bluuebunny/crossref_metadata_2025 huggingface dataset:
Borehole seismic‐source radiation in layered isotropic and anisotropic media: Real data analysis
Authors: Dong Wenjie , Toksöz M. Nafi
Published: October 2002

The source and receiver boreholes in crosshole seismology are usually considered unimportant except for their effects on body wave radiation and reception patterns. We present counter examples by analyzing a real crosswell data set from Buckhorn, Illinois, using computer simulations. The algorithm used is a combination of the boundary element method (for the source borehole) and the borehole coupling theory (for the receiver borehole) in transversely isotropic media. We find that most of the strong events in the data are inexplicable unless both boreholes are included in the modeling. The importance of the boreholes stems from the local geology which consists of highly contrasted sedimentary rocks. At a high‐contrast interface, wave conversion is no longer a negligible secondary effect. In fact, converted waves can be stronger than the primaries.
Crosswell tomographic estimation of elastic constants in heterogeneous transversely isotropic media
Authors: Michelena Reinaldo J. , Harris Jerry M. , Muir Francis
Published: October 2002

The procedure to estimate elastic constants of a transversely isotropic medium from limited‐aperture traveltimes has two steps. First, P‐ and SV‐wave traveltimes are fitted with elliptical velocity functions around one of the axes of symmetry. Second, the parameters that describe the elliptical velocity functions are transformed analytically into elastic constants. When the medium is heterogeneous, the process of fitting the traveltimes with elliptical velocity functions is performed tomographically, and the transformation to elastic constants is performed locally at each position in space. Crosswell synthetic and field data examples show that the procedure is accurate as long as the data aperture is constrained as follows: it should not be too large otherwise the elliptical approximation may not be adequate, and it should not be too small because the tomographic estimation of elliptical velocities fails, even if the medium is actually isotropic.
Frequency‐domain acoustic‐wave modeling and inversion of crosshole data: Part II—Inversion method, synthetic experiments and real‐data results
Authors: Song Zhong‐Min , Williamson Paul R. , Pratt R. Gerhard
Published: October 2002

In full‐wave inversion of seismic data in complex media it is desirable to use finite differences or finite elements for the forward modeling, but such methods are still prohibitively expensive when implemented in 3-D. Full‐wave 2-D inversion schemes are of limited utility even in 2-D media because they do not model 3-D dynamics correctly. Many seismic experiments effectively assume that the geology varies in two dimensions only but generate 3-D (point source) wavefields; that is, they are “two‐and‐one‐half‐dimensional” (2.5-D), and this configuration can be exploited to model 3-D propagation efficiently in such media. We propose a frequency domain full‐wave inversion algorithm which uses a 2.5-D finite difference forward modeling method. The calculated seismogram can be compared directly with real data, which allows the inversion to be iterated. We use a descents‐related method to minimize a least‐squares measure of the wavefield mismatch at the receivers. The acute nonlinearity caused by phase‐wrapping, which corresponds to time‐domain cycle‐skipping, is avoided by the strategy of either starting the inversion using a low frequency component of the data or constructing a starting model using traveltime tomography. The inversion proceeds by stages at successively higher frequencies across the observed bandwidth. The frequency domain is particularly efficient for crosshole configurations and also allows easy incorporation of attenuation, via complex velocities, in both forward modeling and inversion. This also requires the introduction of complex source amplitudes into the inversion as additional unknowns. Synthetic studies show that the iterative scheme enables us to achieve the theoretical maximum resolution for the velocity reconstruction and that strongly attenuative zones can be recovered with reasonable accuracy. Preliminary results from the application of the method to a real data set are also encouraging.
Generalized gravity gradient analysis for 2-D inversion
Authors: Butler Dwain K.
Published: October 2002

Gravity gradient profiles across subsurface structures that are approximately 2-D contain diagnostic information regarding depth, size, and structure (geometry). Gradient space plots, i.e., plots of horizontal gradient versus vertical gradient, present the complete magnitude and phase information in the gradient profiles simultaneously. Considerable previous work demonstrates the possibility for complete structural interpretation of a truncated plate model from the gradient space plot. The qualitative and quantitative diagnostic information contained in gradient space plots is general, however. Examination of the characteristics of gradient space plots reveals that 2-D structures are readily classified as extended or localized. For example, the truncated plate model is an extended model, while the faulted plate model is a localized model. Comparison of measured or calculated gradient space plots to a model gradient space plot catalog allows a rapid, qualitative determination of structure or geometry. “Corners” of a polygonal cross‐section model are then determined as profile points corresponding to maxima on the vertical gradient profile. A generalized approach to structural interpretation from gravity data consists of (1) determining vertical and horizontal gradient profiles perpendicular to the strike of a 2-D gravity anomaly, (2) determining the structural geometry from the gradient space plot, and (3) locating profile positions of structural corners from the vertical gradient profile. This generalized inversion procedure requires no quantitative information or assumption regarding density contrasts. Iterative forward modeling then predicts the density contrasts. Application of this generalized gravity gradient inversion procedure to high quality gravity data results in an effective density prediction consistent with measured near‐surface densities and the known increase in density with depth in deep sedimentary basins.
Heat flow and gravity responses over salt bodies: A comparative model analysis
Authors: Corrigan Jeff , Sweat Mary
Published: October 2002

Two‐dimensional numerical modeling of sea‐floor heat flow and water‐bottom gravity responses to systematic variations in simple subsurface salt body geometries provides insight on the relative usefulness of these two data types for extracting salt geometry information. For a given salt body geometry, diffusion of heat through overlying sediments results in a dramatic decrease in the amplitude of heat flow anomalies as the depth to the top of the salt body increases. For top‐of‐salt depths greater than about 1 km, the heat flow response is insensitive to the length of salt feeder stocks and to the thickness of salt tongues/sheets. This shallow depth‐to‐top‐of‐salt sensitivity range, in addition to a number of environmental factors that can adversely affect interpretation of heat flow anomalies in terms of heat refraction towards and through salt bodies, severely limits the usefulness of sea‐floor heat flow data for constraining aspects of salt body geometry. For gravity data, the critical factor for addressing salt body geometry is the distribution of salt relative to the sediment‐salt density crossover depth (above and below which salt is more and less dense, respectively, than the surrounding sediment). Except when the relevant geometry information being sought (presence and/or length of feeder stock, thickness of salt tongue or sheet) is near the density crossover depth, the geometry‐related information content of the gravity field is greater than that of the heat flow field. Based on these model results, measurement uncertainty considerations, and data limitations, we conclude that gravity data generally offer an order of magnitude greater resolution capability than sea‐floor heat flow data for addressing salt body geometry issues of exploration interest.
Efficient synthetic‐seismogram generation in transversely isotropic, inhomogeneous media
Authors: Alkhalifah Tariq
Published: October 2002

I develop an efficient modeling technique for transversely isotropic, inhomogeneous media using a mix of analytical equations and numerical calculations. The analytic equation for the raypath in a factorized transversely isotropic (FTI) media with linear velocity variation, derived by Shearer and Chapman, is used to trace rays between two points. In addition, I derive an analytical equation for geometrical spreading in FTI media that aids in preserving program efficiency; however, the traveltimes are calculated numerically. I then generalize the method to treat general transversely isotropic (TI) media that are not factorized anisotropic inhomogeneous by perturbing the FTI traveltimes, following the perturbation ideas of Červený and Filho. A Kirchhoff‐summation‐based program relying on Trorey’s diffraction method is used to generate synthetic seismograms for such a medium. For the type of velocity models treated, the program is much more efficient than finite‐difference or general ray‐trace modeling techniques.
Some analyses of 2-D median f-k filters
Authors: Duncan Guy , Beresford Greg
Published: October 2002

Two‐dimensional median filters can be designed so that they have properties similar to f-k fan filters. This is done by using the coefficients of a truncated impulse response of an f-k filter as the weight coefficients for the weighted median process. The filter is called a median f-k filter and can be used to discriminate between events on the basis of apparent velocity. The filter appears suitable as a poststack coherency filter because it produces less distortion at wavefield terminations than conventional f-k fan filters. One‐dimensional weighted median filters that include negative coefficients are a logical starting point for the analysis of median f-k filters since simple numerical techniques may be used to analyze the behavior of these filters. We show that median filters with negative coefficients do not provide an unbiased estimate of the mean and can misplace the position of steps. Faults on a stacked section may be modeled by steps, and therefore applying a median f-k filter to stacked seismic data could change the position of faults. However, the distortion of steps introduced by median f-k filters is shown to be less than the distortion produced by the corresponding linear f-k filter, and the error in step placement is small. We present simple model examples and a seismic field data example to illustrate differences between linear f-k filters and median f-k filters.
Passive seismic imaging using microearthquakes
Authors: Daneshvar M. Reza , Clay Clarence S. , Savage Martha K.
Published: October 2002

We have developed a method of processing seismic signals generated by microearthquakes to image local subsurface structure beneath a recording station. This technique uses the autocorrelation of the vertically traveling earthquake signals to generate pseudoreflection seismograms that can be interpreted for subsurface structure. Processed pseudoreflection data, from microearthquakes recorded in the island of Hawaii, show consistent reflectivity patterns that are interpreted as near‐surface horizontal features. Forward modeling of the pseudoreflection data results in a P‐wave velocity model that shows reasonable agreement with the velocity model derived from a refraction study in the region. Usable signal‐to‐noise ratio is obtained down to 2 s. A shear‐wave velocity model was also generated by applying this technique to horizontal component data.
Orientation of a downhole triaxial geophone
Authors: Greenhalgh Stewart A. , Mason Iain M.
Published: October 2002

The bearing and elevation of a geophone’s axes are unknown when a geophone package is placed in a deep exploration borehole. Auxiliary instruments, such as inclinometers, compasses or gyros, may be incorporated into the downhole package to give some indication of local attitude relative to some external reference field, but these solutions may encounter difficulties because small gyros drift, the geomagnetic field may be distorted locally or screened by casing, and inclinometers add to the cost and weight of the downhole receiver. An alternate approach to determining the orientation of a remote tool is to compare its triaxial response to that expected for a known elastic wavefield. The accuracy of the measurement depends on the quality of the geophone‐to‐formation coupling and on the extent of one’s knowledge of the externally impressed wavefield.
A least‐squares minimization approach to depth determination from numerical horizontal gravity gradients
Authors: Abdelrahman El‐Sayed Mohamed , Sharafeldin Sharafeldin Mahmoud
Published: October 2002

The sphere and the horizontal cylinder models can be very useful in quantitative interpretation of gravity data measured in a small area over buried structures. Several graphical and numerical methods have been developed by many workers for interpreting the residual gravity anomalies caused by these models to find the depth of most geologic structures. Excellent reviews are given in Saxov and Nygaard (1953) and Bowin et al. (1986). The numerical approaches (Odegard and Berg, 1965; Gupta, 1983; Sharma and Geldart, 1968; Lines and Treitel, 1984; and Shaw and Agarwal, 1990) may have advantages in theory and practice over graphical depth estimation techniques (Pick et al., 1973: Nettleton, 1976; Telford et al., 1976). However, effective quantitative interpretation procedures using the least‐squares method based on the analytical expression of simple numerical horizontal gravity gradient anomalies are yet to be developed.
Walk‐away VSP using drill noise as a source
Authors: Haldorsen Jacob B. U. , Miller Douglas E. , Walsh John J.
Published: October 2002

We describe a method for extracting and deconvolving a signal generated by a drill bit and collected by an array of surface geophones. The drill‐noise signature is reduced to an effective impulse by means of a multichannel Wiener deconvolution technique, producing a walk‐away reverse vertical seismic profile (VSP) sampled almost continuously in depth. We show how the multichannel technique accounts for noise and for internal drill‐string reflections, automatically limiting the deconvolved data to frequencies containing significant energy. We have acquired and processed a data set from a well in Germany while drilling at a depth of almost 4000 m. The subsurface image derived from these data compares well with corresponding images from a 3-D surface seismic survey, a zero‐offset VSP survey, and a walk‐away VSP survey acquired using conventional wireline techniques. The effective bandwidth of the deconvolved drill‐noise data is comparable to the bandwidth of surface seismic data but significantly smaller than what can be achieved with wireline VSP techniques. Although the processing algorithm does not require the use of sensors mounted on the drill string, these sensors provide a very economic way to compress the data. The sensors on the drill string were also used for accurate timing of the deconvolved drill‐noise data.
Composite distribution inversion applied to crosshole tomography
Authors: Clippard James D. , Christensen Douglas H. , Rechtien Richard D.
Published: October 2002

Crosshole tomography requires solution of a mixed‐determined inverse problem and addition of a priori information in the form of auxiliary constraints to achieve a stable solution. Composite distribution inversion (CDI) constraints are developed by assuming parameters are drawn from a composite distribution consisting of both normally and uniformly distributed parameters. Nonanomalous parameter estimates are assumed to be Gaussian while anomalous parameters are assumed uniform. The resulting constraints are sensitive to anomaly volume and are an alternative to the usual constraints of minimizing [Formula: see text] solution length or some measure of roughness. Damped least‐squares inversion, which minimizes solution length, distributes anomalous signal through poorly resolved areas to produce in attenuated and smoothed anomalies. Similar regularization methods, such as smoothness or flatness constraints, also degrade small spatial wavelength features and produce diffuse images of distinct anomalies. CDI constraints preserve small spatial wavelength features by encouraging small amplitude anomalies to assume the value of the reference model and by allowing truly anomalous parameter estimates to assume whatever value minimizes prediction error without incurring additional penalty. CDI tomograms are characterized by nearly ideal point‐spread functions, suggesting the possibility of better quantitative parameter estimates than are produced using most existing methods. CDI tomograms of both synthetic and field data are shown to produce less diffuse images with more accurate anomaly amplitude estimates than damped least‐squares methods. The CDI algorithm is potentially applicable to nontomographic inversion problems.
Recognition of Ricker wavelets by syntactic analysis
Authors: Huang Kou‐Yuan , Leu Dar‐Ren
Published: October 2002

Syntactic pattern recognition techniques are applied to the analysis of 1-D seismic traces to classify Ricker wavelets. Seismic Ricker wavelets have structural information, and each wavelet can be represented by a string of symbols. To recognize the strings, we use a finite‐state automaton to identify each string. The automaton can accept strings having substitution, insertion, and deletion errors of the symbols. There are two attributes, terminal symbol and weight, in each transition of the automaton. A minimum‐cost, error‐correcting, finite‐state automaton is proposed to parse the input string.
On: “A 3-D seismic case history evaluating fluvially deposited thin‐bed reservoirs in a gas‐producing property,” by B. A. Hardage, R. A. Levey, V. Pendleton, J. Simmons, and R. Edsen (November 1994 GEOPHYSICS, 59, p. 1650–1665).
Authors: Roksandic Miodrag M.
Published: October 2002

Hardage et al. (1994) conducted a study at Stratton Field with the purpose of detecting thin‐bed compartmented reservoirs in a fluvially deposited system (Oligocene Frio Formation), and rightly concluded that, in order to determine which seismically imaged stratigraphic changes are compartment boundaries, it is necessary to incorporate geologic and reservoir engineering data (particularly reservoir pressure data) into seismic interpretation. Their interpretation philosophy consisted of defining depositional stratal surfaces (I would rather say paleodepositional surfaces), and in analyzing seismic reflection amplitude behavior on such surfaces.
Inversion of the power spectrum from gravity anomalies of prismatic bodies
Authors: García‐Abdeslem Juan
Published: October 2002

I develop methods for modeling and inversion of gravity anomalies produced by prismatic bodies. The forward problem is solved in the wavenumber domain, where the power spectrum of the gravity anomaly is given by the product of independent functions that describe depth, thickness, horizontal dimensions, and density of the source body. The solution of the inverse problem is based on an iterative ridge‐regression algorithm, starting from an initial trial of the geometry and density of the source body. This procedure is assessed in a number of examples with both numeric and field data spectra. The method is first tested on the spectrum obtained from simple gravity anomalies and is found to be capable of recovering the source parameters. Next the method is applied to gravity data from a survey made near Noranda, Quebec, Canada. This interpretation compares favorably with drillhole data and provides an estimation of the mass of the source body which is similar to previous estimates.
Water saturation and permeability from resistivity, dielectric, and porosity logs
Authors: de Lima Olivar A. L.
Published: October 2002

An analytical approach previously proposed to model the electrical properties of shaly sands has been adapted to obtain the cementation exponent (m), the water saturation [Formula: see text], and the shaliness distribution from electromagnetic and porosity log measurements in an oil well. Shaliness is described by the clay volume fraction (p) and by the clay type parameter [Formula: see text] in the sand. For a shale‐coating structure these parameters can be related to the exchange cation density [Formula: see text]. Based on an equivalent flow regime inside this granular model, a Kozeny‐Carman type equation has been derived, expressing the intrinsic permeability (k) of the medium in terms of a porosity‐tortuosity factor [Formula: see text] and of the parameter [Formula: see text]. The power‐law derived expression shows that k decreases with the amount of clay, not only because a high [Formula: see text] implies a narrowing of the pore channels, but also because it modifies the hydraulic tortuosity of the medium. This new equation has been statistically tested with extensive petrophysical laboratory data for different types of sandstones and satisfactorily applied to core and log data of an oil well from Potiguar Basin, Rio Grande do Norte, Brazil.
Air‐gun bubble damping by a screen
Authors: Langhammer Jan , Landrø Martin , Martin James , Berg Eivind
Published: October 2002

A method for damping unwanted bubble oscillations from a seismic air gun is presented. The method exploits the fact that the primary pressure peak generated by an air gun is produced during the first 5–10 ms after firing. The air bubble is destroyed by mounting a perforated screen with an optimal radius about the gun. Once the primary pressure peak has been generated by the bubble, the bubble is destroyed by the screen, leading to a corresponding decrease in the measured pressure amplitude of the secondary bubble oscillations. Controlled near‐field measurements of 40‐cubic inch and 120‐cubic inch air guns with and without damping screens are used. The primary to bubble ratio improves from 1.4 without a screen to 4.4 with a screen in the near‐field. The corresponding values for estimated far‐field signatures are 1.8 to 9.0 when the signatures are filtered with an out‐128 Hz (72 dB/Oct) DFS V filter.
Prestack processing of land data with complex topography
Authors: Rajasekaran Sara , McMechan George A.
Published: October 2002

A new wave‐equation–based prestack seismic processing system is proposed. This system has only two essential elements; velocity analysis and depth migration. This approach applies truly surface‐consistent statics corrections, regardless of the amount of elevation, change or of near‐surface velocity variation. It uses tomography for estimating the details of shallow velocities and a finite‐difference solution of the two‐way wave‐equation both for computation of image times and for data extrapolation in migration. A field data set that violates most of the assumptions in conventional common midpoint (CMP) processing, because of severe elevation changes and near‐surface velocity variations, is successfully processed. The final depth section reveals a complicated fold‐thrust geometry that was not visible after CMP processing.
Lateral prediction for noise attenuation by t-x and f-x techniques
Authors: Abma Ray , Claerbout Jon
Published: October 2002

Attenuating random noise with a prediction filter in the time‐space domain generally produces results similar to those of predictions done in the frequency‐space domain. However, in the presence of moderate‐ to high‐amplitude noise, time‐space or t-x prediction passes less random noise than does frequency‐space, or f-x prediction. The f-x prediction may also produce false events in the presence of parallel events where t-x prediction does not. These advantages of t-x prediction are the result of its ability to control the length of the prediction filter in time. An f-x prediction produces an effective t-x domain filter that is as long in time as the input data. Gulunay’s f-x domain prediction tends to bias the predictions toward the traces nearest the output trace, allowing somewhat more noise to be passed, but this bias may be overcome by modifying the system of equations used to calculate the filter. The 3-D extension to the 2-D t-x and f-x prediction techniques allows improved noise attenuation because more samples are used in the predictions, and the requirement that events be strictly linear is relaxed.
Turning‐ray tomography
Authors: Stefani Joseph P.
Published: October 2002

Turning‐ray tomography is useful for estimating near‐surface velocity structure in areas where conventional refraction statics techniques fail because of poor data or lack of smooth refractor/velocity structure. This paper explores the accuracy and inherent smoothing of turning‐ray tomography in its capacity to estimate absolute near‐surface velocity and the statics times derived from these velocities, and the fidelity with which wavefields collapse to point diffractors when migrated through these estimated velocities. The method comprises nonlinear iterations of forward ray tracing through triangular cells linear in slowness squared, coupled with the LSQR linear inversion algorithm. It is applied to two synthetic finite‐ difference data sets of types that usually foil conventional refraction statics techniques. These models represent a complex hard‐rock overthrust structure with a low‐velocity zone and pinchouts, and a contemporaneous near‐shore marine trench filled with low‐ velocity unconsolidated deposits exhibiting no seismically apparent internal structure. In both cases velocities are estimated accurately to a depth of one‐ fifth the maximum offset, as are the associated statics times. Of equal importance, the velocities are sufficiently accurate to correctly focus synthetic wavefields back to their initial point sources, so migration/datuming applications can also use these velocities. The method is applied to a real data example from the Timbalier Trench in the Gulf of Mexico, which exhibits the same essential features as the marine trench synthetic model. The Timbalier velocity inversion is geologically reasonable and yields long and short wavelength statics that improve the CMP gathers and stack and that correctly align reflections to known well markers. Turning‐ray tomography estimates near‐surface velocities accurately enough for the three purposes of lithology interpretation, statics calculations, and wavefield focusing for shallow migration and datuming.
Estimations of formation velocity, permeability, and shear‐wave anisotropy using acoustic logs
Authors: Cheng Ningya , Cheng Chuen Hon
Published: October 2002

Field data sets collected by an array monopole acoustic logging tool and a shear wave logging tool are processed and interpreted. The P‐ and S‐wave velocities of the formation are determined by threshold detection with cross‐correlation correction from the full waveform and the shear‐wave log, respectively. The array monopole acoustic logging data are also processed using the extended Prony’s method to estimate the borehole Stoneley wave phase velocity and attenuation as a function of frequency. The well formation between depths of 2950 and 3150 ft (899 and 960 m) can be described as an isotropic elastic medium. The inverted [Formula: see text] from the Stoneley wave phase velocity is in excellent agreement with the shear‐wave log results in this section. The well formation between the depths of 3715 and 3780 ft (1132 and 1152 m) can be described as a porous medium with shear‐wave velocity anisotropy about 10% to 20% and with the symmetry axis perpendicular to the borehole axis. The disagreement between the shear‐wave velocity from the Stoneley wave inversion and the direct shear‐wave log velocity in this section is beyond the errors in the measurements. Estimated permeabilities from low‐frequency Stoneley wave velocity and attenuation data are in good agreement with the core measurements. Also it is proven that the formation permeability is not the cause of the discrepancy. From the estimated “shear/pseudo‐Rayleigh” phase velocities in the array monopole log and the 3-D finite‐difference synthetics in the anisotropic formation, the discrepancy can best be explained as shear‐wave anisotropy.
Reconciliation of crosshole seismic velocities with well information in a layered sedimentary environment
Authors: Pratt R. Gerhard , Sams Mark S.
Published: October 2002

In sedimentary environments, horizontal fine layering can cause significant complications when analyzing and comparing seismic data with different frequencies and propagation directions. At the Whitchester test site, three boreholes penetrate upper Carboniferous cyclical sediments, involving interbedded carbonates, sandstones, and mudstones, with seismic velocities ranging from less than 3.0 km/s in the mudstones to over 4.5 km/s in the carbonates. Initial crosshole results from two boreholes approximately 200 m deep and 75 m apart showed a poor correlation with the borehole logs recorded in a third, intermediate borehole. Furthermore, the tomographic images were inconsistent with the expected geology. The objective of this paper is to reconcile the crosshole seismic data with the borehole log information. The borehole information must be upscaled to match the resolution of the crosshole experiment. However, upscaling techniques based on Backus averaging lead to the prediction of significant anisotropy (of the order of 20% in places) with a vertical symmetry axis. This anisotropy is a result of a combination of fine layering and the intrinsic mineral anisotropy measured on the core samples, although it is the layer‐induced anisotropy that dominates at the crosshole frequencies of 400 Hz. This prediction was tested by including anisotropy parameters into the analysis of the crosshole data, using anisotropic velocity tomography. In the central part of the final anisotropic velocity tomograms, where the ray coverage is adequate, these crosshole results are consistent with the anisotropy predictions. Once the anisotropy was properly accounted for, the tomographic images are consistent with the layered nature of the geology at the site, and they show the location and throw of a known fault in the section. We conclude that, at this site and at crosshole frequencies, seismic layer‐induced anisotropy plays a significant role and must be accounted for when processing and interpreting crosshole seismic data.
Polarization measurement uncertainty on three‐component VSP
Authors: Knowlton Kelly B. , Spencer Terry W.
Published: October 2002

To determine the downhole orientation of the sonde on a three‐component vertical seismic profile (VSP) recorded using three offset sources at different azimuths, the azimuth of the direct P arrival for each source with respect to a horizontal geophone axis was found using particle polarization analysis. The azimuthal angles between the sources as determined by polarization analysis were compared with the actual azimuths between the sources to determine the error in polarization measurement. The least scatter in azimuthal measurements for the three sources at a given depth was found to occur for measurements made on a time window over which the particle motion had the most rectilinear behavior. This window occurred after the first break and preceded the peak of maximum amplitude. Uncertainties in azimuthal measurement and signal‐to‐noise (S/N) ratios were found over four depth intervals on the field VSP, with azimuthal uncertainty ranging from 1.0° at a S/N ratio of 9.0 to 5.6° at a S/N ratio of 3.7. Cases were found for which the azimuthal measurement was most likely in error for only one of the three sources at a given depth, and after these errors were removed, the uncertainty in polarization measurement reduced by as much as 60%. Polarization uncertainties on a model consisting of a sinusoidal three‐component signal contaminated with band‐limited Gaussian noise exhibited values similar to those for the field VSP at comparable S/N ratios.
Fresnel zones for broadband data
Authors: Brühl Matthias , Vermeer Gijs J. O. , Kiehn Michael
Published: October 2002

For monochromatic waves, the term “Fresnel zone” is well‐defined even though different authors use different terminology. Most authors use the definition originating from optics. There, the first Fresnel zone is defined as the area of a circular hole in a screen between a light source and an observation point that produces maximum light intensity in the observation point (Figure 1). If the radius of the hole is enlarged, minima and maxima in light intensity alternate. The first maximum is reached if the raypath difference between the direct ray and the ray traveling via the edge of the hole equals half a wavelength. The extension of the definition to energy reflected from a circular disk is straightforward (if we restrict ourselves to ray theory and neglect the angle dependency of the reflection coefficient) and is illustrated in Figure 2 (see also Sheriff, 1991).
Underground imaging by frequency‐domain electromagnetic migration
Authors: Zhdanov Michael S. , Traynin Peter , Booker John R.
Published: October 2002

A new method of the resistivity imaging based on frequency‐domain electromagnetic migration is developed. Electromagnetic (EM) migration involves downward diffusion of observed EM fields whose time flow has been reversed. Unlike downward analytical continuation, migration is a stable procedure that accurately restores the phase of the upgoing field inside the Earth. This method is indented for the processing and interpretation of EM data collected for both TE and TM modes of plane‐wave excitation. Until recently, the method could be applied only for determining the position of anomalous structures and for finding interfaces between layers of different conductivity. There were no well developed approaches to the resistivity imaging, which is the key problem in the inversion of EM data. We provide a novel approach to determining not only the position of anomalous structures but their resistivity as well. The main difficulty in the practical realization of this approach is determining the background resistivity distribution for migration. We discuss the method of the solution of this problem based on differential transformation of apparent resistivity curves. The final goal of migration is to provide a first order interpretation using a computational effort equivalent to a forward modeling calculation.
True‐amplitude migration taking fine layering into account
Authors: Wapenaar C. P. A. , Herrmann F. J.
Published: October 2002

The central process in wave‐equation—based depth migration is inverse wavefield extrapolation. The commonly applied matched filter approach to inverse wavefield extrapolation ignores the angle‐dependent amplitude and phase distortions that are related to fine layering. This may result in dispersed images and erroneous amplitude variation with angle of incidence (AVA) effects. “Power reciprocity” formulates a relation between transmitted and reflected wavefields. It provides the basis for a modified matched filter that does account for the aforementioned distortions. The correction term by which the matched filter is modified can be derived directly from the reflection data. With this modified matched filter, prestack depth migration will yield nondispersed images with correct AVA behavior.
Analysis of higher‐order, finite‐difference schemes in 3-D reverse‐time migration
Authors: Wu Wen‐Jing , Lines Larry R. , Lu Han‐Xing
Published: October 2002

The design and usefulness of practical algorithms for 3-D reverse‐time depth migration is examined, while demonstrating data applications of the algorithm. We evaluate quantitatively the accuracy of the finite‐difference operator from second‐order to eighth‐order for the scalar wave equation by comparing numerical and analytical solutions. The results clearly show the advantage of using higher‐order, finite‐difference schemes, especially from second‐order to fourth‐order for space derivatives. Hence, a finite‐difference method with the accuracy of fourth‐order in space and second‐order in time is applied to 3-D full scalar wave equations in reverse‐time migration. Considerable savings in CPU and memory are obtained by using larger horizontal grid spacings than the vertical spacings. Therefore, we derive dispersion and stability conditions for such unequal grid spacing. The 3-D reverse‐time migration of data from the Hibernia Field shows an image improvement over 2-D migrations, despite the fact that much of the structure was considered to be approximately 2-D. Stable, nondispersive, finite‐difference methods of higher order can allow for tractable and efficient 3-D reverse‐time migration solutions.
Comparison of 2-D numerical viscoelastic waveform modeling with ultrasonic physical modeling
Authors: Chen Genmeng
Published: October 2002

The objective of the study is to test the validity of theoretical models of wave attenuation by comparing their predictions of attenuation against physical model results. The study is confined to a 2-D geometry, and the viscoelastic materials used in physical modeling are those commonly used in the experiment. The physical modeling data of homogeneous media are compared with the numerical results in the frequency domain. The time‐domain comparisons between numerical modeling and physical modeling are also shown by three examples. The theoretical viscoelastic models used in the numerical study are the Kelvin‐Voigt model, the standard linear solid model, and the standard linear solid model with a continuous spectrum of relaxation time. On the comparison of a single model, all the models simulate the physical model fairly well, but the standard linear solid model gives the best result among them. The Kelvin‐Voigt model is easy to use as a quick first‐order simulation of the viscoelastic materials because it has fewer viscosity parameters than the other two models. The disadvantage of the Kelvin‐Voigt model is that it predicts too much attenuation of the high‐frequency components. It is also shown that neglecting the viscosity of some materials like polyvinylcloride plastic (PVC), which has high viscosity, will produce incorrect results in synthetic seismograms.
Fibre optic temperature sensing: A new tool for temperature measurements in boreholes
Authors: Großwig Stephan , Hurtig Eckart , Kühn Katrin
Published: October 2002

Usually, the temperature in boreholes is determined using a standard temperature probe. The logging technique is either “stop and go”, or the probe is lowered as a moving probe into the borehole using a controlled speed. Distributed temperature probe arrays installed permanently in a borehole are an alternative to moving probes and can be applied especially for temperature monitoring even under conditions where moving probes cannot be used. The distributed optical fiber sensing technique represents a new approach for temperature measurements. The basis for this method is given in Boiarski (1993), Dakin et al. (1985), Farries and Rogers (1984), Hartog and Gamble (1991), Rogers (1988), Rogers (1993). First results using fiber optic temperature sensing in boreholes and temperature monitoring for studying geotechnical and environmental problems (e.g., waste deposits) are published in Hurtig et al. (1993; 1994; 1995) and Hurtig and Schrötter (1993).
Groundwater NMR in conductive water
Authors: Shushakov Oleg A.
Published: October 2002

A surface method of groundwater prospecting using nuclear magnetic resonance (NMR) in the Earth’s magnetic field is under study. The technique is employed for hydrogeological surveys down to a depth of about 100 m. The advantage of this method is that an NMR signal can be observed only in the presence of groundwater. A circular wire loop with a diameter of 100 m is laid out on the ground to excite and receive the NMR signal. An oscillating current with a rectangular pulse‐shape is passed through the loop, with the carrier‐frequency being equal to the proton‐resonance frequency in the Earth’s field. The excitation pulse is followed by a nuclear induction emf caused by the free Larmor precession in the Earth’s field. Of practical importance is the effect of the electrical conductivity of the ground on a groundwater NMR survey. Finite‐ground conductivity can result in induced currents that can screen the NMR signal. The calculations of NMR signals are based on the transformation of Maxwell’s equations in terms of magnetic Hertz potentials through use of the reciprocity principle. Groundwater NMR is measured with an instrument designed at the Institute of Chemical Kinetics and Combustion, Russian Academy of Science, Novosibirsk. Experiments were conducted in the Altay region of Russia. Both NMR‐signal amplitude and phase, were measured and compared with the calculated results for horizontally stratified media. Borehole logs and vertical‐resistivity profiles were also used for evaluation of results. The conductivity is shown to affect both phase and amplitude of the NMR signal at resistivities of a few to a few tens of ohm‐m depending on the depth of the water‐saturated layers. There is good agreement between calculated and experimental data. It is also established that the measurements of only NMR amplitude and phase are not sufficient for determining groundwater salinity.
Inversion technique for recovering the elastic constants of transversely isotropic materials
Authors: Okoye Patrick N. , Zhao Ping , Uren Norm F.
Published: October 2002

A least‐squares iterative inversion technique has been developed for the determination of the elastic parameter δ* of any transversely isotropic modeling material in the laboratory. For most applications in petroleum geophysics, the elastic parameter δ* is very important and is the crucial anisotropic parameter for near‐vertical P‐wave propagation. Despite the potential importance of δ* in seismic exploration and for resolution in an anisotropic medium, the conventional procedures adopted in estimating its value unfortunately are faced with many ambiguities and the reliability of its measurement is doubtful prior to the development of this technique. The anisotropic inverse modeling technique finds the best fitting solution. To optimize the accuracy of the results presented in this paper, analytical rather than numerical differentiations were implemented and the modeling procedures allow for controlled iterative adjustments in resolving the parameter δ*. Inversion of the first‐arrival traveltimes obtained for vertical P‐waves through an anisotropic material known as phenolite yield estimates of the elastic parameter δ* as well as the vertical P‐wave velocity [Formula: see text] of the material. Accurate picking of the first‐arrival traveltimes is essential since δ* is found to be very sensitive to small differences between vertical and oblique traveltime picks. The inversion results have been found to be stable and convergent, and they also highlight the need for good angular coverage to determine the anisotropy parameters in materials suspected of being anisotropic.
Elastic wave‐propagation simulation in heterogeneous media by the spectral moments method
Authors: Rousseau Valérie , Benoit Claude , Bayer Roger , Cuer Michel , Poussigue Gérard
Published: October 2002

The second‐order elastic wave propagation equations are solved using the spectral moments method. This numerical method, previously developed in condensed matter physics, allows the computation of Green’s functions for very large systems. The elastic wave equations are transformed in the Fourier domain for time derivatives, and the partial derivatives in space are computed by second‐order finite differencing. The dynamic matrix of the discretized system is built from the medium parameters and the boundary conditions. The Green’s function, calculated for a given source‐receiver couple, is developed as a continued fraction whose coefficients are related to the moments and calculated from the dynamic matrix. The continued fraction coefficients and the moments are computed using a very simple algorithm. We show that the precise estimation of the waveform for the successive waves arriving at the receiver depends on the number of moments used. For long recording times, more moments are needed for an accurate solution. Efficiency and accuracy of the method is illustrated by modeling wave propagation in 1-D acoustic and 2-D elastic media and by comparing the results obtained by the spectral moments method to analytical solutions and classical finite‐difference methods.
Velocity and interface depth determination by tomography of depth migrated gathers
Authors: Kosloff Dan , Sherwood John , Koren Zvi , Machet Elana , Falkovitz Yael
Published: October 2002

A method for velocity and interface depth determination based on tomography of migrated common reflecting point (CRP) gathers is presented. The method is derived from the tomographic principle that relates traveltime change along a given ray to perturbations in slowness and layer depths. The tomographic principle is used to convert depth errors in migrated CRP gathers to time errors along a CRP ray pair and thus enable use of conventional traveltime tomography. It is also used to affect a very fast prestack migration and set up the tomography matrix. The velocity‐depth determination method uses the available offsets of all CRPs and inverts for the parameters of all layers simultaneously. Hand picking of depth errors on CRP gathers is avoided by a method where the tomography matrix operates directly on the migrated gathers. The velocity‐depth determination method is demonstrated on a synthetic example and on a field example from the North Sea.
PC‐based acquisition and processing of high‐resolution marine seismic data
Authors: Lee Ho‐Young , Hyun Byung‐Koo , Kong Young‐Sae
Published: October 2002

We have improved the quality of high‐resolution marine seismic data using a simple PC‐based acquisition and processing system. The system consists of a PC, an A/D converter, and a magneto‐optical disk drive. The system has been designed to acquire single‐channel data at up to 60,000 samples per second and to perform data processing of seismic data by a simple procedure. Test surveys have been carried out off Pohang, southern East Sea of Korea. The seismic systems used for the test were an air gun and a 3.5 kHz sub‐bottom profiling system. Spectral characteristics of the sources were analyzed. Simple digital signal processes which include gain recovery, deconvolution, band‐pass filter, and swell filter were performed. The quality of seismic sections produced by the system is greatly enhanced in comparison to analog sections. The PC‐based system for acquisition and processing of high‐resolution marine seismic data is economical and versatile.