bluuebunny/crossref_metadata_2025 huggingface dataset:
A two‐layer stacking procedure to enhance converted waves
Authors: Kennett B. L. N.
Published: October 2002

For marine seismic sources quite efficient conversion of P‐waves to S‐waves can occur at hard seafloors, e.g., carbonate horizons in tropical waters. The S‐waves are reflected back from structures at depth and are reconverted to P‐waves in the water before detection by the receiver array. Such PSSP reflections can carry useful information on the structure beneath the sea bed but are most significant at large offsets and so are not easily stacked with a conventional normal moveout (NMO) procedure based on a hyperbolic time trajectory. A two‐layer stacking procedure that separates the water layer from the region below the seafloor provides a very effective means of extracting the PSSP arrivals, but also works well for P‐waves. There is no direct analytic form for the stacking trajectories but they can be calculated quite efficiently numerically. A further advantage is that the stacking velocity for S‐waves in the lower layer can be interpreted directly in terms of S‐wave propagation, so that S‐wave interval velocities can be found. Stacking procedures based on such simple physical models are likely to be useful in other cases where attention needs to be focused on a particular aspect of the wavefield.
On: “Imaging salt with turning seismic waves,” by Dave Hale, N. Ross Hill, and Joe Stefani (GEOPHYSICS, 57, 1453–1462 November 1992).
Authors: Graebner Mark
Published: October 2002

Hale et al., have written a very interesting paper showing the impressive results that can be obtained when diving waves are used to image the flanks of salt domes. Unfortunately, they give an erroneous description of the diving wave in the Fourier domain [equation (1) in their paper]. In equation (1) they state that the migrated image is the contribution of the conventionally reflected wave and a second term which represents the contribution of the turning wave [Formula: see text] where [Formula: see text] (I have replaced their k with ωp in the integrand). They state that the diving wave has a frequency‐independent π/2 phase shift. That statement is incorrect.
First‐arrival traveltime calculation for anisotropic media
Authors: Qin Fuhao , Schuster Gerard T.
Published: October 2002

Huygens’s principle is used to derive a method for calculating first arrival traveltimes in anisotropic media. Numerical results for several transverse isotropic (TI) models show that calculated Huygens traveltimes are in good agreement with traveltimes computed by a finite‐difference solution to the anisotropic wave equation. This Huygens method is stable and accurate for the test models and so it may be useful in anisotropic data inversion and wave propagation visualization.
2-D random media with ellipsoidal autocorrelation functions
Authors: Ikelle L. T. , Yung S. K. , Daube F.
Published: October 2002

The integration of surface seismic data with borehole seismic data and well‐log data requires a model of the earth which can explain all these measurements. We have chosen a model that consists of large and small scale inhomogeneities: the large scale inhomogeneities are the mean characteristics of the earth while the small scale inhomogeneities are fluctuations from these mean values. In this paper, we consider a two‐dimensional (2-D) model where the large scale inhomogeneities are represented by a homogeneous medium and small scale inhomogeneities are randomly distributed inside the homogeneous medium. The random distribution is characterized by an ellipsoidal autocorrelation function in the medium properties. The ellipsoidal autocorrelation function allows the parameterization of small scale inhomogeneities by two independent autocorrelation lengths a and b in the horizontal and the vertical Cartesian directions, respectively. Thus we can describe media in which the inhomogeneities are isotropic (a = b), or elongated in a direction parallel to either of the two Cartesian directions (a > b, a < b), or even taken to infinite extent in either dimension (e.g., a = infinity, b = finite: a 1-D medium) by the appropriate choice of the autocorrelation lengths. We also examine the response of seismic waves to this form of inhomogeneity. To do this in an accurate way, we used the finite‐difference technique to simulate seismic waves. Special care is taken to minimize errors due to grid dispersion and grid anisotropy. The source‐receiver configuration consists of receivers distributed along a quarter of a circle centered at the source point, so that the angle between the source‐receiver direction and the vertical Cartesian direction varies from 0 to 90 degrees. Pulse broadening, coda, and anisotropy (transverse isotropy) due to small scale inhomogeneities are clearly apparent in the synthetic seismograms. These properties can be recast as functions of the aspect ratio [Formula: see text] of the medium, especially the anisotropy and coda. For media with zero aspect ratio (1-D media), the coda energy is dominant at large angles. The coda energy gradually becomes uniformly distributed with respect to angle as the aspect ratio increases to unity. Our numerical results also suggest that, for small values of aspect ratio, the anisotropic behavior (i.e., the variations of pulse arrival times with angle) of the 2-D random media is similar to that of a 1-D random medium. The arrival times agree with the effective medium theory. As the aspect ratio increases to unity, the variations of pulse arrival times with angle gradually become isotropic. To retain the anisotropic behavior beyond the geometrical critical angle, we have used a low‐frequency pulse with a nonzero dc component.
Applications of seismic polarization analysis
Authors: Perelberg Azik I. , Hornbostel Scott C.
Published: October 2002

Multicomponent seismic recording contains a wealth of information. A major challenge is to distill this information into something manageable. Polarization analysis is a technique for simplifying the situation by extracting two simple measures: ellipticity and directionality. These measures can be obtained quickly from the data covariance matrices and are sufficient for data analysis or for the design of weighting functions for seismic wave‐type selection. Data analysis using these measures may include the study of ellipticity to discern anisotropy and the use of ellipticity and directionality for a quick survey of the various wave types in the data. Applications of the related weighting functions include the removal of ground roll, the separation of converted waves or refractions, and the removal of out‐of‐plane arrivals.
Slowness adaptive f-k filtering of prestack seismic data
Authors: Duncan Guy , Beresford Greg
Published: October 2002

Frequency‐wavenumber velocity filtering is often applied to prestack seismic data for the attenuation of coherent noise. Although the process often gives excellent results, it can sometimes result in signal smoothing and distortion and poor attenuation of coherent noise. A slowness adaptive f-k filter reduces signal distortion and improves the attenuation characteristics of the filter. The technique uses a time‐ and space‐variant narrow reject‐band f-k filter. Optionally, coherent noise is compressed before application of the filter. The apparent slowness of coherent noise events is estimated using local t-x slant stacks weighted by coherence. A two‐dimensional (2-D) window is moved across the shot record, and at each point on the record slant stacks are taken through the central sample of the window. The slowness value that produces the maximum stack is assigned to the central sample of the window. In this way, an instantaneous slowness image of the shot record is produced. A one‐dimensional (1-D), high‐pass, finite‐duration impulse‐response (FIR) filter is applied in a spatially and temporally varying way across the record on the basis of the instantaneous slowness values. Before filter application, trace‐to‐trace static and amplitude effects are estimated and removed from the data. This results in compression of coherent noise and improved attenuation after filtering. The filtering process has been applied to low‐fold prestack dynamite data from the Surat Basin, Australia. The results indicate that the technique has good attenuation characteristics and produces minimal distortion of seismic signal. The process, however, is computationally expensive.
Green’s equivalent‐layer concept in gravity band‐pass filter design
Authors: Pawlowski Robert S.
Published: October 2002

Green’s concept of an equivalent‐source layer is invoked to construct a data adaptive, zero‐phase, Wiener band‐pass filter for regional‐residual gravity anomaly separation. The observed gravity field’s Fourier power spectrum is modeled with two Green’s equivalent‐source layers, one equivalent layer for the shallower (residual field) geologic sources, and a second equivalent layer for the deeper (regional field) geologic sources. The depths and average physical property contrasts of the two equivalent layers are determined by fitting the observed gravity field’s Fourier power spectrum with a two‐layer spectral model. Each equivalent layer is simulated by a horizontal thin sheet with randomly varying and randomly distributed point density sources spread throughout it. Adopting such a theoretical model for the Fourier power spectrum yields a stable and well‐behaved filter transfer function. Like all band‐pass filtering though, the method is ineffective in the case of insufficient vertical separation between the shallow and deep geologic sources whose gravity anomalies it is desired to separate. The filter design process is simple and easily automated, being well‐suited for modular implementation in a “filter tool kit” applications program running on either a workstation or personal computer. Equally important, the method yields repeatable, interpreter‐independent results.
An experimental investigation of factors influencing compressional‐ and shear‐wave velocities and attenuations in tight gas sandstones
Authors: Tutuncu Azra N. , Podio Augusto L. , Sharma Mukul M.
Published: October 2002

Results are presented for compressional and shear velocities and attenuations in fully brine‐saturated tight gas cores with porosities from 3 to 11.9 percent and clay contents from 1 to 38 percent. The influence of porosity, clay content, frequency, and stress on velocities and attenuations were examined using the amplitude spectra of P‐ and S‐waves in the frequency domain. Attenuations of samples were obtained using the spectral ratio method. For a few selected samples the attenuations were also measured using the length correlation method and these results were compared with the spectral ratio results. In tight gas sandstones, the attenuations obtained were 2 to 5 times greater than the attenuation obtained for Berea sandstone. In general, the presence of clay softens the rock grain contacts causing smaller values of compressional ([Formula: see text] and shear ([Formula: see text]) velocities as the clay content increases. However, the [Formula: see text] ratio was found to increase with clay content. Compressional‐and shear‐wave amplitude spectra exhibited a shift in peak frequency toward lower frequencies for samples with higher clay content when compared to clean samples. Velocities and attenuations were found to be frequency dependent, but the positive slope of both compressional and shear attenuations indicate that scattering starts to dominate at the lower frequency end of the ultrasonic measurements. Both [Formula: see text] and [Formula: see text] increased while both compressional and shear attenuations decreased when stress was increased.
A case study of integrated hydrocarbon exploration through basalt
Authors: Withers Robert , Eggers Dwight , Fox Thomas , Crebs Terry
Published: October 2002

A large unexplored tectonic basin with the potential for significant hydrocarbon accumulations was identified in north‐central Oregon using a variety of geophysical techniques. The basin, informally named after the local town of Heppner, is covered by several thousand feet of Miocene Columbia River Basalt Group (CRBG) but is readily identified by a gravity low against the Blue Mountains Uplift. The Paleocene/Eocene Herren Formation (Pigg, 1961), which outcrops on the Blue Mountains Uplift south of the Heppner Basin, offered good source and reservoir potential. Based on lateral extent, thickness and paleocurrent structures in the Herren Formation, the unit was expected to be present in the basin. Gravity modeling produced nonunique interpretations, thus magnetotelluric (MT) information was used to constrain the CRBG thickness. Static shifts in the MT data were removed using transient electromagnetic (TEM) data before MT data inversion. After extensive experimentation, adequate seismic data were obtained for structural mapping, but the seismic data were interpretable with confidence only after MT determinations of the CRBG thickness. As a result of the favorable geologic and geophysical information, the ARCO Hanna ♯1 well was drilled to 9100 ft (2800 m) near Heppner, Oregon in section 23, T2S, R27E in 1988. The thickness of the CRBG and Oligocene John Day Formation were accurately predicted by the geophysical interpretations. An unanticipated thickness of Eocene Clarno Formation was encountered and drilling ceased in this unit. No Herren Formation was penetrated during drilling. Geophysical well logs indicate the Clarno Formation has densities and resistivities sufficient to account for the gravity and electrical anomalies defining the prospect. Poor seismic quality was explained by the heterogeneous nature of the pre‐CRBG volcanic section encountered in the well.
Electromagnetic response of a discretely grounded circuit—An integral equation solution
Authors: Qian Wei , Boerner David E.
Published: October 2002

We derive an integral equation to describe the electromagnetic response of a discretely grounded circuit. This investigation is relevant to the study of man‐made structures such as metallic fences, grounded powerlines, and pipelines, all of which may fall into the class of discretely grounded conductors. The solution developed here is an extension to existing circuit theory and takes into account the self and mutual interaction of the circuit elements. It is possible to ignore these interactions at low frequencies where the grounding impedances dominate the effective impedance of the circuit. However, at frequencies where the electromagnetic skin depth is comparable to the length between adjacent grounding points, the effective impedance of the circuit is proportional to frequency, and the inductance of the circuit dominates its electromagnetic response. Within the quasi‐static limit (i.e., where displacement currents can be neglected) electromagnetic excitation by either horizontal electric or vertical magnetic dipoles produces a constant primary electric field at high frequencies (far‐field). Thus, the electric current in the discretely grounded circuit will always be inversely proportional to frequency for these types of sources. Horizontal magnetic dipole or vertical electric dipole sources generate primary electric fields that are proportional to the inverse square root of frequency in the high frequency limit of the quasi‐static domain, and thus the current in a circuit excited by such sources will decrease as the inverse of square root of frequency. The integral equation solution derived here can be used to investigate the influence from cultural conductors on actual electromagnetic surveys and also provides further insights into the current channeling response of surficial conductors.
On: “Advantages of using the vertical gradient of gravity for 3-D interpretation,” by I. Marson and E. E. Klingele (November 1993 GEOPHYSICS, 58, p. 1588-1595)
Authors: Ferris Craig
Published: October 2002

I was rather amazed to read “Advantages of using the vertical gradient of gravity for 3-D interpretation”in the November issue of Geophysics. The authors have reinvented the wheel. Vertical gradient interpretations have been used commercially for almost fifty years.
A standard finite‐difference scheme for the time‐domain computation of anelastic wavefields
Authors: Krebes E. S. , Quiroga‐Goode Gerardo
Published: October 2002

We show that the finite‐differencing technique based on the consecutive application of the central difference operator to spatial derivatives, a standard well‐known technique that has been commonly used in the seismological literature for solving the elastic equation of motion, can also be used to obtain a stable time‐domain, finite‐difference scheme for solving the anelastic equation of motion. We compare the results of the scheme for a heterogeneous medium with those of the time‐domain finite‐difference scheme previously developed by Emmerich and Korn and find that they agree very closely. We show, analytically, that in the case of a homogeneous medium, the two schemes give identical numerical results for certain zero initial conditions. The scheme based on the standard technique uses more computer time and memory than the scheme of Emmerich and Korn. However, from a theoretical viewpoint, it is easier to analyze, as it is developed solely with a familiar standard method.
Automated source‐edge location with a new variable pass‐band horizontal‐gradient operator
Authors: Thurston Jeffrey B. , Brown R. James
Published: October 2002

A new class of space‐domain convolution operators permits computation of the components of the horizontal gradient of gridded potential‐field data. These so‐called gradient‐component operators allow one to vary the passband and thus control the frequency content of the resulting horizontal‐gradient map. This facilitates computation of gradient maps that accommodate data of widely varying frequency content. Examination of the transfer functions of these operators suggests that this method of numerical differentiation is well suited to potential‐field data: in particular, the operators suppress long wavelengths and highfrequency noise bands and amplify signal. Maps of the horizontal gradient of certain potential fields (e.g., gravity, pseudogravity) may be combined with algorithms that locate relative maxima, so‐called thresholding, to automate the procedure of source‐body edge detection, which is a useful tool in mapping, for example, basement grain, fault patterns, and igneous intrusive bodies. We apply this new operator, together with an existing thresholding algorithm, to a field example from western Canada and demonstrate its potential for improved imaging of the horizontal‐gradient magnitude and thus improved edge detection.
Hydrocarbon‐generation‐induced microcracking of source rocks
Authors: Vernik Lev
Published: October 2002

Laboratory measurements of ultrasonic velocity and anisotropy in kerogen‐rich black shales of varying maturity suggest that extensive, bedding‐parallel microcracks exist in situ in most mature source rocks undergoing the major stage of hydrocarbon generation and migration. Given the normal faulting regime with the vertical stress being the maximum principal stress typical of most sedimentary basins, this microcrack alignment cannot be accounted for using simplified fracture mechanics concepts. This subhorizontal microcrack alignment is consistent with (1) a model of local principal stress rotation and deviatoric stress reduction within an overpressured formation undergoing hydrocarbon generation, and with (2) a strong mechanical strength anisotropy of kerogen‐rich shales caused by bedding‐parallel alignment of kerogen microlayers. Microcracks originate within kerogen or at kerogen‐illite interfaces when pore pressure exceeds the bedding‐normal total stress by only a few MPa due to the extremely low‐fracture toughness of organic matter. P‐wave and, especially, S‐wave anisotropy of the most mature black shales, measured as a function of confining pressure, indicate the effective closure pressure of these microcracks in the range from 10 to 25 MPa. Estimates of pore pressure cycles in the matrix of the active hydrocarbon‐generating/expelling part of the source rock formation show that microcracks can be maintained open over the sequence of these cycles and hence be detectable via high‐resolution in‐situ sonic/seismic studies.
Time‐domain estimation of MT Impedance tensor
Authors: Spagnolini Umberto
Published: October 2002

The spectral analysis of magnetotelluric (MT) data for impedance tensor estimation requires the stationarity of measured magnetic (H) and electric (E) fields. However, it is well known that noise biases timedomain tensor estimates obtained via an iterative search by a descent algorithm to determine the least‐mean‐square residual between measured and estimated E data obtained from H data. To limit the noise that slows down, or even prevents convergence, the steepest descent step size is based upon the statistics of the residual (Bayes’ estimation). With respect to uncorrelated noise, the time‐domain technique is more robust than frequency‐domain techniques. Furthermore, the technique requires only short‐time stationarity. The time‐domain technique is applied to data sets (Lincoln Line sites) from the EMSLAB Juan de Fuca project (Electromagnetic Sounding of the Lithosphere and Asthenosphere Beneath the Juan de Fuca Plate), as well as to data from a southern Italian site. The results of EMSLAB data analysis are comparable to those obtained by robust remote reference processing where larger data sets were used.
Electric study of fracture anisotropy at Falkenberg, Germany
Authors: al Hagrey Said Attia
Published: October 2002

Electric and electromagnetic methods have been applied for mapping subsurface fractures and the directional dependence of in‐situ electric parameters at the hot dry rock site at Falkenberg, Germany. This study includes the determination of several anisotropy parameters like the mean, longitudinal and transverse resistivity components ([Formula: see text], [Formula: see text] and [Formula: see text], respectively), the anisotropy coefficient λ, and the strike angle Θ. Terrain conductivity measurements using the technique of frequency‐domain electromagnetic induction reveal a dominant anomaly strike of east‐south‐east— west‐north‐west, nearly parallel to the fracturing strike of N110°. With increasing distance from the central borehole, the mise à la masse potential differences exhibit a transition from a direct to a paradoxical relationship to the resistivity anisotropy induced by the fracturing. These observations are explained using a model for an ellipsoidal fracture. The qualitative interpretation of the sounding data of Schlumberger and crossed‐square arrays clearly shows the anisotropy paradox related to the N110° strike. The crossed‐square method applied over a range of electrode spacings of 2.1–447 m yields apparent anisotropy values of [Formula: see text], [Formula: see text], [Formula: see text], λ = 1.15–1.34 and Θ ≈ 110°. The quantitative interpretation of the sounding data shows a conductive fracture zone (ρ ≈ 300–700 ω ⋅ m) embedded in the resistive granitic basement (ρ > 1200 ω ⋅ m) at a depth of 55–85 m. The anisotropy parameters for the fracture zone are ρm = 700 ω ⋅ m, [Formula: see text], ρt = 1225 ω ⋅ m and λ = 1.75. The fact that the anisotropy coefficient of the conductive zone is higher than the apparent λ is in agreement with the conclusion that the anisotropy at the study site is related mainly to fracturing.
Finite‐difference schemes for elastic waves based on the integration approach
Authors: Zahradník Jiri , O’Leary Patrick , Sochacki James
Published: October 2002

The authors present a second order explicit finite‐difference scheme for elastic waves in 2-D nonhomogeneous media. These schemes are based on integrating the equations of motion and the stress‐free surface conditions across the discontinuities before discretizing them on a grid. As an alternative for the free‐surface treatment, a scheme using zero density above the surface is suggested. This scheme is first order and is shown to be a natural consequence of the integrated equations of motion and is called a vacuum formalism. These schemes remove instabilities encountered in earlier integration schemes. The consistency study reveals a close link between the vacuum formalism and the integrated/ discretized stress‐free condition, giving priority to the vacuum formalism when a material discontinuity reaches the free surface. The two presented free‐surface treatments coincide in the sense of the limit (grid size → 0) for lateral homogeneity at or near the free surface.
Electrical resistivity measurement through metal casing
Authors: Schenkel Clifford J. , Morrison H. Frank
Published: October 2002

Methods using dc electrical arrays to measure formation resistivity through casing have relied on approximate forms for the current and potential distributions to derive a simple relationship between the formation resistivity and the transverse resistance calculated from measurements of the potential and its second derivative inside the casing. We have derived a numerical solution for the potentials and their derivatives to examine the accuracy of the approximate forms for casing of finite‐length, annular zones of varying radius, and for vertical discontinuities such as layers or abrupt changes in annular zone radius. For typical conductivity contrasts between the casing and formation, the approximate relationships may be off by as much as 60 percent for long casing and may show variations of 20 to 30 percent as the electrode array moves along the casing. In principle an iterative scheme could be devised to correct the readings if high accuracy was required. The numerical results show that to first order the current flow from the casing is radial, and that all the analytic expressions based on this assumption for evaluating layer resolution and the effects of annular layers are valid. An interesting byproduct of this study has been the discovery that the distortion of the potentials in a nearby well by an annular disk (e.g., an injected steam zone) surrounding the current injection well is greater if the injection well is cased. Crosswell resistivity surveys appear feasible if one of the wells is cased.
Linear and parabolic τ-p transforms revisited
Authors: Zhou Binzhong , Greenhalgh Stewart A.
Published: October 2002

New derivations for the conventional linear and parabolic τ-p transforms in the classic continuous function domain provide useful insight into the discrete τ-p transformations. For the filtering of unwanted waves such as multiples, the derivation of the τ-p transform should define the inverse transform first, and then compute the forward transform. The forward transform usually requires a p‐direction deconvolution to improve the resolution in that direction. It aids the wave filtering by improving the separation of events in the τ-p domain. The p‐direction deconvolution is required for both the linear and curvilinear τ-p transformations for aperture‐limited data. It essentially compensates for the finite length of the array. For the parabolic τ-p transform, the deconvolution is required even if the input data have an infinite aperture. For sampled data, the derived τ-p transform formulas are identical to the DRT equations obtained by other researchers. Numerical examples are presented to demonstrate event focusing in τ-p space after deconvolution.
On the “wraparound” multiple problem of recording seismic reflections in deep water
Authors: McBride J. H. , Hobbs R. W. , Henstock T. J. , White R. S.
Published: October 2002

During continuous seismic recording in deep water, primery reflections may be accompanied by seabed and sediment interbed multiples produced by the previous shot (Tucker and Yorston, 1973). Such “wraparound” multiples (WAMs), because of their repetition in the water column, show little normal moveout (Levin, 1971; Hardy et al., 1989) and produce patterns on the stacked common-midpoint (CMP) section that are not readily recognizable as a multiple reflection series. Once recorded, WAMs are thus extremely difficult to remove and may be mistaken for primary reflectivity (e.g., Taner, 1980; Berryhill and Kim, 1986). The purpose of this study is to provide some insight into the conditions that contribute to contamination of the CMP section by WAMs.
Shallow subsurface mapping by electromagnetic sounding in the 300 kHz to 30 MHz range: Model studies and prototype system assessment
Authors: Stewart Duff C. , Anderson Walter L. , Grover Thomas P. , Labson Victor F.
Published: October 2002

A new instrument designed for frequency‐domain sounding in the depth range 0–10 m uses short coil spacings of 5 m or less and a frequency range of 300 kHz to 30 MHz. In this frequency range, both conduction currents (controlled by electrical conductivity) and displacement currents (controlled by dielectric permittivity) are important. Several surface electromagnetic survey systems commonly used (generally with frequencies less than 60 kHz) are unsuitable for detailed investigation of the upper 5 m of the earth or, as with ground‐penetrating radar, are most effective in relatively resistive environments. Most computer programs written for interpretation of data acquired with the low‐frequency systems neglect displacement currents, and are thus unsuited for accurate high‐frequency modeling and interpretation. New forward and inverse computer programs are described that include displacement currents in layered‐earth models. The computer programs and this new instrument are used to evaluate the effectiveness of shallow high‐frequency soundings based on measurement of the tilt angle and the ellipticity of magnetic fields. Forward model studies indicate that the influence of dielectric permittivity provides the ability to resolve thin layers, especially if the instrument frequency range can be extended to 50 MHz. Field tests of the instrument and the inversion program demonstrate the potential for detailed shallow mapping wherein both the resistivity and the dielectric permittivity of layers are determined. Although data collection and inversion are much slower than for low‐frequency methods, additional information is obtained inasmuch as there usually is a permittivity contrast as well as a resistivity contrast at boundaries between different materials. Determination of dielectric permittivity is particularly important for hazardous waste site characterization because the presence of some contaminants may have little effect on observed resistivity but a large effect on observed permittivity.
Inversion of induction logs based on maximum flatness, maximum oil, and minimum oil algorithms
Authors: Zhang Yong‐chun , Shen Liang C. , Liu Ce
Published: October 2002

Inversion of induction logs does not yield unique solutions of formation resistivity profiles because of noise. In this work, three algorithms are used to invert induction logs. The first algorithm employs the maximum flatness criterion that is aimed at obtaining a formation resistivity profile that is as close to a constant as possible. The second is the maximum oil algorithm that has the objective of yielding the highest resistivity profile in the reservoir. The third is the minimum oil algorithm aimed at finding the lowest resistivity profile. All three algorithms are subject to the condition that the resulting computed log must agree with the field log within a preset value. These algorithms are stable because noise and depth shift in the field log have little effect on the result.
Experimental measurements of seismic attenuation In microfractured sedimentary rock
Authors: Peacock Sheila , McCann Clive , Sothcott Jeremy , Astin Timothy R.
Published: October 2002

Ultrasonic compressional‐ and shear‐wave attenuation in water‐saturated Carrara Marble increase with increasing crack density and decreasing effective pressure. Between 0.4 and 1.0 MHz, empirical linear relationships between 1/Q and crack density CD were found to be: CD = 1.96 ± 0.63 × 1/Q, for compressional waves and CD = 6.7 ± 1.5 × 1/Q, for shear waves.
LIMITING VALUES OF GRAVITATIONAL AND MAGNETIC ANOMALIES DUE TO A SUBTERRANEAN STRUCTURE BOUNDED BY A SINGLE DIFFERENTIAL SURFACE
Authors: Fisher J. W.
Published: October 2002

Calculations are made of gravitational and magnetic anomalies supposed due to local variations in the form of a single differential surface separating an overlying rock from an underlying one, the effective thickness of the latter being infinite. Both layers are homogeneous in density or magnetic susceptibility and only those magnetic effects are considered which are due to induction in the earth’s magnetic field. The maximum possible values of these anomalies and of their first and second horizontal derivatives are calculated under these simplified conditions, and it is pointed out that the numerical readings of these quantities, provided by survey results, may lead to useful estimates of the extreme depth and proportions of the structure responsible.
RESOLUTION CONTROL IN SEISMIC SURVEYS
Authors: Beers Roland F.
Published: October 2002

Development of the concept of velocity stratification by experimental means has revealed the origin of certain reflection horizons. Correlation of these data with subsurface lithology suggests a method of attack, employing seismic means, on the problem of the stratigraphic trap. This paper outlines the problem and describes a method whereby stratigraphic relationships may be measured. It is suggested that a seismic wave may be fitted to the task by altering its wave length in accordance with the degree of resolution desired. Experimental examples of the procedure are given.
AVERAGE VERTICAL VELOCITIES FROM REFRACTION AND REFLECTION PROFILES
Authors: Ransone W. R. , Romberg Frederick
Published: October 2002

This paper presents data from a combined refraction and reflection profile from which vertical velocities are calculated. The accuracy of velocities so obtained is brought out by checking them with velocities obtained from shooting a well drilled two years after the seismograph work. It is also interesting to note that this method offers a means of identifying reflection horizons with refracting horizons, permitting better correlation between geologic and seismic markers. The method is of particular value in areas where little exploration has been done.
THE FUTURE OF GEOPHYSICS
Authors: Born W. T.
Published: October 2002

The petroleum industry may well take pride in the fact that it stands today in a position, shared with few others, of being able to supply easily whatever demands may be made upon it by reason of the present national defense program; and at the same time, can assure the nation that its future supply of oil is insured by a proven domestic reserve greater now than at any time in the past.
THE PROBABLE ERRORS OF DELTA‐T VELOCITIES
Authors: Romberg Frederick
Published: October 2002

It is proposed to test the accuracy of delta‐t velocities by finding the average deviations and probable errors of the delta‐t readings. Data are provided to demonstrate that errors smaller than one per cent can reasonably be expected. The quantitative effect of numbers of observations, shooting set‐ups, quality of data, and computing methods are discussed, and examples shown. Examples of regional and local lateral gradient are added.
COMPARISON OF WELL SURVEY AND REFLECTION “TIME‐DELTA TIME” VELOCITIES
Authors: Steele W. E.
Published: October 2002

Empirical curves are given, comparing directly measured well survey velocities and velocities indirectly determined by means of reflection data. The observed relationships are discussed.
Migration velocity analysis: Accounting for the effects of lateral velocity variations
Authors: Tieman Hans J.
Published: October 2002

The most common migration velocity analysis algorithms, iterative profile migration, focusing analysis, and stack power analysis are based on restrictive subsurface assumptions that may cause the methods to break down in the presence of lateral velocity variations. Typically, the subsurface is assumed to have either a constant velocity, or at most, a depth variable velocity. Recent innovations include the use of traveltime inversion philosophy to invert migration measurements of the curvature as a function of offset exhibited by an event following migration. Traveltime inversion makes few assumptions regarding the subsurface, but is a rather unstable process. Thus, an important question is, “Under what conditions do the traditional methods break down and make the use of traveltime inversion methods mandatory?” Marrying the common migration analysis with tomography results in a set of equations that, while useful for generating updates from migration measurements, are too complex for answering the above question. However, by restricting the subsurface to low relief structures and assuming small angle wave propagation, these updating equations can be approximated by forms that are identical to the traditional updating equations, except for a factor that is dependent upon the magnitude, position, and spatial wavelength of potential lateral velocity variations. These simpler equations indicate that even relatively long wavelength anomalies can cause updates to point in the wrong direction and iterative procedures to diverge, and under certain conditions, cause the accuracy of these updates to decrease dramatically.
An optimal absorbing boundary condition for elastic wave modeling
Authors: Peng Chengbin , Toksöz M. Nafi
Published: October 2002

Absorbing boundary conditions are widely used in numerical modeling of wave propagation in unbounded media to reduce reflections from artificial boundaries (Lindman, 1975; Clayton and Engquist, 1977; Reynolds, 1978; Liao et al., 1984; Cerjan et al., 1985; Randall, 1988; Higdon, 1991). We are interested in a particular absorbing boundary condition that has maximum absorbing ability with a minimum amount of computation and storage. This is practical for 3-D simulation of elastic wave propagation by a finite‐difference method. Peng and Toksöz (1994) developed a method to design a class of optimal absorbing boundary conditions for a given operator length. In this short note, we give a brief introduction to this technique, and we compare the optimal absorbing boundary conditions against those by Reynolds (1978) and Higdon (1991) using examples of 3-D elastic finite‐difference modeling on an nCUBE-2 parallel computer. In the Appendix, we also give explicit formulas for computing coefficients of the optimal absorbing boundary conditions.
Airborne gravity tests over Lake Ontario
Authors: Halpenny John F. , Darbha Dak M.
Published: October 2002

Airborne gravity measurements were made over Lake Ontario in March 1991 to establish the feasibility of using a small survey aircraft for rapid and economical gravity surveys. A LaCoste & Romberg linear dynamic gravity meter, used with a PC lap‐top based digital controller, was operated in a twin‐engine Piper Navajo aircraft. A Trimble 4000 GPS receiver, operated in differential kinematic mode with a nearby base station, provided precise navigation, and radar and barometric altimeters provided additional altitude data. A major gravity low in eastern Lake Ontario, previously outlined by underwater gravity measurements, was surveyed with six lines from two flights at an altitude of 1000 ft (300 m). Large vertical accelerations caused by aircraft motion were almost completely removed using the height measurements, although separate reductions using global positioning system (GPS) and radar heights showed significant differences that indicated that the height measurements could still be improved. A grid formed of the differences between the smoothed air data and 56 underwater stations had a mean of −0.2 mGal and standard deviation of 2.7 mGal.
Ray‐based synthesis of bistatic ground‐penetrating radar profiles
Authors: Cai Jun , McMechan George A.
Published: October 2002

An algorithm has been developed to numerically synthesize 2-D bistatic (common‐offset), ground‐penetrating radar (GPR) profiles using the principles of geometrical ray theory. By assuming nondispersive propagation, kinematic properties of electromagnetic waves are simulated by ray tracing. Dynamic properties are simulated by computing transmitter and receiver directivities, reflection and transmission coefficients, geometrical spreading, and attenuation coefficients. The main limitations are that wave effects, such as diffractions, and offline (3-D) effects are not included. The algorithm is applied to iterative modeling of multioffset, multifrequency GPR data acquired over an outcrop of fractured Austin Chalk in Dallas County in northeast Texas. Modeling is able to simulate realistically the main time and amplitude behaviors observed in GPR reflections at 50, 100, and 200 MHz at each of 1, 3, and 5 meter antenna separations from a single model. Detailed modeling produces quantitative estimates of the spatial distributions of electrical properties that are consistent with the geologic environment.
A multioffset, three‐component VSP study in the Sudbury Basin
Authors: Miao Xiao‐Gui , Moon Wooil M. , Milkereit B.
Published: October 2002

A multioffset, three‐component vertical seismic profiling (VSP) experiment was carried out in the Sudbury Basin, Ontario, as a part of the LITHOPROBE Sudbury Transect. The main objectives were determination of the shallow velocity structure in the middle of the Sudbury Basin, development of an effective VSP data processing flow, correlation of the VSP survey results with the surface seismic reflection data, and demonstration of the usefulness of the VSP method in a crystalline rock environment. The VSP data processing steps included rotation of the horizontal component data, traveltime inversion for velocity analysis, Radon transform for wavefield separation, and preliminary analysis of shear‐wave data. After wavefield separation, the flattened upgoing wavefields for both P‐waves and S‐waves display consistent reflection events from three depth levels. The VSP-CDP transformed section and corridor stacked section correlate well with the high‐resolution surface reflection data. In addition to obtaining realistic velocity models for both P‐ and S‐waves through least‐square inversion and synthetic seismic modeling for the Chelmsford area, the VSP experiment provided an independent estimation for the reflector dip using three component hodogram analysis, which indicates that the dip of the contact between the Chelmsford and Onwatin formations, at an approximate depth of 380 m in the Chelmsford borehole, is approximately 10.5° southeast. This study demonstrates that multioffset, three‐component VSP experiments can provide important constraints and auxiliary information for shallow crustal seismic studies in crystalline terrain. Thus, the VSP technique bridges the gap between the surface seismic‐reflection technique and well‐log surveys.
Rock/water interaction in dielectric properties: Experiments with hydrophobic sandstones
Authors: Knight Rosemary , Abad Ana
Published: October 2002

The dielectric constant of a partially saturated sandstone varies as a function of the level of water saturation. Experimental data indicate that rock/water interaction, at low saturations, has a large effect on the measured dielectric response. To theoretically predict the dielectric constant of the rock/water/air system, this rock/water interaction must be accounted for by including the effect of the water wetting the rock solid. Alternatively, if the rock/water interaction can be eliminated, a three‐component (dry rock, water, and air) mixing law can be used to model the dielectric behavior. In this laboratory study, a chemical treatment is used to change four water‐wet sandstones into hydrophobic sandstones. In three hydrophobic samples the rock/water interaction, seen in the data for the water‐wet samples, is eliminated and the dielectric constant of the partially saturated sandstones can be simply modeled as a dry rock/water/air system. This experimental study illustrates the importance of the chemical state of the rock surfaces in determining the dielectric behavior of sandstones.
Subsurface fault detection using seismic data for hazardous‐waste‐injection well permitting: An example from St. John the Baptist Parish, Louisiana
Authors: Zinni Edward V.
Published: October 2002

The Covington aquifer is used as a major source of drinking water for east St. John the Baptist Parish, Louisiana. A recent subsurface geologic study in Herbert and Hanson (1991) suggested that the Covington aquifer might communicate hydrologically with two hazardous‐waste‐injection reservoirs, thus putting the fresh‐water Covington aquifer in danger of contamination. A case history shows how seismic‐reflection and electric well‐log data were used to determine the possibility of hydrologic communication between the Covington aquifer and the hazardous‐waste‐injection reservoirs. Cross‐sections, net‐sand isopachs, and structure maps are constructed for three hazardous‐waste‐injection reservoirs and the Covington aquifer. Four previously undetected normal faults bisect the three injection reservoirs and the Covington aquifer. The presence of these faults significantly increases the possibility of communication by acting as conduits and allowing vertical migration of fluids along the fault planes, or by causing juxtaposition of sand against sand and allowing leakage to occur across the fault planes. In addition, the upper injection reservoir is part of a fluvial system that occasionally scours down into the Covington aquifer, depositing sand from the injection reservoir interval directly on the Covington aquifer sand. Because of this stratigraphic contact, the injection reservoir is possibly in hydrologic communication with the Covington aquifer. The injection of liquid‐hazardous‐waste into these reservoirs could jeopardize not only the water quality of the Covington aquifer, but possibly the quality of other shallow freshwater aquifers, should vertical migration occur along the fault planes. Without the use of seismic data, the risk of contaminating underground sources of drinking water would not have been properly assessed.
2.5-D wave equations and high‐frequency asymptotics
Authors: Stockwell John W.
Published: October 2002

The need for modeling 3-D seismic data in a 2-D setting has motivated investigators to create so‐called 2.5-D modeling methods. One such method proposed by Liner involves the use of an approximate 2.5-D wave operator for constant‐density media. The traveltimes and amplitudes predicted by high‐frequency asymptotic ray series (WKBJ) analysis of the Liner 2.5-D wave equation match those predicted by Bleistein’s 2.5-D ray‐theoretic development in constant wavespeed media. However, high‐frequency analysis indicates that the Liner 2.5-D variable wavespeed equation will have a maximum amplitude error of ±35% in a linear c(z) model where the wavespeed doubles or halves from the beginning to the end of a raypath. These amplitudes are comparable to those produced by converting 2-D data to 2.5-D using correction factors of the type proposed by Emersoy and Oristaglio and Deregowski and Brown, with the exception being that the Liner equation lacks the half derivative waveform correction present in these operators. An alternate method of constructing 2.5-D wave operators based on the WKBJ analysis is proposed. This method permits variable density (acoustic) 2.5-D wave operators to be derived.