the depth) the errors in velocity and depth are inversely proportional to the square of the spread length. Errors can thus be reduced more effectively at small spread lengths by increasing the maximum offset rather than by including more offsets. 2) Laterally varying velocities can be estimated accurately at all but isolated points in their spatial frequency spectrum, called “wavelengths of maximum ambiguity.” If these ambiguous wavelengths are stabilized by damping them rather than by more traditional lateral smoothing techniques, structural or velocity features smaller than a spread length need not be smeared laterally. 3) A deep velocity anomaly is estimated with lower accuracy than is a shallow one. The theory presented here is a complement to more general methods of velocity inversion, such as tomography, which can be used to solve very complex problems beyond the scope of this analysis.
On: “Sensitivity of reflected and transmitted seismic data,” by Rune Mittet and Tom Houlder (November 1993 GEOPHYSICS, 58, 1621–1628)
Authors: Tarantola Albert
Published: October 2002

Jannane et al. (1989) made a simple numerical demonstration of a well known fact (Claerbout, 1985): that seismic data are sensitive to the large wavelengths of the velocity model and to the short wavelengths of the impedances. Taking a standard geometry (offsets between 250 m and 1750 m, dominant source frequency of 35 Hz, velocities from 1 km/s to 2.5 km/s, 1.5 s of signal), Jannane et al. conclude that standard seismic data are sensitive to impedance perturbations with wavelengths inside the 30 m-60 m bandwidth, and to velocity perturbations for wavelengths larger than 300 m.
2-D Hartley transforms
Authors: Sundararajan N.
Published: October 2002

Two different versions of kernels associated with the 2-D Hartley transforms are investigated in relation to their Fourier counterparts. This newly emerging tool for digital signal processing is an alternate means of analyzing a given function in terms of sinusoids and is an offshoot of Fourier transform. Being a real‐valued function and fully equivalent to the Fourier transform, the Hartley transform is more efficient and economical than its progenitor. Hartley and Fourier pairs of complete orthogonal transforms comprise mathematical twins having definite physical significance. The direct and inverse Hartley transforms possess the same kernel, unlike the Fourier transform, and hence have the dual distinction of being both self reciprocal and having the convenient property of occupying the real domain. Some of the properties of the Hartley transform differ marginally from those of the Fourier transform.
Field trials of a seismoelectric method for detecting massive sulfides
Authors: Kepic Anton W. , Maxwell Michael , Russell R. Don
Published: October 2002

An underground test of a seismoelectric prospecting method for massive sulfides was performed at the Mobrun Mine (Rouyn‐Noranda, Quebec) in June 1991. The method is based upon the conversion of seismic energy to high‐frequency pulses of electromagnetic radiation by sulfide minerals. The delay between shot detonation and detection of the electromagnetic radiation gives a one‐way traveltime for the acoustic wave to reach the zone of seismoelectric conversion, which when combined with P‐wave velocity allows the shot‐to‐ore zone distance to be calculated. A 0.22-kg explosive charge located within 50 m of the orebody provided the seismic excitation, and the resulting electromagnetic emissions were received by electric dipole and induction‐coil antennas. First‐arrival information from a 35‐shot survey above an orebody, the 1100 lens, provides firm evidence that short duration pulses of electromagnetic radiation are produced by the passage of acoustic waves through the orebody. The survey also demonstrated that seismoelectric conversions could be induced at shot‐to‐orebody distances of 50 m and detected at distances of up to 150 m from the orebody. Areas of seismoelectric conversion are highlighted in sections produced by plotting the position of seismic wavefronts during signal reception. The sections show anomalies that correlate with the known structure and location of the orebody and demonstrate the potential of using this seismoelectric phenomenon as an exploration tool.
A new strategy for CCP stacking
Authors: Zhong Benshan , Zhou Xixiang , Liu Xuecai , Jiang Yule
Published: October 2002

Raypaths for P-SV‐converted waves are asymmetrical and the reflected events are not hyperbolic. Consequently, standard routines for NMO correction of P‐waves cannot be applied in a straightforward manner. This is a major difficulty in data processing of P-SV‐converted waves. This paper proposes a new strategy for common conversion point (CCP) stacking. The technique accomplishes reflection point migration, nonhyperbolic moveout, and CCP stacking in one step.
3-D crosswell transmissions: Paraxial ray solutions and reciprocity paradox
Authors: Sun Jianguo
Published: October 2002

Transmission of seismic waves through a 3-D earth model is of fundamental importance in seismology. If the model consists of many layers separated by curved interfaces, the only feasible solution to the transmitted waves is the one given by the geometrical optics approximation. Transmitted rays, transmitted wavefield, and the first Fresnel zone associated with a transmission point can be expressed by four 2 × 2 constant matrices constituting the 4 × 4 linearized ray transformation matrix. Generally, the ray transformation matrix can be constructed by dynamic ray tracing. However, if the layers are homogeneous, it can be formulated in closed form by using elementary vector calculus and coordinate transformations. Using the symplecticity of the ray transformation matrix, transmissions in opposite directions can be formulated by the ray transformation matrix for only one direction, and a reciprocity relation can be established. After the decomposition theorem for the ray transformation matrix, the transmitted wavefield in a model with many curved interfaces can be computed in a cascaded way. Using the [Formula: see text]‐matrix decomposition theorem, the normalized geometrical spreading factor can be expressed by means of the area of the first Fresnel zone of a transmission point. If seismic waves propagate through a locally spherical interface, the reciprocity relation may not hold. Using ray theory, this fact is shown by formulating the transmitted wavefield with the two principal radii of curvature of the transmitted wavefront at the transmission point under consideration. Using wave theory, this fact is shown by analyzing the Debye integral with the method of stationary phase.
Cross‐borehole resistivity tomography of a pilot‐scale, in‐situ vitrification test
Authors: Spies B. R. , Ellis Robert G.
Published: October 2002

Direct current (dc) cross‐borehole resistivity measurements were used to monitor the melting and solidification processes of an in‐situ vitrification (ISV) experiment at Oak Ridge National Laboratory (ORNL) in Tennessee. Six boreholes, 6-m deep, were augured around the ISV site, and five electrodes implanted in each hole. Three sets of crosswell, pole‐pole resistivity data were collected: prior to the melt phase, immediately after power shut‐off, and after the melt zone had solidified and returned to ambient temperature. These three sets of data were inverted using a conjugate‐gradient scheme to produce conductivity images of the melt phase and the vitrified end products. The images obtained depend quite strongly on the model weighting function applied to the inversion. With an optimum weighting function based on a priori spatial constraints, the resistivity images delineate the melt zone and provide a reasonable indication of its geometry. The resistivity data support, but do not require, the existence of the melt zone.
Gravity source moment inversion: A versatile approach to characterize position and 3-D orientation of anomalous bodies
Authors: Medeiros Walter E. , Silva João B.C.
Published: October 2002

We present a 3-D gravity interpretation method based on the inversion of source moments employing the series derived from the gravity anomaly expansion in multipoles and retaining moments up to second order only. It presents the advantages of being essentially linear and allowing a straightforward computer implementation. The method requires neither an explicit assumption about the source geometry nor a priori knowledge about the density contrast distribution, which may even be nonuniform. The method assumes implicitly: (1) that the source fits in the interior of an imaginary sphere whose center coincides with the source center of mass and whose radius is smaller than the depth to the source center of mass relative to the measuring plane, and (2) that the spatial density distribution presents three ortogonal planes of symmetry intersecting at the source center of mass. The first assumption can be met by upward continuing the observed anomaly. When both assumptions are met, the method produces reliable and stable estimates of the total anomalous mass, the coordinates of the center of mass, the three principal axes directions of the anomalous body, and the relative importance among the axes. The method is particularly suited for interpreting compact, isolated or disjoint, but spatially correlated sources. The method is sensitive to an incomplete removal of the regional field (presumably superimposed on the residual anomaly of interest) because the unremoved part of the regional field may strongly affect the estimates of the residual source moments.
On: “Pulse distortion in depth migration,” M. Tygel, J. Schleicher, and P. Hubral (October 1994 GEOPHYSICS, 59, p. 1561–1569).
Authors: Barnes Arthur E.
Published: October 2002

Tygel et al. have written an excellent and rigorous discussion of pulse distortion in seismic reflection data caused by prestack depth migration. Such distortion is easily understood by recognizing that it is more or less the same effect as normal moveout (NMO) stretch combined with frequency shifting due to poststack time migration.
Optimum weighted stack for multiple suppression
Authors: Schoenberger Michael
Published: October 2002

In water deeper than about 300 m, common midpoint stack is moderately effective at suppressing multiple reflections. The stacking process acts as a low‐pass multiple filter, rejecting, to some extent, frequencies greater than the reciprocal of the multiple’s residual moveout on the farthest offset trace. A weighted stack can often be even more effective at attenuating multiples, if the weights are chosen such that the multiple filter response has low amplitudes at those frequencies where the multiple is strong. A least‐squares technique is developed to determine optimum weights, based on the multiple’s frequency content, the water depth, the primary velocity, and on a stabilization factor equivalent to white noise in deconvolution filter design. To be truly optimum, the weights must vary both spatially, to account for water‐depth changes, and temporally, to account for sediment velocity changes and mute patterns; but the filter responses are not very sensitive to the input parameters and need to be redesigned only infrequently. The superiority of weighted stacks to uniform stacks is demonstrated by application to three deep‐water examples. In all cases, weighted stacks attenuate multiples more effectively than do conventional uniform stacks.
Nonuniqueness in traveltime tomography: Ensemble inference and cluster analysis
Authors: Vasco Don W. , Peterson John E. , Majer Ernest L.
Published: October 2002

We examine the nonlinear aspects of seismic traveltime tomography. This is accomplished by completing an extensive set of conjugate gradient inversions on a parallel virtual machine, with each initiated by a different starting model. The goal is an exploratory analysis of a set of conjugate gradient solutions to the traveltime tomography problem. We find that distinct local minima are generated when prior constraints are imposed on traveltime tomographic inverse problems. Methods from cluster analysis determine the number and location of the isolated solutions to the traveltime tomography problem. We apply the cluster analysis techniques to a cross‐borehole traveltime data set gathered at the Gypsy Pilot Site in Pawnee County, Oklahoma. We find that the 1075 final models, satisfying the traveltime data and a model norm penalty, form up to 61 separate solutions. All solutions appear to contain a central low velocity zone bounded above and below by higher velocity layers. Such a structure agrees with well‐logs, hydrological well tests, and a previous seismic inversion.
Some refinements on the finite‐difference method for 3-D dc resistivity modeling
Authors: Zhao Shengkai , Yedlin Matthew J.
Published: October 2002

Two basic refinements of the finite‐difference method for 3-D dc resistivity modeling are presented. The first is a more accurate formula for the source singularity removal. The second is the analytic computation of the source terms that arise from the decomposition of the potential into the primary potential because of the source current and the secondary potential caused by changes in the electrical conductivity. Three examples are presented: a simple two‐layered model, a vertical contact, and a buried sphere. Both accurate and approximate Dirichlet boundary conditions are used to compute the secondary potential. Numerical results show that for all three models, the average percentage error of the apparent resistivity obtained by the modified finite‐difference method with accurate boundary conditions is less than 0.5%. For the vertical contact and the buried sphere models, the error caused by the approximate boundary condition is less than 0.01%.
Comparison of experimental velocity measurements with theoretical results in a solid‐solid composite material
Authors: Zhang Mei , Ebrom Daniel A. , McDonald John A. , Tatham Robert H.
Published: October 2002

Two‐phase composite media were constructed of aluminum powder inclusions embedded randomly in a clear resin matrix. The shapes of the inclusions are elongate, with aspect ratios ranging from 0.1 to 1.0, averaging about 0.25. The effective velocities of ultrasonic elastic waves in the bulk media were measured for these composite materials, and the observed data were compared to the theoretical values calculated using five different numerical models. The observed velocities agree with the theoretical values calculated using a dynamic model under the assumption that the inclusions have an oblate spheroidal shape.
AVO inversion of Troll Field data
Authors: Buland Arild , Landrø Martin , Andersen Mona , Dahl Terje
Published: October 2002

A stratigraphic elastic inversion scheme has been applied to a data set from the Troll East Field, offshore Norway. The objective of the present work is to obtain estimates of the P‐ and S‐wave velocities and densities of the subsurface. The inversion is carried out on τ − p transformed common depth‐point (CMP) gathers. The forward modeling is performed by convolving a wavelet with the reflectivity that includes water‐bottom multiples, transmission effects, and absorption and array effects. A damped Gauss‐Newton algorithm is used to minimize a least‐squares misfit function. Inversion results show good correlation between the estimated [Formula: see text] ratios and the lithologies in the wells. The [Formula: see text] ratio is estimated to 2.1–3.0 for shale and 1.6–2.0 for sandstones. In the reservoir, the [Formula: see text] ratio is estimated to 1.55 in the gas sand and to 1.62 below the fluid contact.