bluuebunny/crossref_metadata_2025 huggingface dataset:
Interactive interpretation of seismic data
Authors: Gerhardstein Anthony C. , Brown Alistair R.
Published: October 2002

Interactive interpretation is urgently needed to increase the productivity of the world’s hard‐pressed seismic interpreters. This paper describes the use of an interactive system that displays seismic data in color on a television screen. The system is easy to use and, by automatically managing the data base for the interpreter, permits him to spend a larger portion of his time on the thoughtful process of interpretation itself. A 3-D data volume can be studied in both vertical or horizontal section form. The system can equally well handle the irregular grid from a 2-D survey. In the course of interpreting a section on the screen, the interpreter may manipulate portions of that section in a variety of ways. He may make composite displays of multiple pieces of data, track horizons in automatic or manual modes, and zoom portions of the data to any desired extent. An interpretation made on one section can later be viewed on other sections marked at the points of intersection. Data can also be flattened to aid in structural and stratigraphic interpretations. Working maps can be produced at any time to check the progress of the interpretation. Final maps can be smoothed and manipulated to yield isochron, isopach, and other map products.
Predictive deconvolution and the zero‐phase source
Authors: Gibson Bruce , Larner Ken
Published: October 2002

Predictive deconvolution is commonly applied to seismic data generated with a Vibroseisr® source. Unfortunately, when this process invokes a minimum‐phase assumption, the phase of the resulting trace will not be correct. Nonetheless, spiking deconvolution is an attractive process because it restores attenuated higher frequencies, thus increasing resolution. For detailed stratigraphic analyses, however, it is desirable that the phase of the data be treated properly as well. The most common solution is to apply a phase‐shifting filter that corrects for errors attributable to a zero‐phase source. The phase correction is given by the minimum‐phase spectrum of the correlated Vibroseis wavelet. Because no minimum‐phase spectrum truly exists for this bandlimited wavelet, white noise is added to its amplitude spectrum in order to design the phase‐correction filter. Different levels of white noise, however, produce markedly different results when field data sections are filtered. A simple argument suggests that the amount of white noise used should match that added in designing the (minimum‐phase) spiking deconvolution operator. This choice, however, also produces inconsistent results; field data again show that the phase treatment is sensitive to the amount of added white noise. Synthetic data tests show that the standard phase‐correction procedure breaks down when earth attenuation is severe. Deterministically reducing the earth‐filter effects before deconvolution improved the resulting phase treatment for the synthetic data. After application of the inverse attenuation filter to the field data, however, phase differences again remain for different levels of added white noise. These inconsistencies are attributable to the phase action of spiking deconvolution. This action is dependent upon the shape of the signal spectrum as well as the spectral shape and level of contaminating noise. Thus, in practice the proper treatment of phase in data-dependent processing requires extensive knowledge of the spectral characteristics of both signal and noise. With such knowledge, one could apply deterministic techniques that either eliminate the need for statistical deconvolution or condition the data so as to satisfy better the statistical model assumed in data‐dependent processing.
Rise times of attenuated seismic pulses detected in both empty and fluid‐filled cylindrical boreholes
Authors: Blair D. P.
Published: October 2002

Fourier‐Bessel theory is used to derive filters representing the influence of both empty and fluid‐filled cylindrical boreholes on particle motion induced in rock by a plane P-wave incident perpendicular to the borehole axis. For wavelengths greater than 10 times the borehole circumference, the effect of the borehole on particle motions is shown to be negligible; thus the results have little relevance for the long wavelengths commonly encountered in earthquake seismology. The results are, however, relevant to the study of stress wave propagation at ultrasonic frequencies in rock masses. For small wavelengths (αa > 3.0) the filter representing particle motion on the wave incident site of an empty borehole reduces to a linear phase filter which increases all amplitudes by a factor of 2 while the filter representing fluid stress at the center of a fluid‐filled borehole may be reduced to simple mathematical expressions. Experimental results were obtained for the interaction of a stress wave with either accelerometers mounted in an empty borehole or a hydrophone located centrally in a fluid‐filled borehole. Both theory and experiment show a similar distortion in the rise time of the pulse traveling past the borehole.
On: “Airborne gravity is here!” by S. Hammer (GEOPHYSICS, v. 48, p. 213–223, February, 1983)
Authors: Herring A. T.
Published: October 2002

There is a serious mistake in Dr. Hammer’s paper. Figure 8 shows upward continued land gravity compared to land‐observed gravity and a helicopter gravity profile over the Big Hill and High Island Salt Domes. The upward continued curve is wrong and seriously misleading. Two years ago I pointed this problem out to Dr. Hammer after his presentation at the Annual Meeting in Los Angeles, and I was not alone in recognizing this error. Thus I am very disappointed to see the same erroneous curve published in the Journal.
A method of editing time series observations
Authors: Halpenny John
Published: October 2002

Data from automatic recording systems often require editing and filtering before they are suitable for computer analysis. The procedure described in this paper produces edited values at regular intervals from input data containing random noise, data gaps, and sudden steps or resets. It uses a Kalman filter with a fixed delay time to estimate the most probable data value at any time, based on information both before and after the time point. Isolated portions of a bad record can be recognized and removed, and steps or offsets are identified and measured. An example is shown of clean output produced from input which suffers from a variety of instrumental problems.
Travel time(like) variables and the solution of velocity inverse problems
Authors: Hagin Frank G. , Gray Samuel H.
Published: October 2002

It was demonstrated earlier that, in the solution of 1-D inverse problems, there is great advantage to changing the independent variable from, say, z (depth) to travel time [Formula: see text]. Essentially the advantage comes from the fact that the acoustic wave equation in travel time has the unknown c(z) appearing in a less critical position. The current paper takes a step toward applying these ideas to the much harder, but more interesting, inverse problem in three dimensions. There is no simple 3-D analog of the above definition of τ. However, it is shown that a surprisingly effective way of decomposing travel time into x, y, z components is straightforward. These are defined via line integrals from, say, (0, 0, 0) to an arbitrary point (x, y, z) along the straight line connecting the points, thus approximating the more natural integrals along the unknown raypath. These line integrals define the new coordinates, and the associated wave equation is derived and then simplified by dropping less important terms. The inverse problem is then attacked in this setting using the 3-D inversion techniques of Cohen and Bleistein (1979). The resulting algorithm is demonstrated to be very similar to those earlier results; however, it is shown that for a single reflecting plane the new results are of “second‐order” accuracy as oppose to first order (when the change in c is small relative to c itself). The algorithm also has some similarity to that of Raz (1982), and some comparisons between these two results are made.
An evaluation of the marine magnetic gradiometer
Authors: Eggers D. E. , Thompson D. T.
Published: October 2002

This study examines the ability of a marine gradiometer to produce diurnal‐free total magnetic intensity data. Using Gulf’s research vessel HOLLIS HEDBERG, an experimental program was carried out in the Santa Barbara channel offshore California. A shore‐based station was operated continuously during the survey period to monitor the temporal magnetic activity. The gradiometer data are adversely affected by the magnetic field of the towing vessel. An analysis of gradiometer bias as a function of ship’s heading and deployment distance shows remarkable agreement with a model of the ship’s magnetic field proposed by Bullard and Mason in 1961. Based upon this model, analytic azimuth correction factors are developed and applied to the gradiometer data. Comparisons are made among single‐sensor, ship‐minus‐shore, and integrated total magnetic intensities. Residual mistie statistics, contour maps, and profile plots form the basis of the comparison. At this location, the ship‐minus‐shore and integrated total intensities produce maps with standard residual misties of 0.55 and 0.62 gammas, respectively. Because of the gradiometer’s operational simplicity and the uncertainty of spatial coherency of diurnal activity, the gradiometer is recommended for routine use in the marine environment. The gradiometer is very effective in eliminating diurnal activity, but it requires moderately more sophisticated data analysis procedures.
Continuation of potential fields between arbitrary surfaces
Authors: Hansen R. O. , Miyazaki Y.
Published: October 2002

An equivalent source algorithm is described for continuing either one‐ or two‐dimensional potential fields between arbitrary surfaces. In the two‐dimensional case, the dipole surface is approximated as a set of plane faces with constant moments over each face. In the one‐dimensional case, the plane faces of the dipole surface reduce to straight line segments. Application of the algorithm to model and field examples of aeromagnetic data shows the method to be effective and accurate even when the terrain has strong topographic relief and is composed of highly magnetic volcanic rocks.
Correlations of radon and carbon isotopic measurements with petroleum and natural gas at Cement, Oklahoma
Authors: Fleischer R. L. , Turner L. G.
Published: October 2002

Radon in soil gas and the carbon isotopic composition of carbon compounds in soil have been measured at 98 sites over the Cement, Oklahoma oil and gas field. Strong correlations were observed between the density of oil and gas wells and storage tanks and soil [Formula: see text], field [Formula: see text], vertical gas flow, and [Formula: see text] gradients. No meaningful correlation is seen with [Formula: see text]. If further study shows that these results are general, the types of measurement done here could facilitate exploratory drilling.
1983 SEG Research Workshops
Authors: Schoenberger M.
Published: October 2002

The SEG Research Workshops have been a regular part of the Annual SEG Meetings for the past 13 years. From 1971 to 1978, during each convention a workshop was held with invited speakers. Starting in 1979, multiple workshops were conducted on the Thursday afternoon following the convention. The workshops were again expanded in 1981 to include not only Thursday afternoon but all day Friday as well. A list of the Research Workshops conducted during the past 13 years follows this introduction.
On the approximation of finite loop sources by two‐dimensional line sources
Authors: Nabighian M. N. , Oristaglio M. L.
Published: October 2002

An appealing feature of time‐domain electromagnetics is that the transient response simplifies considerably at late time, usually tending to a power‐law or exponential decay. In this note, we point out an interesting discrepancy between the late‐time asymptotics of a finite loop source over a half‐space and its natural two‐dimensional (2-D) approximation, which is two line sources of opposite polarity lying on a half‐space. Expressions for the transient responses of both loop (Wait and Ott, 1972) and line sources (Oristaglio, 1982) have been derived before; they show that at late times the voltage induced in a horizontal receiving coil decays as [Formula: see text] for a loop source and [Formula: see text] for a line source. Here we show that the slower decay for the line source is inherently a 2-D effect. To do this, we derive a closed‐form expression for the transient voltage induced by a finite wire of length 2L on a half‐space—a new result, for which we can separately examine the limits [Formula: see text] and [Formula: see text] Surprisingly, these limits are not interchangeable. First taking L to be infinite and then doing the late‐time asymptotic expansion yields the [Formula: see text] decay of a line source; in contrast, first doing the late‐time expansion gives a decay of [Formula: see text] for the finite wire, which is formally unchanged as the length goes to infinity.
COCORP: Utah Area, Part I; and Mojave, California Area
Authors: Kaufman S.
Published: October 2002

The Consortium for Continental Reflection Profiling (COCORP) announces the availability of the data package and tapes for the Utah area, Part I and for the Mojave, California area for the cost of reproduction and shipping. The COCORP operation is part of the U.S. Geodynamics Project sponsored by the National Academy of Sciences and funded by the National Science Foundation. The executive group of the consortium consists of members from Cornell University, Dartmouth College, Princeton University, Rice University, and the University of Wisconsin. Cornell University is the operating institution.
Foreword and Introduction
Authors: Nabighian Misac N.
Published: October 2002

Over the last few years there has been a dramatic surge of interest in time‐domain electromagnetic (TDEM) methods of exploration as evidenced by the proliferation of commercially available time‐domain equipment and contractors, by the extensive research carried out at various companies and universities, and by the large number of papers published on the subject. At the last few SEG Annual Meetings there were special sessions devoted solely to TDEM and there was even a time‐domain electromagnetic workshop at the SEG Annual Meeting in Las Vegas. As such, it is not difficult to understand the motivation for preparing this Special Issue of Geophysics devoted to TDEM techniques: to present recent advances and to give us a glimpse at the future. At this moment it seems appropriate to trace the evolution of transient techniques from their early beginnings to the present. Historically, TD techniques have evolved along two parallel paths, transient EM soundings and transient EM prospecting (or inductive TDEM). The first technique is used primarily to delineate layered structures of interest in oil and groundwater exploration. It is also used in delineating zones of high conductivity within the ground, with the expectation that they might be associated with either geothermal or massive sulfide targets. By contrast, transient EM prospecting is primarily used for locating massive sulfide ore deposits at great depths, and its use is associated almost exclusively with the mining industry.
Approximate calculations of the transient electromagnetic response from buried conductors in a conductive half‐space
Authors: McNeill J. D. , Edwards R. N. , Levy G. M.
Published: October 2002

The transient electromagnetic (TEM) response from a conductive plate buried in a conductive half‐space and energized by a large‐loop transmitter is investigated in a heuristic manner. The vortex and galvanic components are each calculated directly in the time domain using an approximate procedure which ignores the electromagnetic coupling present in the complete solution. In modeling the vortex and galvanic current flows, the plate is replaced with a single‐turn wire loop of appropriate parameters and a distribution of current dipoles, respectively. The results of calculations of the transient magnetic field at the surface of the earth are presented for a few selected cases of practical interest. The relative importance of the vortex and galvanic components varies with the half‐space resistivity. The vortex component dominates if the half‐space is resistive, in which case free‐space algorithms suffice for numerical modeling. Furthermore the measured responses give much useful information about the target, and large depths of exploration should be achieved. As the half‐space resistivity decreases, a significant half‐space response is observed, caused by currents induced in the half‐space itself. This response can be very large. Spatial variations in it caused by relatively small changes in resistivity, i.e., geologic noise, obscure the response from deep targets making them difficult to detect. The effect of the half‐space is also to delay, distort, and reduce the vortex component in comparison with the free‐space response. The behavior of the galvanic component is determined by the haft‐space current flow. The presence of this component explains the large enhancement of overall target response seen at early times over relatively resistive ground and the departure from an exponential decay seen over more conductive ground, again with respect to responses predicted by free‐space modeling. In more conductive ground the galvanic component completely dominates the vortex component, resulting in the loss of useful diagnostic information. Although target location and depth can still be determined, target shape and orientation are poorly defined. Because of galvanic current saturation good conductors are difficult to distinguish from poor ones.
Transient representation of the Sommerfeld‐Weyl integral with application to the point source response from a planar acoustic interface
Authors: Tygel Martin , Hubral Peter
Published: October 2002

Point source responses from a planar acoustic and/or elastic layer boundary (as well as from a stack of planar parallel layers) are generally obtained by using as a starting point the Sommerfeld‐Weyl integral, which can be viewed as decomposing a time‐harmonic spherical source into time‐harmonic homogeneous and inhomogeneous plane waves. This paper gives a powerful extension of this integral by providing a direct decomposition of an arbitrary transient spherical source into homogeneous and inhomogeneous transient plane waves. To demonstrate with an example the usefulness of this new point source integral representation, a transient solution is formulated for the reflected/transmitted response from a planar acoustic reflector. The result is obtained in the form of a relatively simple integral and essentially corresponds to the solution obtained by Bortfeld (1962). It, however, is arrived at in a physically more transparent way by strictly superimposing the reflected/transmitted transient waves leaving the interface in response to the incident transient homogeneous and inhomogeneous plane waves coming from the center of the point source.
The use of summary representation for electromagnetic modeling
Authors: Tarlowski C. Z. , Raiche A. P. , Nabighian M.
Published: October 2002

The method of summary representation developed by G. N. Polozhii is a quasi‐analytical method for solving self‐adjoint, finite‐difference boundary value problems expressed on regular meshes. In principle, the method should allow considerable savings in computing time as well as improved accuracy when compared to commonly used finite‐difference schemes. We have used summary representation as the basis for a new hybrid scheme to solve the two‐dimensional Helmholtz equation for electromagnetic modeling. The theory behind this hybrid scheme is presented. Preliminary results for the two‐dimensional problem show that substantial computing time and storage savings can be made.
GEOPHYSICAL EDUCATION IN A DEPARTMENT OF GEOPHYSICS
Authors: Macelwane, James B.
Published: October 2002

There have been a number of valuable discussions on the subject of geophysical education. Among these is the recent symposium on geophysical education which was sponsored by the A.I.M.M.E. However, the impression made upon one by the study of the various papers published on the subject of geophysical education is a sense of incompleteness, a feeling that the sight of the forest is lost because of preoccupation with the trees.
SOME NEGLECTED ASPECTS OF CHEMICAL EXPLORATION
Authors: Sanderson R. Thomas
Published: October 2002

In a general discussion of geochemical exploration for petroleum, some of the problems particularly related to the soil‐analysis method are considered in detail. It is suggested that with technique now available for gas‐analysis, the older method of soil‐gas sampling may avoid some of the difficulties inherent in the soil‐sampling method.
SHOT HOLE CHARACTERISTICS IN REFLECTION SEISMOLOGY
Authors: McCready H. J.
Published: October 2002

The influence of shot‐hole characteristics on seismograms is discussed in some detail. Modifications in the formations surrounding the shot‐hole caused by successive shots result in compaction or aeration affecting changes in delay times as well as changes in character of the recorded events. The importance of considering these effects in the interpretation of seismograph records and some methods of alleviating these problems by suitable field technique are discussed.
Analysis of a gas hydrate off southwest Mexico using seismic processing techniques and Deep Sea Drilling Project Leg 66 results
Authors: Collins Barclay P. , Watkins Joel S.
Published: October 2002

Analysis of a reprocessed seismic reflection line and results from Deep Sea Drilling Project Leg 66 off the southwest coast of Mexico reveal a bottom simulating reflector (BSR) associated with the equilibrium phase boundary of methane hydrate. Several seismic processing techniques were used to accentuate the lateral continuity of the BSR and to delineate the top and base of a 200-700 m thick concentrated hydrate layer. These results suggest the concentrated hydrate layer extends about 20 km parallel to the slope of the inner trench wall in water depths ranging from 2 250-4 500 m. Direct seismic indicators below the BSR and geochemical evidence imply small amounts of free gas may be trapped beneath the base of the hydrate layer and suggest that low‐permeability hydrated sediments can act as a seal for reservoirs.
Geophone frequency calibration and laser verification
Authors: MacArthur A.
Published: October 2002

A geophone frequency calibration procedure has been developed that provides both amplitude and phase information over a selectable frequency range with a single measurement. It is easy to perform in either a field or laboratory environment using a two‐channel analog input spectrum analyzer, and an inexpensive, easily constructed, shaker table. A second calibration procedure that uses laser interferometry was also developed and implemented to verify the first, more practical method. Agreement between the two procedures is within the uncertainty limits of the test equipment at all frequencies including those near and at physical resonance. Some concepts of digital signal processing were reviewed that affect the interpretation of both calibration and seismic test data.
CPS/SEG 1985 Joint Meeting Report of the CPS/SEG Meeting, April 12–16, 1985 in Beijing
Authors: Kuo John Tsung Fen
Published: October 2002

The Society of Exploration Geophysicists (SEG), the China Geophysics Society (CGS), the China Society of Petroleum Geophysicists, now Committee of Geophysical Exploration of CPS, and the China Petroleum Society (CPS) deserve every credit for bringing exploration geophysicists ever closer together in the last seven years. The Joint Meeting of CPS/SEG in Beijing, The People’s Republic of China, April 12–16, 1985, was the culmination of past efforts put forth by all these societies.
Generalized pulse‐spectrum technique
Authors: Chen Y. M.
Published: October 2002

The generalized pulse‐spectrum technique (GPST) is a versatile and efficient iterative numerical algorithm for solving multiparameter inverse problems of a system of nonlinear partial differential equations. Here it is used to determine bulk modulus, shear modulus, and density variation from seismic data. Numerical simulations of simple but nontrivial examples are carried out on coarse uniform grids and adaptive grids to test and to study the general characteristics of GPST without the real measurements (seismograms). GPST does give reasonably good results. It can be made more efficient by implementing parallelism, automatic adaptive‐grids, and methods of domain decomposition into the numerical algorithm.
Acoustic pulse transmission in half‐spaces and finite‐length cylindrical rods
Authors: Blair D. P.
Published: October 2002

A combination of both dynamic finite‐element modeling (DFEM) and analytical techniques is used to evaluate the geometric attenuation of acoustic pulses propagated in elastic half‐spaces and finite‐length elastic cylindrical rods. Solutions for the half‐space are presented in a scale‐independent form and are relevant to the study of pulse propagation in large rock masses. For example, it is shown that if a surface‐mounted source has most of its spectral output below approximately 20 kHz, then the transmitted acoustic pulse within 1 m of the source exhibits a pulse‐shape distortion due to geometric attenuation that may dominate the distortion due to material attenuation. The results for the cylindrical rod are relevant to the study of pulse propagation in rock cores, and for this case the geometric effect yields a large increase in pulse‐shape distortion as a function of the distance from the source. For an aluminum cylindrical rod 1.0 m long, 0.05 m in diameter, and having a P‐wave velocity of 6 175 m/s, the geometric attenuation of acoustic pulses having rise times of approximately five microseconds is over 20 times larger than the material attenuation obtained for silica dolomite. In all studies, good agreement was found between the DFEM solution and the appropriate analytical solution. Furthermore, good agreement was also found between a DFEM solution and experimental results for acoustic pulse propagation along the cylindrical aluminum bar.
Seismic model studies of the overburden‐bedrock reflection
Authors: Pullan S. E. , Hunter J. A.
Published: October 2002

One aspect of the modification of seismic waves on passage through the Earth is the partitioning of energy at subsurface interfaces as described by the Zoeppritz equations. These equations have been applied to simple two‐ and three‐layer models to determine the variations in the amplitude and phase of a reflection signal at nonnormal angles of incidence. Synthetic seismograms have been produced to illustrate the effect of these variations on a seismic wavelet. It is found that the phase variations can lead to substantial changes in the character of the reflected wavelet as the source‐geophone distance increases. These changes are dependent on the angle of incidence and on the elastic properties of the subsurface layers. In particular, for models approximating an overburden over bedrock situation, the reflected pulse is predicted to “change phase” when the velocity contrast between the two layers is relatively small. This effect has been observed on field records. Geophysicists should be aware of this phenomenon, both in terms of the problems it may cause in observing a bedrock reflection and in terms of the potential it has for indicating subsurface elastic properties.
Seismic data processing: Current industry practice and new directions
Authors: Schultz Philip S.
Published: October 2002

The last ten years have seen an evolution in the state of the art for seismic data processing on a number of fronts. Data transformations investigated have made some types of analyses much more straightforward. Deconvolution has become a sophisticated process which includes statistical, model‐based, and deterministic methods. Vibroseis® processing has led to a greater understanding of the statistical limitations in recovery of the wave‐field amplitude from sign‐bit recording, and the deconvolution of Vibroseis data has improved. Multichannel filtering and analysis in transform domains have resulted in increasingly effective tools for noise reduction and signal enhancement. In statics analysis, surface consistency as a constraint remains a standard, and refraction analysis has become popular as a means of preconditioning data for residual statics estimation. Advances in stacking methodology have come mainly from addressing three effects: ray bending through lateral velocity variations, complex structure, and source‐receiver azimuth variations. Recently many techniques of interpretation processing have been introduced with the goal of improving the estimate of earth properties, rather than generating a stacked or imaged section of higher fidelity. The interaction among related disciplines has increased and this serves to amplify the effectiveness of each discipline because they are now developing in less isolation.
The use of the conjugate‐gradient algorithm in the computation of predictive deconvolution operators
Authors: Koehler Fulton , Taner M. Turhan
Published: October 2002

A number of excellent papers have been published since the introduction of deconvolution by Robinson in the middle 1950s. The application of the Wiener‐Levinson algorithm makes deconvolution a practical and vital part of today’s digital seismic data processing. We review the original formulation of deconvolution, develop the solution from another perspective, and demonstrate a general and rigorous solution that could be implemented. By “general” we mean a deterministic time‐varying and multichannel operator design, and by “rigorous” we mean the straightforward least‐squares error solution without simplifying to a Toeplitz matrix. Also we show that the conjugate‐gradient algorithm used in conjunction with the least‐squares problem leads to a satisfactory simplification; that in the computation of the operators, the square matrix involved in the normal equations need not be computed. Furthermore, the product of this matrix with a column matrix can be obtained directly from the data as a result of two cascaded simple convolutions. The time‐varying deconvolution problem is shown to be equivalent to the multichannel deconvolution problem. Hence, with one simple formulation and associated programming, the procedure can be utilized for time‐constant single‐channel and multichannel deconvolution and time‐varying single‐channel and multichannel deconvolution.
Polarization method for the determination of Poisson’s ratio in boreholes
Authors: Spencer T. W. , Wu Ru Chuan
Published: October 2002

Conventional methods for determining Poisson’s ratio from full‐waveform sonic logs rely on measurement of the shear velocity. This measurement is subject to significant error from contamination by events which do not travel at the shear velocity; it fails in soft formations where the shear head wave and pseudo‐Rayleigh waves are not excited. A new method is presented for determining Poisson’s ratio and shear velocity. This method can be implemented by recording the components of particle displacement on the borehole wall. The particle trajectory in the compressional head wave is rectilinear and is deflected with respect to the direction of propagation. The deflection is a sensitive function of Poisson’s ratio, varying by more than 30 degrees over the range of Poisson’s ratios encountered in rocks. In the polarization method all the measurements are made on the compressional head wave, the spatial resolution is greater than for velocity measurements, and the deflection is greatest in soft formations.
Transient plane‐wave transforms for the point‐source seismogram
Authors: Tygel M. , Hubral P.
Published: October 2002

The point‐source seismogram for a horizontally stratified medium needs—contrary to a generally accepted belief—only to be decomposed into a finite continuous spectrum of plane wave seismograms in order to guarantee its exact recovery from the spectrum of plane wave seismograms. In this short note, we present some new and updated forward and inverse transformation formulas with which exact decomposition and composition of a point‐source seismogram is achieved in the time domain. The new transformation formulas result from combining certain recently observed fundamental properties of point‐source seismograms with well‐known formulas of the existing theory of plane wave decomposition (Müller, 1971; Chapman, 1980; Phinney et al., 1981. The theory of plane‐wave decomposition of point‐source seismograms is now well established in seismic exploration. It is associated with such concepts as the Fourier‐Bessel transform, the Radon transform, backprojection, and slant‐stacking.
Direct estimation of shear‐wave interval velocities from seismic data
Authors: Carrion P. M. , Hassanzadeh S.
Published: October 2002

Conventional velocity analysis of seismic data is based on normal moveout of common‐depth‐point (CDP) traveltime curves. Analysis is done in a hyperbolic framework and, therefore, is limited to using the small‐angle reflections only (muted data). Hence, it can estimate the interval velocities of compressional waves only, since mode conversion is negligible when small‐angle arrivals are concerned. We propose a new method which can estimate the interval velocities of compressional and mode‐converted waves separately. The method is based on slant stacking or plane‐wave decomposition (PWD) of the observed data (seismogram), which transforms the data from the conventional T-X domain into the intercept time‐ray parameter domain. Since PWD places most of the compressional energy into the precritical region of the slant‐stacked seismogram, the compressional‐wave interval velocities can be estimated using the “best ellipse” approximation on the assumption that the elliptic array velocity (stacking velocity) is approximately equal to the root‐mean‐square (rms) velocity. Similarly, shear‐wave interval velocities can be estimated by inverting the traveltime curves in the region of the PWD seismogram, where compressional waves decay exponentially (postcritical region). The method is illustrated by examples using synthetic and real data.
Elastic‐wave velocities and attenuation in an underground granitic repository for nuclear waste
Authors: Paulsson B. N. P. , Cook N. G. W. , McEvilly T. V.
Published: October 2002

The behavior of a quartz monzonite rock mass subjected to a thermal load from emplaced canisters with electric heaters simulating high‐level nuclear waste was monitored by a cross‐hole seismic technique in a drift 340 m below the surface in the Stripa mine facility in Sweden. Traveltimes and amplitudes of 20 to 60 kHz ultrasonic compressional (P) and shear (S) waves were measured over the experiment duration of 750 days, on 2 to 4 m paths between four diamond‐drilled boreholes around a heater. The signals were transmitted between the boreholes in six different directions and at different depths. Path‐averaged P- and S-wave velocities were obtained from the times of flight of pulses of acoustic waves between separate P- and S-wave piezoelectric transmitter and receiver crystals. The attenuation [Formula: see text] was obtained by a spectral ratio technique. When the heater was turned on, the P- and S-wave velocities increased to 4 and 10 percent, respectively, and stabilized at the elevated values. The P-wave velocities along a particular profile were found to increase linearly with the mean temperatures in the profiles sampled. These mean temperatures increased 25° to 55 °C during the course of the experiment. When the heater was turned off after 398 days of heating, the velocities decreased with temperature and finally reached levels in most cases below those observed prior to heating the rock. The highest thermal stress close to the heater was estimated at 55 MPa. [Formula: see text] values increased throughout the heating interval, reaching changes of up to 60 percent shortly after turning off the heater. The Q values reveal no direct correlation with temperature or the closely associated thermal stress, although these phenomena clearly are driving the variation in Q. There is strong evidence relating attenuation properties to fracture closure and pore pressure changes associated with draining of the rock mass.
Simulations of noise rejection and mantissa‐only recording: An experiment in high‐amplitude noise reduction with COCORP data
Authors: Klemperer Simon L. , Brown Larry
Published: October 2002

Vertical stacking of Vibroseis® sweeps for the purposes of reflection data enhancement and data compression often smears sporadic noise bursts into summed traces with degrading effects. One noisy record can easily nullify the signal enhancement benefits of stacking the remaining good sweeps (e.g., Gimlin and Smith, 1980). This paper compares different methods of noise editing before vertical stacking, as applied to deep seismic data collected by COCORP (Consortium for Continental Reflection Profiling) in conditions of heavy traffic noise. Noise editing is of particular concern in crustal seismic work where weak reflections from great depths are often critical to interpretations. In our test, individual sweeps rather than the field sum of these sweeps were recorded separately on tape. This procedure, single‐sweep recording, allowed four computer simulations: (A) recording with no noise editing; (B) recording with a field noise‐rejection (NRJ) system; (C) using a more optimal NRJ system which was computed using the characteristics of the recorded sweeps and applied after recording; and (D) recording using mantissa‐only (MO) format. In this initial, limited experiment, we found significantly greater noise reduction with both tailored, postrecording NRJ systems and MO format than with the simple field NRJ currently employed by COCORP. Normally, these alternative recording methods are not used, either because they entail excessive tape handling and preprocessing, as in the case of single‐sweep recording and application of postrecording NRJ, or because they entail loss of dynamic range, as with MO format recording. However, this study suggests that in the presence of severe traffic noise the benefits of these alternative techniques may outweigh their drawbacks.
Laboratory‐determined transport properties of Berea sandstone
Authors: Daily William D. , Lin Wunan
Published: October 2002

We report laboratory measurements of electrical resistivity ρ, water permeability k, and compressional wave velocity [Formula: see text] for both intact and fractured Berea sandstone samples as functions of temperature from 20°C to 200°C and effective pressure [Formula: see text] from 2.5 MPa to 50 MPa. For the intact sample, [Formula: see text] increases from 3.52 km/s to 4.16 km/s as [Formula: see text] goes from 3 to 50 MPa. With increasing temperature, [Formula: see text] decreases at rates of about 3 percent per 100°C at [Formula: see text] of 5 MPa and about 1.5 percent per 100°C at [Formula: see text] of 38 MPa. Data from the fractured sample are qualitatively similar, but velocities are about 10 percent lower. For both intact and fractured samples, ρ increases less than 15 percent as [Formula: see text] increases from 2.5 MPa to 50 MPa. Although both samples show a larger decrease in resistivity with increasing temperature, most of this change is attributed to the decrease in resistivity of the pore fluid over that temperature range. For both samples, k decreases with increasing pressure and temperature. The intact sample permeability varies from 23 mD at 3 MPa and 20°C to less than 1 mD at 50 MPa and 150°C. The permeability of the fractured sample varies from 676 mD at 3 MPa and 20°C to less than 1 mD at 40 MPa and 190°C. The effect of the fracture on k vanishes after several pressure cycles and above about 100°C. These laboratory data are used to demonstrate the possibility of using resistivity and velocity measurements to estimate in‐situ permeability of a reservoir.
The behavior of the apparent resistivity phase spectrum in the case of two polarizable media
Authors: Soininen Heikki
Published: October 2002

I employed numerical modeling to examine the formation of the apparent resistivity phase spectrum first of a polarizable prism situated in a polarizable half‐space, and second of two polarizable prisms joined in an unpolarizable half‐space. The calculations were done using the integral equation technique. The frequency dependence of the resistivity of the polarizable medium is depicted by means of the Cole‐Cole dispersion model. The effect of a weakly polarizable half‐space may be handled by simply adding the phase angle of the half‐space to the apparent phase due to the body. The apparent spectral parameters can be inverted by fitting the sum of two Cole‐Cole dispersion model phase spectra to the apparent phase spectrum. Of the parameters describing the prism, the apparent chargeability is smaller than the chargeability of the original petrophysical spectrum because of geometric attenuation. The apparent frequency dependence, on the other hand, is very close to the value of the original frequency dependence. The apparent time constant is commonly also near the true time constant of the petrophysical spectrum. The values of the apparent spectral parameters of the polarizable half‐space are all close to their petrophysical or true values. The apparent spectrum of two polarizable prisms builds up in a complex fashion. Nevertheless, by measuring the spectra at a number of points along a profile crossing over two formations differing in time constant, the various components can be discriminated from the apparent spectrum even if the difference in time constant is small. As the conductivity contrast decreases, the share of the spectrum of the formation in the apparent spectrum increases. Similarly, the formation with the smaller time constant is in a more advantageous position than the body with the greater time constant.
Absorbing boundary conditions for acoustic media
Authors: Keys R. G.
Published: October 2002

By decomposing the acoustic wave equation into incoming and outgoing components, an absorbing boundary condition can be derived to eliminate reflections from plane waves according to their direction of propagation. This boundary condition is characterized by a first‐order differential operator. The differential operator, or absorbing boundary operator, is the basic element from which more complicated boundary conditions can be constructed. The absorbing boundary operator can be designed to absorb perfectly plane waves traveling in any two directions. By combining two or more absorption operators, boundary conditions can be created which absorb plane waves propagating in any number of directions. Absorbing boundary operators simplify the task of designing boundary conditions to reduce the detrimental effects of outgoing waves in many wave propagation problems.
Separation of upgoing and downgoing waves in vertical seismic profiling by contour‐slice filtering
Authors: Suprajitno M. , Greenhalgh S. A.
Published: October 2002

The separation of upgoing waves and downgoing waves in vertical seismic profiling is effectively accomplished in the frequency‐wavenumber (f‐k) domain using a new technique of contour‐slice filtering. The response of the filter has no straight edge or boundary and preserves the “natural state” of the spectra. The cutoff contour level can be reduced as low as desirable to remove background noise and other unwanted signals. In this sense it can also be viewed as a noise‐rejection filter. The effectiveness of the process has been clearly demonstrated through synthetic and real examples. Its success with the upgoing waves depends on the degree of coherency of events in the VSP record. To enhance the coherency and to avoid velocity aliasing, we use a novel band‐pass frequency filter which is the result of convolving a boxcar spectral window with a frequency‐shifted Gaussian function.
Inference of manganese nodule coverage from Sea Beam acoustic backscattering data
Authors: Moustier Christian de
Published: October 2002

Normal incidence reflectivity from a manganese nodule field was measured with a 12 kHz Sea Beam multibeam echo‐sounding system, aboard the R/V Thomas Washington and used to infer nodule coverage. A reflectivity map of the area was produced using the intensity of the specular return from each ping. The patchiness of the nodule coverage is evidenced by definite highs and lows in the reflectivity pattern. Ground truth was provided by near‐bottom acoustic measurements and photographs taken with the Deep Tow instrument package of the Marine Physical Laboratory. Agreement between the simple nodule coverage predictions from Sea Beam acoustic data and the bottom photographs taken throughout the area is 98 percent. Although the Sea Beam system is limited in its dynamic range, this paper shows that it can be used very effectively to determine both topography and nodule coverage in potential mining areas.
Applications of elastic forward modeling to seismic interpretation
Authors: Reshef Moshe , Kosloff Dan
Published: October 2002

In the correct processing and interpretation of time sections gathered over complicated heterogeneous structures, acoustic assumptions no longer suffice and elastic effects need to be taken into consideration. This study presents numerical modeling results obtained with the Fourier method. Two classes of important geophysical problems were considered. The first class of problem was wave propagation in structures with both vertical and horizontal heterogeneities. Results of the calculations showed strong generation of converted phases and head waves. Generation of these phases is strongly dependent upon the velocity contrasts in the medium. The second class of problem was wave propagation in structures which contain both fluids and solids. The time sections recorded in the fluid regions again showed strong converted phases which could easily be misinterpreted as genuine acoustic reflections. The numerical results for the fluid‐solid model proved to be in agreement with physical model results. The presence of many phases on the time sections and snapshots requires that the elastic modeling method give accurate amplitudes and distinguish between P‐waves and S‐waves since otherwise their interpretation can become prohibitively complicated. In this respect, the Fourier method appears suitable because of its high accuracy and its ability to distinguish between P‐ and S‐waves through respectively applying the numerical divergence and curl.