bluuebunny/crossref_metadata_2025 huggingface dataset:
Pore‐filling material and its effect on velocity in rocks
Authors: Marion D. , Nur A.
Published: October 2002

A new method estimates the dependence of P and S velocities in rocks on elastic properties of the pore‐filling material by going beyond Gassmann’s relations and accounting for pore‐filling materials with nonzero shear stiffness. The resulting estimates are in good agreement with experimental data and Gassmann’s relations for values of velocity in water‐saturated rocks predicted from velocity measured on dry rocks. The method successfully predicts the temperature dependence of velocity: (1) in frozen rocks at the ice‐water phase transition, and (2) in rocks saturated with heavy hydrocarbon undergoing solid‐liquid phase transition.
Industrial interference magnetotellurics: An example from the Tangshan area, China
Authors: Qian Wei , Pedersen L. B.
Published: October 2002

In order to attack the problem of industrial interference in magnetotellurics, we apply the general theory of Weidelt and Christensen describing the electromagnetic fields of a grounded electric dipole over a horizontally stratified earth. The impedance tensor for dipole fields is proven to be independent of the dipole orientation. Within the near field, the elements of the impedance tensor are real quantities, and are independent of period. Because of this, the logarithm of the apparent resistivity versus the logarithm of period rises with a slope of 45 degrees and the phase is zero. We study the inversion of the controlled‐source impedance tensor to reveal earth structure at a given distance from the source, using a nonlinear least‐squares technique. We present an interpretation of magnetotelluric data in the Tangshan area of China as a practical application of the technique. Sedimentary thicknesses of about 600 m are estimated and agree well with conventional Schlumberger soundings. The upper resistive crust (thickness about 13 km) overlies a more conductive lower crust whose resistivity is below a few hundred Ω⋅m.
Fresnel zones in the light of broadband data
Authors: Knapp R. W.
Published: October 2002

The investigation of zero‐offset response to circular reflectors of increasing Fresnel zone size shows that reflection response is a constant and is independent of reflector size, except when the reflector diameter is so small that the diffractions interfere with the primary reflection. The extent of this effect is dependent upon vertical resolution and the time separation of the primary reflector and the diffraction. Interference occurs for reflectors smaller in diameter than the first Fresnel zone. Migration removes this interference. For broadband data the Fresnel zone solution breaks into two parts: the primary reflector and the edge‐effects diffractor. With broadband seismic data, reflections and diffractions separate in time, except at locations near faults or very small bodies. Reflections are the seismic response to interlayer discontinuity and are independent of reflector size. Diffractions are the seismic response to lateral discontinuities and edges and depend on proximity to—and geometry of—the edge. Except in the locale of an edge, broadband reflections and diffractions are separated physically on the section and mentally by the interpreter. Furthermore, standard CMP processing attenuates diffractions, especially when CMP lateral offset is some distance from the diffractor.
Compact sleeve‐gun source arrays
Authors: Fontana P. M. , Haugland T.‐A.
Published: October 2002

Data derived from far‐field signature measurements have inspired several guidelines for using clustered sleeve guns effectively in tuned marine seismic source arrays. Primarily, these data show that for a given volume the signature produced by a cluster of sleeve guns has a comparable bubble period, increased primary amplitude, and reduced bubble‐pulse amplitude compared to the signature of a single gun. These results agree with those reported for clusters of conventional air guns. However, when the data are analyzed in terms of acoustic and operational efficiency, we find that for array elements with volumes greater than [Formula: see text] two‐gun clusters are more desirable than equivalent volume clusters of several small volume guns. For array elements with volumes up to [Formula: see text], the data show no significant advantages for using clusters instead of single guns. These guidelines have led to the design of sleeve‐gun arrays that produce signatures with temporal and spectral characteristics equal to or exceeding those produced by conventional air‐gun arrays incorporating almost twice the total gun volume. Moreover, these new arrays operate with a total number of individual guns comparable to conventional arrays, thus improving the performance of source arrays on small survey vessels without having to increase air compressor capacity or ancillary source equipment.
Inverse Q filtering by Fourier transform
Authors: Hargreaves N. D. , Calvert A. J.
Published: October 2002

Although some attention has been paid to the idea that seismic migration is equivalent to a type of deconvolution (of the spatial wavelet), less thought has been given to the opposite perspective: that deconvolution (of the earth Q filter) might itself be equivalent to a form of migration. The key point raised in this paper is that a dispersive 1-D backward propagation can form the basis of a number of different algorithms for inverse Q filtering, each of which is akin to a particular migration algorithm. An especially efficient algorithm can be derived by means of a coordinate transformation equivalent to that in the Stolt frequency‐wavenumber migration. This fast algorithm, valid for Q constant with depth, can be extended to accommodate depth‐variable Q by cascading a series of constant Q compensations, as in cascaded migration. By combining a cascaded phase compensation with a windowed approach to amplitude compensation, we obtain an algorithm that is sufficiently efficient to be used routinely for prestack data processing. Data examples compare the results of conventional processing with the more stable phase treatment that can be obtained by including prestack inverse Q filtering in the processing.
Principal component transforms of triaxial recordings by singular value decomposition
Authors: Jackson G. M. , Mason I. M. , Greenhalgh S. A.
Published: October 2002

Polarization analysis can be achieved efficiently by treating a time window of a single‐station triaxial recording as a matrix and doing a singular value decomposition (SVD) of this seismic data matrix. SVD of the triaxial data matrix produces an eigenanalysis of the data covariance (cross‐energy) matrix and a rotation of the data onto the directions given by the eigenanalysis (Karhunen‐Loève transform), all in one step. SVD provides a complete principal components analysis of the data in the analysis time window. Selection of this time window is crucial to the success of the analysis and is governed by three considerations: the window should contain only one arrival; the window should be such that the signal‐to‐noise ratio is maximized; and the window should be long enough to be able to discriminate random noise from signal. The SVD analysis provides estimates of signal, signal polarization directions, and noise. An F‐test is proposed which gives the confidence level for the hypothesis of rectilinear polarization. This paper illustrates the analysis and interpretation of synthetic rectilinearly and elliptically polarized arrivals at a single triaxial station by SVD.
Wave‐equation traveltime inversion
Authors: Luo Y. , Schuster G. T.
Published: October 2002

This paper presents a new traveltime inversion method based on the wave equation. In this new method, designated as wave‐equation traveltime inversion (WT), seismograms are computed by any full‐wave forward modeling method (we use a finite‐difference method). The velocity model is perturbed until the traveltimes from the synthetic seismograms are best fitted to the observed traveltimes in a least squares sense. A gradient optimization method is used and the formula for the Frechét derivative (perturbation of traveltimes with respect to velocity) is derived directly from the wave equation. No traveltime picking or ray tracing is necessary, and there are no high frequency assumptions about the data. Body wave, diffraction, reflection and head wave traveltimes can be incorporated into the inversion. In the high‐frequency limit, WT inversion reduces to ray‐based traveltime tomography. It can also be shown that WT inversion is approximately equivalent to full‐wave inversion when the starting velocity model is “close” to the actual model. Numerical simulations show that WT inversion succeeds for models with up to 80 percent velocity contrasts compared to the failure of full‐wave inversion for some models with no more than 10 percent velocity contrast. We also show that the WT method succeeds in inverting a layered velocity model where a shooting ray‐tracing method fails to compute the correct first arrival times. The disadvantage of the WT method is that it appears to provide less model resolution compared to full‐wave inversion, but this problem can be remedied by a hybrid traveltime + full‐wave inversion method (Luo and Schuster, 1989).
Nonlinear waveform inversion of plane‐wave seismograms in stratified elastic media
Authors: Kormendi F. , Dietrich M.
Published: October 2002

We present a method for determining the elastic parameters of a horizontally stratified medium from its plane‐wave reflectivity. The nonlinear inverse problem is iteratively solved by using a generalized least‐squares formalism. The proposed method uses the (relatively) fast convergence properties of the conjugate gradient algorithm and achieves computational efficiency through analytical solutions for calculating the reference and perturbational wavefields. The solution method is implemented in the frequency‐wave slowness domain and can be readily adapted to various source‐receiver configurations. The behavior of the algorithm conforms to the predictions of generalized least‐squares inverse theory: the inversion scheme yields satisfactory results as long as the correct velocity trends are introduced in the starting model. In practice, the inversion algorithm should be applied first in the precritical region because of the strong nonlinear behavior of postcritical data with respect to velocity perturbations. The suggested inversion strategy consists of first inverting for the density and P‐wave velocity (or P‐wave impedance) by considering plane waves in the low slowness region (near‐normal angles of incidence), then in optimizing for the S‐wave velocity by progressively including contributions from the high slowness region (steep angles of incidence). Numerical experiments performed with noise‐free synthetic data prove that the proposed inversion method satisfactorialy reconstructs the elastic properties of a stratified medium from a limited set of plane‐wave components, at a reasonable computing cost.
The generalized gravity anomaly: Endoscopic microgravity
Authors: Lakshmanan J.
Published: October 2002

Various underground 3-D gravity surveys have necessitated a generalization of the usual gravity corrections and of the Bouguer anomaly. The method presented here compares raw, time‐dependent gravity measurements, to a model’s total theoretical field, including known fields: moon, sun, 1967 Reference Ellipsoid, oceans; partially known fields: due to a single digital terrain model of known geometry but of unknown densities; and unknown fields due to underground structures of unknown shapes and of unknown densities. For a single‐density model, the corresponding first‐degree residual is close in concept to the Bouguer anomaly. To best determine underground structure, generalized inversion then leads to determination of the one or several densities and of one or several “regional” parameters, which minimize residuals. The suggested method is mainly advantageous in special types of gravity surveys, such as rugged terrain, or in the case of underground surveys, where conventional corrections, with a preset terrain density can possibly lead to substantial errors. Two field examples are developed (1) the Cheops pyramid survey, where the processing of gravity measurements inside, above, and around the pyramid led to an evaluation of the structure’s overall density and of density changes in the structure; and (2) the Coche hydroelectric tunnel in the Alps, where the method leads to a 3-D model explaining the very strong gravity anomalies observed in the tunnel and on the mountain above it.
Complex conductivity measurements and fractal nature of porosity
Authors: Ruffet C. , Gueguen Y. , Darot M.
Published: October 2002

Complex resistivity measurements were performed on 22 saturated samples (sandstones, slate, shale, and granites) at room temperature and pressure over a frequency range from 1 Hz to 5 MHz, using a two‐terminal sample holder. Although low‐frequency measurements (from 1 Hz to 1–10 kHz) are perturbed by electrode polarization phenomena, we observed classical behavior for 20 samples, i.e., behavior that can be fitted to a Cole and Cole response function, and different behavior for the other two (two slates). These two last samples exhibit an almost constant imaginary part of the complex resistivity. Since the frequency dependence is caused by interfacial effects, it is possible to characterize the internal surface area from electrical measurements. We use models developed by Le Méhauté and Crépy (1983) and Po Zen Wong (1987) to calculate the fractal dimension d of the internal surface area from experimental data. An independent measurement confirms that the specific surface area correlates with d. The two models contradict each other, however, and we suggest that Le Méhauté and Cré py’s model is appropriate to describe the observed behavior. Fractal dimension varies between 2.02 (Fontainebleau sandstone) and 2.94 (shale).
Seismic trace interpolation in the F-X domain
Authors: Spitz S.
Published: October 2002

Interpolation of seismic traces is an effective means of improving migration when the data set exhibits spatial aliasing. A major difficulty of standard interpolation methods is that they depend on the degree of reliability with which the various geological events can be separated. In this respect, a multichannel interpolation method is described which requires neither a priori knowledge of the directions of lateral coherence of the events, nor estimation of these directions. The method is based on the fact that linear events present in a section made of equally spaced traces may be interpolated exactly, regardless of the original spatial interval, without any attempt to determine their true dips. The predictability of linear events in the f-x domain allows the missing traces to be expressed as the output of a linear system, the input of which consists of the recorded traces. The interpolation operator is obtained by solving a set of linear equations whose coefficients depend only on the spectrum of the spatial prediction filter defined by the recorded traces. Synthetic examples show that this method is insensitive to random noise and that it correctly handles curvatures and lateral amplitude variations. Assessment of the method with a real data set shows that the interpolation yields an improved migrated section.
A nonaliased integral method for dip moveout
Authors: Hale Dave
Published: October 2002

Integral (Kirchhoff‐style) methods for dip moveout (DMO), while possessing several advantages over Fourier transform methods, are prone to problems of spatial aliasing. DMO methods with spatially aliased operators yield significant processing errors, even when applied to data that are not spatially aliased. In particular, such DMO methods may alter the amplitude and phase of horizontal reflections, to which DMO should do nothing. A simple test can be used to detect spatial aliasing in a DMO implementation. First, compute the responses of DMO to an impulse for several source‐receiver offsets and recording times. Second, sum the traces in each of these impulse responses to obtain the corresponding horizontal‐reflection responses. These horizontal‐reflection responses should be identical to the original impulse for all offsets and times. Integral DMO methods with spatial aliasing problems fail this test. Integral DMO methods typically apply a sequence of time compressions to an input seismogram so that each input sample forms an elliptical impulse response. These methods typically yield horizontal‐reflection responses with significant errors in amplitude and phase that vary with source‐receiver offset and recording time. Perhaps surprisingly, the worst errors may occur for small offsets and late times, for which DMO action should be inconsequential. The integral DMO method proposed in this paper applies a sequence of time shifts to each input seismogram. These shifted input traces are mapped to output traces along curved trajectories that enable each sample to form the appropriate elliptical impulse response. The proposed method, by design, passes the test described above. Even the most approximate, highly efficient implementation of this method will improve the imaging of dipping reflections without altering horizontal reflections.
Frequency‐wavenumber inversion of acoustic data
Authors: Amundsen L. , Ursin B.
Published: October 2002

We describe an inversion procedure for obtaining velocities, densities, and layer thicknesses from multioffset reflection seismograms generated by a point source excitation in a stratified acoustic medium. The reference seismograms are decomposed into cylindrical waves in the frequency‐wavenumber domain, where the inversion is performed by minimizing the difference between the reference data and modeled data using a least‐squares technique. The modeling algorithm is based on the classical reflectivity method. We include the effects of limited temporal and spatial apertures by applying a frequency and wavenumber dependent filter in the modeling process. This inversion is suited for Jacobian matrix methods where the first derivatives of the model‐generated data with respect to the model parameters are calculated from analytical expressions together with the modeling. The numerical examples show that the inversion algorithm recovers with reasonable accuracy the parameters used to generate the reference data. The low frequency trends of both the velocity and the density are restored. The convergence of the iterative minimization algorithm is fast.
Borehole stresses created by downhole seismic sources
Authors: Winbow G. A.
Published: October 2002

This paper analyzes the stresses created by downhole acoustic sources used for crosshole or reverse VSP data acquisition. The pressures created by downhole sources may be as large as [Formula: see text] bars which cannot affect intact steel casing. However, such pressures might in principle either (A) crack the cement radially through excessive hoop stress [Formula: see text], or (B) loosen the cement from the casing through an excessive value of shear stress [Formula: see text].
La Primavera caldera (Mexico): Structure inferred from gravity and hydrogeological considerations
Authors: Alatorre‐Zamora M. A. , Campos‐Enríquez J. O.
Published: October 2002

La Primavera geothermal field (Mexico) is associated with a Pleistocene rhyolitic caldera. This gravity study was conducted to assist its development and explotation. Digital processing of the gravity data (upward and downward continuations, vertical derivatives) enabled delineation of the main features of the caldera’s subsurficial structure. A 3-D structural model was established, which could be supported by gravity modeling (2-D and 3-D forward modeling). Accordingly, the caldera is featured by an asymmetric subsurface structure: a major depression in its northern half, and a boomerang‐shaped structural high to the south. Lineaments reflecting the regional northwest‐southeast and northeast‐southwest structural fabric were observed. The basal volcanics units are affected by lineaments of the northwest‐southeast system, whereas the northeast‐southwest system affects only the shallower units. The structural high has a northwest‐southeast trend at the western and southwestern portion of the caldera. From its middle part eastward, it has a northeast‐southwest direction. The actual geothermal production zone is located above this structural high, on the portion where it changes orientation. Correlation with hydrogeological and geochemical data enabled interpreting the different geologic structures in the context of the hydrothermal system: at depth the northwest‐southeast structures seem to control lateral fluid migration, and connect areas of enhanced permeability (i.e., the central production zone and the hydrothermal manifestations located at the caldera’s western rim). Enhanced zones of fracturing favorable for entrapping hydrothermal fluids and structural accidents that may act as conduits (respectively as barriers) for fluids are delineated. In particular, a new target zone, where the production of geothermal fluids may extend, has been identified to the south of the production zone. The structural image elaborated here constitutes a geologic frame for the prevailing hydrogeological conceptual model. This structural information is also useful for the tasks of selecting sites for the reinjection of geothermal brines.
Seismic wave attenuation in volcanic rocks from VSP experiments
Authors: Pujol J. , Smithson S.
Published: October 2002

Seismic wave attenuation in the Columbia Plateau basalts and Snake River Plain volcanics was analyzed using vertical seismic profiling (VSP) data. The computation of attenuation coefficients is based on fitting a straight line to the logarithm of amplitude ratios computed for fixed values of frequency and variable depth. This approach does not require any assumptions on the dependence of Q on frequency. For the Columbia Plateau basalts, the attenuation coefficients obtained from the field data are smaller than those computed from the synthetic VSP generated using the sonic and density logs, indicating that the observed attenuation is related to scattering effects and is substantially larger than the intrinsic attenuation of basalt. Therefore, it is concluded that only a lower bound for Q can be established, in agreement with recent findings by other authors. The effective attenuation of seismic energy in basalts (about [Formula: see text] for the peak frequency) is comparable to the effective attenuation observed in sedimentary rocks (between [Formula: see text] and [Formula: see text]). Results from two VSPs recorded in the Snake River Plain volcanics using air gun and vibrator sources show some frequency‐dependent effects. The depth range analyzed covers two different lithologic units (rhyolitic rocks with interbedded volcanic sediments above more homogeneous rhyodacitic rocks). The air gun energy (with a peak frequency near 15 Hz) clearly detects a difference in the attenuating properties of the two types of rocks. The vibrator energy, on the other hand, also detects this difference, but only for the lower frequencies. For frequencies near the peak frequency (31 Hz), attenuation is almost the same in the two units. The difference in attenuation for the two types of rocks is real and cannot be explained as processing artifacts, because it can be observed for both sources by analyzing the amplitude decay in the time domain. The peak‐frequency attenuation coefficients for the lower section are [Formula: see text] and [Formula: see text] for the vibrator and air gun sources, respectively. For the upper section, the corresponding values are [Formula: see text] and [Formula: see text]. The difference in attenuation implied by the last two coefficients is probably not real, because the decay of energy in the time domain for the two sources is much closer to each other. The Columbia Plateau and Snake River Plain VSPs show that the poor quality of reflection data commonly associated with volcanic rocks cannot be explained by unusually high attenuation.
Airborne gravimetry from a small twin engine aircraft over the Long Island Sound
Authors: Bell Robin E. , Coakley Bernard J. , Stemp Robert W.
Published: October 2002

In January 1990, a test of the feasibility of airborne gravimetry from a small geophysical survey aircraft, a Cessna 404, was conducted over the Long Island Sound using a Bell Aerospace BGM-3 sea gravity meter. Gravity has been measured from large aircraft and specially modified de Havilland Twin Otters but never from small, standard survey aircraft. The gravity field of the Long Island Sound is dominated by an asymmetric positive 30 mGal anomaly which is well constrained by both marine and land gravity measurements. Using a Trimble 4000 GPS receiver to record the aircraft’s horizontal position and radar altimeter elevations to recover the vertical accelerations, gravity anomalies along a total of 65 km were successfully measured. The root mean square (rms) difference between the airborne results and marine measurements within 2 km of the flight path was 2.6 mGal for 15 measured values. The anomalies recovered from airborne gravimetry can also be compared with the gridded regional free air gravity field calculated using all available marine and land gravity measurements. The rms difference between 458 airborne gravity measurements and the regional gravity field is 2.7 mGal. This preliminary experiment demonstrates that gravity anomalies, with wavelengths as short as 5 km, can be measured from small aircraft with accuracies of 2.7 mGal or better. The gravity measurements could be improved by higher quality vertical and horizontal positioning and tuning the gravimeter’s stabilized platform for aircraft use.
Inversion of potential‐field data by iterative forward modeling in the wavenumber domain
Authors: Xia Jianghai , Sprowl Donald R.
Published: October 2002

Direct inversion of potential‐field data is hindered by the nonuniqueness of the general solution. Convergence to a single solution can only be obtained when external constraints are placed on the subsurface geometry. Two such constrained geometries are dealt with here: a single, nonplanar interface between two layers, each of uniform density or magnetization, and the distribution of the density or magnetization contrast within a single layer. Both of these simple geometries have geologic application. Inversion is accomplished by iterative improvement in an initial subsurface model in the wavenumber domain. The inversion process is stable and is efficient for usage on large data sets. Forward calculation of anomalies is by Parker’s (1973) algorithm (Blakely, 1981).
Nonlinear seismic trace interpolation
Authors: Martinson Douglas G. , Hopper John R.
Published: October 2002

The nonlinear correlation technique has been used to guide a seismic trace interpolant to fill gaps in seismic surveys, replace noisy traces, and produce evenly spaced arrays. Given an initial alignment (NMO correction for prestack data and manually inserted correlation lines for post‐stack data), the correlation aligns corresponding features between adjacent seismic traces and quantifies the traveltime difference between the traces on a point‐for‐point basis. This information is used to construct synthetic (interpolated) traces, at any arbitrary distance between the correlated traces, which preserve dip and amplitude changes of the individual reflectors, assuming that such dip and amplitude changes occur linearly (or some other specified functional form) between the correlated traces. The technique is applied to a 48-channel, NMO corrected, CDP gather and to a stacked seismic section to demonstrate its use, sensitivities, and limitations in processing and geologic interpretation studies. Traces synthesized in the CDP gather filling an artificial gap 0.85 km wide reproduce the true traces from the gap with good fidelity (correlation coefficients between the synthetic and real traces average ≳0.85). In another example, ∼85 percent of the variance of the original 48-channel CDP gather is recovered through interpolation by using only 16 channels. A stacked section, with true trace spacing of 25 m, was decimated to 100 m trace spacing, then interpolated to restore the original 25 m spacing. The interpolated traces reproduce the real traces with correlations of ⩾0.95, thus recovering ⩾90 percent of the variance of the original section.
Multilayer dielectric inversion for electromagnetic propagation logging
Authors: Hsu Yupai P.
Published: October 2002

A new method for dielectric constant and conductivity inversion is described in this article. The method consists of a two‐step process. A mathematical operation named the ‘Pinched Second Derivative” is first defined and applied to identify the bed boundaries of a multilayered medium. The operation is based on the fact that when a receiver crosses a bed boundary, the received signal suffers a discontinuity in the second derivative in proportion to the dielectric contrast between the adjacent beds. An oscillating multilayer forward model is then used to parametrically invert the dielectric constant and conductivity of each layer of the medium. The inversion involves a two‐dimensional (2-D)‐moving‐grid search and an error function defined through a Chebyshev type metric.
Array responses for plane and spherical incidence
Authors: Cross Guy M.
Published: October 2002

The design of array filters for attenuation of source‐generated seismic noise is founded, almost without exception, on the assumption of plane‐wave incidence. We examine the theoretical limitations of this assumption and assess their practical significance. A time‐domain array response is developed, incorporating spherical wavefront geometry and divergence. Departures from conventional plane‐wave response are related to systematic differences between the spectral composition of respective waveforms within the array’s aperture. The study also develops a hybrid‐array response based on a modified plane‐wave assumption, retaining plane wavefront geometry but incorporating a model‐consistent approximation to spherical divergence. Deviations between the spherical‐wave response and those arising under plane‐wave assumptions are analyzed in terms of effective implementation errors required to compensate. Analysis reveals that the magnitude of these effective deviations can significantly exceed those expected for actual implementation errors. Findings establish that errors stemming from a plane‐wave approximation are controlled by the ratio of reflector depth to aperture width and diminish as the distance between the source and array midpoint becomes large compared with the former parameters.
Optimum seismic illumination of hydrocarbon reservoirs
Authors: Rietveld W. E. A. , Berkhout A. J. , Wapenaar C. P. A.
Published: October 2002

A method is proposed for the design and application of a wave theory‐based synthesis operator, which combines shot records (2-D or 3-D) for the illumination of a specific part of the subsurface (target, reservoir) with a predefined source wavefield. After application of the synthesis operator to the surface data, the procedure is completed by downward extrapolation of the receivers. The output simulates a seismic experiment at the target, carried out with an optimum source wavefield. These data can be further processed by migration and/or inversion. The main advantage of the proposed method is that control of the source wavefield is put at the target, in contrast with the conventional wave stack procedures, where control of the source wavefield is put at the surface. Moreover, the proposed method allows true amplitude, three‐dimensional (3-D), prestack migration that can be economically handled on the current generation of supercomputers.
Linear velocity function: Reflection times for specific ranges
Authors: Richardson Riley P.
Published: October 2002

In raypath modeling a linear‐with‐depth velocity function is often assumed since this assumption is reasonably accurate over limited depths in many sedimentary basins. Moreover, the resultant theoretical raypaths are especially simple, being arcs of circles.
Detection of subbottom ice‐bonded permafrost on the Canadian Beaufort Shelf by ground electromagnetic measurements
Authors: Palacky G. J. , Stephens L. E.
Published: October 2002

Accurate knowledge of the location of subbottom ice‐bonded permafrost is essential in planning major infrastructure projects such as pipelines in the Arctic. A combination of refraction and shallow reflection seismic surveys and drilling has been considered the most effective means of permafrost detection in offshore environments. Field experiments recently carried out on the Beaufort Shelf by the Geological Survey of Canada indicate that multifrequency horizontal‐loop electromagnetic (HLEM) measurements are a fast and cost‐effective alternative. During test surveys, 60 to 80 soundings were completed in a day. Analysis of HLEM field data shows that a four‐layer model is appropriate for the local geologic conditions. The first layer in the model, sea ice, is highly resistive (5000 Ω ⋅ m). The second layer consists of seawater and seawater‐saturated sediments (estimated resistivity 1 Ω ⋅ m). Because their true resistivities are not significantly different, it was not possible to separate the two highly conductive layers by inversion. The target of the survey, subbottom ice‐bonded permafrost, was the third layer in the section (resistivity 5000 Ω ⋅ m). The bottom layer is formed by unfrozen sediments (average resistivity 2 Ω ⋅ m). Comparison with seismic and drilling results indicates that detection of ice‐bonded permafrost by the HLEM method is reliable; the outlines of its upper boundary given by seismic and HLEM measurements are in good agreement. At the only borehole available, unconstrained inversion of HLEM data provided an accurate estimate of permafrost depth and thickness, but there is not enough drilling information to objectively assess the reliability of the technique.
The zero‐velocity layer: Migration from irregular surfaces
Authors: Beasley Craig , Lynn Walt
Published: October 2002

Seismic data acquired in areas with irregular topography are usually corrected to a flat datum before migration. A time‐honored technique for handling elevation changes is to time shift the data before application of migration. This simple time shift, or elevation‐static correction, cannot properly represent wide‐angle or dipping reflections as they would have been recorded at the datum. As a result, when elevation varies significantly, accuracy in event positioning may be compromised for migration and other wave‐equation processes, such as dip moveout processing (DMO). Traditionally, such over‐ and under‐migration artifacts have been dealt with by increasing or decreasing the migration velocity. However, simple adjustment of the migration velocity cannot undo the wave‐field distortions induced in seismic data acquired over varying elevations. More sophisticated and accurate solutions such as wave‐equation datuming are too computationally demanding for routine use. Here, we propose an efficient and accurate technique for doing migration from irregular surfaces using conventional migration algorithms. As in elevation‐static corrections, surface‐recorded data are time‐shifted to a horizontal datum; for our process, we choose to have that datum elevation lie at or above the highest elevation in the survey. After migration, the datum elevation can always be adjusted to any other level by means of a bulk time shift. In the migration step, the velocity is set to zero (or some very small value) in the layer between the surface and the datum; below the original surface, the interval velocity represents the best estimate of the subsurface geology. By adding a zero‐velocity layer, the migration algorithm is applied to the data from the flat datum and no lateral propagation is allowed until a nonzero velocity is encountered at the recording surface. Synthetic and field data examples demonstrate that use of the “zero‐velocity layer” significantly improves imaging accuracy relative to conventional migration from a flat datum. Moreover, the geologically derived migration‐velocity field need not be adjusted to compensate for shortcomings in the datum‐static procedure. The technique can be extended to prestack processes such as DMO, shot‐ and receiver‐gather downward extrapolation, and migration and thus suggests a unified approach to processing data from irregular surfaces.
Imaging and velocity estimation with depth‐focusing analysis
Authors: MacKay Scott , Abma Ray
Published: October 2002

Prestack depth migration uses two imaging conditions, zero time and zero offset, during downward continuation to form a migrated depth section. When the migration velocities are exact, the two imaging conditions act in a complementary fashion to yield a focused image. When the migration velocities are in error, reflected energy collapses to zero offset at depths that are inconsistent with the zero‐time imaging condition. The result is a deteriorated seismic image. However, by interpreting the nonzero times at which focusing actually occurs, the migration velocities can be updated iteratively in a process called depth‐focusing analysis. To produce a well‐focused seismic image, the goal of depth‐focusing analysis must be the elimination of focusing errors; however, practical considerations can prevent this goal from being achieved. Therefore, to relax the sensitivity of the migrated image to focusing errors, we introduce a nonzero‐time imaging condition by extracting the data along the interpreted surface of focusing from the depth‐focusing analysis volume. This method, called focal‐surface imaging, estimates the results of prestack depth migration using the updated velocities. Depth‐focusing analysis is shown to be a robust approach to velocity estimation and imaging. Limitations arising from constant‐velocity and low‐dip approximations are reduced in the presence of increasing velocities with depth. Lateral velocity errors, sources of exaggerated focusing errors and diverging velocity solutions, can also be addressed by applying a damping factor to the interpreted depth errors. Velocity estimation and focal‐surface imgaging, using iterative prestack depth migration, were applied to a southern North Sea data set. Starting with a regional velocity function, the first iteration provided an updated velocity field that more accurately conformed to the known lithologies. The focal‐surface image, formed from the same iteration, contained significantly more focused energy than the conventional section formed by prestack depth migration. However, structural differences between the two sections indicated the need for another iteration of migration using the updated velocities. The second iteration indicated smaller velocity errors and enough similarity between the migrated section and the new focal‐surface image to indicate that further iterations were unnecessary.
Application of diffraction tomography to fracture detection
Authors: Tura M. Ali C. , Johnson Lane R. , Majer Ernest L. , Peterson John E.
Published: October 2002

Two diffraction tomography techniques are applied to crosshole field data to detect fractures in granitic rock. The techniques used are the conventional back‐propagation method and a new quadratic programming method incorporating constraints. In this formulation, the Born approximation is used for linearization of the inverse problem. Two dimensional (2-D) pseudo spectral finite‐difference synthetic data are generated to demonstrate the inversion methods and justify use of the Born approximation. Also, using 2-D Born synthetic data, the velocity sensitivity of the inversion algorithm and reduction of fracture generated tube waves and S‐waves are investigated. The inversion methods are applied to field data from the Grimsel test site in Switzerland. The data are collected from a [Formula: see text] rectangular area where fractures are known to exist. Data acquisition with 0.5 m spacing of three component receivers and a piezoelectric source is carried out so as to obtain a nearly complete coverage of the region. Crosshole inversions are performed on data from the receiver components in the plane of the rectangular region and normal to its boundary. As the result of a separate experiment conducted in a homogeneous region of the granitic rock, a cosine function was found to best fit the source radiation pattern. A background attenuation value is estimated for the region, using a simple statistical approach, and estimates of the wavelet are found by common source gathers, common receiver gathers, and averages of all traces. The preprocessing steps are: (1) source radiation correction, (2) attenuation correction, (3) removal of the incident wavefield, (4) muting beginning of the traces and windowing the ends, (5) wavelet deconvolution, and (6) two‐and‐a‐half dimensional (2.5-D) corrections. This preprocessing is designed to enhance scattered P‐waves that are used in the inversions. Images obtained from the application of back‐propagation and quadratic programming methods to the preprocessed data show possible fracture zones that agree well at the boundaries of the region with the fracture sets observed from core samples taken from the boreholes. Although the quadratic programming method is an order of magnitude slower than the back‐propagation method, as demonstrated by the synthetic examples, it proves useful by yielding high resolution images when constraints can be imposed. Transmission ray tomography is also applied to the crosshole data, and although the resolution is not as high, general agreement with the wave equation based methods is obtained.
Use of transient electromagnetics to define local hydrogeology in an arid alluvial environment
Authors: Taylor Kendrick , Widmer Michael , Chesley Matthew
Published: October 2002

To characterize a local groundwater system in an arid alluvial environment, a transient electromagnetic (TEM) survey was performed with a dense station spacing along intersecting profiles. Structural features of significance to groundwater development were clearly evident in the color modulated apparent resistivity versus time pseudosections that were used to display the apparent resistivity data. Modeling results in conjunction with pseudosections were used to determine exploratory drill sites. Comparing the modeled results with electric logs from test holes demonstrated that 1-D modeling is not suitable for areas where rapid lateral change occurs. These areas are easily identified in the apparent resistivity versus time pseudosections. Sites not located in fault zones with a resistivity greater than a few tens of ohm‐m were considered to be preferred locations for production wells. Hydraulic testing confirmed this interpretation philosophy. The survey demonstrates the utility of closely spaced TEM soundings to characterize local hydrogeology and to define areas suitable for groundwater development.
P-SV stacking charts and binning periodicity
Authors: Eaton David W. S. , Lawton Donald C.
Published: October 2002

A surface stacking chart (Sheriff and Geldart, 1982; Yilmaz, 1987) is a convenient graphical representation, in source‐receiver space, of the trace geometry for a seismic survey. In this display format, collections of traces corresponding to common‐receiver, common‐source, common‐offset, and common‐midpoint (CMP) gathers are aligned along vertical, horizontal, 45 degree, and 135 degree azimuths (θ), with respect to the receiver axis. Provided that the source spacing (Δs) is an integer multiple of the receiver interval (Δr), traces are exactly aligned in these directions (neglecting skids, detours, etc.). The subsurface multiplicity (fold) along lines of constant CMP is then given by the well known expression.
3-D finite‐difference modeling of elastic waves in borehole environments
Authors: Yoon Kwi‐Hyon , McMechan George A.
Published: October 2002

Full‐wavefield sonic logs may be simulated for arbitrarily complicated three‐dimensional (3-D) borehole environments using 3-D elastic finite differences. To ensure reliability through large contrasts in Poisson’s ratio (across mud‐casing‐lithology contacts), a staggered grid finite‐difference formulation is used. Cylindrical symmetry is not assumed so the responses of features such as asymmetrical washouts and dipping structure are easily obtained. When features are asymmetrical, seismic responses vary significantly at various points around the hole circumference at any depth. Even for simple hole geometries, observed responses are complicated because of coupling between waves inside and outside the hole. Observations are also sensitive to the source‐receiver separation. Output formats include fixed‐time snapshots of displacement, divergence, and curl, and seismograms for the center and edge of a borehole; this allows detailed arrival identification and interpretation. To our knowledge, this is the most comprehensive and flexible scheme for modeling borehole waves that has been implemented to date.
Filtration fronts in pressure compliant reservoirs
Authors: Dvorkin Jack , Nur Amos
Published: October 2002

Hydrofracturing is believed to be the major mechanism responsible for the migration of crust fluids and for the transport of hydrocarbons from overpressured fluid compartments. In their analysis of the process of porosity reduction in the earth’s crust and crustal pore pressure generation, Walder and Nur (1984) point out that elevated pore pressure generated as a result of local porosity reduction may lead to brittle failure with partial relief of high pore pressure. Such episodic fracturing accompanied by crack healing, may be a common process throughout most of the crust. Hunt (1990) also speculates that the generation of oil and gas within the compartments plus the thermal expansion of pore fluids eventually causes fracturing of the top compartment seal during periods of basin sinking. Seal fracturing causes a pressure drop with compartment fluids rushing to the breakout point. Episodic cycles of resealing and breakout may occur in intervals of thousands of years in rapidly sinking basins such as the United States Gulf coast.
On: “The generalized gravity anomaly: Endoscopic microgravity” by J. Lakshmanan (GEOPHYSICS, 56, 712–723, May 1991)
Authors: MacQueen J. D. , LaFehr T. R.
Published: October 2002

We disagree with Lakshmanan’s characterization of conventional reduction techniques, his comparison of “conventional” corrections with his method, and his apparent use of gravity inversion.