bluuebunny/crossref_metadata_2025 huggingface dataset:
Compact gravity inversion
Authors: Last B. J. , Kubik K.
Published: October 2002

We present a new criterion for the inversion of gravity data. The principle employed is to minimize the volume of the causative body, which is equivalent to maximizing its compactness. The anomalous density distribution is obtained using an iterative technique which is numerically stable and rapidly convergent. The principle can also be adapted to include modeling of gravity anomalies by single‐density sources. The advantage of this approach is that desirable geologic characteristics are automatically incorporated into the model with a minimum of subjective judgments on the part of the interpreter. The treatment of noise in the data fits naturally into the formulation of the inversion procedure. The method is illustrated by the inversion of noise‐free and noisy data generated from a two‐dimensional model consisting of a regular array of identical rectangular blocks whose densities can be individually specified. In every case the algorithm successfully recovers the correct density distribution from the data. In the case of noise‐contaminated data, a complete separation of the noise from the signal is achieved. The practical effectiveness of the method is demonstrated by the inversion of published gravity data. The results obtained are compared with existing models and with available drilling information.
Negative transient voltage and magnetic field responses for a half‐space with a Cole‐Cole impedance
Authors: Raiche A. P.
Published: October 2002

In a recent paper, Lee (1981) developed an asymptotic formula for the coincident loop transient electromagnetic (TEM) response of a polarizable half‐space having a Cole‐Cole impedance. By using parameters corresponding to three different mineral deposits, Lee showed that negative transients would be obtained for delay times of 0.4 to 1.1 msec. The method developed by Knight and Raiche (1982) to calculate the transient response of layered earths was used to check these results for three reasons.
Analysis of seismic data using horizontal cross‐sections
Authors: Bone Marion R. , Giles Ben F. , Tegland Edward R.
Published: October 2002

Three‐dimensional data provide the interpreter with a new dimension for interpretation: horizontal cross‐sections. This article suggests formats for the display of the data and techniques for deriving contour maps of horizons, strike directions, dip angles, thicknesses of formations, and fault blocks.
Numerical methods for the migration of constant‐offset sections in homogeneous and horizontally layered media
Authors: Dubrulle Augustin A.
Published: October 2002

This paper describes schemes for the frequency‐domain migration of constant‐offset sections in homogeneous and horizontally layered media. Their main advantage is that individual constant‐offset sections can be processed in isolation. The basis for these schemes is a generalization of the following plane‐wave analysis of migration for the zero‐offset case. Migration can be viewed in the time domain as a process whereby the plane waves echoed from a source to a geophone by a reflecting point are shifted in time to a common intersection. The shift applicable to each plane wave can be derived from the knowledge of the diffraction curve for the reflecting point in a neighborhood of the geophone abscissa. This time shift in turn determines the phase shift applicable in the frequency domain to the associated Fourier component of the seismic section. The central part of our methods is the construction of those elements of the diffraction curve pertinent to the shifts from the velocity map of the time section. The result is a table of shifts for an appropriate sample of plane‐wave directions, to be used for interpolation in the migration process. The effectiveness of the method is illustrated by computer experiments with synthetic data.
Computational experience with Marple’s algorithm for autoregressive spectrum analysis
Authors: Barrodale I. , Delves L. M. , Erickson R. E. , Zala C. A.
Published: October 2002

In 1980 two different recursive algorithms were published, complete with Fortran programs, for autoregressive (AR) spectral estimation, based on least‐squares solutions for the AR parameters using forward and backward linear prediction. The first of these to appear, by Barrodale and Erickson (1980a, b) forms the normal equations for the Mth order AR parameters from the corresponding normal equations for the [Formula: see text] order parameters. However, for each value of M = 1, 2, …, MMAX, the normal equations are solved by Cholesky’s method ab initio, i.e., without reference to the solution of the previous normal equations of lower order. In contrast, the later algorithm by Marple (1980) calculates the Mth order AR parameters in a recursive manner from the [Formula: see text] order parameters.
On: “Seismic data display and reflection perceptibility,” by Frank J. Feagin (GEOPHYSICS, v. 46, February 1981, p. 106–120).
Authors: Brown Alistair
Published: October 2002

Mr. Feagin’s article was refreshingly different and certainly addressed an important subject for seismic interpreters. However, he left untouched the subject of the perceptibility of seismic character and stratigraphic detail. The use of color to enhance the interpretability of various kinds of seismic section has been an emerging subject for some time and is a natural extension to Mr. Feagin’s work.
An analysis leading to the reduction of scattered noise on deep marine seismic records
Authors: Tsai C. James
Published: October 2002

The rough and uneven basaltic basement in deep marine environments functions as a potential source of scattering. The source‐generated noise not only interferes with the weak and deep primary reflections, but often resembles primary reflections, thereby confusing the seismic interpretation. An analysis of the arrival time of scattered noise indicates that NMO and apparent dip of scattered noise exhibit azimuth dependence. For isotropic scattering, in‐line scattering at ranges near 0 degree azimuth and sideswipe from 90 degree azimuth should dominate the seismic section. Actual marine data displayed in various data gathers illustrate these characteristics of scattered noise and also correct a common misconception about the appearance of scattered noise. The analysis suggests the use of CDP stacking and velocity filtering to attenuate the scattered noise. The combined effect shows a total reduction of scattered noise by 21 dB for the frequency bandwidth from 20 to 32 Hz.
The influence of pore geometry on the dielectric dispersion of clean sandstones
Authors: Korringa J.
Published: October 2002

In an idealized description a clean sandstone is an isotropic two‐component medium consisting of a solid with dielectric constant [Formula: see text] and a pore fluid (brine) with dielectric constant [Formula: see text] and conductivity [Formula: see text]. On the basis of D. Bergman’s work [Annals of Physics 138, 78, 1982] the dielectric response of such a medium is expressed in terms of the dc conductivity and of a real, positive and bounded function, defined in 〈0, 1〉 and independent of [Formula: see text], [Formula: see text], and [Formula: see text]. This function is restricted by three relations which express weighted averages of it in terms of the asymptotic value of the dielectric response at high frequency, the porosity, and the dc conductivity. The formalism is exact for two‐component media of the type specified. It supplements an approximate treatment by Lysne (1983) and can, without much difficulty, be generalized to any isotropic two‐component composites.
On: “Magnetic anomalies of two‐dimensional bodies in a magnetic half‐space” by E. E. S. Sampaio (GEOPHYSICS, v. 47, p. 1229–1234, August, 1982).
Authors: Eskola L.
Published: October 2002

In a recent paper Sampaio presented an analytic solution of the magnetic field problem for a circular magnetized cylinder embedded in a homogeneous magnetized half‐space. In his paper, Sampaio also stated that the numerical method for solving magnetostatic problems by Eskola and Tervo (1980) doesn’t take into consideration the susceptibility contrast between the half‐space and the air. The model treated by Sampaio doesn’t actually exist, however. For a magnetized environment, in addition to the upper boundary, there is also a lower boundary, i.e., where the rock loses its magnetization (at least at the Curie point). This boundary holds an additional source of magnetic field that is of the same order of strength as the field caused by the upper boundary, if the horizontal dimensions of the magnetized environment are large. If the horizontal dimensions are not large, the effect of the vertical boundaries of the environment must also be taken into consideration. Eskola and Tervo (1980) find no difficulty in taking into consideration all the boundaries by means of their method.
On: “Simulating true amplitude reflections by stacking shot records,” by P. Hubral (GEOPHYSICS, v. 49, p. 303–306, 1984).
Authors: Krey Th.
Published: October 2002

Quite recently Peter Hubral published a short note in which he described a special, very perspicuous stacking method which, starting from the records of a line survey, produces true amplitude reflections for “normal waves,” as defined in his Introduction. In the following I want to supplement Hubral’s note by showing the analytical connection with Hubral’s earlier paper (Hubral, 1983) and the additional short note by Krey (Krey, 1983). My present investigation will be two‐dimensional (2-D) as is that in the subject paper; an extension to the three‐dimensional (3-D) case is conceptionally easy for the following analytical derivation as well as for Hubral’s note. Besides a basic confirmation of Hubral’s findings, I shall show that the result of Hubral’s method has still to be multiplied by [Formula: see text] in the 2-D case and by [Formula: see text] in the 3-D case in order to obtain the precise result. Here ω is the frequency. Moreover the angle of emergence α of the zero‐offset raypath has to be taken into account.
A comparison of common‐midpoint, single‐shot, and plane‐wave depth migration
Authors: Temme P.
Published: October 2002

A comparison of common‐midpoint (CMP), single‐shot, and plane‐wave migration was made for simple two‐dimensional structures such as a syncline and a horizontal reflector with a laterally variable reflection coefficient by using synthetic seismograms. The seismograms were calculated employing the finite‐difference technique. CMP sections were simulated by 18-fold stacking and plane‐wave sections by slant stacking. By applying a finite‐difference scheme, the synthetic wave field was continued downward. The usual imaging condition of CMP migration was extended in order to carry out migration of single‐shot and plane‐wave sections. The reflection coefficient was reconstructed by comparing the migrated wave field with the incident wave field at the reflector. The results are: (1) all three migration techniques succeeded in reconstructing the reflector position; (2) as a consequence of the finite aperture of the geophone spread, only segments of the reflector could be reconstructed by single‐shot and plane‐wave migration; (3) for single‐shot and plane‐wave migration the reflection coefficient could be obtained; and (4) CMP migration may lead to incorrect conclusions regarding the reflection coefficient.
The relationship between Born inversion and migration for common‐midpoint stacked data
Authors: Cheng Guan , Coen Shimon
Published: October 2002

The relationship between Born inversion and wave‐equation migration of common‐midpoint (CMP) stacked seismic reflection data is analytically determined. The three‐dimensional (3-D) velocity distribution obtained by Born inversion is shown to be directly related to the 3-D reflectivity function obtained by wave‐equation migration for full bandwidth or band‐limited data. The relationship is obtained by the reformulations of migration and Born inversion methods as inverse source problems for the 3-D wave equation. The reformulation leads to a definition of the reflectivity function as the source function for the wave equation. It also leads to determination of the Born inversion results by applying the algorithm for wave‐equation migration to modified surface data. The modified data are simply related to the CMP stacked data. Alternatively, Born inversion results may be obtained directly from the migrated section. Results from synthetic and recorded data are presented and found to be consistent with the theoretical developments.
On‐bottom gravity profile across the East Pacific Rise crest at 21° north
Authors: Luyendyk Bruce P.
Published: October 2002

Twenty on‐bottom gravity stations were taken with a LaCoste‐Romberg model G gravity meter inside the submersible Alvin, over a 6 km long cross‐strike profile which straddles the East Pacific Rise spreading center at 21°N. The residual Bouguer anomaly shows a 1.5 mGal low with a total half‐width of 1.7 km centered about the spreading axis and hydrothermal vents. Uncertainties in the ocean depth are believed to contribute a standard error of about 0.3 to 0.4 mGal for the gravity data. The observed elevation factor of 0.1552 mGal/m indicates that the upper ocean crust bulk density is [Formula: see text]. On the other hand, 90 rock samples give a density of [Formula: see text]. This suggests that the largescale porosity of the upper crust is 14 percent. The gravity anomaly was inverted to find the ideal body with the smallest maximum density contrast which satisfies the data and their associated errors. This body extends from the sea floor to 2 500 m depth and has a density contrast of [Formula: see text]. If the density contrast is allowed to be as large as [Formula: see text], the top of the ideal body can be as deep as 300 m and its bottom as deep as 1 300 m; alternatively with the body top at the sea floor, the bottom of the body can be no shallower than 500 m. The ideal body is, therefore, restricted to the dikes and flows of the upper ocean crust. The body is not thought to represent magma in the shallow crust, but rather fractured crust filled with heated seawater.
A microcomputer based system for small‐scale geophysical surveys
Authors: Sowerbutts W. T. C. , Mason R. W. I.
Published: October 2002

Manufacturers of geophysical instruments have kept abreast of the many developments in microelectronics that have taken place in the last decade, and have made important developments in geophysical instrumentation. The instruments they have produced are not only smaller and more robust than their predecessors, but are easier to operate, produce a quicker response, and often perform additional functions.
An experimental test of P-wave anisotropy in stratified media
Authors: Melia Patrick J. , Carlson Richard L.
Published: October 2002

In theory, stratified media in which the layers are elastically homogeneous and isotropic approximate transversely isotropic media with an axis of symmetry perpendicular to layering when the seismic wavelength is sufficiently longer than the layer spacing. The phenomenon has apparently been observed in field measurements, and acoustic anisotropy in deep‐sea sediments, measured in the laboratory, has been attributed to fine‐scale bedding laminations. However, to the best of our knowledge, no rigorous test of the theory has been made. We have made a partial test by making laboratory measurements of compressional‐wave velocities parallel and perpendicular to layering in fabricated samples consisting of glass and epoxy. We found no statistically significant difference between observation and theory in this limited test. Further, having used several frequencies, we found that the velocities progressively change from the long‐wave values toward those predicted by the time‐average relation, as expected. Finally, it has been proposed that the long‐wave approximation holds when the ratio of the seismic wavelength to layer thickness (λ/d) is 10–100. We found that the minimum ratio was highest in the midrange of composition (half glass, half epoxy), even though the samples in that range have the smallest combined layer thickness. This result suggests that the frequency dependence of anisotropy in layered media is a function of the proportions of the materials as well as the thickness of the layers.
The horizontal loop electromagnetic response of a thin plate in a conductive earth: Part I—Computational method
Authors: Hanneson J. E. , West G. F.
Published: October 2002

The frequency‐domain EM response of a thin vertical tabular conductor situated in a two‐layer earth was obtained by solving the electric field integral equation numerically. The method of point collocation and expansion of the scattering current components in global polynomic basis functions was employed. Selection of polynomials which obey a boundary condition of no current flow from the edges had a beneficial effect on the results. As the number of basis functions increases, the solution converges to a stationary value which is valid over a sufficiently wide range of parameters to permit the computation of horizontal loop (Slingram) profiles over a plate which can be large enough to simulate a semi‐infinite half‐plane. The solution gradually worsens as the conductivity contrast between the plate and the host increases, and it eventually fails in situations where magnetic induction effects are strong and electric effects are negligible. The solution also worsens as the plate size increases or as the vertical magnetic dipole source approaches the plate.
Principle of reverse‐time migration
Authors: Levin Stewart A.
Published: October 2002

At the Migration Research Workshop of the 52nd SEG Meeting in Dallas (1982), an elegant method of wave‐equation migration, known as reverse‐time migration, was presented by Whitmore. Shortly thereafter McMechan (1982, 1983) and Baysal (1982) described virtually identical approaches to migration. This, along with further development by Baysal et al (1983a,b) and Whitmore (1983), has sparked much interest and discussion. In May 1983, Loewenthal and Mufti also published a migration algorithm, quite different from the above, that they call “reversed time migration” and developed as early as 1977. In this short note I present a general principle underlying reverse‐time migration and discuss the various reverse‐time migration procedures in light of it.
Grain shape effects on dielectric and electrical properties of rocks
Authors: Sen P. N.
Published: October 2002

Recently there has been a considerable interest in the effect of anisotropy in the grain shape in the electrical and dielectrical properties of rocks and other inhomogeneous media (Sen 1981a, b; Sen et al, 1981; Mendelson and Cohen, 1982; and Kenyon, 1983). In this note I point out that equation (34) of Mendelson and Cohen (MC) is incorrect. The dc limit of MC equation (34) for the conductivity of rock σ, in terms of porosity ϕ and water conductivity [Formula: see text], gives [Formula: see text] or [Formula: see text] where [Formula: see text] L is the depolarization factor along the principal axis of spheroidal grain and 〈 〉 denotes an average over the distribution in L. This value of [Formula: see text] is in disagreement with the correct value of m in equation (28) of MC [equation (6) below]. [When the sign mistakes in equations MC (33)–(34) are corrected, [Formula: see text]. This agrees with equation (6) below for the case when L has a single value and averaging is redundant.] This inconsistency arises from an incorrect replacement of the inverse of an average in MC equation (33) by an average of inverses. The corrected form of MC equation (33) is [Formula: see text] where ε and [Formula: see text] are the dielectric constants of the mixture and of the matrix, respectively. The dielectric constant [Formula: see text] is complex, [Formula: see text] is real, [Formula: see text] is the permittivity of vacuum, σ the conductivity, ω the angular frequency. The last factor in the right‐hand side of the equation was replaced incorrectly by the average of the inverse, which is incorrect in general. Note that in the dc limit equation (4) above gives [Formula: see text] and, by integration, [Formula: see text] where [Formula: see text] is the dc conductivity of water, σ(0) is the dc conductivity of formation, and [Formula: see text]
Interactive seismic mapping of net producible gas sand in the Gulf of Mexico
Authors: Brown Alistair R. , Wright Roger M. , Burkart Keith D. , Abriel William L.
Published: October 2002

In the Garden Banks area of offshore Louisiana several gas sands have been drilled and found productive. However, the sands are laterally variable in thickness and effectiveness. An improved understanding of the spatial distribution of net producible gas sand is highly desirable for reservoir management. The bright reflections from the top and the base of each sand were tracked automatically on an interactive interpretation system. This yielded time structure maps and hence isochron maps for each gross sand interval. The horizon Seiscrop™ sections diplaying amplitudes over the sand interfaces were then summed, adjusted for tuning effects, and smoothed to yield estimates of net gas/gross sand ratio over the area under study. By combining these with the corresponding isochron maps and an appropriate gas sand interval velocity, we obtained net gas sand isopach maps which tie acceptably with well data. Integration of these provided total reservoir volumes. ™Trademark of Geophysical Service Inc.
Two‐point ray tracing in a three‐dimensional medium consisting of homogeneous nonisotropic layers separated by plane interfaces
Authors: Stöckli R. F.
Published: October 2002

The ray‐tracing problem is considered the solution to a minimum travel time problem for media where each layer may be regarded as a transversely isotropic homogeneous solid. The wave surface‐wavefront at t = 1 s, corresponding to a wave generated at the point source, associated with each layer’s anisotropy is approximated by surfaces which are not more difficult to handle, from a computational point of view, than ellipsoidal surfaces. These approximating surfaces are those used in ray‐tracing computation; a ray being a true ray approximation is thus obtained.
Mineral interfacial processes in the method of induced polarization
Authors: Klein James D. , Biegler Tom , Horne M.D.
Published: October 2002

A phenomenological laboratory investigation has been conducted of the IP response of pyrite, chalcopyrite, and chalcocite. The technique that was used is standard in electrochemistry and employs rotating disk electrodes. The effect of rotation is to stir the electrolyte and thus to restrict the maximum distance available for diffusion of electroactive aqueous species. For high rotation speed and low excitation frequencies, the mean diffusion length exceeds the thickness of the diffusion layer. The net effect is to reduce the electrode impedance at low frequency. The thickness of the diffusion layer and thus the impedance at low frequency can be controlled by the rotation speed. Measurements using rotating disk electrodes have been conducted in both the time domain and the frequency domain. For both pyrite and chalcopyrite, the results were the same: no dependence on rotation was observed. For frequency domain measurements with chalcocite, a strong dependence on rotation was observed. The interpreted diffusion layer thickness was found to depend on rotation speed to the [Formula: see text] power, in agreement with results predicted by hydrodynamic theory. The results of this study imply that there are two physical processes responsible for electrode polarization in the IP method. For chalcocite and perhaps other related copper sulfide minerals, the probable mechanism is diffusion of copper ions in the groundwater. In case, the phenomenon is correctly described by the Warburg impedance. Chalcocite’s distinctive response is thought to be related to its forming a reversible oxidation‐reduction couple with cupric ions in solution. No other common sulfide mineral forms a reversible couple with its cations in solution. For the other minerals of this study, the lack of dependence on rotation implies that diffusion of active species in the electrolyte is not the controlling process. Possible alternate mechanisms include surface controlled processes such as surface diffusion or adsorption phenomena. Ancillary data obtained during this study indicate the interface impedance of chalcopyrite is proportional to the electrode potential which in turn can be controlled by rotation speed, electrolyte composition, or application of an external dc current or voltage. This implies that the surface concentration of active species is dependent on electrode potential.
Seismic versus sonic velocities: A vertical seismic profiling study
Authors: Stewart Robert R. , Huddleston Phil D. , Kan Tze Kong
Published: October 2002

Vertical seismic profiling (VSP) techniques provide a method to measure accurately the seismic velocity and lithologic structure near the borehole. The analysis of a VSP survey can also provide insight into seismic‐wave propagation especially when related to sonic measurements. But VSP and sonic log velocities (or traveltimes) are often found to disagree. Recent field evidence of these differences suggests that the VSP traveltimes are delayed with respect to the integrated sonic times, especially in the deep section (>3000 ft), by about 2.0 ms/1000 ft on the average. The VSP has numerous applications in exploration geophysics, such as calibrating the sonic log. It is thus important to understand why the two measurements differ. Differences in the geometries, source frequencies, and instrumental errors of the two surveys are reviewed. More detailed analysis of seismic wave propagation in the VSP shows that short‐path multiples and velocity dispersion can have a significant delaying effect on the seismic traveltimes. One‐dimensional, wide‐band VSP synthetic seismograms are generated in the frequency domain to study these effects. Different parameters (bandwidth, signal‐to‐noise, layer thickness, multiples, attenuation, dispersion) are varied in the synthetic seismograms. A comparative display of synthetic VSP traveltime minus the integrated sonic time is used to view the effects of these parameters on the synthetic traces. Reasonable variation in noise, layer thickness, bandwidth, and picking method have a small effect on traveltimes. Field data from the Anadarko Basin (4 wells) and an East Texas well are examined with the same technique. From the modeling and field examples, it is found that short‐path multiples can cause a seismic pulse delay of up to 2.0 ms/1000 ft with respect to the integrated sonic log in highly cyclically stratified sections. Velocity dispersion associated with attenuation can have a larger effect, causing up to 7.0 ms/1000 ft delay of the VSP traveltimes with respect to the integrated sonic. These wave propagation effects can explain the observed discrepancy between VSP and integrated sonic times in the deep section.
Radiation patterns of torsionally vibrating seismic sources
Authors: Dorn Geoffrey A.
Published: October 2002

The radiation patterns available in the literature for seismic surface sources are restricted to far‐field, low‐frequency solutions for undamped half‐space models. A theoretical study of the radiation pattern of a circular baseplate vibrating torsionally on the surface of an N-layered anelastic medium demonstrates that the patterns in the literature do not reasonably represent the radiated field of a source on a damped layered medium. The radiation pattern of a source is a measure of the strength of the output signal as a function of direction and is determined by calculating the displacements at points along the arc of a circle at a specified radial distance from the source. The solution for the displacement due to a vibrator or a plane‐layered anelastic medium is obtained by solving the elastodynamic wave equation using Fourier and Hankel transforms. The displacement is expressed in terms of an inverse Hankel transform which is performed numerically. The shape of the torsional radiation pattern in a half‐space is a function of frequency, baseplate radius, shear‐wave velocity, and radial distance between the source and the observation points. As frequency increases, the source beams more energy in a near‐vertical direction, and less in directions near the horizontal. Vertical beaming also increases as baseplate radius increases and as shear‐wave velocity decreases. The radiated field approaches an asymptotic far‐field radiation pattern at radial distances in excess of 5 to 8 baseplate radii. In a layered medium, constructive and destructive interference of direct, reflected, and refracted waves results in a characteristic lobate structure in the radiated field. A minimum occurs just below the layer interface due to the combined effects of reflections and refractions. As frequency increases, a greater proportion of energy penetrates the layer interface and enters the underlying medium. At frequencies typically used in exploration seismology the majority of the signal from a torsional source is trapped in the surface layer. The shape of the radiation pattern at a given distance from the source is a function of the density, velocity, and Q contrasts between layers. For a given acoustic impedance contrast, more energy is trapped in the surface layer if the contrast is caused primarily by a velocity rather than a density change since the velocity contrast results not only in reflections but also in refractions and Love waves. A variation in Q between layers further distorts the radiated field. The pattern for a layered model does not asymptote to a far‐field pattern until the radial distance is much greater than the total thickness of the layers in the model. Radiation pattern modeling for layered media may help resolve the discrepancies between theoretical patterns and field measurements and may provide a means of optimizing the radiated field of a surface source or source array in terms of its directivity and the partitioning of energy between various wave types.
A method for computation of velocity profiles by inversion of large‐offset records
Authors: Carrion Philip M. , Kuo John T.
Published: October 2002

This paper describes a new method for recovering velocity profiles utilizing both phase and amplitude information including wide‐angle arrivals, post‐ and precritical reflections. This method is based on a double spatial transformation with a minimization procedure. The first transformation is slant stacking of the observed wave field (seismogram). The second is projecting the slant stacked wave field into the domain of horizontal slowness p and depth z. In this domain the inverse problem is reduced to finding the critical path [Formula: see text] where V(z) is the true velocity of the compressional waves. A numerical algorithm based on a minimization technique is used to find the critical path, which is equivalent to the set of turning points of the critically reflected rays. When this path is found, then the following criteria are satisfied: (1) most of the energy is concentrated away from the precritical region; (2) the computed reflection coefficients reach their maximum on this path; and (3) for horizontally stratified media or CMP data, the reflectors are aligned in the p-z domain. In tests, this method has been shown to recover the velocity profile from both synthetic and real data. It is shown that the method is able to recover accurately velocity profiles even if only part of the data are given. For example, only part of the data are available when low‐ and high‐frequency components are missing or when the data are truncated in lateral extent due to the finite length of the recording system. Moreover, the method is able to handle virtually any vertical velocity gradients in a medium; therefore, it can be applied to complicated geologic structures. The method does not require elimination of multiples, but it is not applicable to the case of a medium with a large lateral velocity gradient. It can be used even for an elastic medium when the mode‐converted energy is not small.
Simultaneous inversion modeling of gravity and aeromagnetic data applied to a geothermal study in Utah
Authors: Serpa Laura F. , Cook Kenneth L.
Published: October 2002

Aeromagnetic and gravity surveys were conducted in the Black Rock Desert, Utah to assess the geothermal potential of the Meadow‐Hatton Known Geothermal Resource Area (KGRA). The presence of basalt flows less than 1000 yr old and a 400 000 yr old rhyolite dome suggested that a hot intrusive body, which should be detectable in both types of potential field data, may provide the heat source for hot springs in the study area. A simultaneous inversion computer program was developed as part of this study to model these potential field data. The resulting models indicate hydrothermal alteration about the hot springs extending to a depth of approximately 1 km. Normal faults above a low‐angle detachment appear to reach a depth of approximately 4 km and provide a path for the circulation of groundwater in the area. No evidence for a buried igneous body was found in the study area, and it is therefore concluded that the migration of fluids along the deep faults is sufficient to account for the water temperatures estimated for the KGRA.
On the half‐slope and straight‐slope methods of basement depth determination
Authors: Rao D. Atchuta , Babu H. V. Ram
Published: October 2002

Although a great variety of interpretation techniques for basement depth determination has been developed during the past two or three decades, the half‐slope and straight‐slope methods are still popular due to their simplicity and general reliability in manual interpretation and are widely used in oil exploration work (Nettleton, 1976). The half‐slope and straight‐slope rules are derived for a particular set of geologic/geophysical conditions and care should be taken in applying them in a more general way. For example, the half‐slope method of Peters (1949) was derived for magnetic anomalies over vertical dikes with vertical polarization. The straight‐slope method uses the horizontal projection of the straight‐line part of the steepest gradient at the inflection point on the anomaly curve as the depth estimator. This rule is purely empirical because mathematically there is no straightline part on the anomaly curve. Vacquier et al. (1951) made an exhaustive study of the straight‐slope method and presented several depth indices measured on different flanks of anomalies due to prismatic bodies.
On the reflection and transmission of viscoelastic waves—Some numerical results
Authors: Krebes E. S.
Published: October 2002

The mathematical theory which is typically used to model the intrinsic anelasticity of the earth is the linear theory of viscoelasticity. The effects of anelasticity on wave propagation, such as absorption and dispersion, are often described using one‐dimensional (1-D) plane waves of the form [Formula: see text] with k complex and frequency‐dependent. These waves are solutions of the 1-D viscoelastic wave equation. The reflection and transmission of plane waves in a layered viscoelastic medium is, however, a 2-D or 3-D problem. The solutions to the 2-D or 3-D viscoelastic wave equation are the so‐called general plane waves, which are classified as homogeneous or inhomogeneous depending upon whether or not the planes of constant phase, i.e., wavefronts, coincide with the planes of constant amplitude (the 1-D plane waves mentioned above are strictly homogeneous).
Shallow seismic reflection mapping of the overburden‐bedrock interface with the engineering seismograph—Some simple techniques
Authors: Hunter J. A. , Pullan S. E. , Burns R. A. , Gagne R. M. , Good R. L.
Published: October 2002

Where unconsolidated overburden exceeds 20 m in thickness, the reflection method may be efficiently used with a 12‐channel engineering seismograph to map topography on the overburden‐bedrock interface as well as possible structure within the overburden. The two techniques we suggest are the simplest forms of reflection profiling which can be applied with a minimum amount of equipment which we suggest is a 12‐channel enhancement seismograph, a 12‐geophone array, and a hammer source. These techniques require good transmission characteristics of the overburden as well as a sharp velocity discontinuity at the overburden‐bedrock interface. For data processing and display a microcomputer is essential.
Euler’s differential equation and the identification of the magnetic point‐pole and point‐dipole sources
Authors: Barongo J. O.
Published: October 2002

The concept of point‐pole and point‐dipole in interpretation of magnetic data is often employed in the analysis of magnetic anomalies (or their derivatives) caused by geologic bodies whose geometric shapes approach those of (1) narrow prisms of infinite depth extent aligned, more or less, in the direction of the inducing earth’s magnetic field, and (2) spheres, respectively. The two geologic bodies are assumed to be magnetically polarized in the direction of the Earth’s total magnetic field vector (Figure 1). One problem that perhaps is not realized when interpretations are carried out on such anomalies, especially in regions of high magnetic latitudes (45–90 degrees), is that of being unable to differentiate an anomaly due to a point‐pole from that due to a point‐dipole source. The two anomalies look more or less alike at those latitudes (Figure 2). Hood (1971) presented a graphical procedure of determining depth to the top/center of the point pole/dipole in which he assumed prior knowledge of the anomaly type. While it is essential and mandatory to make an assumption such as this, it is very important to go a step further and carry out a test on the anomaly to check whether the assumption made is correct. The procedure to do this is the main subject of this note. I start off by first using some method that does not involve Euler’s differential equation to determine depth to the top/center of the suspected causative body. Then I employ the determined depth to identify the causative body from the graphical diagram of Hood (1971, Figure 26).
VELOCITY STRATIFICATION AS AN AID TO CORRELATION
Authors: Beers Roland F.
Published: October 2002

This paper outlines the historical development of the concept “velocity stratification” from the field data of refraction and reflection seismographs. The concept is shown to form the basis of the phenomenon known as “seismic reflections.” Experimental data are interpreted to show approximately the magnitude of reflection coefficients. Employment of velocity stratification as a means of stratigraphic correlation is suggested. Means whereby data for these correlations may be collected are discussed. Finally the use of the method for attack on stratigraphic problems is outlined.
SEISMIC REFRACTION STUDIES IN THE ORINOCO BASIN OF DEPOSITION, VENEZUELA
Authors: Kamen‐Kaye M.
Published: October 2002

The progress of seismic refraction methods in the Orinoco embayment is briefly sketched. Uniformity of post‐basement velocities and relations to compaction are noted, unusual velocities being analyzed. The author’s use of flexure‐time contours is described and the effect of basement velocity variation on flexure time is presented empirically.
First results of the MOSES experiment: Sea sediment conductivity and thickness determination, Bute Inlet, British Columbia, by magnetometric offshore electrical sounding
Authors: Edwards R. N. , Law L. K. , Wolfgram P. A. , Nobes D. C. , Bone M. N. , Trigg D. F. , DeLaurier J. M.
Published: October 2002

A static electrical method has been developed to determine the electrical resistivity of crustal rock beneath the sea. The transmitter is a vertical, long‐wire bipole, extending from the sea surface to the sea floor. A commutated current, generated on the ship, is fed to two large electrodes: one near the sea surface, the other at the end of a long insulated wire. The return path for the current is through the sea and the subjacent crust. The receiver is a self‐contained, remote, microprocessor‐controlled magnetometer which is deployed from the ship to the sea floor and subsequently recovered. The data are measurements of the azimuthal component of the magnetic field as a function of transmitter‐receiver horizontal separation. The acronym MOSES has been coined for the method. The choice of the name MOSES is appropriate because the system geometry is carefully arranged to remove many of the adverse effects of the relatively conductive sea water. In particular, accurate estimates of sea floor resistivity are possible because the data are proportional to the transmitted current from the source into the crustal material. A sea test of the method in a water depth of 640 m was conducted in the “V” shaped Bute Inlet, British Columbia. Transmitted power was 1.25 kW; averaging time at each transmitter location was 1 hour. Transmitter‐receiver separations ranged from 150 to 2 000 m. The resistivity and thickness of a sedimentary section beneath the sea were determined as 1.9⋅Ω m and 560 m, respectively. The interpretation was accomplished both by matching the data converted to apparent resistivity to corresponding model type curves and by generalized linear inverse theory. Errors in the final parameters were estimated at about 9.2 percent using a parameter eigenvector analysis. The interpreted resistivity is in accord with direct measurement on core samples of sediment porosity. The interpreted thickness is less than an upper limit determined by extrapolating local inlet topography beneath the sea.
Field testing of explosive surface seismic sources in the Canadian thrust belt
Authors: Davis Thomas L. , Lawton Don C.
Published: October 2002

Field tests of explosive surface seismic sources conducted in the foothills of Alberta illustrate that data quality obtained from surface shooting can be improved significantly by source‐array design and the placement of detonation points within the array. A source‐array design which maximizes the data quality in the Alberta foothills is a simple, 50 kg, cross‐configuration of linear surface explosives with detonation from all ends. No significant difference occurs between shooting at the ground surface versus elevating and detonating the cross‐configuration a few meters off the ground. Ove‐rshooting can occur by improper design of weighted cross‐configuration arrays. Multifold profiling with the cross‐configuration illustrates that quality seismic data can be derived in the Alberta thrust belt through the use of surface seismic sources.
Deconvolution by autocepstral windowing
Authors: Scheuer T. E. , Wagner D. E.
Published: October 2002

The autocepstrum of a reflection seismogram is defined by the cepstrum of its autocorrelation function. Using the autocepstrum extends the basic deconvolution method for removing a minimum‐phase source wavelet to unmask subsurface reflectivity. When we record only the seismic trace and assume a minimumphase source wavelet, deconvolution reduces to estimating the wavelet autocorrelation. In practice, a portion of the seismic trace autocorrelation is used as an estimate of the wavelet autocorrelation. This can be justified by assuming a random reflectivity series with a white power spectrum. However, in cases where the reflectivity spectrum is not white, a preferred wavelet autocorrelation may be obtained by low‐pass windowing the trace autocepstrum. This approach liberates the selection of various deconvolution parameters such as filter length and design window length that are typically chosen to reinforce the assumption of a white reflectivity spectrum. For problems that require short, deconvolution‐filter design windows, and thus nonwhite reflectivity spectra, windowing the trace autocepstrum is an appropriate alternative to the conventional practice of windowing the trace autocorrelation.
COCORP: Nevada areas, Part I and Part II
Authors: Kaufman S.
Published: October 2002

The Consortium for Continental Reflection Profiling (COCORP) announces the availability of the data packages and digital tapes for two areas: Nevada area, Part 1, lines 4, 5, and 6 covering 270.1 line‐km; and Nevada area, Part II, lines 1, 2, 3, and 7 covering 273 line‐km. The costs are the costs of reproduction and shipping, only. The COCORP operation is part of the U.S. Geodynamics Program sponsored by the National Academy of Sciences and funded by the National Science Foundation. The executive group of the consortium consists of representatives from Cornell University, Dartmouth College, Princeton University, Rice University, and the University of Wisconsin. Cornell University is the operating institution. The line locations for the two areas are shown in Figure 1. Also shown is Nevada line 8 which is not yet ready for distribution but which will be part of the N. Cal‐Nevada package to be issued shortly. Petty‐Ray was the contractor for the data acquisition. Processing was done on the Megaseis system at Cornell by students and staff of the Department of Geological Sciences.
Fifty years of case histories
Authors: Kettle Robert W.
Published: October 2002

Fifty years of case histories naturally reflect different and ever‐changing technology, but the underlying theme remains unchanged: to show that careful surface measurements can reveal physical properties of the subsurface. Finding a salt dome with a torsion balance in the 1920s and defining reservoir properties with shear waves in the 1980s involve different techniques, but in both the physical properties of the rocks are interpreted from geophysical measurements to solve geologic problems. Most early case histories attempted to establish the validity of geophysical tools; an example would be attempting to show the usefulness of refraction fan shooting by comparing its results with drill hole information. Later tools would in turn be compared with established tools. Reflection seismic interpretations were compared with torsion balance and refraction fan shooting results. As the science of geophysics became more accepted, case histories focused on more detailed aspects of technology and on discovery of subtle traps. Early papers dwelt at length on the application of geophysics to large features and on comparison of results with drill information. Later case histories continued to use geologic information to test geophysical interpretation, but they emphasized the subtle and hidden aspects rather than the obvious. The most recent case histories seldom deal with large structures. Instead they demonstrate the source wavelet required to resolve a stratigraphic trap or the effect that changing the reservoir fluid has on the reflection character so that hydrocarbons can be identified, or inferred, directly from the seismic data. Geophysical exploration has evolved and case histories reflect that evolution.
Low‐frequency electrical properties
Authors: Olhoeft G. R.
Published: October 2002

In the interpretation of induced polarization data, it is commonly assumed that metallic mineral polarization dominantly or solely causes the observed response. However, at low frequencies, there is a variety of active chemical processes which involve the movement or transfer of electrical charge. Measurements of electrical properties at low frequencies (such as induced polarization) observe such movement of charge and thus monitor many geochemical processes at a distance. Examples in which this has been done include oxidation‐reduction of metallic minerals such as sulfides, cation exchange on clays, and a variety of clay‐organic reactions relevant to problems in toxic waste disposal and petroleum exploration. By using both the frequency dependence and nonlinear character of the complex resistivity spectrum, these reactions may be distinguished from each other and from barren or reactionless materials.