bluuebunny/crossref_metadata_2025 huggingface dataset:
Source processes of industrially‐induced earthquakes at The Geysers geothermal area, California
Authors: Ross Alwyn , Foulger G. R. , Julian Bruce R.
Published: October 2002

Microearthquake activity at The Geysers geothermal area, California, mirrors the steam production rate, suggesting that the earthquakes are industrially induced. A 15-station network of digital, three‐component seismic stations was operated for one month in 1991, and 3,900 earthquakes were recorded. Highly‐accurate moment tensors were derived for 30 of the best recorded earthquakes by tracing rays through tomographically derived 3-D [Formula: see text] and [Formula: see text] structures, and inverting P- and S-wave polarities and amplitude ratios. The orientations of the P- and T-axes are very scattered, suggesting that there is no strong, systematic deviatoric stress field in the reservoir, which could explain why the earthquakes are not large. Most of the events had significant non‐double‐couple (non-DC) components in their source mechanisms with volumetric components up to ∼30% of the total moment. Explosive and implosive sources were observed in approximately equal numbers, and must be caused by cavity creation (or expansion) and collapse. It is likely that there is a causal relationship between these processes and fluid reinjection and steam withdrawal. Compensated linear vector dipole (CLVD) components were up to 100% of the deviatoric component. Combinations of opening cracks and shear faults cannot explain all the observations, and rapid fluid flow may also be involved. The pattern of non-DC failure at The Geysers contrasts with that of the Hengill‐Grensdalur area in Iceland, a largely unexploited water‐dominated field in an extensional stress regime. These differences are poorly understood but may be linked to the contrasting regional stress regimes and the industrial exploitation at The Geysers.
Seismic anisotropy in the upper 500 m of the Southern Sydney Basin
Authors: Urosević Milovan , Juhlin Christopher
Published: October 2002

An analysis of seismic anisotropy at a BHP mining site in the Southern Sydney Basin by combined use of crosshole and vertical seismic profiling (VSP) data is presented. The upper 250 m in this area is highly heterogeneous and has a major impact on the analysis of P-wave traveltimes. It is shown that using P-wave information solely would not, at least in this case, lead to any reasonable estimate of the elastic constants, in particular C13, even if the measurements contained a full range of incident angles. However, if the measurements of SV-waves are available, even over a small range of incident angles, then C13 is determined more accurately. P-wave velocities measured in the vertical and horizontal directions show that anisotropy is present in the area. Additional measurements, along different incident angles, indicate that the rock down to 500 m depth is predominantly transversely isotropic (TI) with a vertical axis of symmetry. The P-wave anisotropy can be approximated as elliptical. Using the elastic constants estimated from the data analyses, synthetic seismograms for heterogeneous TI media were generated. Comparison of the seismic modeling with real crosshole data shows that it is necessary to include both fault zones and gas accumulations in the model to qualitatively match the real data. By using SV-waves in the multioffset VSP data, reflectors are mapped more accurately than by using P-waves, even under the assumption of isotropy and in the presence of heterogeneity. Mapping of converted P-SV waves by a straight ray approach also produced better results than the corresponding isotropic P-wave mapping. Inclusion of elliptical anisotropy into Kirchhoff migration resulted in better P-wave images than using an isotopic migration code. We conclude that both P-wave VSP multioffset mapping and tomographic inversion methods need to account for anisotropy to be accurate in this area, while SV-waves may be handled using isotropic codes. The same is true for crosshole and surface seismic data.
Using surface deformation to image reservoir dynamics
Authors: Vasco Don W. , Karasaki Kenzi , Doughty Christine
Published: October 2002

The inversion of surface deformation data such as tilt, displacement, or strain provides a noninvasive method for monitoring subsurface volume change. Reservoir volume change is related directly to processes such as pressure variations induced by injection and withdrawal. The inversion procedure is illustrated by an application to tiltmeter data from the Hijiori test site in Japan. An inversion of surface tilt data allows us to image flow processes in a fractured granodiorite. Approximately 650 barrels of water, injected 2 km below the surface, produces a peak surface tilt of the order of 0.8 microradians. We find that the pattern of volume change in the granodiorite is very asymmetrical, elongated in a north‐northwesterly direction, and the maximum volume change is offset by more than 0.7 km to the east of the pumping well. The inversion of a suite of leveling data from the Wilmington oil field in Long Beach, California, images large‐scale reservoir volume changes in 12 one‐ to two‐year increments from 1976 to 1996. The influence of various production strategies is seen in the reservoir volume changes. In particular, a steam flood in fault block II in the northwest portion of the field produced a sudden decrease in reservoir volume.
Poroelastic model to relate seismic wave attenuation and dispersion to permeability anisotropy
Authors: Parra Jorge O.
Published: October 2002

A transversely isotropic model with a horizontal axis of symmetry, based on the Biot and squirt‐flow mechanisms, predicts seismic waves in poroelastic media. The model estimates velocity dispersion and attenuation of waves propagating in the frequency range of crosswell and high‐resolution reverse vertical seismic profiling (VSP) (250–1250 Hz) for vertical permeability values much greater than horizontal permeability parameters. The model assumes the principal axes of the stiffness constant tensor are aligned with the axes of the permeability and squirt‐flow tensors. In addition, the unified Biot and squirt‐flow mechanism (BISQ) model is adapted to simulate cracks in permeable media. Under these conditions, the model simulations demonstrate that the preferential direction of fluid flow in a reservoir containing fluid‐filled cracks can be determined by analyzing the phase velocity and attenuation of seismic waves propagating at different azimuth and incident angles. As a result, the fast compressional wave can be related to permeability anisotropy in a reservoir. The model results demonstrate that for a fast quasi-P-wave propagating perpendicular to fluid‐filled cracks, the attenuation is greater than when the wave propagates parallel to the plane of the crack. Theoretical predictions and velocity dispersion of inter‐well seismic waves in the Kankakee Limestone Formation at the Buckhorn test site (Illinois) demonstrate that the permeable rock matrix surrounding a low‐velocity heterogeneity contains vertical cracks.
Determining bed boundaries from inversion of EM logging data using general measures of model structure and data misfit
Authors: Zhang Zhiyi , Chunduru Raghu K. , Jervis Michael A.
Published: October 2002

In this paper we propose a new method to locate bed boundaries by carrying out a 1-D nonlinear inversion of electromagnetic (EM) logging data. We first solve for 1-D resistivity structure in which the earth is modeled using layers of constant thickness. This thickness is determined based upon the tool resolution and the desired resolution from the user. We use general measures of data misfit and model structure in the inversion to construct piecewise‐constant models through the iteratively reweighted least‐squares (IRLS) procedure, and we minimize a generic global model objective function subject to data constraints. The general measure includes the traditional [Formula: see text] norm as a special case. The recovered piecewise‐constant models simulate traditional models comprised of a few uniform layers and hence permit easy determination of bed boundaries. The generic model objective function allows the incorporation of prior geological information and provides measures of the closeness to the reference model and the amount of structure in the recovered model. We develop an efficient scheme to incorporate the IRLS procedure into the search for the appropriate regularization parameter at each iteration. The iterations required by the IRLS algorithm are combined with those for the linearized, iterative inversion procedure. The applicability of the algorithm is demonstrated using synthetic and field data examples.
Time‐lapse (4-D) seismic monitoring of primary production of turbidite reservoirs at South Timbalier Block 295, offshore Louisiana, Gulf of Mexico
Authors: Burkhart Tucker , Hoover Andrew R. , Flemings Peter B.
Published: October 2002

Two seismic surveys acquired over South Timbalier Block 295 field (offshore Louisiana) record significant differences in amplitude that are correlated to hydrocarbon production at multiple reservoir levels. The K8 sand, a solution‐gas‐drive reservoir, shows increases in seismic amplitude associated with gas exsolution. The K40 sand, a water‐drive reservoir, shows decreases in seismic amplitude associated with increases in water saturation. A methodology is presented to optimize the correlation between two seismic surveys after they have been individually processed (poststack) This methodology includes rebinning, crosscorrelation, band‐pass filtering, and cross‐equalization. A statistical approach is developed to characterize the correlation between the seismic surveys. This statistical analysis is used to discriminate seismic amplitude differences that record change in rock and fluid properties from those that could be the result of miscorrelation of the seismic data. Time‐lapse seismic analysis provides an important new approach to imaging hydrocarbon production; it may be used to improve reservoir characterization and guide production decisions.
Infinite boundary element absorbing boundary for wave propagation simulations
Authors: Fu Li‐Yun , Wu Ru‐Shan
Published: October 2002

In the boundary element (BE) solution of wave propagation, infinite absorbing elements are introduced to minimize diffractions from truncated edges of models. This leads to a significant simplification and reduction of computational effort, especially for 3-D problems. The infinite BE absorbing boundary condition has a general form for both 2-D and 3-D problems and for both acoustic and elastic cases. Its implementation is facilitated by the introduction of the corresponding shape functions. Numerical experiments illustrate a nearly perfect absorption of unwanted diffractions. The approach overcomes some of the difficulties encountered in conventional absorbing techniques and takes less memory space and less computing time.
Effective AVO crossplot modeling: A tutorial
Authors: Ross Christopher P.
Published: October 2002

The ability to crossplot attributes from a 3-D seismic volume permits a geophysicist to identify and high grade subsets of the 3-D volume that warrant detailed inspection. In the case of amplitude‐variation‐with‐offset (AVO) crossplotting, the seismic attributes are derived from CDP data. Crossplotting has become a fundamental process in AVO analysis, just as it is in petrophysical analysis. Comprehending the intricacies and selection of attributes is essential for successful AVO analysis and improved seismic interpretation. AVO crossplotting of modeled seismic data derived from well logs with the Biot‐Gassmann equations provides a basis for understanding fluid substitution effects on AVO attribute interactions when crossplotting. With these model‐based understandings, improved multi‐attribute interpretation processes can commence with AVO crossplotting of seismic volumes.
Topographic migration of georadar data: Implications for acquisition and processing
Authors: Lehmann Frank , Green Alan G.
Published: October 2002

Application of conventional elevation static corrections and migration to wavefield data recorded on irregular surfaces may result in poor reconstructions of complex subsurface features. Particulary poor images may be obtained at locations where the depths to target structures are comparable to undulations in the surface topography. For example, topographic relief of only 1-2 m may be important for the processing of georadar data. We describe an algorithm that allows georadar data to be migrated directly from gently to highly irregular acquisition surfaces. When applied to a variety of complicated synthetic data sets, topographically migrated images are observed to be markedly superior to those produced by two standard processing schemes. Extensive tests demonstrate that topographic migration should be considered in regions characterized by surface gradients ≫10% (i.e., dips ≫6°). For effective topographic migration, lateral and vertical coordinates of the georadar antennas should be determined to better than 10% of the dominant georadar wavelength, and velocities should be known to within 10–20% (e.g., 0.01–0.02 m/ns) of their true values. When applied to data collected across a moderately dipping (∼14°) rock glacier in the Swiss Alps, georadar sections resulting from two standard processing schemes have reflectors with depths and dips that differ by a significant 10–15% from those in the topographically migrated images.
Near‐surface VSP surveys using the seismic cone penetrometer
Authors: Jarvis Kevin D. , Knight Rosemary
Published: October 2002

We have found that high‐quality vertical seismic profile (VSP) data can be collected for near‐surface applications using the seismic cone penetrometer. Cone‐mounted accelerometers are used as the VSP receivers, and a sledgehammer against the cone truck baseplate is used as a source. This technique eliminates the need to drill a borehole, thereby reducing the cost of the survey, and results in a less invasive means of obtaining VSP data. Two SH-wave VSP surveys were acquired over a deltaic sand/silt sequence and compared to an SH-wave common‐depth‐point (CDP) reflection profile. The VSP data were processed using a combination of singular‐value‐decomposition filtering, deconvolution, and f-k filtering to produce the final VSP extracted traces. The VSP traces correlate well with cone geotechnical logs and the CDP surface‐seismic data. The first breaks from the VSP can be used to generate shear‐wave velocity profiles that are important for time‐to‐depth conversion and the velocity correction of the CDP surface data.
Color display of the localized spectrum
Authors: Theophanis Stephen , Queen John
Published: October 2002

This paper develops a new display technique for seismic cross‐sections, called spectral color. The need to visualize frequency information in seismic data is recognized uniformly and often is accomplished through the color display of instantaneous frequency. The spectral content of a reflected event can carry information about the reflecting horizon’s characteristics which will not be resolved in the instantaneous state of the record. Spectral color is devised to overcome the problem of displaying an entire localized spectrum at each time sample and offset of a seismic section. The localized spectrum is calculated with a relatively new time‐frequency representation called the S-transform, which combines a Fourier technique with adaptive windowing in the frequency domain. A color (RGB triplet) based on the localized spectral content is calculated and the pixel is displayed at the appropriate position in the seismic section. As a result, the seismic cross‐section is displayed in an intuitive manner that is much the way we see the world around us. Strongly reflecting or well‐lit objects appear to us as bright, and the color tells us about the frequency content of the reflected energy. Spectral color is applied to ultrasonic laboratory data acquired over a thin anisotropic disk. It reveals a change in color (spectral content) with azimuth where no significant amplitude variation with azimuth was observed. Spectral color is illustrated further by application to a 3-D field data set and is compared to other, more standard, color displays.
Airborne geophysical surveying for hazardous waste site characterization on the Oak Ridge Reservation, Tennessee
Authors: Doll W. E. , Nyquist J. E. , Beard L. P. , Gamey T. J.
Published: October 2002

Airborne geophysical methods that were developed for mineral and petroleum exploration can, with some modification, be applied to environmental problems where large areas must be characterized. A helicopter survey that deployed magnetic, electromagnetic, and radiometric sensors carried out one of the first large‐scale airborne environmental surveys at a U.S. government facility at Oak Ridge, Tennessee in 1993–1994. The survey included testing of a new airborne electromagnetic system designed specifically for environmental applications and for controlled field tests of magnetic systems. Helicopter‐borne magnetic measurements were capable of discriminating groups of as few as ten metallic 208-liter (55-gallon) storage drums under representative field conditions. Magnetic and electromagnetic sensors were able to distinguish groups of metal‐filled waste disposal trenches within disposal sites, but were unable to resolve individual trenches. Electromagnetic data proved to be the most effective airborne technique for geological mapping in this portion of the Appalachian fold‐and‐thrust belt and for locating karst features. Radiometric measurements were useful both in geological mapping and in detecting zones of high radiation related to hazardous waste. The Oak Ridge survey proved valuable for quickly screening large areas and for locating anomalies for subsequent ground follow‐up. On‐board video was used to reduce the number of instances of ground follow‐up by allowing the visual screening of anomalous areas.
Noncausal spatial prediction filtering for random noise reduction on 3-D poststack data
Authors: Gülünay Necati
Published: October 2002

A common practice in random noise reduction for 2-D data is to use pseudononcausal (PNC) 1-D prediction filters at each temporal frequency. A 1-D PNC filter is a filter that is forced to be two sided by placing a conjugate‐reversed version of a 1-D causal filter in front of itself with a zero between the two. For 3-D data, a similar practice is to solve for two 2-D (causal) one‐quadrant filters at each frequency slice. A 2-D PNC filter is formed by putting a conjugate flipped version of each quadrant filter in a quadrant opposite itself. The center sample of a 2-D PNC filter is zero. This paper suggests the use of 1-D and 2-D noncausal (NC) prediction filters instead of PNC filters for random noise attenuation, where an NC filter is a two‐sided filter solved from one set of normal equations. The number of negative and positive lags in the NC filter is the same. The center sample of the filter is zero. The NC prediction filters are more center loaded than PNC filters. They are conjugate symmetric as PNC filters. Also, NC filters are less sensitive than PNC filters to the size of the gate used in their derivation. They can handle amplitude variations along dip directions better than PNC filters. While a PNC prediction filter suppresses more random noise, it damages more signal. On the other hand, NC prediction filters preserve more of the signal and reject less noise for the same total filter length. For high S/N ratio data, a 2-D NC prediction filter preserves geologic features that do not vary in one of the spatial dimensions. In‐line and cross‐line vertical faults are also well preserved with such filters. When faults are obliquely oriented, the filter coefficients adapt to the fault. Spectral properties of PNC and NC filters are very similar.
Drill‐bit signal separation for RVSP using statistical independence
Authors: Poletto Flavio , Rocca Fabio L. , Bertelli Luca
Published: October 2002

Drill‐bit noise and RVSPs. Time and depth migration of seismic data usually is based on velocities estimated from surface measurements. The actual velocities, which calibrate the depth‐migrated seismic to the well data, are determined after the well is drilled using check shots and VSPs. By then, it may be too late to correct for seismic migration velocity imprecision and impact the well’s success. The earlier we can obtain such velocity information, the more time will be available for reprocessing (remigration of either the seismic or any depth maps), if necessary. It is therefore very useful to determine the velocities to calibrate seismic and well velocities while drilling. This allows us to better locate the drill bit in time on the seismic section; to redirect the well, if necessary; and to gather as much further information as possible on the rocks ahead of the bit.
Anisotropic approximations for mudrocks: A seismic laboratory study
Authors: Jakobsen Morten , Johansen Tor Arne
Published: October 2002

An experimental technique for determining the in‐situ elastic properties of mudrocks with (horizontal) alignments in the microstructure is used to study the accuracy of a set of three nested scalar anisotropic approximations for transversely isotropic (TI) media. Each subsequent approximation adds one more velocity parameter and includes the previous as a special case. These approximations are convenient and robust because of their close relationship to standard geophysical measurements. There exists no good theory to predict the effects of an imposed stress on the elasticity of mudrocks. In this study, the tensor of elastic moduli of a single test specimen of mudrock subjected to an anisotropic stress field is determined from ultrasonic group velocity measurements involving pointlike transducers. The mechanical characterization (performed at constant pore pressure) is accompanied by detailed microscopic observations and analysis. The method was used to obtain accurate elastic constants for five well‐defined mudrocks with different degrees of anisotropy. These elastic constants were then used to study the accuracy of the three anisotropic approximations. We have found that mudrocks can be significantly anelliptic. The agreement between the second anisotropic approximation and the exact phase velocity was found to be very good for qP-waves. For qSV-waves agreement was not as good, and the third anisotropic approximation is required to obtain reasonable accuracy.
Quasi‐analytical approximations and series in electromagnetic modeling
Authors: Zhdanov Michael S. , Dmitriev Vladimir I. , Fang Sheng , Hursán Gábor
Published: October 2002

The quasi‐linear approximation for electromagnetic forward modeling is based on the assumption that the anomalous electrical field within an inhomogeneous domain is linearly proportional to the background (normal) field through an electrical reflectivity tensor λ⁁. In the original formulation of the quasi‐linear approximation, λ⁁ was determined by solving a minimization problem based on an integral equation for the scattering currents. This approach is much less time‐consuming than the full integral equation method; however, it still requires solution of the corresponding system of linear equations. In this paper, we present a new approach to the approximate solution of the integral equation using λ⁁ through construction of quasi‐analytical expressions for the anomalous electromagnetic field for 3-D and 2-D models. Quasi‐analytical solutions reduce dramatically the computational effort related to forward electromagnetic modeling of inhomogeneous geoelectrical structures. In the last sections of this paper, we extend the quasi‐analytical method using iterations and develop higher order approximations resulting in quasi‐analytical series which provide improved accuracy. Computation of these series is based on repetitive application of the given integral contraction operator, which insures rapid convergence to the correct result. Numerical studies demonstrate that quasi‐analytical series can be treated as a new powerful method of fast but rigorous forward modeling solution.
Advanced inversion methods for airborne electromagnetic exploration
Authors: Sengpiel Klaus‐Peter , Siemon Bernhard
Published: October 2002

Airborne electromagnetic (AEM) surveys can contribute substantially to geologic mapping and target identification if good‐quality multifrequency data are produced, properly evaluated, and displayed. A set of multifrequency EM data is transformed into a set of apparent resistivity ([Formula: see text]) and centroid depth ([Formula: see text]) values, which then are plotted as a sounding curve. These [Formula: see text] curves commonly provide a smoothed picture of the vertical resistivity distribution at the sounding site. We have developed and checked methods to enhance the sensitivity of sounding curves to vertical resistivity changes by using new definitions for apparent resistivity and centroid depth. One of these new sounding curves with enhanced sensitivity to vertical resistivity contrasts is plotted from [Formula: see text] [Formula: see text] values derived from differentiation of the [Formula: see text] curve with respect to the frequency f. This approach is similar to the Niblett‐Bostick transform used in magnetotellurics. It not only enhances vertical changes in resistivity but also increases the depth of investigation. Sounding curves can be calculated directly from EM survey data and can be used to generate a resistivity‐depth parasection. Based on such a section, it can be decided whether a Marquardt‐type inversion of the AEM data into a 1-D layered half‐space model is adequate. Each sounding curve can be transformed into an initial step model of resistivity as required for the Marquardt inversion. We have inverted data from sedimentary sequences with good results. For data from a dipping conducting layer and a dipping plate, we have found that the results depend on the right choice of the starting model, in which the number of layers should be large rather than too small. Complex resistivity structures, however, often are represented better by using the sounding‐curve results than with the parameters of a layered half‐space.
An application for removing cultural noise from aeromagnetic data
Authors: Muszala Stefan , Stoffa Paul L. , Lawver L. A.
Published: October 2002

A high‐resolution aeromagnetic survey collected over the North Slope of Alaska by World Geoscience was acquired in response to a need for highly detailed data in an area where traditional geophysical techniques are expensive and prohibitive (McConnell, 1995) (Figure 1). These data were recently released to the University of Texas’ Institute for Geophysics and provide a unique opportunity to investigate the problem of cultural noise suppression in aeromagnetic data. The data contain isolated magnetic anomalies that are presumably from the many drill platforms and their accompanying cultural objects such as buildings and pipe repositories (Figure 2). We present a new, automated method to reduce the amplitude of these cultural anomalies without affecting the magnetic signal from the surrounding geology.
VTI documented
Authors: Winterstein D. F. , De G. S.
Published: October 2002

Shear‐wave (S‐wave) seismic data indicate that rocks of McElroy oil field on the Central Basin Platform of West Texas are transversely isotropic with a vertical symmetry axis (VTI). Although geophysicists from time to time had anticipated or assumed that sedimentary rock was VTI, no one had shown that any sedimentary section actually was VTI. The proof at McElroy comes from a nine‐component, near‐offset vertical seismic profile (VSP) combined with a ring of 15 offset VSPs spaced about 24° apart at offsets of about 460 m. S-wave splitting at VSP frequencies was negligible for vertical propagation to a depth of 885 m but reached about 12 ms for nonvertical propagation from the offset source locations. Crossed‐dipole log data supported the VSP result for vertical propagation but found two layers of vertically birefringent rock at depth whose thicknesses were below VSP resolution.
Recent advances in seismic lithologic analysis
Authors: Castagna John P.
Published: October 2002

An objective of seismic analysis is to quantitatively extract lithology, porosity, and pore fluid content directly from seismic data. Rock physics provides the fundamental basis for seismic lithology determination. Beyond conventional poststack inversion, the most important seismic lithologic analysis tool is amplitude‐variation‐with‐offset (AVO) analysis. In this paper, I review recent progress in these two key aspects of seismic lithologic analysis.
Multicomponent seismology—The next wave
Authors: Davis Thomas L.
Published: October 2002

Multicomponent seismology requires recording of seismic data with three‐ and sometimes four‐component receivers. The three components measure displacement of the ground, usually in two horizontal and one vertical directions. The fourth component is a measurement of pressure, which is used in sea‐bottom surveys. Measuring three components of ground displacement enables the recording of compressional (P) and shear (S) waves which represent the full complement of “body” waves in seismology. Earthquake seismologists have been using the full complement for years to interpret the structure of our living planet; however, exploration seismologists have been slow to bring multicomponent seismology to the forefront of their measurement system. This is finally changing. Thanks to new seismic acquisition recording systems, it is now feasible to economically record multicomponent seismic data in both land and marine (sea‐bottom) settings. In the future, all land or sea‐bottom seismic data will be recorded by multicomponent technology, thereby bringing us the next wave of exploration geophysics as we begin to “see the unseen.”
Mise‐a‐la‐masse survey for an auriferous sulfide deposit
Authors: Bhattacharya B. B. , Gupta Dinesh , Banerjee Buddhadeb , Shalivahan
Published: October 2002

A mise‐a‐la‐masse survey was carried out in Bhukia area, Banswara district, Rajasthan, India for auriferous sulfide occurrences. This area was originally surveyed for copper mineralization. Exploratory drilling, however, proved it to be economically not viable. The area was reopened for geophysical surveys when grab samples indicated the presence of gold. Initial geophysical surveys for copper mineralization showed electromagnetic, induced polarization, and resistivity anomalies. At first, one borehole was drilled for gold exploration on the basis of initial geophysical surveys. It encountered massive sulfide mineralization in association with gold. Borehole logging and a mise‐a‐la‐masse survey were carried out in this borehole. Three further boreholes drilled on the basis of the mise‐a‐la‐masse results encountered massive sulfide mineralization in association with gold. One of the three boreholes, 100 m from the first borehole along strike, was used for another set of mise‐a‐la‐masse measurements. A composite equipotential map was prepared using the results of mise‐a‐la‐masse results of both the boreholes. The equipotential contours show a north‐northwest‐south‐southeast trend of mineralization. The boreholes drilled on the basis of the mise‐a‐la‐masse results have delineated a strike length of more than 500 m of gold‐bearing sulfide mineralization. The sulfide content ranges from 10 to 40% and gold concentration ranges from 2 to 6 ppm. The dip and plunge of the lode, as anticipated from the mise‐a‐la‐masse results, are toward the west and north, respectively. Mise‐a‐la‐masse surveys are continuing in the adjoining areas.
Kirchhoff imaging beyond aliasing
Authors: Biondi Biondo
Published: October 2002

Crucial image resolution may be lost when spatially aliased data are imaged with Kirchhoff algorithms that employ standard antialiasing methods. To maximize resolution, I introduce a method that enables the proper imaging of some aliased components in the data, while avoiding aliasing artifacts. The proposed method is based on a detailed analysis of the different types of aliasing that affect Kirchhoff imaging. In particular, it is based on the observation that operator aliasing depends on the dip spectrum of the data. A priori knowledge on the characteristics of the dip spectrum of the data, in particular on its asymmetry, can thus be exploited to enable “imaging beyond aliasing.” The method is not of general applicability, but it successfully improves the image resolution when a priori assumptions on the data dips are realistic. The imaging of salt‐dome flanks in the Gulf of Mexico has been enhanced by the application of the proposed method.
Reply by the authors to the discussion by Hocine Tabti, Leiv‐J. Gelius, and Isabelle Lecomte
Authors: Chen Jing , Schuster Gerard T.
Published: October 2002

Different survey geometries give different resolution limits of migrated images, but limitations on paper length restricted our range of examples. Equation (18) in our original paper applies to a fixed source and receiver distribution, such as the one performed by Morey and Schuster (1999) in their 3-D imaging experiment. However, Tabti et al.’s request for the moving array formula is excellent, so the derivation is given below for a moving receiver array.
Depth‐domain velocity analysis in VTI media using surfaceP-wave data: Is it feasible?
Authors: Stunff Yves Le , Grechka Vladimir , Tsvankin Ilya
Published: October 2002

The main difficulties in anisotropic velocity analysis and inversion using surface seismic data are associated with the multiparameter nature of the problem and inherent trade‐offs between the model parameters. For the most common anisotropic model, transverse isotropy with a vertical symmetry axis (VTI media), P-wave kinematic signatures are controlled by the vertical velocity V0and the anisotropic parameters ε and δ. However, only two combinations of these parameters—NMO velocity from a horizontal reflector Vnmo(0) and the anellipticity coefficient η—can be determined from P-wave reflection traveltimes if the medium above the reflector is laterally homogeneous. While Vnmo(0) and η are sufficient for time‐domain imaging in VTI media, they cannot be used to resolve the vertical velocity and build velocity models needed for depth migration. Here, we demonstrate that P-wave reflection data can be inverted for all three relevant VTI parameters (V0, ε and δ) if the model contains nonhorizontal intermediate interfaces. Using anisotropic reflection tomography, we carry out parameter estimation for a two‐layer medium with a curved intermediate interface and reconstruct the correct anisotropic depth model. To explain the success of this inversion procedure, we present an analytic study of reflection traveltimes for this model and show that the information about the vertical velocity and reflector depth was contained in the reflected rays which crossed the dipping intermediate interface. The results of this work are especially encouraging because the need for depth imaging (such as prestack depth migration) arises mostly in laterally heterogeneous media. Still, we restricted this study to a relatively simple model and constrained the inversion by assuming that one of the layers is isotropic. In general, although lateral heterogeneity does create a dependence of P-wave reflection traveltimes on the vertical velocity, there is no guarantee that for more complicated models all anisotropic parameters can be resolved in a unique fashion.
On: “A new insight into the reciprocity principle” (Børge Arntsen and José M. Carcione, GEOPHYSICS, 65, 1604–1612).
Authors: de Hoop Adrianus T.
Published: October 2002

Reciprocity is an important property of elastodynamic, electromagnetic, and acoustic wavefields. Combined with optimization techniques, reciprocity theorems can be regarded as providing the basic ingredients to imaging and inversion methods in geophysical exploration and remote sensing (de Hoop, M. V. and de Hoop, A. T., 2000). Furthermore, reciprocity serves as the basis for the elimination procedures of surface‐related multiples in marine seismic data processing (Fokkema and van den Berg, 1993). In view of all this, a thorough and elucidating discussion on the configurations to and the conditions under which reciprocity applies, and what reciprocity leads to, like the recent paper by Arnsten and Carcione (2000), serves a useful purpose. In this paper, also a number of interesting applications are worked out in detail. The aim of this discussion is to indicate briefly how some of the results obtained in that paper are related to the ones that the present author has presented in de Hoop (1995), a reference that does not appear in Arntsen and Carcione (2000).
SIMPLIFICATION OF TIDAL CORRECTIONS FOR GRAVITY METER SURVEYS
Authors: Adler Joseph L.
Published: October 2002

Recent papers have called attention to the considerable effect of lunar and solar tidal forces on the vertical component of gravity but have given no instructions for the computation of this effect. Material published in English on this subject, so far as the author has been able to discover, does not present a rapid, practical method of computation which could be used economically as a matter of routine in gravimeter work. In this paper he attempts to analyze the degree of accuracy required in these computations, and develops a chart from which the tidal correction for any gravimeter obervation in the United States can be obtained directly in less than a minute with an average error of less than 0.01 milligal.
REFLECTED REFRACTIONS
Authors: Swartz C. A. , Lindsey R. W.
Published: October 2002

A number of very steep reflections were observed while making a seismograph survey of a salt dome. A time plot of events recorded along a typical profile is shown. The steep reflections came both from the sides of the dome material and from the adjacent upturned sedimentary strata. A particular event of very infrequent occurrence, a reflection from the vertical side of the dome traveling essentially horizontally along a refracted trajectory, is discussed in detail.
INTERPRETATION OF THE TRANSIENT BEHAVIOR OF THE REFLECTION SEISMOGRAPH
Authors: Piety R. G.
Published: October 2002

This paper discusses a method of design for the reflection seismograph based on the galvanometer response when the geophone is subject to a unit impulse. A graphical method of obtaining the actual response to an arbitrary geophone motion in terms of the impulsive response is given. A family of desirable types of impulsive response is obtained by analyzing the implications of this graphical computation when strong low and high frequency interference is present. The application of this method of computation to the evaluation of corrections in the apparent arrival time of reflections obtained with different instruments is outlined.
PROBLEMS IN TEMPERATURE CONTROL OF GRAVIMETERS
Authors: Clewell Dayton H.
Published: October 2002

The theoretical considerations entering into the design of thermostatically controlled ovens for gravimeters are presented. The author points out that to obtain the most uniform temperature distribution without the use of excessive amounts of material, plural thermostat control points on the heater element are desirable. To reduce long‐time fluctuations it is recommended that dual thermostating be employed in which an inner controlled‐shell is surrounded by an outer thermostated layer held at an approximately constant temperature several degrees lower than the temperature of the inner shell.
AN ANALYTICAL METHOD OF MAKING WEATHERING CORRECTIONS
Authors: Dobrin M. B.
Published: October 2002

A method of weathering is described by which intercept times can be rapidly and accurately computed from first arrival times without the plotting of time‐distance curves. The velocities are determined by a mechanical procedure, based on least squares theory, which normally requires no exercise of judgment on the part of the computer. The application of the method to actual field set‐ups is illustrated by sample calculations.
Discussion on “An Analysis of Abnormal Reflexions”
Authors: Dix Hewitt
Published: October 2002

We have observed this kind of effect on several occasions both in California and in the Mid‐Continent area. Although the entire record referred to in Fig. 2 as “seismogram from down thrown side of fault” cannot be seen completely, it appears that there is at least a suggestion of a reflection following “reflection from fault plane” about 0.130 seconds later and parallel with the “fault” reflection. If this second event is verified by inspection of the record, then either secondary shot disturbance or a reflected refraction involving two types of waves is required. The two types of waves might be both compressional from two different fault planes, or they might be compressional and shear waves from the same fault plane. The shear waves to be expected are those generated by the refraction process. The separation of the two reflections would then depend on the Poisson ratio and the length of path in the material traversed by both compressional and shear waves. Reference might well be made in this instance to the tables included in the paper of Muskat and Meres (Geophysics, Vol. V, pp. 115–148, 1940) to get a rough idea of the relative amplitudes to be expected.
ASSOCIATION OF MAGNETIC AND DENSITY CONTRASTS WITH IGNEOUS ROCK CLASSIFICATIONS
Authors: Nettleton L. L. , Elkins T. A.
Published: October 2002

The frequent co‐existence of strong magnetic and gravity anomalies directs attention to the properties of igneous rocks which may account for them. Density data are direct and unambiguous but magnetic data are much less certain. On the assumption that it is determined by disseminated magnetite, the susceptibility can be estimated from the proportion of magnetite in the rock. The possibility of estimating magnetite content from chemical analyses is examined and while not very satisfactory, it probably gives relative values of some merit. From published chemical analyses the magnetite content and probable intensity of magnetization are calculated for some 1450 samples of igneous rocks, for which density values are also given. The values and resulting contrasts are analyzed statistically and tabulated for the C.I.P.W. and the Iddings methods of classification. The contrasts, thus determined, are in reasonable agreement with expectations from the analysis of magnetometer and gravimeter surveys.
Seismic monitoring of fluid fronts: An experimental study
Authors: Wulff Angelika‐M. , Mjaaland Svein
Published: October 2002

Seismic signatures of time‐dependent reservoir processes, necessary for the interpretation of 4‐D seismic data, are still insufficiently described. This experiment was designed to monitor fluid‐front movements and saturation changes and to identify the related seismic signatures.

Ultrasonic P‐ and S‐wave transmission and reflection measurements were used to monitor the waterflooding of a porous sandstone. The sandstone was flooded in steps by filling a tank in which the room‐dry cubic (50‐cm side) block of rock was placed.

Waterflooding caused the velocity, amplitude, and frequency of the transmitted waves to diminish significantly; however, the changes were reversible by drying. The maximum reduction of the velocities was 7% and 12% for P‐ and S‐waves, respectively. The velocity and amplitude behavior can be explained by the Biot‐Gassmann's theory, local fluid flow, and grain‐surface effects. The correct interpretation of seismic signatures of fluid processes in reservoirs thus involves a knowledge of rock physical relations and attenuation mechanisms.

Even at small saturations, reflections from the block bottom were strongly attenuated, but those from the upgoing water front could be monitored. The latter reflections were best observed in differential seismic traces, confirming that seismic monitoring can observe moving fronts directly.
Biot slow‐wave effects in stratified rock
Authors: Pride Steven R. , Tromeur Eric , G. Berryman James
Published: October 2002

The transmission of P‐waves through the stratified layers of a sedimentary basin is modeled numerically using Biot theory. The effects on the transmissivity of frequency, angle of incidence, layer thickness, permeability and elastic compliance of the rocks are all considered. Consistent with previous analytical work, it is found that the equilibration of fluid pressure between the fine layers of a sedimentary sequence can produce significant P‐wave attenuation at low frequencies. For this attenuation mechanism to act within the surface‐seismic band (say, 3–300 Hz), we find that there must be layering present at the scale of centimeters to tens of centimeters. If the layering is restricted to layers of roughly 1 m thickness or greater, then for typical sandstone formations, the attenuation caused by the interlayer flow occurs below the seismic band of interest. Such low‐frequency interlayer flow is called Biot slow‐wave diffusion in the context of Biot theory and is likely to be the dominant source of low‐frequency attenuation in a sedimentary basin, even for relatively tight and stiff reservoir rock; however, the effect is enhanced in more compliant materials. At higher frequencies, the generation of slow‐waves at interfaces is also shown to significantly affect the P‐wave scattering so long as the layers are sufficiently thin and sufficiently compliant. This effect on the P‐wave scattering is shown to increase with increasing angle of incidence. Our work is limited to performing numerical experiments, with care given to making realistic estimates of all the material properties required. No attempt is made here to define an equivalent viscoelastic solid that allows for such slow‐wave effects.
Atmospheric sources for audio-magnetotelluric (AMT) sounding
Authors: Garcia Xavier , Jones Alan G.
Published: October 2002

The energy sources for natural-source magnetotelluric (MT) frequencies > 1 Hz are electromagnetic (EM) waves caused by distant lightning storms and which propagate within the Earth–ionosphere waveguide. The properties of this waveguide display diurnal, seasonal, and 11-year solar-cycle fluctuations, and these temporal fluctuations cause significant signal amplitude attenuation variations—especially at frequencies in the 1- to 5-kHz so-called audiomagnetotelluric (AMT) dead band. In the northern hemisphere these variations increase in amplitude during the nighttime and the summer months, and they correspondingly decrease during the daytime and the winter months. Thus, one problem associated with applying the AMT method for shallow (<3 km) exploration can be the lack of signal in certain frequency bands during the desired acquisition interval. In this paper we analyze the time variations of high-frequency EM fields to assess the limitations of the efficient applicability of the AMT method. We demonstrate that magnetic field sensors need to become two orders of magnitude more sensitive than they are currently to acquire an adequate signal at all times. We present a proposal for improving AMT acquisition involving continuous profiling of the telluric field only during the daytime and AMT acquisition at a few base stations through the night.