bluuebunny/crossref_metadata_2025 huggingface dataset:
GEOPHYSICAL INVESTIGATIONS OF GROUNDWATER RESOURCES IN THE KALAHARI BASIN
Authors: Worthington Paul F.
Published: October 2002

A regional geoelectrical survey of a semiarid development area of about [Formula: see text], approximately 300 km NNE of Windhoek, has indicated a quadripartite subdivision of the Kalahari sediments of late Cretaceous to Tertiary age, with a variable development due to considerable undulations of the Damara basement. The Karroo formations are apparently less well represented. A two‐fold interpretative procedure, in which an in itial semiqualitative interpretation involving total longitudinal conductance forms the basis for a subsequent quantitative interpretation, has made it possible to map confidently the major lithological units despite the dearth of reliable stratigraphic control. By relating the geoelectrical data to the groundwater head levels, the degree of penetration and production characteristics of boreholes, and the chemical analyses of water samples, the calcareous sands and sandstones of presumed Middle Kalahari age have been seen to constitute the major aquifer in terms of higher sustained yields of good quality water. Encouraging correlations have been noted between these same production characteristics and the geoelectrically derived thickness, resistivity and transverse resistance of this aquifer. It has therefore been possible to delineate the most promising areas for future groundwater development. The Dar Zarrouk parameters form the basis of a suggested procedure for optimizing the initial stages of groundwater exploration in similar arid and semiarid regions where there is a general absence of lithostratigraphic information.
NUMERICAL TRACING OF CRITICALLY REFRACTED RAYS
Authors: Chander Ramesh
Published: October 2002

The need to trace critically refracted rays through arbitrarily stratified velocity models arises frequently during seismic refraction interpretation. But the means to do this routinely have been lacking so far. The main difficulty encountered while tracing such a ray using laws of reflection and refraction is that the direction in which the ray should leave the source is unknown. Determination of this direction is regarded here as a problem of solving two coupled nonlinear equations in two unknowns (namely, the azimuthal and colatitudinal angles describing the direction) for which several techniques are available. Details of the algorithm actually tested, and the procedure for tracing the critically refracted ray once the direction at source is known, are discussed below.
MAGNETIC ANOMALY DUE TO A VERTICAL RIGHT CIRCULAR CYLINDER WITH ARBITRARY POLARIZATION
Authors: Singh Shri Krishna , Sabina Federico J.
Published: October 2002

A closed form solution for the total anomalous magnetic field due to a vertical right circular cylinder with arbitrary polarization is derived under the assumption that the magnetization is uniform. As expected, the computed field is similar to the field due to a “similar” prism‐shaped body.
EXTENDED ARRAYS FOR MARINE SEISMIC ACQUISITION
Authors: Lofthouse J. H. , Bennett G. T.
Published: October 2002

In‐line arrays for both source and receiver have been implemented for marine seismic reflection data acquisition. The in‐line array dimensions (variable within limits) are considerably greater than any previously used system of which we are aware. The arrays were designed to attenuate extremely strong sea‐bottom multiples during the data acquisition phase. The source comprised 25 airguns arranged in five identical in‐line subarrays. Each subarray produced a signal of better than 6 barmeters acoustic intensity with a primary‐to‐bubble ratio of approximately 4.4 from guns totaling 297 cu in. When this source was delivered in 1973, it constituted the most powerful production airgun source for which we had seen calibration measurements. Receiver arrays were implemented by a “weighting‐mixing” box (which formed part of the DFS IV instrument), the input to which comprised 53 channels of data each from a 50 m live section in the streamer cable. Processing techniques which are complementary to the field procedures have been developed. Comparisons with “conventional” data (and such data processed to simulate field arrays) show significant improvements in “data quality” from the new field techniques, that is, the new data are easier to interpret geologically because interfering multiples have been attenuated relative to desired energy. Whilst the large outgoing signal amplitude will have made some contribution to the data quality, the major improvement is believed to result from the use of arrays in the recording phase. This system, first used for production in August 1973, was subsequently used successfully during recording of 17,000 km of offshore seismic data from Eastern Canada, the North Sea, and the Mediterranean.
A COMPARISON BETWEEN SONOBUOY AND OCEAN BOTTOM SEISMOGRAPH DATA AND CRUSTAL STRUCTURE OF THE TEXAS SHELF ZONE
Authors: Ibrahim Abou‐Bakr K. , Latham Gary V.
Published: October 2002

Three refraction profiles were shot in the Gulf of Mexico 100 nautical miles south of Galveston. Recordings were made using free floating and anchored sonobuoys deployed near an ocean bottom seismograph (OBS). Explosions and airgun sources were used. Vertical component geophones with resonant frequencies of 4.5 Hz and 8 Hz were used in the OBS system. We compared seismic data recorded by the conventional sonobuoy and the OBS. The signal‐to‐noise ratio for the OBS is about three to four times better than that of the sonobuoy, facilitating the interpretation of the three profiles. Based on the first and later arrivals obtained from the OBS records, traveltime curves and an amplitude distance curve are constructed. The results show that the velocities at the experimental sites vary between 1.7 km/sec at the surface to 7.7 km/sec at a depth of 14.5 km. The 7.7 km/sec layer may be lower crust or anomalous mantle. The low velocity sediments (1.7 – 3.3 km/sec) thicken westward from the deployment point.
ELECTROMAGNETIC SCATTERING FROM BURIED WIRES
Authors: Watts Raymond D.
Published: October 2002

The fast Fourier transform is used to compute the electromagnetic fields scattered by a long straight conductor buried in a layered earth. The source field is a transverse‐electric plane wave. A conductor buried less than 0.25 skin‐depths disturbs the ambient fields significantly. The position and depth of the conductor can be determined from the observed fields. A method for including several conductors, including their interactions, is discussed.
Reply by author to discussion by Shri Krishna Singh (GEOPHYSICS, April 1977, p. 661–662), on “Time Dependent Electromagnetic Fields of an Infinite, Conducting Cylinder Excited by a Long Current‐Carrying Cable” by S. K. Verma (GEOPHYSICS, April 1973, p. 369—379)
Authors: Verma Saurabh K.
Published: October 2002

I thank Dr. Singh for taking further interest in my paper. At the time of writing my reply (Verma, 1974) none of the publications: Hill and Wait, (1972) (in reality published after the publication of December 1973 issue of Radio Science) Wait and Hill (1973) or Wait (1974) had arrived in the library of my institute. These publications, particularly Wait (1974) wherein Dr. Wait contradicts his earlier explanation (Wait, 1973), obviously invalidate the physical argument in question.
Orthonormal lattice filter—a multistage multichannel estimation technique
Authors: Alam M. Aftab
Published: October 2002

The orthonormal lattice filter algorithm is a unified recursive technique to estimate the least‐squares coefficients of a broad class of linear models. It consists of a sequence of two recursions. First, the Gram‐Schmidt orthonormalization procedure estimates a set of inner products from multiple signal vectors, for example, multichannel sampled observations. A lattice type network structure, suitable for adaptive applications, implements the Gram‐Schmidt procedure. Second, a recursion introduced in this paper transforms the inner products into model coefficients. In contrast with conventional algorithms, this algorithm does not require the covariance matrix to design the filters but directly filters the observations. We consider a number of special cases of the multichannel model, for example the rational polynomial model, the autoregressive model, and the autoregressive‐moving average model with known and unknown inputs. In addition, we introduce the autoregressive‐orthonormal model for the unknown input case. We also transform the coefficients of the autoregressive‐orthonormal model to estimate the poles and zeros of a signal transfer function and thereby obtain the amplitude and phase spectra of the signal. For each model, we appropriately simplify the general orthonormal lattice filter algorithm and also the filter network structure. It turns out that for the autoregressive model the simplified algorithm is the Itakura‐Saito algorithm which is a direct version of the Levinson recursion and the simplified structure is a lattice type ladder. With the example of a noisy truncated signal, we compare the actual and estimated values of (a) the coefficients of a known‐input autoregressive‐moving average model, and (b) the autoregressive‐orthonormal power density spectrum of the signal.
A quadripole resistivity survey of the Imperial Valley, California
Authors: Harthill Norman
Published: October 2002

A quadripole resistivity survey of the Imperial Valley, California was carried out from the Salton Sea in the north to the Mexican border in the south. The east and west boundaries of the survey were the topographic limits of the valley. The quadripole resistivity method consists of sequentially energizing two orthogonal bipole sources with a square wave of electric current and measuring the resultant electric fields with a pair of orthogonal wire receivers. Two resultant electric fields are measured and by combining them in different proportions, their resultant can be made to rotate through 360 degrees. By performing this procedure, an ellipse of resistivity can be calculated at each measurement location. The arithmetic mean of the maximum and minimum axes of the ellipse is a tensor invariant resistivity. It was this value which was used to define the variation of electrical resistivity over the Imperial Valley. The quadripole survey of the Imperial Valley was undertaken to compare its known geothermal fields with previously unsurveyed areas. The results show that the geothermal fields of the Imperial Valley have distinctive resistivity characteristics. A large circular area south of the city of Brawley was found which has the same characteristics as the known geothermal fields. From the resistivity data, it is predicted that this south Brawley resistivity anomaly represents a geothermal field which will produce water with a temperature of approximately 200°C and with a salinity between 20,000 and 50,000 ppm. A well to test the prospect was scheduled to be drilled early in 1978.
COCORP southwestern Wyoming data now available
Authors: Kaufman S.
Published: October 2002

The data packages for three seismic lines run in the Wind River uplift area of Wyoming are now available for the cost of reproduction and shipping. These data were obtained by the Consortium for Continental Reflection Profiling (COCORP), who are applying sophisticated continuous seismic reflection techniques to the solution of geologic problems of the crust and upper mantle of the earth. The 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 committee of the consortium consists of members from Cornell University, University of Houston, Princeton University, University of Texas at Austin, University of Wisconsin, and Shell Oil Co. Cornell University has the operational responsibility.
Gravity interpretation with the aid of quadratic programming
Authors: Fisher N. J. , Howard L. E.
Published: October 2002

The inverse gravity problem is posed as a linear least‐squares problem with the variables being densities of two‐dimensional prisms. Upper and lower bounds on the densities are prescribed so that the problem becomes a linearly constrained least‐squares problem, which is solved using a quadratic programming algorithm designed for upper and lower bound‐type constraints. The solution to any problem is smoothed by damping, using the singular value decomposition of the matrix of gravitational attractions. If the solution is required to be monotonically increasing with depth, then this feature can be incorporated. The method is applied to both field and theoretical data. The results are plotted for (1) undamped, nonmonotonic, (2) damped, nonmonotonic, and (3) damped, monotonic solutions; these conditions illustrate the composite approach of interpretation where both damping techniques and linear constraints are used in refining a solution which at first is unacceptable on geologic grounds while fitting the observed data well.
On: “The effect of subsurface sampling on one‐dimensional synthetic seismograms,” by M. Schoenberger and F. K. Levin (GEOPHYSICS, November 1979, p. 1813–1829).
Authors: Rüter H. , Schepers R.
Published: October 2002

In one part of their paper, Schoenberger and Levin describe time delays measured between specified phases of the transmitted wavelet and the calculated traveltime of the first onset. They point out that this time delay depends upon the time sampling, and it increases if the sample interval is decreased from 4 to [Formula: see text] msec. The proper traveltime to compare to it, in our opinion, is not the traveltime of the first onset calculated by summing transit times, but the traveltime of the selected phase calculated with infinitely small sample intervals. Calculating time delays in this way, the time delay decreases with decreasing sample interval as the time delay between synthetic seismograms, and field seismograms decrease by calculating the synthetic ones with smaller sample intervals.
Restrictions on the use of Cole‐Cole dispersion models in complex resistivity interpretation
Authors: Major J. , Silic J.
Published: October 2002

The Cole‐Cole dispersion approximates the behavior of inductive coupling over only a part of the frequency range. We show that the interaction between the induced polarization (IP) and inductive coupling (IC) effects will, in general, be more complicated than a multiplication of two Cole‐Cole dispersions representing the separate IP and IC effects. The inversion of published data shows that the time constant [Formula: see text] of the IP effect is often a poorly defined parameter. Hence, small changes in the parameters used to approximate inductive coupling by a Cole‐Cole dispersion can produce very large changes in the calculated value of [Formula: see text]. From these considerations, guidelines are developed to show when the Cole‐Cole dispersion may be successfully used to approximate inductive coupling and define [Formula: see text].
Fault location by channel wave seismology in United Kingdom coal seams
Authors: Buchanan D. J. , Davis R. , Jackson P. J. , Taylor P. M.
Published: October 2002

About 80 percent of British deep mined coal is obtained from advance longwall faces. This mining method is highly capital intensive. Therefore, it is important to know the geologic structure prior to mining. Small faults can have a large effect on production rates. Channel wave seismology offers a possible method for locating these faults. A coal seam resembles a dispersive acoustic duct, and energy from a seismic source within the seam can be coupled into channel wave modes. Scattered waves from defects within the seam can be recorded and the data analyzed to produce a map of the seam. It has been shown that dispersion does not present a fundamental limitation to the resolution that can be obtained. In addition, broadband numerical holographic mapping procedures in the form of lag sums can be implemented. These techniques have been described previously by Mason et al. (1980a). This paper is divided into two main parts. First, we show how the basic lag‐sum techniques have been extended to produce an adaptive‐lag‐sum (ALS) mapping procedure. Second, we describe a variant to standard common‐depth‐point (CDP) stacking. This variation, called dynamic trace gathering (DTG), stacks only those traces with similar reflection points. Both ALS and DTG are extremely flexible. Targets at large angles to the geophone line may be imaged. Both processes can be used to analyze transmission and reflection seismograms, with or without mode conversion at any target. Dispersive propagation may also be included. The techniques are illustrated using data from underground field trials at three collieries.
Three‐dimensional resistivity inversion using alpha centers
Authors: Petrick Wm. R. , Sill Wm. R. , Wards S. H.
Published: October 2002

The method of alpha centers represents a class of solutions to the three‐dimensional (3-D) dc conduction equation based on certain nonlinear substitutions for electric potential and earth conductivity. A solution is obtained which is sufficiently fast to make the inversion of 3-D resistivity data practical. The inversion routine results in a conductivity distribution defined by α centers which simultaneously fits the data from several parallel or perpendicular dipole‐dipole profiles. To illustrate the characteristics of this modeling approach, we apply the inversion algorithm to three theoretical and four field data sets. The four field data sets represent samples from massive sulfide and geothermal environments. The technique, when applied to theoretical data from prismatic bodies, gives fairly good estimates of the positions of conductive inhomogeneities but poor estimates of their actual conductivities. The entire inversion algorithm requires less than 15,000 words of computer memory, thus making it tractable for small computers. We envision two major applications. The first is for in‐field data interpretation to site drilling locations or to guide further exploration. The second is for obtaining a good initial guess for more sophisticated and costly multidimensional inversion schemes.
A new technique for layered earth magnetotelluric inversion
Authors: Larsen J. C.
Published: October 2002

A new one‐dimensional (1-D) inverse method for layered‐earth interpretation of magnetotelluric (MT) response curves is based on the method of Schmucker (1972). It involves transforming the E over B response into a nondimensional complex logarithm response, computing the partial derivatives from a new algorithm for the logarithm response, and iteratively solving (by damped least squares) for the logarithm of the conductivity contrast between layers. Error bars for the layer conductivities are estimated by a simple application of propagation of errors assuming random and independent response errors. Backus‐Gilbert type smoothing kernels are also computed in order to specify the upper and lower depth limits on the conductivity model and to examine whether the layer conductivities are locally averaged values. The kernels are found to be an important aid in model interpretation and emphasize the fact that layer conductivities are averages. The method is illustrated using artificial and real data.
Induced polarization in disseminated sulfide ores containing elongated mineralization
Authors: Wong J. , Strangway D. W.
Published: October 2002

A disseminated sulfide ore is represented by a two‐component system in which metallically conducting prolate spheroidal particles (simulating elongated mineralization) are randomly scattered throughout an electrolytic host. The Helmholtz equations describing the spatial and frequency dependence of anions and cations in the electrolyte near the surface of a particle are solved in prolate spheroidal coordinates. Expressions for the frequency‐dependent dipole moment induced on the particle by external electric fields transverse or parallel to the long axis of the particle are found by examining boundary conditions related to electrochemical charge transfer between the metallic particle and the electrolyte. The dipole moments of individual particles can be used to determine the effective conductivity spectrum of the mixture as a whole via the simple Maxwell formula or a novel recursive calculation which is accurate for large‐volume fractions of particles. Examples of conductivity spectra from this electrochemical model incorporating elongated particles are presented, and comparison of results with appropriate experimental data indicates good agreement.
Windowing and estimation variance in deconvolution
Authors: Sicking Charles J.
Published: October 2002

Spiking deconvolution operators are computed using the Levinson and orthonormal lattice filter algorithms. By comparing the amplitude and phase spectra of the operators, the differences between the two methods are shown to be strictly tied to the differences in how the algorithms handle the windowing problem. Also, the two methods are equivalent if the windowing problem can be overcome through the use of multipass deconvolution prior to the application of an interpretive wavelet. The most significant effect of windowing is to introduce errors in the estimate of the phase spectrum. When a filter is computed from a single trace, estimation variance in the filter is high. Spatial averaging in the filter design process can overcome a large part of this problem and produce sections with better lateral continuity. The parameters averaged spatially are the autocorrelations and the inner products for the Levinson and orthonormal lattice filter algorithms, respectively. This suggests that short window, time, and space varying deconvolution is undesirable and that the geology is better defined by using operators which result from averaging over several independent estimates. Analysis of the phase spectra of the deconvolution operators computed for synthetic and real data indicates that selection of the interpretive wavelet based on the phase spectrum of the deconvolution operator is the best approach to reject the frequencies which will have a large phase error in the deconvolved section.
The sensitivity of Born inversion to the choice of reference velocity: A simple example
Authors: Weglein A. B. , Gray S. H.
Published: October 2002

We examine the sensitivity of the Born model to the input background velocity. We use a one‐dimensional analytic example to point out the difference between a corrective procedure and merely a perturbative one. We examine various aspects of the sensitivity issue, including the trade‐off between velocity determination and mapping of reflector location. Although this problem is discussed within the context of the Born model, it is an issue common to all perturbative methods (e.g., migration methods) which transform surface reflection data into a map of subsurface reflectors.
Approximate method for direct interpretation of gravity anomalies caused by surface three‐dimensional geologic structures
Authors: Švancara Jan
Published: October 2002

An efficient method for direct transformation of gravity anomalies into contour lines of depth is proposed. The method is applicable for surface three‐dimensional bodies whose shape may be approximated by a cone at the first step. Hypothetical and field data evaluations presented here prove the efficiency of the method.
Continuously time‐variable recursive digital band‐pass filters for seismic signal processing
Authors: Stein R. A. , Bartley N. R.
Published: October 2002

A design technique is described for continuously time‐variable recursive digital band‐pass filters for seismic signal processing. Two types of band‐pass filters are considered: a cascade of a low‐pass and a high‐pass filter, and a direct band‐pass filter, with all filters being derived from a continuous unit‐bandwidth Butterworth low‐pass prototype. Linear interpolation of the filter coefficients between points at which they are known exactly is used to reduce the computational overhead. Data are given for determining the length of the interpolation interval to meet prescribed worst case magnitude and frequency error criteria. A zero‐phase response is achieved by filtering in the forward time direction followed by filtering in the reverse time direction. An example is included.
Homomorphic deconvolution
Authors: Jin D. J. , Rogers J. R.
Published: October 2002

The advantages of homomorphic deconvolution are that it does not require the assumptions of minimum‐phase wavelet and of a white random reflection coefficient series. Disadvantages of the method which have been recognized in the public domain are difficulties in unwrapping the phase, in dealing with band‐limited signals, and in handling mixed‐phase reflection coefficient series. These difficulties may be respectively overcome by using an “adaptive numerical integration algorithm” (Tribolet, 1977), frequency transformations (Tribolet, 1979), and exponential weighting of the signal (Tribolet, 1979). There seems to have been some understanding in the literature and among exploration researchers that additive noise would affect the performance of homomorphic deconvolution. However, to the best of our knowledge there have not appeared in the literature any analytical expressions or experiments conclusively showing how additive noise affects homomorphic deconvolution. Analytic and experimental analyses demonstrated that additive noise plays a critical role in homomorphic deconvolution such that homomorphic deconvolution is unreliable whenever the spectral amplitudes of the signal are very small over certain frequency bands and even a small amount of noise is present. This unreliability of the method overshadows its advantages.
Acoustic reciprocity—A paradox
Authors: Eisner E.
Published: October 2002

A claim that reciprocity fails to hold in some particular situation is usually treated with the same skepticism that would greet a claim of failure of energy conservation. Hence my response to claims that in real field situations reciprocity had been observed to fail was to look for improper application of the theorem. To convince myself that I understood how to apply the theorem, I considered the simplest case, i.e., the scalar wave equation, and attempted to construct a counter example. Finding the flaw in the counter example would serve to renew my belief that I understood how to apply reciprocity correctly.
The effect of velocity variations on the beamwidth of a seismic wave
Authors: Sutton G. R.
Published: October 2002

The effect of depth and lateral velocity variations on the width of a seismic beam propagating through the earth is investigated. The model used is unusual and provides an interesting alternative insight to the conventional models appearing in the geophysical literature. Some basic calculations show that, at frequencies and depths of interest in oil exploration, a significant lateral shift in the center of the seismic beam occurs in addition to the expected broadening of the seismic beam.
On: “Automated identification of rock boundaries: an application of the Walsh transform to geophysical well‐log analysis” by E. N. Lanning and D. M. Johnson (GEOPHYSICS, v. 48, p. 197–205, February, 1983).
Authors: Chan David S. K.
Published: October 2002

Lanning and Johnson’s paper presents the technique of Walsh transform domain low‐pass filtering as a means of enhancing transitions in well logs prior to boundary picking. They argue that existing fast means of computing the Walsh transform (fast Walsh transform algorithms) make this procedure particularly efficient. In this discussion it is shown that, for all practical cases of interest, the results of their paper can be obtained without any mention of the Walsh transform. In fact, using the Walsh transform unnecessarily increases computational complexity. Specifically, it is shown that the low‐pass sequency filtering described for obtaining a stepped version of the original signal with a given step width is equivalent to segmenting simply the signal into equal length segments and replacing all values in each segment by the segment average.
On: “Dip limitations on migrated sections as a function of line length and recording time” by H. B. Lynn and S. Deregowski (GEOPHYSICS, v. 46, p. 1362, October, 1981).
Authors: Levin Stewart A.
Published: October 2002

Dr. Lynn and Dr. Deregowski nicely analyzed how recording time and line length limit the detection of dipping beds and the resolution of deeper diffractors. For migration and synthetic seismogram generation I exploited this decrease in dip spectrum at late recording times both to supress migration artifacts and to decrease the cost of migration.
On: “Comparison of apparent resistivity functions for transient electromagnetic methods” by A. P. Raiche (GEOPHYSICS, v. 48, p. 787–789).
Authors: Macnae J. , Lamontagne Y.
Published: October 2002

We have read with great interest this short note essentially showing some advantages of measuring the step response rather than the impulse response for depth sounding purposes. We have reached very similar conclusions in the interpretation of UTEM® transient data in depth sounding applications compared to pulse EM data: generally the step response itself or derived apparent resistivities show the effect of either deep or very conductive layers at earlier sampling times than does the pulse response, and the apparent resistivities are most often simpler in shape and single valued. For measurements outside the transmitter loop, both step and pulse responses are more complex, but in general the step response is relatively simpler and has its characteristic points at earlier times. To give two quantitative examples: for a vertical dipole source, the zero crossing for a thin horizontal conductive layer response occurs √3 times earlier for the step response; for a half‐space response, the ratio is quite close to 3 to 1. The asymptotic behavior used to define apparent resistivities by Raiche is therefore reached at earlier sampling times for the step response in spite of the generally smaller amplitude of the pulse response at late times. We are thus in complete agreement with the main thrust of the short note.
Dip‐moveout by Fourier transform
Authors: Hale Dave
Published: October 2002

The conventional normal‐moveout (NMO) and common‐midpoint (CMP) stacking process enhances reflections having a particular moveout velocity, while attenuating events (such as multiple reflections) having different moveout velocities. Unfortunately, this process also acts as a dip filter applied to the CMP stack. In other words, NMO and stacking enhances reflections having a particular slope in the CMP stack, while attenuating reflections having different slopes. NMO and stacking, like any dip filter, degrades lateral resolution. Fortunately, this dip‐filtering action can be suppressed by applying, in addition to NMO, a prestack process known variously as DEVILISH, prestack partial migration, and dip‐moveout. As the latter term implies, this process is a dip‐dependent moveout correction that enables reflections from both horizontal and dipping reflectors to be stacked with the same NMO velocity. Stated another way, NMO velocities estimated from dip‐moveout‐corrected seismograms are independent of the dips of subsurface reflectors. Dip‐moveout by Fourier transform is a method for performing dip‐moveout (DMO) correction in the frequency‐wavenumber domain. The implementation of this method, which resembles the implementation of a discrete Fourier transform, is quite different from and compares favorably with previously published finite‐difference DMO algorithms. DMO by Fourier transform, unlike DMO by finite‐differences, is accurate for all offsets and all dips, provided that velocity is constant. Because velocity is never constant, some accuracy is inevitably lost; but the application of DMO by Fourier transform to recorded seismograms demonstrates the ability of this process to enhance (1) the dip bandwidth of CMP stacks and (2) the accuracy of velocity estimates.
Geophysical and geochemical anomaly in northeastern New York
Authors: Fleischer Robert L. , Turner Larry G.
Published: October 2002

Trapped concentrations of oil and natural gas in the earth will sometimes release vapors, whose presence and motion might be used to help infer where resources are located. Tests for carbon anomalies may be applied to search for hydrocarbons in the earth, and alterations in [Formula: see text] in the earth can be used to locate regions of subsurface gas flow. In a recent report we have observed both such anomalies over the Cement, Oklahoma oil and gas fields (Fleischer and Turner, 1984). Because of this result, whose generality is not yet known, it is of interest to know where other similar carbon and gas‐flow anomalies exist. (We should note that extensive examples of carbon anomalies in the soil have been presented; many of these are referenced in Fleischer and Turner, 1984.) This short note describes one such geochemical anomaly that has been observed using three methods of mapping—one radiometric, one isotopic, and one chemical. Testing whether this anomaly is associated with hydrocarbons is a logical next step that has not been taken.
Seismic indicators of stratigraphy
Authors: Sinvhal A. , Khattri K. N. , Sinvhal H. , Awasthi A. K.
Published: October 2002

An attempt has been made to predict relative contents of sand, shale, and coal in a sequence of lithologies as traversed and depicted by a seismogram, using discriminant analysis. Monte Carlo simulations and Markov chains have been used to model the subsurface, and discriminant analysis was first tried on these and then tested on field data. Subsurface lithology was predicted by this technique and shows great potential in future exploration programs.
Magnetotelluric responses of three‐dimensional bodies in layered earths
Authors: Wannamaker Philip E. , Hohmann Gerald W. , Ward Stanley H.
Published: October 2002

The electromagnetic fields scattered by a three‐dimensional (3-D) inhomogeneity in the earth are affected strongly by boundary charges. Boundary charges cause normalized electric field magnitudes, and thus tensor magnetotelluric (MT) apparent resistivities, to remain anomalous as frequency approaches zero. However, these E‐field distortions below certain frequencies are essentially in‐phase with the incident electric field. Moreover, normalized secondary magnetic field amplitudes over a body ultimately decline in proportion to the plane‐wave impedance of the layered host. It follows that tipper element magnitudes and all MT function phases become minimally affected at low frequencies by an inhomogeneity. Resistivity structure in nature is a collection of inhomogeneities of various scales, and the small structures in this collection can have MT responses as strong locally as those of the large structures. Hence, any telluric distortion in overlying small‐scale extraneous structure can be superimposed to arbitrarily low frequencies upon the apparent resistivities of buried targets. On the other hand, the MT responses of small and large bodies have frequency dependencies that are separated approximately as the square of the geometric scale factor distinguishing the different bodies. Therefore, tipper element magnitudes as well as the phases of all MT functions due to small‐scale extraneous structure will be limited to high frequencies, so that one may “see through” such structure with these functions to target responses occurring at lower frequencies. About a 3-D conductive body near the surface, interpretation using 1-D or 2-D TE modeling routines of the apparent resistivity and impedance phase identified as transverse electric (TE) can imply false low resistivities at depth. This is because these routines do not account for the effects of boundary charges. Furthermore, 3-D bodies in typical layered hosts, with layer resistivities that increase with depth in the upper several kilometers, are even less amenable to 2-D TE interpretation than are similar 3-D bodies in uniform half‐spaces. However, centrally located profiles across geometrically regular, elongate 3-D prisms may be modeled accurately with a 2-D transverse magnetic (TM) algorithm, which implicitly includes boundary charges in its formulation. In defining apparent resistivity and impedance phase for TM modeling of such bodies, we recommend a fixed coordinate system derived using tipper‐strike, calculated at the frequency for which tipper magnitude due to the inhomogeneity of interest is large relative to that due to any nearby extraneous structure.
TRANSIENT TESTING OF SEISMIC RECORDING APPARATUS
Authors: Washburn Harold , Olesen Raymond
Published: October 2002

A transient generator and shaking table suitable for testing response characteristics of seismometers and associated amplifier and recording equipment is described. From variable‐area sound‐on‐film records representing idealized transients or actual field seismograph recordings, the generator and shaking table provide a high‐fidelity conversion to mechanical motion of the test table. The special experimental truck used in obtaining test records is also described briefly.
Velocity determination in layered systems with arbitrarily curved interfaces by means of wave field extrapolation of CMP data
Authors: Wapenaar C. P. A. , Berkhout A. J.
Published: October 2002

The paper starts with a brief review of conventional velocity determination procedures for plane‐layered systems. These methods assume hyperbolic moveout curves in common midpoint (CMP) data. It is shown that in layered systems with arbitrarily curved interfaces these methods fail since the moveout curves are nonhyperbolic. The subject of this paper is a wave‐theoretical approach to velocity determination. By means of wave field extrapolation of CMP data, nonhyperbolic moveout curves are transformed into hyperbolic ones. The proposed process is called velocity replacement (VR) since an inhomogeneous overburden is replaced by a homogeneous velocity medium. The effect of VR is illustrated on synthetic data. From the results it may be concluded that velocity determination after VR yields significantly more accurate results than velocity determination before VR. The technique of VR is also proposed as a preprocessing tool prior to stack in situations of nonhyperbolic moveout curves.
Applications of algebraic reconstruction techniques to crosshole seismic data
Authors: Peterson John E. , Paulsson Bjorn N. P. , McEvilly Thomas V.
Published: October 2002

Tomographic imaging techniques were applied to two crosshole data sets to determine the velocity structures and the reliability and resolution of the algorithms on real data. The experiments were carried out at the Retsoff salt mine in New York and at the underground radioactive waste study site at the Stripa mine facility in Sweden. The traveltimes at Restoff were high quality and were obtained over raypaths of up to 500 m in length. The structure was quite complicated with velocity contrasts up to 50 percent. The Stripa site was in granitic rock with velocity contrasts of only a few percent. The dimensions of the experiment were small with maximum ray lengths of just over 10 m. The data at this site were collected with very high accuracy, source and receiver locations were measured to better than 1.0 mm, and traveltimes were read to 0.001 ms. A number of algorithms similar to the algebraic reconstruction techniques (ART) used in medical imaging have been applied to the data. Some modifications of the algorithms, such as the application of weighting schemes, damping parameters, and curved raypaths, were performed. The resulting velocity fields were compared to the known fields and with each other to determine an optimal method. The algorithms were found to be a rapid, reliable means of reconstructing the slowness field of real data. Low‐velocity zones were recovered with accuracy in location and value. It was also found that great care was necessary in application of the techniques to ensure that proper damping parameters are used and the proper number of iterations taken; otherwise poor reconstructions will result.
On: “The half‐slope and straight‐slope methods of basement depth determination” by D. Atchuta Rao and H. V. Ram Babu (GEOPHYSICS, 49, 1365–1368, August, 1984).
Authors: Åm Knut
Published: October 2002

There appear to be some mistakes in the subject paper. The authors seem not to have realized that the anti‐symmetric contact anomaly represents one of the flanks of a symmetrical dike anomaly of infinite width, and that the symmetric contact anomaly belongs to the family of anti‐symmetric dike anomalies. This explains the peculiar looking curve for contacts (R = ∞ curve) in their Figure 2 where the contact curve is seen to intersect the dike curves. To be correct the R = ∞ curve has to be turned around a vertical axis so that the value for θ = 90 ends up at θ = 0 and vice versa. It may have added to the authors’ confusion that they used two different sources for the mathematical expression of contact and dike anomalies without realizing that the angle θ has different definitions in the papers cited. In the formulas given by Åm (1972) the angle θ has a value of 90 degrees for the symmetric and 0 degrees for the antisymmetric dike anomaly, whereas in Gay (1963) the angle θ has a value of 0 degrees for the symmetric and —90 degrees for the antisymmetric case.
Columbia River Basalt Plateau—An integrated approach to interpretation of basalt‐covered areas
Authors: Prieto Corine , Perkins Carolyn , Berkman Ernest
Published: October 2002

An interpretation is presented of a 219 km regional profile which traverses the eastern Columbia River Plateau in Washington State. Aeromagnetic, magnetotelluric (MT), and gravity data were first interpreted separately. All three data sets then were satisfied by a single geologic model. The objective of this case study is to illustrate the individual contributions derived from these three geophysical data sets to a final integrated interpretation. The aeromagnetic interpretation has produced regional structural information and data from which rock compositions can be inferred. The MT interpretation shows that the basalt/sediment interface can be determined, and thus a relative sediment thickness can be inferred. The gravity interpretation is dependent upon an additional method to determine either the basalt or basement horizon. In order for the gravity interpretation to approximate depth to basement or sedimentary thickness, the base of the basalt must be determined from another scientific method. From comparison of the regional structural results of the three geophysical techniques we conclude that aeromagnetic or MT data can be used to determine major structural trends. Reasonable rock compositions are also determined from the combined data sets. The interpreter must be aware of the different rock properties measured by each tool when performing an integrated interpretation; comparisons between the various techniques must be based upon similar assumptions. We recommend that detailed, integrated models be included for a thorough evaluation of any basalt‐covered area. The analysis of rock composition and regional structural information thus derived provides a sound basis for a regional tectonic interpretation and subsequent prospect evaluation.
A nonreflecting boundary condition for discrete acoustic and elastic wave equations
Authors: Cerjan Charles , Kosloff Dan , Kosloff Ronnie , Reshef Moshe
Published: October 2002

One of the nagging problems which arises in application of discrete solution methods for wave‐propagation calculations is the presence of reflections or wraparound from the boundaries of the numerical mesh. These undesired events eventually override the actual seismic signals which propagate in the modeled region. The solution to avoiding boundary effects used to be to enlarge the numerical mesh, thus delaying the side reflections and wraparound longer than the range of times involved in the modeling. Obviously this solution considerably increases the expense of computation. More recently, nonreflecting boundary conditions were introduced for the finite‐difference method (Clayton and Enquist, 1977; Reynolds, 1978). These boundary conditions are based on replacing the wave equation in the boundary region by one‐way wave equations which do not permit energy to propagate from the boundaries into the numerical mesh. This approach has been relatively successful, except that its effectiveness degrades for events which impinge on the boundaries at shallow angles. It is also not clear how to apply this type of boundary condition to global discrete methods such as the Fourier method for which all grid points are coupled.
Numerical computation of individual far‐field arrivals excited by an acoustic source in a borehole
Authors: Kurkjian Andrew L.
Published: October 2002

In this paper, I model the acoustic logging problem and numerically compute individual arrivals at far‐field receivers. The ability to compute individual arrivals is useful for examining the sensitivities of each arrival to various factors of interest, as opposed to examining the full waveform as a whole. While the numerical computation of the mode arrivals (Peterson, 1974) and the numerical computation of the first head waves (Tsang and Rader, 1979) have been previously reported, the numerical computation of the entire set of head‐wave arrivals is new and is the major contribution of this paper. Following Roever et al. (1974) and others, the full wave field is represented as a sum of contributions from both poles and branchcuts in the complex wavenumber plane. The pole contributions correspond to mode arrivals while the branch cuts are associated with the body waves (i.e., head waves). Both the pole and branch cut contributions are computed numerically and results are presented for the cases of a slow and a fast formation. The shear event in the slow formation is found to be relatively small, consistent with observations in measured data. Contrary to existing knowledge, the shear event in the fast formation is also relatively small. The apparent strong shear arrival in the full waveforms is due primarily to the trapped mode pole in the vicinity of cutoff.
On: “Kirchhoff elastic wave migration for the case of noncoincident source and receiver,” by John R. Kuo and Ting‐fan Dai (GEOPHYSICS, 49, 1223–1238, August, 1984).
Authors: Rechtien Richard D.
Published: October 2002

Upon reading the work of Kuo and Dai, I have a question concerning the validity of their formulation for wave field extrapolation. They present a Kirchhoff‐Helmholtz type integral formulation for the downward extrapolation (continuation) of an elastic wave field. Mathematically, the formulation is exceedingly complex, since scalar images standardly employed in the acoustic situation are replaced by their dyadic counterparts. The result of this complexity is that derivatives of the dependent‐field variables are not subject to elimination from the resulting extrapolation expressions, as they are for the acoustic condition. Thus one is required not only to specify the values of the particle velocity, or displacement, on the data boundary, but also the values of most of their derivatives.