bluuebunny/crossref_metadata_2025 huggingface dataset:
Cross‐borehole delineation of a conductive ore deposit in a resistive host—experimental design
Authors: Johnson David , Cherkaev Elena , Furse Cynthia , Tripp Alan C.
Published: October 2002

The finite‐difference time‐domain method is used for high‐resolution full‐wave analysis of cross‐borehole electromagnetic surveys of buried nickel sulfide deposits. The method is validated against analytical methods for simple cases, but is shown to be a valuable tool for analysis of complicated geological structures such as faulted or layered regions. The magnetic fields generated by a wire loop in a borehole near a nickel sulfide deposit are presented for several cases. The full‐wave solution is obtained up to 200 MHz, where quasi‐static methods would have failed. The dielectric response is included in the solution, and the diffractive nature of the field is observed. The sensitivity of each receiver in a vertical line in the cross borehole is presented and analyzed to provide an optimal weighting for receivers that can be applied to an experimental study.
Polarization filter with singular value decomposition
Authors: de Franco Roberto , Musacchio Gemma
Published: October 2002

We present a singular value decomposition (SVD) based algorithm for polarization filtering of triaxial seismic recordings based on the assumption that the particle motion trajectory is essentially 2-D (elliptical polarization). The filter is the sum of the first two eigenimages of the SVD on the signal matrix. Weighing functions, which are strictly dependent on the intensity (linearity and planarity) of the polarization, are applied. The efficiency of the filter is tested on synthetic traces and on real data, and found to be superior to solely covariance‐based filter algorithms. Although SVD and covariance‐based methods have similar theoretical approach to the solution of the eigenvalue problem, SVD does not require any further rotation along the polarization ellipsoid principal axes. The algorithm presented here is a robust and fast filter that properly reproduces polarization attributes, amplitude, and phase of the original signal. A major novelty is the enhancement of both elliptical and linear polarized signals. Moreover as SVD preserves the amplitude ratios across the triaxial recordings, the particle motion ellipse before and after filtering retains a correct orientation, overcoming a typical artifact of the covariance‐based methods.
Attenuating primaries and free‐surface multiples of vertical cable (VC) data while preserving receiver ghosts of primaries
Authors: Ikelle Luc T.
Published: October 2002

Marine vertical cable (VC) data contain primaries, receiver ghosts, free‐surface multiples, and internal multiples just like towed‐streamer data. However, the imaging of towed‐streamer data is based on primary reflections, while the emerging imaging algorithms of VC data tend to use the receiver ghosts of primary reflections instead of the primaries themselves. I present an algorithm for attenuating primaries, free‐surface multiples, and the receiver ghosts of free‐surface multiples while preserving the receiver ghosts of primaries. My multiple attenuation algorithm of VC data is based on an inverse scattering approach known, which is a predict‐then‐subtract method. It assumes that surface seismic data are available or that they can be computed from VC data after an up/down wavefield separation at the receiver locations (streamer data add to VC data some of the wave paths needed for multiple attenuation). The combination of surface seismic data with VC data allows one to predict free‐surface multiples and receiver ghosts as well as the receiver ghosts of primary reflections. However, if the direct wave arrivals are removed from the VC data, this combination will not predict the receiver ghosts of primary reflections. I use this property to attenuate primaries, free‐surface multiples, and receiver ghosts from VC data, preserving only the receiver ghosts of primaries. This method can be used for multicomponent ocean bottom cable data (i.e., arrays of sea‐floor geophones and hydrophones) without any modification to attenuate primaries, free‐surface multiples, and the receiver ghosts of free‐surface multiples while preserving the receiver ghosts of primaries.
TEST OF A QUANTITATIVE MOUNTAIN BUILDING THEORY BY APPALACHIAN STRUCTURAL DIMENSIONS
Authors: Elkins Thomas A.
Published: October 2002

A mathematical paper by Ross Gunn (Journal of the Franklin Institute, volume 224, pages 19–53, 1937) gives a mountain building theory which makes possible the calculation of certain structural dimensions in terms of thickness and strength of the geologic elements involved. The present paper applies various formulae from Gunn’s paper to the calculation of width and amplitude of folds, to basin widths and depths, and to mountain heights of the Appalachian Mountain system. The numerical values resulting are in surprisingly good agreement with previously published values measured or estimated from the known geology of the area. A brief discussion is also given of the theoretical interpretation, by Gunn’s theory, of the large isostatic gravity anomaly here as due to possible crustal stress.
A STUDY OF ELECTRICAL EARTH NOISE
Authors: Neuenschwander E. F. , Metcalf D. F.
Published: October 2002

A method of investigation into the characteristics of electrical ground noise is described. The factors involved in the origin and distribution of ground static and the value of noise level surveys as applied to geophysical prospecting are also given.
THE LIMITING SENSITIVITY OF SEISMIC DETECTORS
Authors: Wolf Alfred
Published: October 2002

The theory of Brownian motion is applied to the problem of determination of the limiting sensitivity of seismic detectors. It is shown that a certain minimum suspended mass is requird for the recording of small ground motions. Electromagnetic geophones are studied in detail, and it is shown how spontaneous thermal fluctuations in voltage limit the performance of these instruments. Under certain conditions, a geophone may be treated as a power generator, and the necessary suspended mass is then determined by the power requirements of the recording apparatus.
THE CROSBYTON ANOMALY, SOUTHEASTERN CROSBY COUNTY, TEXAS
Authors: McLemore Ethel Ward , Weaver Paul , Barton Donald C.
Published: October 2002

The purpose of this study has been the interpretation of the most probable structure producing the slightly subcircular anomaly at Crosbyton, as mapped by the pendulum, torsion balance, magnetometer, and dip needle. Work was begun in 1935 under the direction of Dr. Barton, who was to have written the geological considerations. All calculations were completed in 1938, but unfortunately the geological interpretation was not written by Dr. Barton before his death, July 8, 1939. At my request, since he had discussed the Crosbyton anomaly with Dr. Barton many times, and was quite familiar with the work we were doing, Mr. Weaver kindly consented to write the geological interpretation and help me with the completion of the paper. It is due to his knowledge and able assistance that the paper has been brought up to date in the light of subsequent drilling by the Gulf Oil Corporation.
SYMPOSIUM—APPLICATION OF GEOLOGY AND GEOPHYSICS TO WAR AND POST‐WAR PROBLEMS OF THE PETROLEUM INDUSTRY: GEOPHYSICS ON THE PACIFIC COAST
Authors: Salvatori Henry
Published: October 2002

In recent years practically all operators in California have largely confined their geophysical work to the reflection seismograph method. There has been a relatively small amount of gravity, magnetic and geochemical work, but with some minor exceptions the reflection method has been and still continues to represent by far the major geophysical method of exploration utilized in this area. But for one doubtful exception, all geophysical discoveries in recent years can be attributed to this method of exploration.
CONTRIBUTION OF GEOPHYSICS TO THE NATIONAL EFFORT
Authors: Hoover Herbert
Published: October 2002

In the present emergency of the war, applied geophysics faces many complex responsibilities. I believe that these responsibilities can be divided into two groups: First—we must do everything possible to increase our petroleum reserves. I would like to delay my comments on that subject until the latter part of my paper. Second—we have the obligation to make available our experience and resources, wherever they may be most needed, on problems that have arisen outside the petroleum industry.
On: “THE RELATIVE PLACE OF EMPIRICAL AND ANALYTICAL METHODS OF GEOPHYSICAL INTERPRETATION”
Authors: Nettleton L. L.
Published: October 2002

Mr. Weaver’s paper is very timely in bringing our attention back to considerations which probably have been overlooked or forgotten in the routine application of geophysical surveys. In the early days of a method, careful thought is apt to be given to the various controlling factors and limits which can influence the results. When a method becomes well established, these controls and factors tend to be forgotten in routine applications and these applications may well run beyond reasonable limits. Thus, in the later stages of development we probably tend to become more empirical and less analytical and therefore Mr. Weaver’s remarks should serve to direct our thoughtful attention back again to the analytical phases of our work. This is true to the extent that a definite distinction can be made between analytical and empirical approaches, both of which have a large part and a proper balance in any well conducted geophysical (or other scientific) activity.
THE IMPEDANCE OF A GROUNDED WIRE
Authors: Wolf Alfred
Published: October 2002

The impedance of an insulated wire stretched along the surface of the earth, regarded as a homogeneous conductor, is a function of frequency and of the conducitivity of the earth. Formulas are given for the inductance and the resistance of such a wire which are applicable under conditions met with in geophysical prospecting.
SEISMOGRAPH EVIDENCE ON THE DEPTH OF THE SALT IN SOUTHERN MISSISSIPPI
Authors: Swartz C. A.
Published: October 2002

Reflection seismograph records shot over the tops of shallow salt domes in Southern Mississippi show deep events which can reasonably be identified as reflections from the base of the salt. Calculations reveal the depth to the bottom of the salt mass to be approximately 22,000 to 26,000 feet.
THE MUTUAL IMPEDANCE OF COLLINEAR GROUNDED WIRES
Authors: West S. S.
Published: October 2002

The formula of R. M. Foster for the mutual impedance of grounded wires lying on the surface of the earth has been evaluated for the case where the wires are grounded at points which are collinear and equally spaced upon the surface of a homogeneous earth. Curves are given showing mutual impedance and phase shift as functions of resistivity and frequency.
SOME EFFECTS OF GEOLOGIC STRUCTURE ON RADIO RECEPTION
Authors: Howell, B. F.
Published: October 2002

Radio fields at standard broadcast frequencies were examined over faults to determine how the geologic conditions affected the fields. The fields in rugged and densely inhabited areas were found to be too irregular to yield understandable patterns. Very small variations in the directions of the fields were suggested but not proven over the San Jacinto fault. No variations in intensity were found over the San Andreas fault, but variations were observed at some places over the Piedmont fault in New Jersey. Such variations might be due to higher conductivity of the fault zone, or to the addition of waves reflected and refracted at the fault surface. Weak areas near San Bernardino were noted, but could not be correlated with faults. It seems improbable that surveys using standard broadcast signals will be of much value in studying radio weak spots, although other approaches may be more revealing.
Geophysical Investigations in the Tri‐State Zinc and Lead Mining District. J. J. Jakosky, R. M. Dreyer, and Clyde H. Wilson
Authors: Jakosky J. J.
Published: October 2002

Early in 1941 the University of Kansas Engineering Experiment Station and the State Geological Survey of Kansas were invited, by several governmental agencies, to investigate the feasibility of using geophysical methods as a guide to ore prospecting in the Tri‐State zinc and lead mining district of Kansas, Oklahoma, and Missouri. The increased demand for zinc and lead in the production of war materials made it appear desirable to attempt to develop some geophysical method or combination of methods which would be more rapid and less expensive than the present method of exploration by churn drilling now commonly employed in the Tri‐State district.
THE GEOPHYSICIST AS A FORECASTER
Authors: Weaver Paul
Published: October 2002

The commercial geophysicist has hitherto been little concerned with a realm of geophysics which deals with the detection of non‐equilibrium by measuring the time‐rate of change of a physical property in a localized area, in order to forecast when a critical point will appear in the system, and what magnitude of transition is probable. The author has selected some examples of such geophysical problems, which he considers of great economic importance, and which would appear amenable to our present technique or to some modification thereof.
Aerial Photographs: Their Use and Interpretation. A. J. Eardley. Harper and Brothers, N. Y. (1942), 200 pp., $2.75.
Authors: Bible John L.
Published: October 2002

This book is confined to the presentation of photogrammetric techniques which are necessary in the general use and interpretation of aerial photographs. “Aerial Photographs” has been thoroughly tested by the author in class‐room use, and represents the greatly expanded eighth edition of the author’s “Interpretation of Geologic Maps and Aerial Photos” which has heretofore been available only in lithoprint form.
FAULT LOCATION BY ELECTRICAL PROSPECTING—AN EXAMPLE
Authors: Hawley Paul F.
Published: October 2002

Electrical resistivity surveys were run in an area near Campbell, Hunt County, Texas, across a known fault of the Mexia system. Lateral inhomogeneity of earth resistivity was investigated by employing constant‐depth surveys, using the Wenner configuration with 1000‐foot spacing between adjacent electrodes. Five depth surveys were made using the Wenner configuration at electrode spacings up to 4000 feet. These depth surveys were interpreted using 3‐layer curves interpolated from the Wetzel‐McMurry data. The interpretations checked reasonably well against resistivity logs in two 350‐foot core holes in the region. The constant‐depth profiles cannot be said to have definitely indicated the presence of the fault, but a thin high resistance bed participating in the faulting was clearly delineated by the depth surveys and could be used to locate the fault.
MAGNETIC INTERPRETATION
Authors: Wantland Dart
Published: October 2002

It is common knowledge that magnetic anomalies do not always correspond to structural uplifts. The author discusses this problem under the headings of: (1) Examples, (2) The magnetic character of sediments, (3) Possibilities of magnetic stratigraphic studies, (4) The place of magnetic surveying.
GEOPHYSICAL EXPLORATION IN POLAND
Authors: Wir S. M.
Published: October 2002

With reference to the paper by Dr. Mitera on the “Present Status and Future Aspects of Geophysical Exploration in Poland” and to that of Dr. Heiland “Notes on Reflections from Steeply Dipping Beds,” printed in Geophysics, I would like to add further details concerned mainly with seismic prospecting carried out in Poland in the years 1934–1937, and to give some description of the work of the period from 1937–1939, i.e., to the outbreak of the present war.
GEOPHYSICAL SURVEY OF THE ARKANSAS BAUXITE REGION
Authors: Malamphy Mark C. , Vallely James L.
Published: October 2002

Magnetic and gravimetric surveys were conducted over an area of approximately 1400 square miles in the bauxite district of central Arkansas. The primary purpose of these surveys was to discover any possible buried and hitherto unknown syenite masses favorable for the occurrence of bauxite and to determine the approximate position of the buried flanks of the known syenite masses which might offer conditions favorable for the discovery of new ore bodies. These surveys indicated that the various syenite outcrops are domes or bosses on a large batholith and that other similar domes occur on the batholith but do not outcrop. Drilling on the local geophysical anomalies proved the presence of 10 buried domes, but only 2 were found to project above the upper surface of the Midway clays, a requisite of conditions favorable for the occurrence of bauxite ore bodies. The geophysical data indicated the approximate configuration of the buried flanks of the known syenite outcrops, and the portions of these flanks that project above the Midway have now been outlined more accurately by drilling. The geophysical surveys have produced evidence permitting the elimination of a large area as unfavorable for the occurrence of bauxite. Magnetic surveys extending along the Midway‐Wilcox contact from Gurdon in Clark County on the southwest to Searcy in White County on the northeast have proved the improbability of the existence of other syenite masses similar to those found in Pulaski and Saline Counties. A detailed magnetic survey of the Magnet Cove area in Hot Spring County has proved that the syenite mass exposed in that locality is an isolated intrusion and entirely unrelated to those of Pulaski and Saline Counties. This syenite mass does not occur under conditions believed to be favorable for the occurrence of bauxite.
ANALYTICAL AND EXPERIMENTAL DATA CONCERNING A SOLID HYDROCARBON SURVEY OF THE FORT COLLINS ANTICLINE
Authors: Tripp R. Maurice
Published: October 2002

An analysis is made of the properties of sediments and their derived products which pertain to the migration of gaseous hydrocarbons. From a thermodynamic consideration the zone of secondary mineralization is localized and the appearance of radioactive highs over structure is possibly explained. A distinction is made between the types of soil waxes, and an hypothesis is advanced to explain the origin of the petroleum derived waxes. The results of a soil wax survey of a producing structure are presented and discussed. No implication is intended regarding the general usefulness of the technique.
DESCRIPTION OF ESMWT COURSE IN PETROLEUM GEOPHYSICS CONDUCTED IN WASHINGTON, D.C.
Authors: Dobrin Milton B.
Published: October 2002

[In response to a request by the Editor, Mr. M. B. Dobrin, who acted as instructor in an ESMWT course in petroleum geophysics which was presented in Washington this Spring and Summer, has prepared the following account of the course for the assistance of those who may be interested in organizing similar courses in the geophysical centers.]
Analysis and interpretation of seismic data from thin reservoirs: Northwest Java Basin, Indonesia
Authors: Adriansyah A. , McMechan George A.
Published: October 2002

A detailed analysis and interpretation is performed of a 2‐D seismic line over a sequence of thin reservoirs in the upper Cibulakan formation in the Northwest Java Basin. Most well sites in this area are selected to be structures near fault zones as faults are assumed to be the main hydrocarbon migration paths. Amplitude variation with offset analysis is little used because of contamination by thin‐layer tuning effects. Attribute analysis, impedance inversion, and full‐wavefield modeling suggest that gas reservoirs are detectable even when they are less than their tuning thickness, as they correspond to acoustic impedance anomalies and low instantaneous frequency. The presence of hydrocarbons can also be detected by anomalous behavior in crossplots of acoustic impedance versus density and P‐wave velocity; sandstone reservoirs show low velocity and low impedance. Two‐dimensional P‐velocity and density distributions resulting from impedance inversion produce synthetic elastic common‐source gathers that display reflection behaviors that are qualitatively similar to the corresponding field data.
Electromagnetic traveltime tomography using an approximate wavefield transform
Authors: Lee Tae Jong , Suh Jung Hee , Kim Hee Joon , Song Yoonho , Lee Ki Ha
Published: October 2002

We present an imaging scheme for mapping cross‐hole electrical conductivity using nonlinear traveltime tomography. Data used are peak arrival time estimates based on an approximate wavefield transform of the synthetic frequency‐domain electromagnetic (EM) field. Direct transformation of frequency‐domain EM fields to wavefields is known to be an ill‐posed problem because the kernel of integral transform is highly damped. In this study, instead of solving such an unstable problem, we approximate the wavefield in the transformed domain via a ray series expansion. If reflected and refracted energy is weak compared to that of direct wave, picking of the peak arrival time may be reduced to estimating the coefficients of the leading term in the ray series expansion. This simplification is valid when the conductivity contrast between background medium and the target anomalous body is small. The first three terms in the expansion are identical to the closed‐form solution for the vertical magnetic field caused by a vertical magnetic dipole source in a homogeneous whole‐space. An adaptive simulated annealing scheme is used to estimate the coefficients of ray series. For a whole‐space, exact traveltime can be extracted using only four frequency samples in our approach, whereas the direct numerical wavefield transform needed at least ten frequencies to construct a reasonable waveform. Nonlinear traveltime tomography using thusly‐extracted peak arrivals from synthetic data shows a reasonable image of the conductivity structure between boreholes.
IP interpretation in environmental investigations
Authors: Slater Lee D. , Lesmes David
Published: October 2002

The induced polarization (IP) response of rocks and soils is a function of lithology and fluid conductivity. IP measurements are sensitive to the low‐frequency capacitive properties of rocks and soils, which are controlled by diffusion polarization mechanisms operating at the grain‐fluid interface. IP interpretation typically is in terms of the conventional field IP parameters: chargeability, percentage frequency effect, and phase angle. These parameters are dependent upon both surface polarization mechanisms and bulk (volumetric) conduction mechanisms. Consequently, they afford a poor quantification of surface polarization processes of interest to the field geophysicist. A parameter that quantifies the magnitude of surface polarization is the normalized chargeability, defined as the chargeability divided by the resistivity magnitude. This parameter is proportional to the quadrature conductivity measured in the complex resistivity method. For nonmetallic minerals, the quadrature conductivity and normalized chargeability are closely related to lithology (through the specific surface area) and surface chemistry. Laboratory and field experiments were performed to determine the dependence of the standard IP parameters and the normalized chargeability on two important environmental parameters: salinity and clay content. The laboratory experiments illustrate that the chargeability is strongly correlated with the sample resistivity, which depends on salinity, porosity, saturation, and clay content. The normalized chargeability is shown to be independent of the sample resistivity and it is proportional to the quadrature conductivity, which is directly related to the surface polarization processes. Laboratory‐derived relationships between conductivity and salinity, and normalized chargeability and clay content, are extended to the interpretation of 1‐D and 2‐D field‐IP surveys. In the 2‐D survey, the apparent conductivity and normalized chargeability data are used to segment the images into relatively clay‐free and clay‐rich zones. A similar approach can eventually be used to predict relative variations in the subsurface clay content, salinity and, perhaps, contaminant concentrations.
Arbitrary source and receiver positioning in finite‐difference schemes using Kaiser windowed sinc functions
Authors: Hicks Graham. J.
Published: October 2002

In finite‐difference methods a seismic source can be implemented using either initial wavefield values or body forces. However, body forces can only be specified at finite‐difference nodes, and, if using initial values, a source cannot be located close to a reflecting boundary or interface in the model. Hence, difficulties can exist with these schemes when the region surrounding a source is heterogeneous or when a source either is positioned between nodes or is arbitrarily close to a free surface.

A completely general solution to these problems can be obtained by using Kaiser windowed sinc functions to define a small region around the true source location that contains several nodal body forces. Both monopole and dipole point sources can be defined, enabling many source types to be implemented in either acoustic or elastic media. Such a function can also be used to arbitrarily locate receivers. If the number of finite‐difference nodes per wavelength is four or more (and with a source region half‐width of only four nodes) this scheme results in insignificant phase errors and in amplitude errors of no more than 0.1%. Numerical examples for sources located less than one node from either a free surface or an image source demonstrate that the scheme can be used successfully for any surface‐source or multisource configuration.
Local‐trace zeroing and spike zeroing: Two short automated noise‐rejection routines to remove noise and spikes on seismic traces
Authors: Stanghellini Giuseppe , Bonazzi Claudia
Published: October 2002

The acquisition of marine seismic data is often affected by noise that introduces spurious signals. Due to the length of the receiver streamer, bad weather conditions can produce low‐frequency, high‐intensity incoherent noise and/or spikes that can be difficult to remove by means of conventional mathematical filters.

In this paper we present two Fortran routines suitable to locate and remove the noise in the low and very low frequency ranges and to locate and suppress spikes. These two routines are designed to run without user interaction once the processing parameters are selected. Both routines are simple and compact, and can be included in any processing software.
P‐wave velocity and permeability distribution of sandstones from a fractured tight gas reservoir
Authors: Dürrast Helmut , Rasolofosaon P. N. J. , Siegesmund Siegfried
Published: October 2002

Fractures are an important fabric element in many tight gas reservoirs because they provide the necessary channels for fluid flow in rocks which usually have low matrix permeabilities. Several sandstone samples of such a reservoir type were chosen for a combined study of rock fabric elements and petrophysical properties. Geological investigations of the distribution and orientation of the fractures and sedimentary layering were performed. In addition, laboratory measurements were carried out to determine the directional dependence of the permeability and P‐wave velocities. Higher permeability values are generally in the plane of the nearly horizontal sedimentary layering with regard to the core axis. With the occurrence of subvertical fractures, however, the highest permeabilities were determined to be parallel to the core axis.

Compressional wave velocities were measured on spherical samples in more than 100 directions to get the VPsymmetry without prior assumptions. Below 50 MPa confining pressure, all samples show a monoclinic symmetry of the P wave velocity distribution, caused by sedimentary layering, fractures, and crossbedding. At higher confining pressure, sedimentary layering is approximately the only effective fabric element, resulting in a more transverse isotropic VPsymmetry. Using the geological‐petrophysical model introduced here, the complex symmetry of the VPdistributions can only be explained by the rock fabric elements. Furthermore, water saturation increases the velocities and decreases the anisotropy but does not change VPsymmetry. This indicates that at this state, all fabric elements, including the fractures, have an influence on P‐wave velocity distribution.
Effect of fluid viscosity on elastic wave attenuation in porous rocks
Authors: Gurevich Boris
Published: October 2002

Attenuation and dispersion of elastic waves in fluid‐saturated rocks due to pore fluid viscosity is investigated using an idealized exactly solvable example of a system of alternating solid and viscous fluid layers. Waves in periodic layered systems at low frequencies can be studied using an asymptotic analysis of Rytov's exact dispersion equations. Since the wavelength of the shear wave in the fluid (viscous skin depth) is much smaller than the wavelength of the shear or compressional waves in the solid, the presence of viscous fluid layers requires a consideration of higher‐order terms in the low‐frequency asymptotic expansions. This expansion leads to asymptotic low‐frequency dispersion equations. For a shear wave with the directions of propagation and of particle motion in the bedding plane, the dispersion equation yields the low‐frequency attenuation (inverse quality factor) as a sum of two terms which are both proportional to frequency ω but have different dependencies on viscosity η: one term is proportional to ω/η, the other to ωη. The low‐frequency dispersion equation for compressional waves allows for the propagation of two waves corresponding to Biot's fast and slow waves. Attenuation of the fast wave has the same two‐term structure as that of the shear wave. The slow wave is a rapidly attenuating diffusion‐type wave, whose squared complex velocity again consists of two terms which scale with iω/η and iωη.

For all three waves, the terms proportional to η are responsible for the viscoelastc phenomena (viscous shear relaxation), whereas the terms proportional to η−1 account for the visco‐inertial (poroelastic) mechanism of Biot's type. Furthermore, the characteristic frequencies of visco‐elastic ωV and poroelastic ωB attenuation mechanisms obey the relation ωVωB = AωR2, where ωR is the resonant frequency of the layered system, and A is a dimensionless constant of order 1. This result explains why the visco‐elastic and poroelastic mechanisms are usually treated separately in the context of macroscopic theories that imply ω << ωR. The poroelastic mechanism dominates over the visco‐elastic one when the frequency‐indepenent parameter B = ωB/ωV = 12η2/μsρfhf2 << 1, and vice versa, where hf is the fluid layer thickness, ρf the fluid density, and μs represents the shear modulus of the solid.
AVO and AVA inversion for fractured reservoir characterization
Authors: Beretta Matteo Mario , Bernasconi Giancarlo , Drufuca Giuseppe
Published: October 2002

Seismic wave reflection amplitudes are used to detect fluids and fracture properties in reservoirs. This paper studies the characterization of a vertically fractured fluid‐filled reservoir by analyzing the reflection amplitudes of P‐waves with varying incident and azimuthal angles. The reservoir is modeled as a horizontal transversely isotropic medium embedded in an isotropic background, and the linearized P‐waves reflection coefficient are considered. The conditioning of the inverse problem is analyzed, and fracture density is found to be the best conditioned parameter. Using diffraction tomography under the Born approximation, an inversion procedure is proposed in the transformed k–ω domain to detect fracture density variations within the reservoir. Seismic data are rearranged in pairs of incident and reflected plane waves, enlightening only one spectral component of the fracture density field at a time. Only the observable spectral components are inverted. Moreover, working in the transformed domain, picking reflection amplitudes is not required. An example of the inversion applied to a synthetic data set is presented. The limitation of source and receiver numbers and the finite bandwidth of the wavelet produce a loss of resolution, but the overall fracture density variations are recovered correctly.
A multioffset vertical seismic profiling experiment for anisotropy analysis and depth imaging
Authors: Grech M. Graziella Kirtland , Lawton Don C. , Gray Samuel H.
Published: October 2002

A multioffset vertical seismic profile (VSP) was carried out in the Rocky Mountain foothills of southern Alberta, Canada. The purpose of this experiment was to investigate whether the dipping shale strata exhibit P‐wave velocity anisotropy and, if so, to calculate the Thomsen anisotropy parameters for use in anisotropic depth migration. Traveltime inversion of first‐arrival data from the multioffset VSP revealed that the dipping Mesozoic clastics in the area exhibit seismic velocity anisotropy of about 10%. The anisotropy parameters derived from this experiment were then used in anisotropic prestack depth migration of data from a surface seismic line close to the VSP well. Comparison of the anisotropic migration with the corresponding isotropic prestack depth migration showed that the target was imaged incorrectly in the isotropic case; a lateral shift of 180 m in the updip direction of the overlying beds was observed. The image obtained with an anisotropic velocity model was also better focused than that obtained assuming isotropic velocities.
Azimuthal offset‐dependent attributes applied to fracture detection in a carbonate reservoir
Authors: Shen Feng , Sierra Jesus , Burns Daniel R. , Toksöz M. Nafi
Published: October 2002

Offset‐dependent attributes—amplitude versus offset (AVO) and frequency versus offset—are extracted from 2‐D P‐wave seismic data using the multiple signal classification technique. These attributes are used to detect fracture orientation in a carbonate reservoir located in the Maporal field in the Barinas basin of southwestern Venezuela. In the fracture normal direction, P‐wave reflectivity is characterized by a large increase of amplitude with offset (large positive AVO gradient) and a large decrease of frequency with offset (large negative frequency versus offset gradient). In the fracture strike direction, P‐wave reflectivity shows a scattered variation in AVO but a small variation in frequency with offset. Our results also show that the reservoir heterogeneity can lead to large variations of AVO signatures and that using azimuthal offset‐dependent frequency attributes can help lessen the ambiguity when detecting fracture orientation.
Prestack depth migration by symmetric nonstationary phase shift
Authors: Ferguson Robert J. , Margrave Gary F.
Published: October 2002

A new depth migration method suitable for heterogeneous media is presented. The well‐known phase shift plus interpolation (PSPI) method and the recently introduced nonstationary phase‐shift (NSPS) method are combined into a single symmetric operator with improved accuracy and stability and with similar computational effort. For prestack depth migration, the symmetric operator is used in a recursive wavefield extrapolation to compute incident and reflected wavefields at any desired depth, and the ratio of the incident and reflected wavefields at a particular depth is used to estimate seismic reflectivity. When the velocity model is made piecewise constant laterally, the symmetric extrapolation operator can be computed efficiently using ordinary phase‐shift extrapolation for a series of reference velocities and appropriate spatial windowing. Migration of the Marmousi synthetic data set by symmetric nonstationary phase shift (SNPS) provides an image that compares favorably with an image of the zero‐offset reflectivity derived from the Marmousi velocity model.
On: “Shallow Stratigraphic Variations over Gulf Coast Structures”
Authors: Gardescu I. I.
Published: October 2002

Lithological studies of the sediments deposited in the vicinity of some of the Gulf Coast Salt Dome structures reflect certain tendencies that would corroborate Dr. Rosaire’s observations of geophysical anomalies.
On: “Shallow Stratigraphic Variations over Gulf Coast Structures”
Authors: Judson S. A.
Published: October 2002

One of the important features of this paper is that it brings attention to the presence of areas of shallow mineralization lying above structures and oil fields of small amounts of uplift and outlines methods of examination to detect such areas.
Author’s Reply
Authors: Van Orstrand C. E.
Published: October 2002

Dr. Weaver’s discussion brings up the question of the causes of local variations of temperature. There are a number of angles to this problem, but for the purposes of this discussion it will perhaps suffice to consider only three of them, namely: (1) excess of radium in granite as compared with that in the over‐lying sediments; (2) differences in thermal conductivity along and perpendicular to the planes of stratification; and (3) recent differential uplift and rapid erosion.
On: Magnetic Interpretation by D. Wantland
Authors: Collingwood D. M.
Published: October 2002

In the field of petroleum exploration, progress in the technique of interpretation of magnetic surveys appears to be following to a great extent the recognition and application of the fact that variations in the magnetic susceptibility of sedimentary formations may cause mappable variations in the magnetic intensity of the earth’s field.
On: “Dimensions and Units of Electromagnetic Quantities”
Authors: Wenner Frank
Published: October 2002

Since the author refers to the actions of the International Electrotechnical Commission (I.E.C.), I would like to point out more specifically what was and was not done in 1930. The important conclusions reached at that time were the following.
International Scientific Co‐operation
Authors: Stratton F. J. M.
Published: October 2002

The gradual casing of restrictions on travel and on the general release of scientific discoveries has made it practicable once more to arrange international meetings of men of science. The general Assembly of the International Council of Scientific Unions will meet in London on July 22–24, 1946. Meanwhile its Executive Committee has held a meeting in London attended by representatives of most of the International Unions adhering to the Council.
On: “THE RELATIVE PLACE OF EMPIRICAL AND ANALYTICAL METHODS OF GEOPHYSICAL INTERPRETATION”
Authors: Born W. T.
Published: October 2002

Mr. Weaver has very ably presented the weakness of the empirical method of approach to geophysical prospecting problems. I am in hearty accord with his plea for the more intensive use of analysis in the consideration of both new and old prospecting methods.
Reply by author to preceding discussion
Authors: Hawkins L. V.
Published: October 2002

I thank Professor Hagedoorn for his comments to which I have the following response. The article, although appearing under “Review Articles” was not intended to be a comprehensive and objective review of all methods used in shallow refraction work. It describes the “Reciprocal Method” and comments on related methods of detailed shallow seismic investigation.
Reply to Dr. Kaarsberg
Authors: Hicks Warren G.
Published: October 2002

Paragraph II: Kaarsberg states that mechanical stress relief should not be ruled out as an explanation of low velocity adjacent the borehole. If the volume of the material stressed increases, the elastic moduli decrease. Therefore, velocity immediately adjacent the bore decreases. This is indeed a powerful point and properly should have been discussed more fully.
Reply by author to discussion by William C. Kellogg
Authors: Steenland Nelson C.
Published: October 2002

Those of us engaged in the pursuit of “high sensitivity” aeromagnetics are fully aware of, undoubtedly most aware of, the problems in its development. Some of these appear in Mr. Kellogg’s letter of 18 March 1966. It should be equally obvious that the advantages outweigh the disadvantages. Solutions to the problems Mr. Kellogg describes are being sought rather than simply accepting them as inalterable. “Oil Fields and Aeromagnetic Anomalies” made no attempt to describe these solutions, but set forth the problem as a new frontier in geophysical exploration.
Reply by author to discussion by Stephen Thyssen‐Bornemisza
Authors: Elkins Thomas A.
Published: October 2002

I have received from you the comments of Dr. Stephen Thyssen‐Bornemisza on my short note “Vertical Gradient of Gravity on Axis for Hollow and Solid Cylinders” (Geophysics, 31, 816–820, 1966).
Reply by authors to discussion by Norman S. Neidell
Authors: Smith Stanley T. , Dougherty Elmer L.
Published: October 2002

In regard to Dr. Neidell’s comments on our paper entitled, “The use of Linear Programming to Filter Digitized Map Data”, we wish to emphasize our stated purpose, namely, to detect “wild points” in the data. We selected the “[Formula: see text]” Norm for the following reasons.
Reply by authors to discussion by J. G. Negi
Authors: Lowrie W. , West G. F.
Published: October 2002

We should like to express our appreciation to Dr. Negi for his interest in our paper. Our comments on his points are the following.
Reply by author to discussion by H. A. Meinardus
Authors: Paul M. K.
Published: October 2002

My comments on Dr. Meinardus’s points are as below.
Reply by Author to Discussion by Kristjansson and Wright
Authors: Meidav Tsvi
Published: October 2002

Kristjansson and Wright are right in their comment that equations (16) or (17) are correct “only if either point A or B is held fixed and this fixed point is used to define the origin for the problem.” Hence, the suggested approach cannot be used as the general solution for the determination of the dip from seismic reflection data.
Reply by authors to discussion by Carl H. Savit
Authors: Dunkin J. W. , Levin F. K.
Published: October 2002

The authors thank Mr. Savit for calling the readers’ attention to the patents he mentions. We anticipated having Walton’s paper appear before or at the same time as our own. When publication of Walton’s paper was delayed, the Associate Editors and referees requested the short description of the method that appears as Appendix B of our work. We made no attempt to include a historical background, since that background logically accompanies Walton’s discussion.
Reply by Authors to Discussion by J. H. Moran
Authors: Roy A. , Dhar R. L.
Published: October 2002

Just the title of our paper, apart from its text, should make it clear that we are not concerned with “geometrical factors” for evaluation of total response in any inhomogeneous configuration. The paper gives a new definition for the radius or depth of investigation of logging sondes and computes the same for a homogeneous medium. The Schlumberger Well Surveying Corporation also considered only a homogeneous medium while evaluating radii of investigation for normal and lateral devices (not laterolog) in their Documents 4 and 8. We consider that our definition is physically clearer and less arbitrary. The most significant result of our analysis is that the radius of investigation of the focused seven‐electrode laterolog is substantially smaller than those of the nonfocused two‐ and three electrode normal and lateral devices.
Author’s Reply to discussion by Thyssen‐Bornemisza
Authors: Hammer Sigmund
Published: October 2002

The foregoing discussion by Dr. Thyssen‐Bornemisza calls attention to interesting extensions of the rather simplified theoretical analysis in my paper. Dr. Thyssen‐Bornemisza has published extensively on the subject of the vertical gradient of gravity and its applications in exploration. My paper was limited to a very specific topic, namely the variability of the vertical gradient along a vertical profile above an anomalous mass.
Reply by author to discussion by Abner Blackman
Authors: Faas R. W.
Published: October 2002

Mr. Blackman has raised an objection to my use of the term “empirical” in describing the relationship between acoustic reflectivity R and sediment porosity η. There may be a point in his argument inasmuch as the parameter R is not measured directly, but is calculated according to my equation (9) [his equation (1)]. However, whether the regression equation [his equation (2)] should be considered theoretical as he states seems also questionable inasmuch as measured values of sound velocity and sediment density were used to construct the reflection coefficient, which was then correlated with measured values of sediment porosity. It seems we may be involved in a problem with semantics.
Reply by author to discussion by William C. Kellogg
Authors: Jensen Homer
Published: October 2002

It is with some hesitation that I take issue with the statements in Mr. Kellogg’s letter regarding my paper in the October 1965 edition of Geophysics, for if other companies in the earth survey profession choose not to accept the advantages of doppler navigation guidance, constant scale tape, and digital presentation, we will be benefitted. However, some comments are required.
Reply by authors to discussion by Sven‐Erick Hjelt
Authors: Wait James R. , Spies Kenneth P.
Published: October 2002

Dr. Hjelt’s discussion on our note has been read with interest. We agree that a technique involving separation of time and space variables is more direct for the specific problem being treated. This is particularly the case when we know the expected form of the final solution at the outset [i.e., his equation (1)]. Our analysis showed that the integral representation is fully equivalent to his exponential series solution. Therefore, there is no area of disagreement.
Reply by author to discussion by A. Roy
Authors: Dieter K. D. , Paterson N. R. , Grant F. S.
Published: October 2002

I will reply to Dr. Roy’s points in the order in which they are raised.
Reply to W. T. Valenta by Ben B. Thigpen.
Authors: Thigpen Ben B.
Published: October 2002

The recommended convention conforms with roughly 85 percent of the users. The field operators are accustomed to this convention. The major geophone manufacturers have marked their geophones in such a way that an upward movement of the geophone case will produce a downward deflection on a microammeter connected to the geophone terminals when the microammeter is connected to conform to the marked polarity.
Reply by Author to Discussion by M. S. Reford
Authors: Bhattacharyya B. K.
Published: October 2002

I appreciate very much the interest shown by Mr. M. S. Reford in my paper. This note is written to clarify some of the points raised by Mr. Reford.
Reply by Roland G. Henderson to discussion by K. N. Khattri and A. N. Datta.
Authors: Henderson Ronald G.
Published: October 2002

It is gratifying to know that our paper has stimulated interest in the reduction problem. The discussion by K. N. Khattri and A. N. Datta, however, treats the obvious and hardly requires a reply. It is well known that any energy in frequencies higher than the Nyquist frequency will be aliased back into the lower frequencies. I would be more interested in any thoughts they might have on how to estimate the error due to aliasing when the space‐domain data are unevenly spaced on an irregular surface.
Reply by the author to Discussion by Tsvi Meidav
Authors: Harthill Norman
Published: October 2002

In his discussion of the subject paper, Meidav expresses a number of opinions about the use of bipole‐dipole surveys which differ from mine. I should like to expand upon the technical merits of some of the comments that he makes.
Reply by K. L. Cook to Discussion by W. C. Kellogg
Authors: Cook Kenneth L.
Published: October 2002

We appreciate the interest shown by William C. Kellogg in our paper and welcome the opportunity to respond to his comments.
Reply by authors to S. Parker Gay, Jr.
Authors: Pitkin James A. , Duval Joseph S.
Published: October 2002

Mr. Gay’s criticism of our estimate of the cost difference between total‐count and spectrometric surveys appears to refer to the cost of flying the survey only, i.e., acquiring the data. Our estimate was intended in a broader sense and includes not only flying the survey, but also reducing the data for interpretation.