bluuebunny/crossref_metadata_2025 huggingface dataset:
Array analysis of reflector heterogeneity
Authors: Louie J. N. , Vidale J. E.
Published: October 2002

In deep crustal reflection study, as in conventional exploration seismology, it is important to determine the geometry of the physical contrasts between rocks that cause reflections, to make reliable geologic interpretations. Fundamentally different reflecting structures produce similar signatures in stacked seismic sections. We have developed a method that uses prestack records to differentiate lateral structural variations from lateral reflectivity variations and laterally homogeneous structures. Full‐wave acoustic multioffset synthetics of canonical 2-D reflector configurations, analyzed by statistically enhanced slant‐stack processes, show that lateral heterogeneity such as a wavy reflector can be identified from changes in slowness across a receiver array as a function of time. Application of these methods to deep crustal reflections, recorded in the Mojave Desert of southern California, identifies laterally heterogeneous midcrustal structures and is consistent with a laterally homogeneous Moho.
Gravity meter calibration at LaCoste and Romberg
Authors: Valliant H. D.
Published: October 2002

An ideal gravity meter has a linear response so that its calibration is represented by a single scale factor. Microscopic variations in manufacturing lead to significant departures from this ideal. These variations from linearity are measured by observing the response of the gravity meter to the addition of a test mass to the gravity meter beam at various intervals throughout the meter’s range. This procedure measures the slope of the calibration curve at these points and therefore serves to define its shape. A compromise between accuracy, reading resolution, and cost needs to be reached. High accuracy requires a large mass difference which causes low resolution, requiring many overlapping observations which in turn leads to higher cost. The current procedure provides precision and resolution consistent with a precision of 10 μGals [Formula: see text] over short ranges. The resulting calibration curve is scaled to gravity units with repeated observations over a 241.9‐mGal gravity range at Cloudcroft, New Mexico. This gravity interval is now known to be too small by a few parts in 10 000, and updating it awaits absolute gravity measurements at the site. For consistency the old value will continue to be used until it can be replaced with a more accurate absolute value. Periodic calibration errors, referred to as circular errors, due to irregularities in manufacturing the measuring screw and gear train, are measured at three points along the travel of the screw. A screw is rejected if the peak‐to‐peak amplitude of the periodic error exceeds 40 and 8 μGals for model G and model D meters, respectively.
Gravity terrain corrections using Gaussian surfaces
Authors: Herrera‐Barrientos J. , Fernandez R.
Published: October 2002

Gravity terrain corrections impose some limitations on the accuracy of gravity surveys. Several numerical techniques have been developed in recent years, which have reduced these errors and at the same time have made the process of gravity data reduction for local and regional studies less tedious. We present here a method to perform gravity terrain corrections with reasonable accuracy. The topography is fitted as a set of Gaussian basis functions, in a manner suitable for implementation in a personal computer. The automated straightforward application of Hammer’s chart to this surface has yielded good estimates of terrain corrections, by comparing results from an analytic solution and a field example.
Radar detection of subglacial sulfides
Authors: Hammond W. R. , Sprenke K. F.
Published: October 2002

Using an ice radar system, we detected anomalous reflection strengths over subglacial disseminated sulfide zones beneath the Mt. Henry Clay Glacier in southeast Alaska. The subglacial sulfide zones, which were verified by drill holes, were not detected by previous magnetic, helicopter EM, or ground‐based time‐domain EM surveys. The sulfide zones were mapped by measuring lateral variations in the strength of radar echoes from the ice‐bedrock interface at the base of the glacier. The reflected power from these disseminated occurrences ranged from 20 percent to 60 percent of the theoretically predicted reflected power from a perfect conductor at the base of the ice. The empirical results of this experiment suggest that ice radar may be a useful tool for direct mineral exploration in ice‐covered terrain.
Assessment of 2-D resistivity structures using 1-D inversion
Authors: Beard L. P. , Morgan F. D.
Published: October 2002

Schlumberger and Wenner array resistivity soundings over 2-D resistivity structures are interpreted using apparent resistivity pseudosections and cross‐sections constructed from 1-D inversions in order to determine the effectiveness of 1-D interpretations over such structures. Cross‐sections contoured from resistivities of inverted “layers” show distinct differences from the apparent resistivity pseudosections and may be used as interpretational aids. Contour lines in the cross‐sections locate the horizontal interfaces of the 2-D structures quite well. The vertically oriented segments of the cross‐section contours are relatively undistorted in the inversion process and are similar to the vertically oriented portions of contours in the apparent resistivity pseudosection. A simple, empirically determined formula is used to separate the sections into resistive and conductive zones and helps to define the geometry of the anomaly. In order to apply the formula, it is necessary to know whether the target is a relative conductor or a relative resistor. Except for the case of a square prism, the Schlumberger array appears to hold advantages over the Wenner in qualitatively assessing an anomaly. The primary drawback of the Wenner array is that its expanding potential electrodes create false anomalous zones and complicate interpretation. As might be expected, structures with long horizontal interfaces, i.e. those more nearly 1-D, yield the most accurate interpretations.
Bedrock topography from magnetic anomalie—An aid for groundwater exploration in hard‐rock terrains
Authors: Ram Babu H. V. , Rao N. Kameswara , Kumar V. Vijay
Published: October 2002

The role of the magnetic method in groundwater exploration is to locate structures such as dikes, faults, fractures, etc., that control the accumulation and movement of groundwater. In hard‐rock terrains, the thickness of the weathered layer is an important parameter that determines the quantity of groundwater accumulated in the unconfined aquifer above the basement. The basement rock, in the process of weathering, loses its magnetic properties and becomes much less magnetic. Therefore, the magnetic response is mostly due to the unweathered hard basement rock, and the depths of magnetic sources obtained from the analysis give us the top of the basement. Information about the thickness of the weathered layer would help in assessing the groundwater potential of the region.
Fault interpretation from high‐resolution seismic data in the northern Negev, Israel
Authors: Bruner Ilan , Landa Eugeny
Published: October 2002

Detection and investigation of fault zones are important tools for tectonic analysis and geological studies. A fault zone inferred on high‐resolution seismic lines has been interpreted using a method of detection of diffracted waves utilizing the main kinematic and dynamic properties of the wavefield. The application of the method to field data from the northern Negev in Israel shows that it provides a good estimate of results and, when used in conjunction with the final stacked data, can give the suspected location of the fault, its sense (reverse or normal), and the amount of “low amplitude” displacement (in an order of the wavelength or even less).
Imaging with deep‐water multiples
Authors: Reiter Edmund C. , Toksöz M. Nafi , Keho Timothy H. , Purdy G. M.
Published: October 2002

Acquisition of on‐bottom hydrophone data recording of a near‐surface source provides an opportunity to treat water column multiples as useful signal. A ray‐equation based Kirchhoff depth migration is used to image primary reflections and deep‐water multiples recorded on an Ocean Bottom Hydrophone (OBH). The image of the subbottom sediments is shown to be improved by inclusion of the deep‐water multiple in the imaging process. Field data, jointly acquired by Woods Hole Oceanographic Institute and University of Texas Institute for Geophysics at Austin and consisting of an OBH (2300 m depth) recording a 10 800 cubic inch air gun array, are used to illustrate the feasibility of the technique. Images are obtained from both the primary reflections and from energy that has undergone an additional passage through the water column. Comparison of these images reveals an excellent correlation of reflectors with the predicted polarity reversal observed in the multiple’s image. Synthetic data are used to examine the difficulties in identifying the true path of the water column multiple. For flat‐layered media there are two different multiple paths — one that reflects beneath the source and one that reflects over the receiver — which have identical traveltimes. They do not, however, have the same amplitude, and it can be shown that their amplitudes differ sufficiently to allow a reliable image to be extracted from the energy that reflects over the receiver. As a final step, the image obtained from the multiple is corrected for the π phase shift from the free surface and added to the image from the primary reflection. This approach is limited to areas where water depths allow reliable separation of primary reflections from water column multiples. Application of this technique allows the utilization of coherent deep water multiples and results in both extended lateral coverage and an increased signal‐to‐noise ratio in the final image.
Prestack depth imaging of three‐dimensional shot gathers
Authors: Reshef M.
Published: October 2002

A full three‐dimensional prestack depth imaging method that operates on the entire shot gather is presented. The imaging can be performed in areas with lateral as well as vertical velocity variations. The method is suitable for a true 3-D survey where receivers are spread in both horizontal directions. Irregularities in shooting geometry are handled with no extra cost and do not require any special treatment. The imaging is based on the solution of the 3-D eikonal equation. An efficient computational algorithm that utilizes parallel processing was successfully tested on existing hardware.
An exact solution for the gravity curvature (Bullard B) correction
Authors: LaFehr T. R.
Published: October 2002

The complete Bouguer reduction includes, in addition to the simple Bouguer slab correction (Bullard A), both curvature (Bullard B) and terrain (Bullard C) corrections. A new closed‐form formula for the curvature correction is derived for which the calculated values differ from those published by Swick by more than 0.5 mGal for high elevations. These corrections reduce those of an infinite slab (Bullard A) to that of a spherical cap having a surface radius of 166.7 km. The spherical cap produces a lesser effect than the infinite slab because of the “truncation” of that part of the slab above the earth and extending to infinity, but it produces a greater effect than the slab because of subslab earth resulting from curvature. The physical significance of the correction lies in the combination of these two differences, which are each a function of elevation. The Bullard B surface radius (166.7 km: outer radius of the Hayford‐Bowie system) is reaffirmed by the exact formula to be appropriate for exploration surveys. Three series approximations are presented and compared, but the exact Bullard B formula is very efficient and easy to program for routine data processing.
RAYPT—A new inversion technique for geotomographic data
Authors: Singh R. P. , Singh Y. P.
Published: October 2002

In geophysical tomography, accurate determination of medium parameters using crosshole data is a serious problem. This is due to the fact that the distance matrix [Formula: see text] is highly ill‐conditioned, large, sparse, and nearly singular. The matrix equations may either require underdetermined or overdetermined solutions and may become inconsistent due to the presence of system noise. As a result, direct inversion methods invariably fail to give unique solutions. Many image reconstruction techniques have their own limitations and are not governed by the constraints of background data. To reduce nonuniqueness and to improve the quality of the reconstructed image, many workers have suggested that either one should add more diverse data or apply constraints based on preliminary information about the medium. We use a new image reconstruction technique “RAYPT” (ray‐projection technique), similar to algebraic reconstruction technique (ART), to compute average slowness of each ray. Based on average slowness value of the rays, constraints are applied to the rays, helping to accurately estimate the image parameters. By taking rays one by one from the first to the last, the iteration continues until the desired rms error value is obtained. We find that this technique is not only computationally very efficient but also gives very good quality images.
P-SV reflection moveouts for transversely isotropic media with a vertical symmetry axis
Authors: Seriff A. J. , Sriram K. P.
Published: October 2002

In a recently published short note, F. K. Levin (1989) discusses the relation between the “moveout velocities” of P-P, P-SV, and SV-SV reflections from the bottom of a transversely isotropic layer with a vertical symmetry axis. We refer to such a medium as one exhibiting “polar anisotropy.” Levin’s note was prompted by a paper of Tessmer and Behle (1988), and it is relevant to a paper by Iverson and others (1989), both of which discuss the computation of shear velocities from moveout velocities obtained with P-P and P-S reflections. Levin’s note addresses the practically important question of the use of this method in the presence of polar anisotropy, a phenomenon which we believe occurs almost universally in the sedimentary layers of the real earth. Levin suggests that polar anisotropy of “typical” magnitude must be considered in this problem. He uses as an estimate of typical magnitudes data given by Thomsen (1986) and concludes from numerical examples that the method of estimating shear velocities proposed by Tessmer and Behle and by Iverson may be subject to unacceptably large errors in many real cases. Moreover, Levin suggests that the source of these errors is mysterious.
A computer implementation of 2.5-D common‐shot inversion
Authors: Dong Wenjie , Emanuel Mark Jeffrey , Bording Phillip , Bleistein Norman
Published: October 2002

The computer implementation of a two‐and‐one‐half dimensional (2.5-D) constant density prestack inversion formalism with laterally and depth‐dependent background propagation speed is a Kirchhoff‐type inversion, summing data from a line of receivers over traveltime curves in the depth‐dependent background medium with weights determined from Born/Kirchhoff inversion theory. This theory predicts that the output will be a reflector map with peak amplitudes on each reflector being in known proportion to the angularly dependent geometrical optics reflection coefficient. The 2.5-D feature provides for out‐of‐plane spreading correction consistent with the prescribed background medium. The method is applied to a synthetic data set and to a physically modeled data set generated at the Seismic Acoustic Laboratory. The graphical output demonstrates the validity of the formalism as a Kirchhoff migration. Parameter estimation for the synthetic data confirmed the theory. Parameter estimation for the experimental data was less successful, partially due to problems with amplitude control in the original experiment and partially due to the limited aperture of the common‐shot data, thereby suggesting that a common‐offset inversion might be more useful for parameter estimation.
Mixed‐delay wavelet deconvolution of the point‐source seismogram
Authors: Tygel M. , Huck H. , Hubral P.
Published: October 2002

The problem of extracting a mixed‐delay source wavelet from a point‐source seismogram for an acoustic, horizontally stratified medium (bounded by a free surface above and a half‐space below or between two half‐spaces) can be completely solved without any further assumptions about the source pulse or the model parameters. The solution relies on information contained in the so‐called evanescent part of the point‐source seismogram, which can be extracted via a plane‐wave decomposition, i.e., by a transformation of the point‐source seismogram from the time‐space domain into the frequency‐rayparameter domain.
The electromagnetic response of an azimuthally anisotropic half‐space
Authors: Li Xiaobo , Pedersen Laust B.
Published: October 2002

Fracture zones are pervasive in crystalline areas. When the earth is seen over a sufficiently large volume fracture zones may be too thin to be individually identified. If they have preferred directions in that volume, the volume can be considered to be an azimuthally anisotropic medium. We have formulated the electromagnetic fields induced by a horizontal electric dipole on the surface of a homogeneous half‐space with azimuthally anisotropic conductivity. The field components are expressed by the two‐dimensional Fourier transform which can be computed by a fast Hankel transform method. The impedance tensor and tipper functions of controlled source tensor magnetotellurics are derived by exciting the dipole source in two different directions. We show the behavior of impedance tensor, tipper functions and their derived quantities: rotational invariants and Parkinson vectors. All transfer functions clearly show anisotropic characteristics. Contours of rotationally invariant apparent resistivities and phases for fixed frequencies are elongated in the direction of maximum conductivity, and Parkinson’s vectors tend to point in the same direction.
Three‐dimensional Fourier gravity inversion with arbitrary density contrast
Authors: Guspí F.
Published: October 2002

The use of variable‐density contrasts in gravity inversion has gained increasing importance in recent years due to the necessity of constructing more realistic models of geophysical structures such as sedimentary basins. Linear, quadratic, and exponential variations, either in the space or in the frequency domain, are the basis of several methods. See, among others, the papers by Granser (1987), Chai and Hinze (1988), Reamer and Ferguson (1989), and Rao et al. (1990). Guspí (1990) used polynomial density‐depth functions for inverting gravity anomalies into 2-D polygons in the space domain.
Solving for the thickness and velocity of the weathering layer using 2-D refraction tomography
Authors: Docherty Paul
Published: October 2002

The weathering layer may be the most variable of all layers; yet in seismic processing, it is routinely taken to be uniform either in thickness or velocity. Datuming through an incorrect weathering model can corrupt the stack, possibly introducing false structure into deep reflectors. Recognizing that direct measurements of the weathering properties may not, in practice, be readily available, this paper investigates the feasibility of computing the weathering model from the travel-times of refracted first arrivals. The problem is formulated in terms of the difference in arrival time at adjacent receivers, resulting in a much sparser matrix for inversion. Lateral variations in both the weathering thickness and velocity are sought; however, analysis indicates a fundamental long wavelength ambiguity in the solution, with gradual changes in velocity trading‐off with gradual changes in thickness. In most cases, it is necessary to include a small number of constraints to obtain the true weathering model. Any roughness in the solution that is not required to fit the data is most effectively removed using a second difference smoothing technique.
Linear filters for solving the direct problem of potential fields
Authors: Bezvoda Vaclav , Hrabe Jan , Segeth Karel
Published: October 2002

One of the major advances of computer science in recent years is the introduction of parallel processors. The efficiency of such devices depends strongly on the symmetry of the algorithms implemented; probably the most efficient are fast Fourier transform‐type (FFT‐type) algorithms. It is possible to show that for these algorithms a SIMD (Single Instruction Multiple Data) processor reduces the number of operations from [Formula: see text] to [Formula: see text] (where N is the number of input data and P is the number of processors) provided that both N and P are integer powers of 2. It thus seems advantageous to reconsider the possible uses of the fast algorithms in geophysics, including those not successful on nonparallel computers (Bezvoda et al., 1986).
Imaging salt with turning seismic waves
Authors: Hale Dave , Hill N. Ross , Stefani Joe
Published: October 2002

Turning seismic waves, which first travel downward and then upward before (and after) reflection, have been recorded in a 3-D seismic survey conducted over an overhanging salt dome. Careful processing of these turning waves enables the imaging of the underside of the salt dome and of intrusions of salt into vertical faults radiating from the dome. When seismic wave velocity increases with depth, waves that initially travel downward are reflected and may turn so as to travel upward before reflection. A simple geometrical argument suggests that these turning waves are likely to exhibit abnormal moveout in common‐midpoint (CMP) gathers, in that reflection time decreases with increasing source‐receiver offset. This abnormal moveout and the attenuation of turning waves by most migration methods suggest that conventional seismic processing does not properly image turning waves. The most important step in imaging turning waves, assuming that they have been recorded, is the migration process. Simple and inexpensive modifications to the conventional phase‐shift migration method enable turning waves to be imaged for little additional computational cost. The examples provided in this paper suggest that these and other such modifications to conventional processing should be used routinely when imaging salt domes.
Location of subsurface targets in geophysical data using neural networks
Authors: Poulton Mary M. , Sternberg Ben K. , Glass Charles E.
Published: October 2002

Neural networks were used to estimate the offset, depth, and conductivity‐area product of a conductive target given an electromagnetic ellipticity image of the target. Five different neural network paradigms and five different representations of the ellipticity image were compared. The networks were trained with synthetic images of the target and tested on field data and more synthetic data. The extrapolation capabilities of the networks were also tested with synthetic data lying outside the spatial limits of the training set. The data representations consisted of the whole image, the subsampled image, the peak and adjacent troughs, the peak, and components from a two‐dimensional (2-D) fast Fourier transform. The paradigms tested were standard back propagation, directed random search, functional link, extended delta bar delta, and the hybrid combination of self‐organizing map and back propagation. For input patterns with less than 100 elements, the directed random search and functional link networks gave the best results. For patterns with more than 100 elements, self‐organizing map to back propagation was most accurate. Using the whole ellipticity image gave the most accurate results for all the network paradigms. The fast Fourier transform data representation also yielded good results with a much faster computation time. Average accuracies of offset, depth, and conductivity‐area product as high as 97 percent could be achieved for test and field data and 88 percent for extrapolation data.
On: “Seismic amplitude anomalies and AVO analyses at Mestena Grande Field, Jim Hogg Co., Texas” by R. C. Burnett (August 1990 GEOPHYSICS, p. 1015–1025).
Authors: Juhlin Christopher
Published: October 2002

I read with interest R. C. Burnett’s paper on amplitude and AVO analyses and found it surprising that, when using a low Poisson’s ratio for the gas sands, there was not a greater difference in the AVO response compared to that of using a high Poisson’s ratio (Figure 11 in his paper). This is contrary to what I would expect based on my experience with AVO modeling. I have found that when there is a large change in Poisson’s ratio (greater than 0.2) between layers, this usually has a significant effect on the AVO response even if the layers are thin.
SMAP revisited and revised
Authors: Brumbaugh David L.
Published: October 2002

The original seismic mis‐tie adjustment procedure (SMAP) is restricted to uniform grids of 2-D seismic lines. In many areas, uniform grids do not occur, especially with multiple‐vintage coverage. An alternative method is proposed still using linear programming techniques to simultaneously minimize the maximum error and absolute error sum, but extending the technique to handle any 2-D seismic grid orientation. Horizons can be treated individually or in combination, with certain restrictions. The original procedure also has some poorly imposed constraints. SAS/OR™ software improves upon the original SMAP results. A comparison is made between a 2-D seismic interpretation corrected with the new algorithm and a 3-D seismic interpretation covering the same area, showing an excellent correlation between the two structural pictures. 2-D seismic interpretation is still a viable tool, and improved maps can be made through this linear programming technique for minimizing mis‐ties.
Automated interpretation of horizontal magnetic gradient profile data
Authors: Marcotte D. L. , Hardwick C. D. , Nelson J. B.
Published: October 2002

A new technique has been developed to estimate the vertical magnetic gradient [Formula: see text] along a profile from measurements of the two orthogonal horizontal magnetic gradients [Formula: see text]. In addition, a means of identifying two‐dimensional anomaly sources and the angle of the structure relative to the profile direction is shown to be a function of [Formula: see text]. This angle can be used to correct interpretations from vertical gradient or total field inversion methods which assume source structures oriented perpendicular to the profile direction. A modified Werner deconvolution algorithm for vertical gradient data incorporating these features has been applied to both real and simulated horizontal gradient data.
Velocity modeling of a long‐period static anomaly, West Cameron Block 225, a Gulf of Mexico case history
Authors: Pickard James E.
Published: October 2002

The local structure at West Cameron Block 225, offshore Louisiana, was originally interpreted as a double‐crested anticlinal feature. Production was established on both crests. A deviated well was later drilled toward the intervening saddle; the well was structurally higher than the supposed crestal wells and found three additional pay sands. The revised interpretation assumed that the structural saddle was a long period static anomaly. This interpretation produced a single anticlinal feature with substantial attic reserves. In drilling the well to tap these attic reserves, many of the expected pay sands were found to be wet or missing. The well was structurally lower relative to the previous wells at nearly all the marker beds. Velocity sag was indeed present; however, only 60 percent of the correction applied to the seismic data could be accounted for in this manner. The stacking velocity profile was modeled for the maximum velocity sag interpretation using two horizons. The modeled profiles exhibit the anticipated fluctuations, but the magnitude of the anomalies is far in excess of the observed values. Therefore, the assumption of a large long‐period static anomaly is inconsistent with the stacking velocity information, and this interpretation must be rejected. This inexpensive technique can be applied in an exploration mode, and its use is not confined to near‐surface anomalies. Accurate estimates of the width and delay of the anomaly can be obtained through iterative modeling.
Viscoelastic attenuation, anisotropy, and AVO
Authors: Samec Pierre , Blangy J. P.
Published: October 2002

The classical interpretation relating Amplitude Versus Offset (AVO) to Poisson’s ratio and other petrophysical properties is based on the assumptions of elasticity and isotropy. We extend this interpretation to a layered medium with anisotropic and/or viscoelastic properties, using a Fourier Pseudo‐Spectral method to solve the wave equation. Both viscoelasticity and anisotropy are key factors for the quantitative interpretation of AVO trends, because they contribute to the seismic energy partition at geological interfaces (the reflection coefficients), and because they continually induce propagation effects. We show that: 1) Reflection coefficients at an interface are strongly dependent on the elastic anisotropy of both the overlying and the underlying media. The AVO effect is further complicated by materials with viscoelastic properties. 2) Propagation effects are due to elastic anisotropic energy focusing and viscoelastic dissipation that distort the energy and phase distribution of the incident and reflected wavefronts. These two phenomena can be of the same order of magnitude as variations in reflection amplitudes with offset and can make it difficult to recover reflection coefficients along an interface from seismic data. In theory, they make the amplitude determination of a seismic event somewhat dependent on wavelet phase changes that occur continually as the wavefront propagates. In practice, they create anisotropic radiation patterns and differentially focus the seismic energy distribution along the wavefront. For these reasons, all detailed reservoir characterizations based on modeling and interpretation work should attempt to account for anisotropy and viscoelastic attenuation; this is not an easy task in the real world because of the difficulty in prescribing appropriate physical parameters.
Transformation to zero offset for mode‐converted waves
Authors: Alfaraj Mohammed , Larner Ken
Published: October 2002

The transformation to zero offset (TZO) of prestack seismic data for a constant‐velocity medium is well understood and is readily implemented when dealing with either P‐waves or S‐waves. TZO is achieved by inserting a dip moveout (DMO) process to correct data for the influence of dip, either before or after normal moveout (NMO) correction (Hale, 1984; Forel and Gardner, 1988). The TZO process transforms prestack seismic data in such a way that common‐midpoint (CMP) gathers are closer to being common reflection point gathers after the transformation.
Surface‐to‐borehole fault imaging in a buried horizontal waveguide—An experimental study in the southern North Sea
Authors: Mason I. M. , Jackson G. M. , Lee D. , Campbell D. S.
Published: October 2002

One fifth of the United Kingdom’s energy is derived from the gas fields of the southern North Sea. The area is divided into large fault blocks, which are laced with minor faults. Surface reflection seismic fault maps of the main producing horizon are distorted by salt rollers or pillows in the overlying high velocity Zechstein evaporites. We have developed an inexpensive method of confirming the presence of minor faults in the relatively flat base of the Zechstein around a well that would simplify production planning. Deep well sonic logs indicate that seismic velocity drops abruptly beneath the base Zechstein and then rises gradually with depth. This profile is characteristic of a whispering gallery, or a buried, variable‐index beam waveguide. If base Zechstein faults scatter energy efficiently into the waveguide, then the faults might be mapped by moving a seismic source over the surface and recording the mode converted arrivals at a geophone within the guide. To test this possibility, over 3000 shots were fired at 27-m intervals in thirteen lines spaced by ≈500 m from a 4436-cu in. (0.075-cu m) marine air gun array into a 3 km deep triaxial VSP geophone, stationed beneath the Rotliegendes. Secondary arrivals were found to move out across the raw field geophone gathers in the manner expected of down‐going compressional to both guided compressional and guided shear vertical mode conversions [P-P and P-SV] at the base of the Zechstein. Prestack migration yielded a map of mode conversion activity at the edges of known faults with throws of 30 + m over an area of [Formula: see text]. We conclude that there is an extensive seismic waveguide beneath the Zechstein in the southern North Sea. Faults scatter down‐going P waves efficiently into the guide; noise levels are low at the bottom of a borehole. Vertical reverberation follows a surface source as it moves away from the wellhead. Horizontal waveguiding reduces geometric spreading loss. These factors make it possible to map small reservoir faults with high‐resolution at long ranges from an exploration well.
Reflection tomography in the postmigrated domain
Authors: Stork Christof
Published: October 2002

Reflection tomography is an inversion method that adjusts a velocity and reflector depth model to be consistent with the prestack time data. This tomography approach minimizes the misfit of the data and model in the premigrated domain. Generally, the data are represented by the traveltimes of reflection events, which has made the technique problematic and unpopular. Techniques generally known as “migration velocity analysis” have a similar objective but use the postmigrated domain. For a variety of practical reasons, this postmigrated domain has advantages over the premigrated domain. With slight modifications, the reflection tomography approach can be implemented in the postmigrated domain. In this domain, a model is determined by optimizing the consistency of imaged reflection events on what has been called a common reflection point (CRP) gather. Extending reflection tomography to the postmigrated domain allows much of the knowledge developed for migration velocity analysis to be coupled with that of reflection tomography. As a result, many of the practical techniques developed for migration velocity analysis can be used to improve the robustness and efficiency of reflection tomography. Similarly, much of the reflection tomography work done on analysis and solution of the linear system can be applied to postmigrated domain optimization.
Similarity and the inverse Q filter: Some simple algorithms for inverse Q filtering
Authors: Hargreaves Neil D.
Published: October 2002

A number of authors have addressed the issue of the time variance of the forward and inverse Q filter on the assumption of a frequency‐constant or nearly frequency‐constant Q factor. Early papers by Futterman (1962) and Carpenter (1966) derived time‐domain solutions for the forward Q filter, given an assumed behavior of Q at low and high frequencies, while Kjartansson (1979) used an assumed stress‐strain relationship to derive the forward Q filter for a precisely frequency‐constant Q. Bickel and Natarajan (1985) applied these earlier results to the derivation of various expressions for the inverse Q filter.
Two‐dimensional inversion filters in magnetic prospecting: Application to the exploration for buried antiquities
Authors: Tsokas Gregory N. , Papazachos Constantinos B.
Published: October 2002

The magnetic total field anomaly is considered as the convolution of two analytically determined functions. One of them is chosen such that it modifies the amplitude of the anomaly while the other controls the anomaly pattern. Using the anomaly of vertical‐sided finite prisms, the inverse of the shape function is computed in the Wiener mode. This is an optimum filter operator, in the least‐square sense, to be convolved with the data. The majority of the structures which are the targets of exploration at archaeological sites can be represented as assemblages of vertical‐sided prisms. This fact motivates the selection of the basic model. On that basis the filtering scheme results in anomalies centered at the epicenter of the disturbing bodies, delineates their lateral extent fairly well, and gives a measure of their magnetization. Applications of synthetic and actual data clarify the merits and disadvantages of the technique.
The relationship between semblance and eigenstructure velocity estimators
Authors: Kirlin R. Lynn
Published: October 2002

This short presentation gives for the first time a formulation of the semblance coefficient in terms of data covariance matrix eigenstructure. Because the high‐resolution wavefront or spectral eigenstructure methods have received so much interest over the past decade, it is necessary to analytically tie the conventionally used semblance produce to eigenstructure, thereby allowing the seismic signal analyst an opportunity to relate the various displays of velocity spectra using more than visual appearance. The eigenstructure form of semblance is compared to a number of the now well‐known eigenstructure‐based spectral estimators that separate signal and noise (vector) subspaces. Because of the inclusion of noise‐space energy in the coherence measure, conventional semblance does not have the resolving power of the newer methods. We suggest an enhanced semblance, based on the signal and noise subspace separation concept. A brief simulation verifies the visual improvement in the velocity spectrum obtained from the enhanced version.
The transient EM step response of a dipping plate in a conductive half‐space
Authors: Hanneson James E.
Published: October 2002

An algorithm for computing the transient electromagnetic (TEM) response of a dipping plate in a conductive half‐space has been developed. For a stationary [Formula: see text] current loop source, calculated profiles simulate the response of the University of Toronto electromagnetic system (UTEM) over a plate in a 1000 Ω ⋅ m half‐space. The objective is to add to knowledge of the galvanic process (causing poloidal plate currents) and the local induction process (causing toroidal currents) by studying host and plate currents with respect to surface profiles. Both processes can occur during TEM surveys. Plates are all [Formula: see text] thick with various depths, dips, and conductances. Calculated host and plate currents provide quantitative examples of several effects. For sufficiently conductive plates, the late time currents are toroidal as for a free‐space host. At earlier times, or at all times for poorly conducting plates, the plate currents are poloidal, and the transitions to toroidal currents, if they occur, are gradual. At very late times, poloidal currents again dominate any toroidal currents but this effect is rarely observed. Stripped, point‐normalized profiles, which reflect secondary fields caused by the anomalous plate currents, illustrate effects such as early time blanking (caused by noninstantaneous diffusion of fields into the target), mid‐time anomaly enhancement (caused by galvanic currents), and late time plate‐in‐free‐space asymptotic behavior.
Modeling elastic fields across irregular boundaries
Authors: Muir Francis , Dellinger Joe , Etgen John , Nichols Dave
Published: October 2002

Geologists often see the earth as homogeneous blocks separated by smoothly curving boundaries. In contrast, computer modeling algorithms based on finite‐difference schemes require elastic constants to be specified on the vertices of a regular rectangular grid. How can we convert a continuous geological model into a form suitable for a finite‐difference grid? One common way is to lay the finite‐difference grid down on the continuous geological model and use whatever elastic constants happen to lie beneath each of the grid points.
Cross‐borehole tomography in anisotropic media
Authors: Carrion Philip , Costa Jesse , Ferrer Pinheiro Jose E. , Schoenberg Michael
Published: October 2002

Anisotropy has significant effect on traveltime cross‐borehole tomography. Even relatively weak anisotropy cannot be ignored if accurate velocity estimates are desired, since isotropic traveltime tomography treats anisotropy as inhomogeneity. Traveltime data in our examples were synthetically generated by a ray‐tracing code for anisotropic media, and the computed quasi‐P‐wave traveltimes were subsequently inverted using the “dual tomography” technique (Carrion, 1991). The results of the tomographic inversion show typical artifacts due to the anisotropy, and that accurate imaging is impossible without taking the anisotropy into account.
Modeling studies of interference reflections in thin‐layered media bounded by coal seams
Authors: Gochioco Lawrence M.
Published: October 2002

High‐resolution seismic data collected over a major U.S. coal basin indicated potential complex problems associated with interference reflections. These problems differed from those normally encountered in the exploration of oil and gas because of differences in the geologic boundary conditions. Modeling studies were conducted to investigate the effects of overlapping primary reflections and the composite reflection that result from stacking individual wavelets. A modified empirical formula of Lindseth’s linear relationship between acoustic impedance and velocity is used to extrapolate velocity information from density logs to provide appropriate geophysical properties for modeling. The synthetic seismograms generated from density and synthetic sonic logs correlated well with the processed seismic data. A 150-Hz Ricker wavelet is used to convolve with the computer models, and the models showed that certain anomalous composite reflections result from the superposition of overlapping primary reflections. Depending on the traveltime delay of latter primary reflections, constructive or destructive interference could significantly alter the signature of the initial reflection associated with the bed of interest, which may lead to misinterpretations if not properly identified. The stratigraphic modeling technique further enhances the interpretation process and shows a close correlation with the seismic data, suggesting that more precise analytical methods need to be used to interpret, sometimes complex, high‐resolution seismic data.
Comparison of vertical seismic profiling techniques
Authors: Zimmerman Linda J. , Chen Sen T.
Published: October 2002

To study the imaging characteristics of various vertical seismic profiling techniques, two vertical seismic profiles (VSP) and a reversed vertical seismic profile (RVSP), where source and receiver positions are interchanged, were collected in the Loudon Oil Field in Illinois. Both VSPs were collected using a line of dynamite charges on the surface as sources. One was collected with geophones and the other with hydrophones as downhole receivers. The RVSP was collected by detonating 25 gram explosive charges in a well and detecting the seismic response with geophones at the surface. Three subsurface images (VSP with geophones, VSP with hydrophones, and RVSP) were produced using VSP-CDP transforms. For comparison, a surface seismic profile was collected along the same line with dynamite sources and vertical geophone receivers. The RVSP and hydrophone VSP stacked sections both produced higher frequency images at shallower depths than did the geophone VSP stacked section. However, the lower frequency geophone VSP stacked section produced an interpretable subsurface image at much greater depths than either the RVSP or the hydrophone VSP sections. The differences are due in part to the more powerful surface sources that were used for the VSPs than the downhole sources used for the RVSP. Furthermore, tube‐wave noise was a more severe problem for both the RVSP and the hydrophone VSP than for the geophone VSP. The results of this experiment demonstrate that if tube‐wave noise could be suppressed, hydrophone VSPs would provide attractive alternatives to geophone VSPs, because it is much easier and cheaper to deploy multilevel hydrophones downhole than geophones. Also, if a high‐powered, nondestructive source is developed, RVSP could be a practical alternative to VSP since one can easily lay out numerous receivers on the surface to record multioffset or three‐dimensional (3-D) VSP data.
Bullwinkle: A unique 3-D experiment
Authors: O’Connell James K. , Kohli Madhu , Amos Scott
Published: October 2002

In the spring of 1988, Shell Offshore Inc. acquired two orthogonal three‐dimensional (3-D) surveys at Prospect Bullwinkle, located in the Green Canyon area of the Gulf of Mexico, to aid the development program which began later that year. Two surveys were acquired because of the complexity of the salt structures in the vicinity of the prospect. The independent acquisition and processing of two surveys shot perpendicular to each other provided a unique data set for checking the quality and accuracy of standard 3-D techniques. The high development cost of this deep water (410 m) turbidite field supported the acquisition of two 3-D data sets to provide a valuable redundancy for stratigraphic interpretation. This large scale 3-D experiment has been analyzed in terms of interpretive impact. Detailed comparisons of the seismic images away from the salt complex show good agreement between the two surveys and verify the relative accuracy and repeatability of the acquisition, processing, and interpretation techniques. Structural comparisons between the surveys show that acquisition oriented in a strike direction to the primary salt face yields a superior sediment image, particularly near overhung salt. An examination of the effects of shooting direction on small scale stratigraphic resolution illustrates the importance of fine sampling in the dip direction to the features of interest. Amplitude maps extracted for the main bright spot level show differences in areal continuity that are large enough to affect the geologic model of the prospect.
On the use of localization theory to characterize elastic wave propagation in randomly stratified 1-D media
Authors: Scales John A.
Published: October 2002

In a series of papers Benjamin White, Ping Sheng, George Papanicolaou and others have described the application of localization theory to the study of elastic wave propagation in randomly stratified one‐dimensional (1-D) media. They describe methods for computing the localization parameters and apply the results to sonic well‐log data. In this note, I will show that, at least in the seismic band, the results of their calculations disagree with effective medium theory. This disagreement may to be due to the lack of low‐frequency information in exploration seismic data.
Amplitude map analysis using forward modeling in sandstone and carbonate reservoirs
Authors: Neff Dennis B.
Published: October 2002

The extent to which seismic amplitude maps can contribute to the analysis of hydrocarbon reservoirs was investigated for clastic and carbonate reservoirs worldwide. By using a petrophysical‐based, forward modeling process called incremental pay thickness (IPT) modeling, five lithology types were quantitatively analyzed for the interplay of seismic amplitude versus lithology, porosity, hydrocarbon pore fluid saturation, bedding geometries, and reservoir thickness. The studies identified three common tuning curve shapes (concave, convex, and bilinear) that were primarily dependent upon the lithology model type and the average net porosity therein. While the reliability of pay and porosity predictions from amplitude maps varied for each model type, all analyses showed a limited thickness range for which amplitude data could successfully predict net porosity thickness or hydrocarbon pore volume. The investigation showed that systematic forward modeling is required before amplitude maps can be properly interpreted.