bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of ray and Fourier methods for modeling monostatic ground‐penetrating radar profiles
Authors: Zeng Xiaoxian , McMechan George A. , Cai Jun , Chen How‐Wei
Published: October 2002

With increasing emphasis on shallow, high‐resolution geophysical techniques for environmental and engineering applications, it has become important to implement and evaluate tools for quantitative interpretation of ground‐penetrating radar (GPR) data. Both ray and Fourier algorithms are viable for numerical simulation of 2-D monostatic GPR data, but they have different characteristics. The ray algorithm uses geometrically complicated layers, where within each the dielectric permittivity and attenuation are constant. The algorithm produces accurate amplitudes for reflection but does not include wave effects such as diffractions from layer truncations. The Fourier algorithm uses a gridded parameterization in which reflections are constructed by superposition of diffractions in a background of constant dielectric permittivity and constant attenuation. This technique includes all wave effects, but it does not contain the antenna directivities. Both algorithms are able to simulate the main features in two representative, but very different, field data sets: one from a Quaternary fluvial/aeolian environment and one from a Cretaceous marine carbonate environment.
Multicomponent 3-D characterization of a coalbed methane reservoir
Authors: Shuck Edward L. , Davis Thomas L. , Benson Robert D.
Published: October 2002

Methane is produced from fractured coalbed reservoirs at Cedar Hill Field in the San Juan Basin. Fracturing and local stress are critical to production because of the absence of matrix permeability in the coals. Knowledge of the direction of open fractures, the degree of fracturing, reservoir pressure, and compartmentalization is required to understand the flow of fluids through the reservoir. A multicomponent 3-D seismic survey was acquired to aid in coalbed methane reservoir characterization. Coalbed reservoir heterogeneities, including isolated pressure cells, zones of increased fracture density, and variable fracture directions, have been interpreted through the analysis of the multicomponent data and integration with petrophysical and reservoir engineering studies. Strike‐slip faults, which compartmentalize the reservoir, have been identified by structural interpretation of the 3-D P‐wave seismic data. These faults form boundaries for pressure cells that have been identified by P‐wave reflection amplitude anomalies. The analysis of polarizations, traveltimes, and reflection amplitudes from the shear‐wave seismic data has allowed the identification of zones of variable fracture direction and fracture density. There is good agreement between stresses inferred from the structural interpretation and those indicated by the shear‐wave polarizations. Reflection amplitudes have been calibrated to seismic velocities and reservoir pressures through the use of petrophysical data taken from core samples. New methods have been developed for the statistical analysis of prestack shear‐wave polarizations, poststack polarizations, and the accurate determination of traveltime anisotropy. The prestack polarization analysis method allows for rapid and efficient determination of a dominant polarization direction. Shear‐wave anisotropy has been quantified over the reservoir zone using both traveltime and thin‐bed reflection response with excellent agreement between the two methods. Crack densities computed from the anisotropy show two regions of high crack density, one coinciding with a sealed overpressured cell and the other in the region of the Hamilton ♯3 well. This indicates the potential for monitoring production of coalbed methane reservoirs using multicomponent seismology.
High‐resolution detection of geologic boundaries from potential‐field anomalies: An enhanced analytic signal technique
Authors: Hsu Shu‐Kun , Sibuet Jean‐Claude , Shyu Chuen‐Tien
Published: October 2002

A high‐resolution technique is developed to image geologic boundaries such as contacts and faults. The outlines of the geologic boundaries can be determined by tracing the maximum amplitudes of an enhanced analytic signal composed of the nth‐order vertical derivative values of two horizontal gradients and one vertical gradient. The locations of the maximum amplitudes are independent of the ambient potential field. This technique is particularly suitable when interference effects are considerable and/or when both induced and remanent magnetizations are not negligible. The corresponding depth to each geologic boundary can be estimated from the amplitude ratio of the enhanced and the simple analytic signals, which provides a simple estimation Such a method has been applied to magnetic data acquired in the Ilan Plain of Taiwan located at the southwestern end of the Okinawa Trough. The quantitative analysis shows that the underlying geologic boundaries deepen southward and slightly eastward. The enlargement of the Ilan Plain in the direction of the Okinawa Trough and the existence of north‐northwest and west‐northwest trending faults near the city of Ilan reveal a discontinuity between the Okinawa Trough backarc extension and the compressional process in Taiwan.
An optimal 9‐point, finite‐difference, frequency‐space, 2-D scalar wave extrapolator
Authors: Jo Churl‐Hyun , Shin Changsoo , Suh Jung Hee
Published: October 2002

In this study, a new finite‐difference technique is designed to reduce the number of grid points needed in frequency‐space domain modeling. The new algorithm uses optimal nine‐point operators for the approximation of the Laplacian and the mass acceleration terms. The coefficients can be found by using the steepest descent method so that the best normalized phase curves can be obtained. This method reduces the number of grid points per wavelength to 4 or less, with consequent reductions of computer memory and CPU time that are factors of tens less than those involved in the conventional second‐order approximation formula when a band type solver is used on a scalar machine.
The 1949 algorithm for the rapid interpretation of aeromagnetic data
Authors: Steenland Nelson C.
Published: October 2002

In the early 1920s, magnetic anomalies were thought to be caused by topographic relief at the base of the sedimentary section. The drilling of the Hobbs anomaly on the Central Basin Platform in West Texas was successful, but the anomaly, primarily intrabasement in origin, could not quantitatively be related to the structure that was found. Moreover the drilling of similar anomalies in the general area proved unsuccessful as far as oil and gas were concerned, so the method acquired a bad reputation rather quickly (Heiland, 1940).
A limitation of well velocity surveys in highly deviated wells drilled parallel to bedding
Authors: Smidt Jesper M.
Published: October 2002

Well velocity surveys (or check‐shot surveys) in vertical or moderately deviated wells provide average velocities and formation interval velocities by assuming a horizontally layered subsurface. The interval velocity, [Formula: see text], is calculated using the simple formula, [Formula: see text]where DZ is the vertical depth interval between two consecutive well geophone locations, [Formula: see text] and [Formula: see text], as shown in Figure 1a, and DT, the difference in the seismic traveltime from a source S at the wellhead to these two locations (Telford et al., 1976, 347). This quantity is the average interval velocity of Al‐Chalabi (1974) (see also Sheriff, 1991, 317). Vertical incidence of the seismic energy at the geophone is assumed, i.e., the source is located vertically above the geophone location. This assumption is made throughout this note; it would only serve to confuse the issue to correct for the usually small horizontal offset between source and receiver locations. I also assume throughout this note that no refraction takes place at any layer interface. Since only two‐layer cases are dealt with, this assumption is hardly a serious limitation.
Dewarping the composite aeromagnetic anomaly map of Australia using control traverses and base stations
Authors: Tarlowski C. , McEwin A. J. , Reeves C. V. , Barton C. E.
Published: October 2002

The composite aeromagnetic anomaly map of Australia, like similar maps of other continent‐size regions, contains spurious trends, warps, and base‐level shifts. These arise from the arbitrary nature of aeromagnetic data processing methods and edge matching of disparate surveys. Corrections have been applied to the Australian map based on long aeromagnetic control traverses flown around the country in January and February 1990. The control traverses were timed to coincide with a continent‐wide magnetometer array study, thus enabling accurate corrections to be made for transient (diurnal) variations of the geomagnetic field. Magnetic observatory data spanning several months each side of the traverse interval were used to correct for long‐term displacement of the total field from its normal value. The transient contribution to total field was modeled as a set of one‐minute time slices over the whole country, and then used to reduce the control‐traverse data to normal values. Absolute anomaly values were generated along traverses by subtraction of IGRF 1990, then compared with corresponding anomalies given by the uncorrected composite map. The resulting anomaly difference values were interpolated and extrapolated over the whole country and used to relevel the composite magnetic anomaly map. The corrections applied range typically between −200 and +200 nT, with a maximum of about +400 nT in the far western region and a minimum of about −450 nT in South Australia. These corrections are smaller than expected, but are still large compared with the amplitude of long‐wavelength magnetic anomalies.
Wavefronts in dissipative anisotropic media: Comparison of the plane‐wave theory with numerical modeling
Authors: Carcione José M. , Quiroga‐Goode Gerardo , Cavallini Fabio
Published: October 2002

In a previous work, Carcione investigated the characteristics of wavefronts in a dissipative anisotropic (orthorhombic) medium by means of time‐harmonic, homogeneous, plane‐wave analysis. Here, we extend the theoretical analysis to a monoclinic medium and perform numerical experiments to investigate the transient response. Given a line source the forward modeling algorithm computes pure shear waves in the symmetry plane of the medium. The results of the theoretical analysis agree with those of our transient wave simulation. For instance, the onset of the wave field coincides with the theoretical wavefront calculated with the unrelaxed (high‐frequency limit) energy velocity. Moreover, while the wavefronts computed with the theoretical envelope and energy velocities coincide practically with a numerical evaluation of the energy location, using the group velocity yields a wrong prediction. Finally, as an interesting example, we present a medium where the wave behaves isotropically (cylindrical wavefronts) at a given frequency, but is strongly anisotropic at the low‐ and high‐frequency limits.
Electromagnetic velocity inversion using 2-D Maxwell’s equations
Authors: Cai Wenying , Qin Fuhao , Schuster Gerard T.
Published: October 2002

We adapt the wave‐equation traveltime inversion (WT) method to the reconstruction of the dielectric distribution from crosswell radar traveltime data. The data misfit gradient is computed using finite‐difference solutions to the 2-D Maxwell’s equations. An advantage of the wave‐equation method over ray‐tracing radar tomography is that it accounts for scattering and diffusion effects and works well in both resistive and moderately conductive rocks. Comparisons with ray‐tracing tomography show that the wave equation method is more robust and accurate when the rock conductivity is larger than .002 S/m. The methods are about equally effective when the conductivity is less than or equal to .001 S/m. The major disadvantage of the wave equation scheme is that it generally requires at least several orders of magnitude more computational time than ray tracing. We also derive the general equation for the waveform radar inversion method, which is closely related to the equations for the WT method and prestack radar migration.
Simulation of ground‐penetrating radar waves in a 2-D soil model
Authors: Casper David A. , Kung K ‐J. Samuel
Published: October 2002

We have developed a pseudospectral forward modeling algorithm for ground‐penetrating radar (GPR) based on an explicit solution of the 2-D lossy electromagnetic wave equation. Complex soil structures can be accommodated with heterogeneous spatial distributions of both wave velocity and electrical conductivity. This algorithm uses a Gaussian line source with uniform directivity, and there are conductive buffer regions surrounding the soil model to approximate absorbing boundary conditions. Three soil models are used to illustrate different aspects of radar wave propagation. The first model is lossless with homogeneous layers imbedded in a homogeneous background medium, the second model has the same lossless layers in a lossy background medium, and the third model is lossless and uses a nonsaturated water flow simulation to create a complex spatial velocity distribution. Two separate simulations with different source frequencies are presented for each soil model. Results indicate that higher frequency GPR will produce a sharper wavelet and can map soil layering structures with high resolution. In a conductive soil, however, higher frequencies attenuate more rapidly and the radar may not detect deeper layers.
Tomographic estimation of the spatial distribution of statics
Authors: Rajasekaran Sara , McMechan George A.
Published: October 2002

Nonuniqueness associated with statics estimation using reflection or refraction data alone can be largely overcome by using both data types together. The use of both reflection and refraction data has the effect of increasing the effective aperture of the observations, and hence generating better constrained velocity estimations. A tomographic inversion for velocity allows direct incorporation of both data types and topography in a single formulation. The resulting velocity distribution can be the basis of static corrections for all angles of propagation.
Conservative modeling of 3-D electromagnetic fields, Part I: Properties and error analysis
Authors: Smith J. Torquil
Published: October 2002

Conservation of electric current and magnetic flux can be explicitly enforced by modeling Maxwell’s equations on a staggered grid, where the different field components are sampled at points offset relative to each other. A staggered finite‐difference (SFD) approximation gives divergence‐free magnetic fields and electric currents ensuring good behavior at all periods. Comparisons of SFD solutions with 2-D quasi‐analytic solutions are very good (∼1% rms error). When a modeled region can be subdivided into uniform subdomains, comparison of analytic solutions and SFD approximations show that the greatest differences occur near the Nyquist wavenumbers; the SFD solutions do not attenuate in space as rapidly as the analytic solutions. The accuracy of a computed SFD solution can be estimated from its wavenumber content. For test cases the accuracy estimates are surprisingly close to the actual accuracies. Grid requirements for modeling short horizontal wavelength components of a solution seem more demanding than for modeling the infinite wavelength (plane‐wave) component: for 2% accuracy 4π samples are required per horizontal wavelength compared to two samples per skin depth for the plane‐wave component.
Wavelet transform inversion with prior scale information
Authors: Li Xin-Gong , Sacchi Mauricio D. , Ulrych Tadeusz J.
Published: October 2002

The concept of scale is of central importance in all problems related to the interpretation of earth data. Inversion of geophysical data does not at present, however, allow the incorporation of scale‐dependent information directly. This article presents an application of the wavelet transform to traveltime inversion where we are specifically concerned with the selection and application of a priori scale information in the wavelet domain to reconstruct tomographic models. Because of the nature of the problem, the wavelet basis chosen for this example is the Haar basis. The results are compared with the classic damped least‐squares (DLS) method, which is a special case of the wavelet transform approach when all the scales are treated equally. We show that with scale‐dependent prior information, most of the artifacts created by the DLS technique are diminished considerably.
Prestack migration by split‐step DSR
Authors: Popovici Alexander Mihai
Published: October 2002

The double‐square‐root (DSR) prestack migration equation, though defined for depth variable velocity, can be used to image media with strong velocity variations using a phase‐shift plus interpolation (PSPI) or split‐step correction. The split‐step method is based on applying a phase‐shift correction to the extrapolated wavefield—a correction that attempts to compensate for the lateral velocity variations. I show how to extend DSR prestack migration to lateral velocity media and exemplify the method by applying the new algorithm to the Marmousi data set. The split‐step DSR migration is very fast and offers excellent imaging results.
The fluid factor as an AVO indicator
Authors: Smith George C. , Sutherland Robin A.
Published: October 2002

Castagna and Smith (1994) presented a very interesting comparison of some of the different amplitude variation with offset (AVO) indicators used in the industry today. They used a worldwide sample of data to compute their examples. In their paper, they alluded to the “fluid factor” of Smith and Gidlow (1987) but they did not compute this attribute for their data. In the following, we suggest how the fluid factor can be computed, and compare the results with the [Formula: see text] and (A + B)/2 techniques. We have used Castagna and Smith’s (1994) notation.
Removal of scattered surface waves using multicomponent seismic data
Authors: Blonk Bastian , Herman Gérard C.
Published: October 2002

In many exploration areas, the shallow subsurface is strongly heterogeneous. The heterogeneities can give rise to scattering of surface waves. These scattered waves can depreciate the quality of land seismic data when they mask the body‐wave reflections from the deeper part of the subsurface. Surface waves scattered near a line of receivers (inline‐scattered waves) can be removed by well‐known filtering techniques (see e.g., Yilmaz, 1987, section 1.6.2). However, surface waves scattered far from the receiver line (crossline‐scattered waves) are left intact partially by filtering because these waves can resemble body‐wave reflections. In previous papers, we have discussed an inverse scattering method for removing scattered surface waves from simulated data (Blonk and Herman, 1994), as well as from field data (Blonk et al., 1995). So far, we have limited our attention to the vertical components of the particle velocity which implies that surface waves and body‐wave reflections can be distinguished on the basis of their respective differences in phase velocity.
Shooting direction and crosswell seismic data acquisition
Authors: Liner Christopher L. , Bozkurt Gokay , Cox V. Dale
Published: October 2002

Four crosswell seismic surveys were acquired in the Glenn Pool Field of northeastern Oklahoma as part of a multidisciplinary reservoir characterization project. The acquisition goal was to generate data suitable for tomographic traveltime inversion. Acquisition parameters and shooting geometry were selected by conducting a parameter test at the site. Following the parameter test, the first survey resulted in high quality data showing clear first arrivals, low ambient noise, some reflection events, and strong source‐generated tube waves. The second survey involved a different receiver well and encountered high ambient noise levels. The noise was strong enough to prohibit first‐arrival picking for much of the data. On‐site analysis of the second survey revealed tube waves emanating from a perforated interval in the receiver well. This well was shut in and was not flowing fluid or gas at the surface. We interpret the source of ambient tube waves as borehole‐to‐formation fluid flow (circulation) associated with the perforations. Since this image plane was important for characterization of the reservoir, the survey was reshot (third survey) by reversing sources and receivers in the two wells. The resulting high‐quality data indicates that shooting direction can be an important factor in crosswell seismic acquisition. This experience influenced acquisition of a previously planned fourth survey so that the ambient noise problem would be avoided.
3-D seismic reflection tomography on top of the GOCAD depth modeler
Authors: Guiziou Jean Luc , Mallet Jean Laurent , Madariaga Raül
Published: October 2002

The estimation of velocity macro‐models by seismic reflection tomography is studied in three‐dimensions. Inversion techniques based on the kinematics of seismic data require an appropriate parameterization of the geological macro‐model, in particular as far as the velocity field is concerned. The step toward structurally complex geological models is achieved by exploiting a new approach to 3-D depth modeling: GOCAD. The peculiarities inherent to GOCAD triangulated surfaces and its associated discrete smooth interpolator (DSI) have led to the development of original ray‐tracing techniques. By exploiting intensively the topology of the triangulated surfaces, these new algorithms make it possible to reach a good balance between accuracy and computation performance. To build a 3-D macro‐model estimation tool, ray‐tracing is then associated with a least‐squares inversion of depth parameters and velocity parameters from 3-D zero‐offset traveltimes and stacking velocities, or multi‐offset prestack traveltimes from 2-D seismic lines.
Wave tilt of radio waves propagating over a layered ground
Authors: Wait James R. , Nabulsi Khalid A.
Published: October 2002

We call attention here to the limitation of the “Q approximation” in characterizing the complex wave tilt W over a layered half‐space. Using an improved formulation the frequency dependence of W is shown to be a discriminator of the electrical characteristic of the subsurface. The height dependence of the observer is also illustrated.
Ground‐penetrating radar: Wave theory and numerical simulation in lossy anisotropic media
Authors: Carcione José M.
Published: October 2002

Subsurface georadar is a high‐resolution technique based on the propagation of high‐frequency radio waves. Modeling radio waves in a realistic medium requires the simulation of the complete wavefield and the correct description of the petrophysical properties, such as conductivity and dielectric relaxation. Here, the theory is developed for 2-D transverse magnetic (TM) waves, with a different relaxation function associated to each principal permittivity and conductivity component. In this way, the wave characteristics (e.g., wavefront and attenuation) are anisotropic and have a general frequency dependence. These characteristics are investigated through a plane‐wave analysis that gives the expressions of measurable quantities such as the quality factor and the energy velocity. The numerical solution for arbitrary heterogeneous media is obtained by a grid method that uses a time‐splitting algorithm to circumvent the stiffness of the differential equations. The modeling correctly reproduces the amplitude and the wavefront shape predicted by the plane‐wave analysis for homogeneous media, confirming, in this way, both the theoretical analysis and the numerical algorithm. Finally, the modeling is applied to the evaluation of the electromagnetic response of contaminant pools in a sand aquifer. The results indicate the degree of resolution (radar frequency) necessary to identify the pools and the differences between the anisotropic and isotropic radargrams versus the source‐receiver distance.
Swept impact seismic technique (SIST)
Authors: Park Choon Byong , Miller Richard D. , Steeples Don W. , Black Ross A.
Published: October 2002

A coded seismic technique is developed that can result in a higher signal‐to‐noise ratio than a conventional single‐pulse method does. The technique is cost‐effective and time‐efficient and therefore well suited for shallow‐reflection surveys where high resolution and cost‐effectiveness are critical. A low‐power impact source transmits a few to several hundred high‐frequency broad‐band seismic pulses during several seconds of recording time according to a deterministic coding scheme. The coding scheme consists of a time‐encoded impact sequence in which the rate of impact (cycles/s) changes linearly with time providing a broad range of impact rates. Impact times used during the decoding process are recorded on one channel of the seismograph. The coding concept combines the vibroseis swept‐frequency and the Mini‐Sosie random impact concepts. The swept‐frequency concept greatly improves the suppression of correlation noise with much fewer impacts than normally used in the Mini‐Sosie technique. The impact concept makes the technique simple and efficient in generating high‐resolution seismic data especially in the presence of noise. The transfer function of the impact sequence simulates a low‐cut filter with the cutoff frequency the same as the lowest impact rate. This property can be used to attenuate low‐frequency ground‐roll noise without using an analog low‐cut filter or a spatial source (or receiver) array as is necessary with a conventional single‐pulse method. Because of the discontinuous coding scheme, the decoding process is accomplished by a “shift‐and‐stacking” method that is much simpler and quicker than cross‐correlation. The simplicity of the coding allows the mechanical design of the source to remain simple. Several different types of mechanical systems could be adapted to generate a linear impact sweep. In addition, the simplicity of the coding also allows the technique to be used with conventional acquisition systems, with only minor modifications.
Minimum traveltime calculation in 3-D graph theory
Authors: Cheng Ningya , House Leigh
Published: October 2002

Traveltime calculation is a crucial part of seismic migration schemes, especially prestack migration. There are many different ways to compute traveltimes. These methods can be divided into three categories: (1) Ray tracing (Julian and Gubbins, 1977; Červený et al., 1977). These treat the problem as a initial value problem by shooting rays from the source to the receivers. Or they can also treat the problem as a two‐point boundary value problem. An initial raypath is bent using perturbation theory until Fermat’s principle is satisfied. Nichols (1994) also computed traveltimes with the amplitude information attached to it in two dimensions. (2) Finite‐difference methods (Reshel and Kosloff, 1986; Vidale, 1988; van Trier and Symes, 1991). These solve the eikonal equation directly by using different numerical schemes such as the Runge‐Kutta method, wavefront expansion, or upwind finite difference. (3) Graph theory (Moser, 1991; Fisher and Lees, 1993; Meng et al. 1994). This method recasts the traveltime problem into a shortest path search over a network, which is constructed from the velocity model. This method is guaranteed to find a stable minimum traveltime with any velocity model.
Imaging in 3-D DMO, Part II: Amplitude effects
Authors: Collins Charles L.
Published: October 2002

The amplitude of the dip‐moveout (DMO) output for a planar reflector at a point P and half offset k is a function of the curvature of the midpoint curve of the DMO input traces, the dip magnitude, and the dip direction. No single weighting function will preserve the amplitude of dipping events. Seismic synthetics on a circle of midpoints minimal data set verify that this data set has 6 dB of amplitude variation with azimuth for a 15° dipping reflector and a 14 dB variation for a 45° reflector. The theory shows that the 3DDMO weight for an arbitrary minimal data set is equal to the impulse response weight multiplied by the density of the normal segments at the dip being imaged. Aliasing is also a function of the normal segment density at the dip being imaged. If the spatial sampling is fine enough to prevent aliasing, DMO can be performed on a dip by dip basis and, by correcting for the normal segment density, these amplitude variations may be corrected.
Where is the zero‐velocity layer?
Authors: Gray Samuel H.
Published: October 2002

The zero‐velocity layer was introduced in Higginbotham et al. (1985) to increase the maximum dip imaging capability of finite‐difference depth migration. Beasley and Lynn (1992) adapted the idea to improve the imaging, again using finite‐difference depth migration, of seismic data acquired in areas of irregular topography. Beasley and Lynn's application improves upon the conventional method of processing, which is to time shift the data from the acquisition surface to a horizontal datum, and then migrate using the near‐surface velocity above the surface and the best estimate of seismic velocity below the surface. The conventional procedure typically produces artifacts in the shallow part of the section that are characteristic of overmigration. To reduce these artifacts, velocities are often reduced for the migration step. The use of the zero‐velocity layer overcomes the need to adjust the migration velocities. Here, a component of the migration velocity is set to zero in the layer between the datum and the surface. The function of the zero‐velocity layer in migration is to remove the elevation‐static correction applied in shifting the data to the flat datum. Only after the data have migrated through the zero‐velocity layer to the irregular recording surface does the migration begin to act in its customary sense, moving energy from trace to trace.
Tensor CSAMT survey over the Sulphur Springs thermal area, Valles Caldera, New Mexico, United States of America, Part I: Implications for structure of the western caldera
Authors: Wannamaker Philip E.
Published: October 2002

An extensive tensor controlled‐source audiomagnetotelluric (CSAMT) survey has been carried out over the Sulphur Springs geothermal area, Valles Caldera, New Mexico. Forty‐five sites were acquired using two crossed transmitter bipoles placed approximately 13 km south of the center of the survey. The soundings in the Sulphur Springs area were arranged in four profiles to cross major structural features. To curtail spatial aliasing, the electric bipoles along each profile were deployed contiguously. The frequency range of acquisition was 4096 Hz down to 1 Hz for the central line, but only down to 4 Hz for most sites of the other lines. CSAMT and magnetotelluric (MT) data taken outside Valles Caldera were constrained by drill logs and imply resistive Bandelier Tuff, underlain by conductive Paleozoic sediments, and further underlain by resistive, primarily Precambrian crystalline rocks. Model cross‐sections within the caldera were derived using 2-D parameterized inversion constrained by drilling, with layered‐earth inversion for starting models. Southeast of the Sulphur Creek fault, the upper 200 m of the section are of relatively low resistivity and correspond to unconsolidated land‐slide and debris flows. The Bandelier Tuff below exhibits higher but variable resistivities because of alteration controlled by local faulting. Beneath the Bandelier Tuff, the Paleozoic sedimentary layer is only moderately less resistive than it is outside the caldera, with the lowest values occuring northwest of Sulphur Creek. Its low resistivity per se does not necessarily represent a hydrothermal aquifer. The Sulphur Creek fault appears to be a locus of substantial change in structural relief; up‐throw of stratigraphy and basement to its west appears to be about 400–500 m. A major normal fault down to the southeast is located under the topographic expression of Freelove Canyon, which is up to 1 km farther southeast than suggested by previous geologic sections. High resistivities possibly corresponding to a vapor zone in the upper 500 m near VC-2B and VC-2A are not consistent with the CSAMT data.
Tensor CSAMT survey over the Sulphur Springs thermal area, Valles Caldera, New Mexico, United States of America, Part II: Implications for CSAMT methodology
Authors: Wannamaker Philip E.
Published: October 2002

The resistivity model for the Sulphur Springs area in the companion paper (Part I) plus the availability of overlapping controlled‐source audiomagnetotelluric (CSAMT) and magnetotelluric (MT) data has allowed study of far‐field to near‐field transitions, source field geometries over the survey area, and scalar‐tensor impedance discrepancies. The regional setting of conductive Paleozoic sediments over resistive basement seriously reduced depth of exploration within the plane‐wave regime to about 1/20th of the transmitter‐receiver separation, rather than the traditional 1/3rd to 1/5th based on half‐space models. As frequency falls to where skin depth in the sedimentary layer exceeds its thickness, transmitter electromagnetic (EM) fields enter the resistive basement and may diffuse to the receiver with relatively little attenuation, promoting near‐field behavior. Comparisons are made of observed electric (E) and magnetic (H) fields inside and outside the caldera with EM fields computed from layered resistivity models derived from local 1-D inversion of the ρa and θ, and from simple 3-D models. First, the comparisons indicate that small‐scale structure near the transmitter does not lead to overprint effects in the impedance data at the receiver but, instead, acts as an equivalent far‐field source. Second, at both high and low frequencies, the observed E and H fields can depart substantially from those predicted by local layered models. In fact, an effective regional layering appears to control the magnetic field amplitudes and the far‐to near‐field transition in this survey area. The observed electric fields, on the other hand, are controlled by all scales of geology. When heterogeneity is important, significant departures between scalar and tensor CSAMT data can be expected, and are exacerbated when the source field is poorly coupled to the sensors. The problem is much reduced for vector CSAMT measurements where all horizontal field components are measured and the maximally coupled results are defined, but mode identification is more difficult for multidimensional structures.
Characterization of geothermal reservoir crack patterns using shear‐wave splitting
Authors: Lou Min , Rial J. A.
Published: October 2002

Microearthquakes recorded by a downhole, three‐component seismic network deployed around the Coso, California, geothermal reservoir since 1992 display distinctive shear‐wave splitting and clear polarization directions. From the polarizations we estimated three predominant subsurface fracture directions, and from the time delays of the split waves we determined tomographically the 3-D fracture density distribution in the reservoir.
Hybrid migration: A cost‐effective 3-D depth‐imaging technique
Authors: Kim Young C. , Hurt, Worth B. , Maher Louis J. , Starich Patrick J.
Published: October 2002

The transformation of surface seismic data into a subsurface image can be separated into two components—focusing and positioning. Focusing is associated with ensuring the data from different offsets are contributing constructively to the same event. Positioning involves the transformation of the focused events into a depth image consistent with a given velocity model. In prestack depth migration, both of these operations are achieved simultaneously; however, for 3-D data, the cost is significant. Prestack time migration is much more economical and focuses events well even in the presence of moderate velocity variations, but suffers from mispositioning problems. Hybrid migration is a cost‐effective depth‐imaging approach that uses prestack time migration for focusing; inverse migration for the removal of positioning errors; and poststack depth migration for proper positioning. When lateral velocity changes are moderate, the hybrid technique can generate a depth image that is consistent with a velocity field. For very complex structures that require prestack depth migration, the results of the hybrid technique can be used to create a starting velocity model, thereby reducing the number of iterations for velocity model building.
Estimation of shear‐wave splitting time using orthogonal transformation
Authors: Shieh Chiou‐Fen
Published: October 2002

Laboratory studies of rock samples show that many mineral assemblages exhibit anisotropic elastic properties. Additionally, it has been known for a long time that stacked isotropic layers exhibit the same properties (Thomson, 1950; Thomsen, 1986). Oil shale, for example, presents such anisotropy. There's no doubt, then, that seismic anisotropy can be considered a promising method of studying the interior of the Earth (Babuska and Cara, 1991).
On: “The magnetotelluric method in the exploration of sedimentary basins,” by K. Vozoff (February 1972 GEOPHYSICS, 37, pages 98-141)
Authors: Esparza Francisco J. , Gómez‐Trebiño Enrique
Published: October 2002

The paper by Vozoff (1972) is largely responsible for the variety of applications of the magnetoteluric method that we are witnessing today. The paper discusses field procedures, data processing techniques, modeling results, and, for the fast time, important definitions and properties of basic magnetotelluric quantities. One of these definitions concerns the transfer function that relates the vertical magnetic field [Formula: see text] to the horizontal components [Formula: see text] and [Formula: see text]. The general relationship between these components at any given frequency can written as [Formula: see text]
Magnetic and gravity anomaly patterns related to hydrocarbon fields in northern West Siberia
Authors: Piskarev Alexey L. , Tchernyshev Mikhail Yu
Published: October 2002

A study of the features of gravity and magnetic fields in the vicinity of oil and gas reservoirs in West Siberia demonstrated a spatial relationship with the hydrocarbon deposits. The relevant magnetic and gravity anomalies cover approximately [Formula: see text] in northern West Siberia. Amplitude and frequency were investigated initially using double Fourier spectrum (DFS) analysis. This was followed by (1) application of transformations, filtering, and “moving windows” analysis; (2) compilation of maps of regional and local anomalies, and potential field derivatives; and (3) investigation of the distribution of parameters in areas of known deposits. Hydrocarbon deposits are located mostly at the slopes of positive regional gravity and magnetic anomalies which are interpreted as relating to deep riftogenic structures. At the same time, it is established that the location of hydrocarbon depositions coincides commonly with local gravity and magnetic minima generated by lows in basement density and magnetization. All known hydrocarbon deposits in northern West Siberia are in areas characterized by comparatively high gradients of constituent of gravity anomalies with a wavelength of about 90–100 km. These newly revealed links between reservoirs and potential field parameters may be a means to predict new discoveries in poorly explored territories and seas, primarily in Russia's Arctic shelf.
Hopfield neural networks, and mean field annealing for seismic deconvolution and multiple attenuation
Authors: Calderón‐Macías Carlos , Sen Mrinal K. , Stoffa Paul L.
Published: October 2002

We describe a global optimization method called mean field annealing (MFA) and its application to two basic problems in seismic data processing: Seismic deconvolution and surface related multiple attenuation. MFA replaces the stochastic nature of the simulated annealing method with a set of deterministic update rules that act on the average value of the variables rather than on the variables themselves, based on the mean field approximation. As the temperature is lowered, the MFA rules update the variables in terms of their values at a previous temperature. By minimizing averages, it is possible to converge to an equilibrium state considerably faster than a standard simulated annealing method. The update rules are dependent on the form of the cost function and are obtained easily when the cost function resembles the energy function of a Hopfield network. The mapping of a problem onto a Hopfield network is not a precondition for using MFA, but it makes analytic calculations simpler. The seismic deconvolution problem can be mapped onto a Hopfield network by parameterizing the source and the reflectivity in terms of binary neurons. In this context, the solution of the problem is obtained when the neurons of the network reach their stable states. By minimizing the cost function of the network with MFA and using an appropriate cooling schedule, it is possible to escape local minima. A similar idea can also be applied to design an operator that attenuates surface related multiple reflections from plane‐wave transformed seismograms assuming a 1-D earth. The cost function for the multiple elimination problem is based on the criterion of minimum energy of the multiple suppressed data.
3-D magnetic imaging using conjugate gradients
Authors: Pilkington Mark
Published: October 2002

A 3-D inversion approach is outlined that determines a distribution of susceptibility that produces a given magnetic anomaly. The subsurface model consists of a 3-D array of rectangular blocks, each with a constant susceptibility. The inversion incorporates a model norm that allows smoothing and depth‐weighting of the solution. Since the number of parameters can be many thousands, even for small problems, the linear system of equations is inverted using a preconditioned conjugate gradient approach. This reduces memory requirements and avoids large matrix multiplications. The method is used to determine the 3-D susceptibility distribution responsible for the Temagami magnetic anomaly in southern Ontario, Canada.
Kinematics of 3-D DMO operators in transversely isotropic media
Authors: Alkhalifah Tariq
Published: October 2002

The 3-D dip‐moveout (DMO) operator in homogeneous transversely isotropic media with vertical symmetry axis (VTI media), unlike that in homogeneous isotropic media, has an out‐of‐plane (cross‐line) component. In general, this component has a shape that is opposite to that of the cross‐line component of the isotropic v(z) saddle‐shaped operator. The width of the cross‐line component of the VTI operator is also smaller overall than that associated with isotropic v(z) media. When both typical anisotropy and inhomogeneity are combined, the net result is an operator with a smaller cross‐line component and a shape that is influenced more by the v(z) behavior. The large cost of a 3-D DMO operator, as well as the generally small cross‐line components associated with the DMO operator in VTI media, suggests the possibility, as is usually done for isotropic v(z) media, of just ignoring the contribution of the out‐of‐plane portion of the operator.
Anisotropic parameters and P‐wave velocity for orthorhombic media
Authors: Tsvankin Ilya
Published: October 2002

Although orthorhombic (or orthotropic) symmetry is believed to be common for fractured reservoirs, the difficulties in dealing with nine independent elastic constants have precluded this model from being used in seismology. A notation introduced in this work is designed to help make seismic inversion and processing for orthorhombic media more practical by simplifying the description of a wide range of seismic signatures. Taking advantage of the fact that the Christoffel equation has the same form in the symmetry planes of orthorhombic and transversely isotropic (TI) media, we can replace the stiffness coefficients by two vertical (P and S) velocities and seven dimensionless parameters that represent an extension of Thomsen's anisotropy coefficients to orthorhombic models. By design, this notation provides a uniform description of anisotropic media with both orthorhombic and TI symmetry. The dimensionless anisotropic parameters introduced here preserve all attractive features of Thomsen notation in treating wave propagation and performing 2-D processing in the symmetry planes of orthorhombic media. The new notation has proved useful in describing seismic signatures outside the symmetry planes as well, especially for P‐waves. Linearization of P‐wave phase velocity in the anisotropic coefficients leads to a concise weak‐anisotropy approximation that provides good accuracy even for models with pronounced polar and azimuthal velocity variations. This approximation can be used efficiently to build analytic solutions for various seismic signatures. One of the most important advantages of the new notation is the reduction in the number of parameters responsible for P‐wave velocities and traveltimes. All kinematic signatures of P‐waves in orthorhombic media depend on just the vertical velocity [Formula: see text] and five anisotropic parameters, with [Formula: see text] serving as a scaling coefficient in homogeneous media. This conclusion, which holds even for orthorhombic models with strong velocity anisotropy, provides an analytic basis for application of P‐wave traveltime inversion and data processing algorithms in orthorhombic media.
Time‐domain electromagnetic survey for gold exploration, Nevada
Authors: Aktarakçi Hasan Kâmil , Harthill Norman , Blohm Mark W.
Published: October 2002

A time‐domain electromagnetic (TDEM) sounding survey was carried out on the western basin‐range boundary of Smith Creek Valley, Nevada. The survey achieved its goal of mapping depth to volcanic basement along five traverses that crossed the boundary. In addition to mapping the electrical basement, the TDEM survey outlined significant cross faulting within the alluvium. Seventy five soundings were made over a five day period using a Geonics EM37 system that shows the usefulness and effectiveness of the TDEM technique to determine depth to electrical basement under heterogeneous range‐front alluvium.
Seismic traveltime analysis for azimuthally anisotropic media: Theory and experiment
Authors: Sayers Colin M. , Ebrom Daniel A.
Published: October 2002

Natural fractures in reservoirs, and in the caprock overlying the reservoir, play an important role in determining fluid flow during production. The density and orientation of sets of fractures is therefore of great interest. Rocks possessing an anisotropic fabric and a preferred orientation of fractures display both polar and azimuthal anisotropy. Sedimentary rocks containing several sets of vertical fractures may be approximated as having monoclinic symmetry with symmetry plane parallel to the layers if, in the absence of fractures, the rock is transversely isotropic with symmetry axis perpendicular to the bedding plane. A nonhyperbolic traveltime equation, which can be used in the presence of azimuthally anisotropic layered media, can be obtained from an expansion of the inverse‐squared ray velocity in spherical harmonics. For a single set of aligned fractures, application of this equation to traveltime data acquired at a sufficient number of azimuths allows the strike of the fractures to be estimated. Analysis of the traveltimes measured in a physical model simulation of a reverse vertical seismic profile in an azimuthally anisotropic medium shows the medium to be orthorhombic with principal axes in agreement with those given by an independent shear‐wave experiment. In contrast to previous work, no knowledge of the orientation of the symmetry planes is required. The method is therefore applicable to P‐wave data collected at multiple azimuths using multiple offset vertical seismic profiling (VSP) techniques.