bluuebunny/crossref_metadata_2025 huggingface dataset:
STRUCTURE OF THE SEG POINT DATA EXCHANGE AND FIELD FORMATS (A PROPOSED SEG DIGITAL RECORDING STANDARD)—IMPLEMENTATION
Authors: Dampney C. N. G.
Published: October 2002

The implementation of the SEG standard described in the companion paper, “Structure of the SEG Point Data Exchange and Field Formats (A Proposed SEG Digital Recording Standard)—Specification” is best done as a hierarchy of routines. Each hierarchical level progressively deals with increasingly specific data conversion operations until the whole process from pure characters to user specified parameters is complete. In this way a robust design is completed which clearly separates aspects of the standard. For example, design decisions related to character representation, control record structure, and data record conversion are carefully separated. The user can specify the data fields required, yet also retain control over storage allocated to buffers and formatting arrays. The listing of Fortran code shows the use of the SEGPD I/O routines by example utility programs. Machine dependent primitive I/O and data conversion routines are carefully specified.
ASSESSMENT OF LANDSAT FILTERS FOR ROCK TYPE DISCRIMINATION, BASED ON INTRINSIC INFORMATION IN LABORATORY SPECTRA
Authors: Hunt Graham R. , Salisbury John W.
Published: October 2002

Ratio values of the reflected energy available through different Landsat band‐pass filters were formed using laboratory spectra of 284 particulate rock and soil specimens, and the potential for using these data to discriminate among rock types was examined. Based exclusively on the intrinsic spectral information in the multispectral scanner (MSS) bands, thereby excluding textural, geomorphic, and vegerational effects that may occur in the field, it was concluded that the ratios can generally be used to discriminate different lithologic units. Further, only mafic and ultramafic rocks, and alteration involving ferric oxide staining, typically display sufficiently characteristic spectral behavior to allow specific identification. Certain unique features, such as those produced by chrominum in some chloritic quartzites can be used for discrimination of a few rare rock types.
Discussion on “Diffraction response for nonzero separation of source and receiver,” by John R. Berryhill (GEOPHYSICS, October 1977, p. 1158–76, and Discussion on “Diffractions for arbitrary source‐receiver locations,” by A. W. Trorey (GEOPHYSICS, October 1977, p. 1177–82)
Authors: Kuhn Milos J.
Published: October 2002

Study of simple synthetic seismograms convinced both authors that the diffracted pulse shape shows virtually no variation with source‐receiver offset if the midpoint is held constant. First, I wish to corroborate their findings by means of a simple example and at the same time suggest that conclusions of both authors apply strictly only to planar (and planar dipping) reflectors. Secondly, I wish to show that because of the invariant behavior of the pure diffraction pulse and because of the equally invariant behavior of the pure reflection pulse, the shape of the reflected pulse for depth points close to the edge of the reflector can not be offset invariant.
Deconvolution with the ℓ1 norm
Authors: Taylor Howard L. , Banks Stephen C. , McCoy John F.
Published: October 2002

Given a wavelet w and a noisy trace t + s * w + n, an approximation ŝ of the spike train s can be obtained using the [Formula: see text] norm. This extraction has the advantage of preserving isolated spikes in s. On some types of data the spike train ŝ can represent s as a sparse series of spikes, which may be sampled at a rate higher than the sample rate of the data trace t. The extracted spike train ŝ may be qualitatively much different than those commonly extracted using the [Formula: see text] norm. The [Formula: see text] norm can also be used to extract a wavelet ŵ from a trace t when a spike train s is known. This wavelet extraction can be constrained to give a smooth wavelet which integrates to zero and goes to zero at the ends. Given a trace t and an initial approximation for either s or w, it is possible to alternately extract spike trains and wavelets to improve the representation of trace t. Although special algorithms have been developed to solve [Formula: see text] problems, all of the calculations can be performed using a general linear programming system. Proper weighting procedures allow these methods to be used on ungained data.
Conductor identification in tropical terrains—Case histories from the Itapicuru greenstone belt, Bahia, Brazil
Authors: Palacky G. J. , Sena F. O.
Published: October 2002

An airborne electromagnetic (EM) and magnetic survey was conducted to prospect for massive sulfide ore bodies in the Itapicuru greenstone belt in the state of Bahia, Brazil. Because tropical weathering is intense in this region, most of the recorded airborne EM anomalies were caused by a conductive overburden. Standard procedures to select targets for ground investigation (follow‐up) were found only partially satisfactory because such procedures resulted in anomalies in conductive environment being largely ignored. Because of this difficulty, a new scheme of target selection was developed which is based on a statistical analysis of conductance within a given lithologic unit. Four case histories of ground geophysical surveys, which were carried out during the follow‐up of airborne anomalies are described. The case histories focus on problems likely to be encountered in environments of intense tropical weathering. In areas covered by a conductive overburden, the EM response is sensitive to a proper choice of field equipment, frequency, and coil separation. As a result of the geophysical survey, bodies of volcanogenic massive sulfides were found in the Itapicuru greenstone belt, but so far none is of economic importance.
Near‐infrared (1.3–2.4) μm spectra of alteration minerals—Potential for use in remote sensing
Authors: Hunt Graham R.
Published: October 2002

Reflection spectra of particulate samples of minerals that commonly occur in hydrothermally altered rocks and soils were recorded to display their features at their natural spectral bandwidths in the near‐infrared from 1.3 to 2.4 μm. Atmospheric transmission spectra were recorded over limited wavelength segments in the same region to demonstrate the availability of some of the diagnostic mineral bands that occur close to regions of intense absorption. Changes occur in the appearance of all these spectra caused by instrumental factors such as less than adequate spectral resolution and response time. The features in the mineral spectra are sufficiently characteristic to be used for analytical purposes, especially including those near 1.4 μm which are unavailable for remote‐sensing activities because of atmospheric obscuration. For remote‐sensing purposes, the features in the 2.2-μm region are emphasized as particularly valuable because they are common to alteration minerals and allow discrimination from nonalteration minerals which provide features only as close as 2.4 μm. Detection of unique features near 1.76 μm that occur only in alunite and gypsum is possible through the atmosphere and so provides diagnostic potential. The location and shape of mineral spectral features are retained unaltered in the spectra of rocks, and intensity with which they appear is governed by the accessibility of the particular mineral in the rock to the interacting radiation. For remote‐sensing purposes, it appears that at least two 0.1-μm wide filters in the 2.2-μm region would be necessary to unambiguously identify the presence or absence of alteration minerals, and that judicious selection of the exact location of filters could provide finer discrimination.
Attenuation of seismic waves in dry and saturated rocks: I. Laboratory measurements
Authors: Toksöz M. N. , Johnston D. H. , Timur A.
Published: October 2002

The attenuation of compressional (P) and shear (S) waves in dry, saturated, and frozen rocks is measured in the laboratory at ultrasonic frequencies. A pulse transmission technique and spectral ratios are used to determine attenuation coefficients and quality factor (Q) values relative to a reference sample with very low attenuation. In the frequency range of about 0.1–1.0 MHz, the attenuation coefficient is linearly proportional to frequency (constant Q) both for P‐ and S‐waves. In dry rocks, [Formula: see text] of compressional waves is slightly smaller than [Formula: see text] of shear waves. In brine and water‐saturated rocks, [Formula: see text] is larger than [Formula: see text]. Attenuation decreases substantially (Q values increase) with increasing differential pressure for both P‐ and S‐waves.
A general solution for a spherical conductor in a magnetic dipole field
Authors: Best Melvyn E. , Shammas Basil R.
Published: October 2002

In electromagnetic (EM) prospecting for volcanogenic massive sulfide ore deposits, a significant number of the responses are associated with compact conductors. As a first approximation, these bodies are studied using a conducting sphere model. An exact solution is given for a spherical conductor excited by a magnetic dipole field in free space for arbitrary transmitter‐receiver (T-R) configurations with receiver positions inside or outside the conductor. In this general approach, it is possible to investigate the lateral attenuation of EM systems. In particular, the effects of flight‐line displacement from the center of the spherical conductor on several airborne EM responses are presented. For example, at normal flying heights, the standard Dighem system has a lateral attenuation 50 times larger than the EM-30 system (for a sphere of 100 m radius). Field results from the Clearwater deposit in New Brunswick are compared to the spherical model attenuations for the Dighem, Otter, and F-500 systems. The behavior of the total magnetic fields [Formula: see text] and [Formula: see text] inside the conductor are presented in the form of magnitude and phase contours. The [Formula: see text] amplitude was found to be approximately the same inside and outside the sphere; the [Formula: see text] amplitude, however, differs significantly in these two regions. Observations such as these may provide some guidance in subdividing anomalous inhomogeneities in future numerical modeling.
Inversion of the geophysical inverse problem for n layers with nonuniqueness reduced to n cases
Authors: Thornton Barry S.
Published: October 2002

A new method is described for reducing nonuniqueness in the geophysical inverse problem for n layers to n cases, provided the ratio of characteristic impedances of sequential layers is of the order of unity and upper and lower layer boundary conditions are known. The method is based on impedance concepts which allow application to a number of different electromagnetic or acoustic experimental methods. Using transformations, the method reduces the nonlinear problem to a linear one as an eigenvalue formulation without approximation. This preserves the nonlinear nature of the problem and increases the significance of the results. Experimental results are given for several electromagnetic applications where the complex permittivity ε = ε′ − iε″ is determined for shallow (35 m) stratified geosections. The frequency range is dictated by multiples of the electrical or acoustic “quarter‐wave” thickness of the layers and, in the particular electromagnetic (EM) studies made, it has been 1 to 8 MHz. Application to ocean bottom sub‐layering is also proposed, arising from the feasibility of experiments under a conductive lake to which mathematical equivalence is shown.
Resolution studies in airborne resistivity surveying at VLF
Authors: Arcone Steven A.
Published: October 2002

Airborne resistivity surveys, which involve the wavetilt phenomena of radiowaves, are used as a reconnaissance exploration technique in both engineering and geologic investigations. Survey results are usually presented as resistivity flight‐line profiles or as contour maps from which the interpretation or site selection process must be initiated. Attempts to interpret the airborne results and correlate them with similar measurements made on the ground have indicated a need to understand better the relationship between ground and airborne resistivity measurements. To aid in this understanding, an analysis was performed to determine a very‐low‐frequency (VLF) airborne system’s response to modeled resistivity anomalies. The analysis considers a VLF plane wave to be incident at a near‐grazing angle upon a model flat earth. The earth is characterized by a uniform surface impedance, except for a circular area upon which the surface impedance assumes a different but uniform value. This change is assumed to result from some unspecified subsurface resistivity inhomogeneity. The change in the surface impedance is manifested by an amplitude change in the earth tangential electric field component. The perturbed two‐dimensional (2-D) plane‐wave fields are then computed at a standard survey altitude and are used to derive the airborne resistivity patterns above the change in surface impedance. The effects of survey altitude, anomaly size, and average ground resistivity upon the airborne values are analyzed. The results show that surface impedance changes representing resistive inhomogeneities are more prominent than those representing conductive inhomogeneities at survey altitudes. Curves are presented to separate the effects of resistivity contrast and radius of the anomalous surface area. The results also demonstrate the large attenuation observed at altitude for some ground resistivity anomalies.
Phase‐compensating filtering to reduce distortion caused by in‐aircraft reactive integrators
Authors: Jensen Oliver G. , Becker Alex
Published: October 2002

Low‐pass filters based on reactive integrators necessarily introduce temporal phase delays into data. The use of such filters in airborne geophysical surveys, therefore, produces equivalent spatial phase delays which displace the high‐wavenumber Fourier components of anomalies downstream along the flight line, causing distortion of the anomaly shapes. A linear filter is described which eliminates this distortion by introducing a compensating, frequency‐dependent phase advance. The restored data provide anomalies as would be seen from an aircraft moving with zero speed. Exact phase compensation for each stage of reactive low‐pass filtering requires an acausal filter whose coefficients are a weighted sum of zeroth‐ and first‐order modified Hankel functions, the weighting being determined by the integrator’s time constant. A very short and useful approximation to the ideal phase‐compensating filter is also described and subsequently applied in the restoration of data obtained from an airborne electromagnetic (EM) survey flown near Hawksbury, Ontario. The example clearly demonstrates the usefulness of applying phase compensation to geophysical data obtained in high‐resolution airborne surveys.
Integrated radon mapping in the earth—Assessment of the 220Rn signal and its exclusion
Authors: Fleischer R. L. , Mogro‐Campero A.
Published: October 2002

In many searches for uranium in the earth by mapping alpha decay activity from [Formula: see text], there is a background of [Formula: see text] activity due to thorium. Both sources of activity are measured, unless special measures are taken to exclude the alpha particles from [Formula: see text] decay. Laboratory experiments and field observations are used to demonstrate that the signal including [Formula: see text] activity is commonly about twice that from [Formula: see text] activity alone, but that the [Formula: see text] activity ratio can vary widely. Use of anti‐ [Formula: see text] membranes is, therefore, important in making meaningful surveys of [Formula: see text] activity. In some cases, anomalously high field readings arise from unusually high local radon emanation. These cases can be recognized by high [Formula: see text] activity measured in the laboratory using separate soil samples from each site used for a reading in the field.
Our Society
Authors: Domenico S. N. , Silverman D.
Published: October 2002

In commemoration of the 50th anniversary of the Society, the 1978–1979 SEG Executive Committee authorized preparation of this history of the Society accompanied by histories of geophysical methods. This endeavor appears especially appropriate at the end of the first half‐century of our Society—a time to chronicle developments and accomplishments, and to attempt predictions of the future.
Gravity method
Authors: LaFehr T. R.
Published: October 2002

Developments in the field of gravity exploration have been significant from Galileo to the recent adaptation of inertial navigation systems. The acquiescence of the torsion balance to the gravity meter came during one of the most exciting times in geophysical exploration, when discoveries were as frequent as once a week. Gravity meters have been made to operate remotely on the water bottom, on the sea surface, in the air, and in boreholes. The final accuracies are generally limited by errors in positioning data rather than the precision of gravity instruments. The achievement of microgal precision (a few parts per billion) is one of the most remarkable engineering developments. Interpretation has nearly kept pace with instrument development, but reports on methods and successes seriously lag (by at least 10 years) in the literature. The problem of ambiguity with respect to mass distributions has been clearly reported. This problem, however, is not adequately treated with respect to anomaly separation. Nonetheless, many excellent papers dealing with anomaly resolution are scattered throughout the literature, from ring operators to fast Fourier transforms. Much discussion also exists on forward and inverse calculations, but little attention has been given in the literature to practical interpretation, especially in the integration of gravity with seismic, magnetic, and borehole geophysical data.
Nomogram for Fresnel‐zone calculation
Authors: Sheriff R. E.
Published: October 2002

Geophysicists commonly recognize that a sizeable portion of a reflector is involved in causing a reflection as seen on a seismic trace, but the areal extent is usually not calculated and hence not appreciated. Most commonly, concepts are simply transferred from classical physical optics and called Fresnel‐zone effects. Confusion sometimes results due to the failure to distinguish whether incident waves are plane or spherical. An excellent explanation (but one which does not use the term “Fresnel zone”) is given by Hagedoorn (1954), from which Figure 1 is taken.
On: “Long‐wave elastic anisotropy in transversely isotropic media” by James G. Berryman (GEOPHYSICS, May 1979, p. 896–917).
Authors: Helbig K.
Published: October 2002

Berryman shows elegantly that the “inequality [Formula: see text] is true for any horizontally stratified, homogeneous material whose constituent layers are isotropic…” However, the final clause of this sentence “…, i.e., any homogeneous, transversely isotropic material,” is, if taken at face value, misleading. It is clear from the proof in the section “A fundamental inequality” that this statement is only shown to hold for lamellated media with isotropic lamellae, and that Berryman chooses arbitrarily and without any warning the phrase homogeneous, transversely isotropic to stand as a synonym for what Backus (1962) painstakingly describes as “smoothed, transversely isotropic, long‐wave equivalent (STILWE).” In view of the fact that even within the context of exploration seismics transverse isotropy can be due to causes other than horizontal stratification with isotropic constituents (e.g., schists can be intrinsically anisotropic, anisotropy might be due to preferential orientation of sandgrains or joints), I believe this choice to be unfortunate. It leads the unsuspecting reader to assume a wider applicability of the fundamental inequality than Berryman really intends to claim, and thus makes it unnecessarily difficult to understand this significant contribution.
Crosshole seismic testing—Procedures and pitfalls
Authors: Butler Dwain K. , Curro Joseph R.
Published: October 2002

Crosshole seismic testing is a valuable technique for determining seismic velocity profiles for critical structure siting investigations. Improper field procedures, however, can result in the acceptance of apparent velocities as true velocities, leading to unconservative design values. Common pitfalls in crosshole seismic testing are (1) the use of source‐receiver borehole spacings too large to enable the determination of true formation velocities, (2) the use of in‐hole station spacings too large to allow resolution of thin velocity layers of interest, (3) the use of incremental traveltimes between widely spaced receiver boreholes, and (4) the naive assumption that boreholes are vertical and hence parallel. A procedure is presented whereby field programs can be more rationally planned.
Recursive removal and estimation of minimum‐phase wavelet
Authors: Alam M. Aftab , Sicking Charles J.
Published: October 2002

The Gram‐Schmidt orthogonalization procedure is simplified under the assumption of stationarity and implemented to perform recursive predictive deconvolution. This process is called the orthonormal lattice filter. The results of deconvolution by this method are compared to the results of deconvolution using the maximum entropy and Levinson algorithms. The orthonormal lattice and maximum entropy algorithms are direct methods and estimate the filter from the data, while the Levinson algorithm is an indirect method and estimates the filter from the autocorrelation function. Results from synthetic and real data show that the direct methods have fewer windowing problems, higher resolving power, and are more suited for use in a time‐varying manner than the indirect method. Results from real data show that optimally weighted space averaging and zero‐phase band‐pass filtering after time‐varying direct deconvolution produces highly resolved and spatially coherent seismic sections.
Densities determined from surface and subsurface gravity measurements
Authors: Sissons B. A.
Published: October 2002

A least‐squares method for the direct inversion of surface and subsurface gravity measurements to obtain in situ density estimates is presented. The method is applied to a set of measurements made in a tunnel through the flank of an andesitic volcano. Densities obtained are [Formula: see text] for material in the top 100 m increasing to [Formula: see text] at about 200 m depth. The average density for rocks penetrated by the tunnel is, from laboratory measurements, [Formula: see text] i.e., about 4 percent higher. The difference is ascribed to joints and voids present in situ and not sampled in the laboratory specimens.
Depth of investigation of collinear electrode arrays over homogeneous anisotropic half‐space in direct current methods
Authors: Bhattacharya B. B. , Sen M. K.
Published: October 2002

The definition of depth of investigation as suggested by Evjen (1938) [subsequently used by Roy and Apparao (1971) also for the study of depth of investigation of electrode arrays in direct current methods for homogeneous isotropic earth] has been used to study the depth of investigation of various collinear electrode arrays for a homogeneous anisotropic half‐space. It has been shown that some simple transformations are to be applied to the expressions of normalized depth of investigation characteristic (NDIC) of the same arrays over homogeneous isotropic earth to obtain normalized depth of investigation characteristic of various arrays placed over homogeneous anisotropic earth. The novelty of anisotropy is that the depth of investigation of collinear electrode arrays over homogeneous anisotropic half‐space bears an inverse relationship with the coefficient of anisotropy and also depends upon array length and dip of the plane of stratification. The effect of the coefficient of anisotropy is most pronounced for horizontally stratified anisotropic earth and is independent of it for vertically stratified anisotropic earth—entirely consistent with the concept of the “;paradox of anisotropy.” The depth of investigation of all the collinear arrays for inclined stratification lies somewhere between the values obtained for horizontal and vertical stratifications.
The airborne electromagnetic discovery of the Detour zinc‐copper‐silver deposit, northwestern Québec
Authors: Reed L. E.
Published: October 2002

In June 1974, a diamond drill operated for Selco Mining Corp. intersected zinc‐copper sulfides in Brouillan Township in northwestern Québec. To date, two bodies have been outlined. These bodies were discovered during a ground follow‐up of a Mark VI Input® electromagnetic (EM) survey. The Input survey covered an area selected on the basis of regional geology and local outcrops of acid volcanic rocks. Conductors were identified that appeared to be associated with potentially favorable geology. They were selected for ground follow‐up. One was the discovery zone. The airborne responses over the zone were less encouraging than those often observed over highly conductive massive sulfides. The low apparent conductivity‐thickness (5 mhos) was suggestive of conductive overburden. However, the character of the profiles suggested a bedrock source. Ground geophysical confirmation identified a drill target. Subsequent to the discovery, more intensive geophysical surveys, both ground and airborne, were carried out. The best EM response suggested a confined source within a much larger mineralized halo. Weaker ground EM response from the halo correlated with the early channel response of the Input system. An airborne EM survey conducted in 1958 over the same area identified both conductive zones. However, they were not followed up. Only with later advances in exploration philosophy, geologic appreciation, and instrumentation were the conductive zones recognized as viable exploration targets.
Accuracy of the linear filter coefficients determined by the iteration of the least‐squares method
Authors: Murakami Yutaka , Uchida Toshihiro
Published: October 2002

The linear filter method is a powerful tool for the estimation of some convolution integrals which are encountered in many aspects of geophysical problems. The accuracy of the method is affected by two factors, apart from the sampling interval. Because of the slowly decaying oscillations in the filter tail, the tail must be truncated at some point. Because of the extensive numerical calculations required for the determination of the filter coefficients, the resultant filter cannot be free from round‐off errors. The Wiener filter analogy, pointed out by Koefoed and Dirks (1979), gives a straightforward and efficient answer to this problem. We show that the Wiener filter technique with the iterative application of the Levinson’s algorithm not only enhances the accuracy of the filter coefficients, but also greatly reduces the oscillations in the filter tail. Because of the very rapid decay of the filter tail, there is no need to truncate it.
Vp/Vs and lithology
Authors: Tatham Robert H.
Published: October 2002

Published laboratory investigations suggest an association exists between the ratio of seismic compressional and shear‐wave velocities [Formula: see text] and sedimentary rock lithology. Comparisons of some theoretical models with these laboratory studies suggest that crack, or pore, geometry has a stronger effect on observed [Formula: see text] values than elastic constants of the minerals comprising the matrix. Further, it can be inferred that the observed association between lithology and [Formula: see text] is a result of an association between lithology and distribution of pore and crack shapes. Direct observation of crack shapes for a variety of lithologies is a next step in strengthening these inferences. The present study reviews the empirical relations between [Formula: see text] and lithology and examines two published theoretical models of cracked elastic media. The models suggest that seismic velocities of sandstones may be controlled by cracks and pores with aspect ratios in the range of [Formula: see text] to 1, dolomite in the range of [Formula: see text] to [Formula: see text], and dense limestones, of generally low porosity, in the range of [Formula: see text] to [Formula: see text]. Direct observations of the aspect ratio of cracks and pores in sedimentary rocks would test these inferences and offer a basis for physical and geologic insight into lithologic interpretations of [Formula: see text] ratios.
Acoustic wave propagation in air‐bubble curtains in water—Part I: History and theory
Authors: Domenico S. N.
Published: October 2002

Air bubbles in water increase the compressibility several orders of magnitude above that in bubble‐free water, thereby greatly reducing the velocity and increasing attenuation of acoustic waves. The effect of air bubbles in water on acoustic wave propagation was studied extensively during World War II as part of an overall effort to apply underwater sound in submarine warfare. Currently, air bubble curtains are used to prevent damage of submerged structures (e.g., dams) by shock waves from submarine explosives. Also, air‐bubble curtains are used to reduce damage to water‐filled tanks in which metals are formed by explosives. Since World War II, research has progressed less feverishly in government and university laboratories. Published results of laboratory experiments generally confirm theoretical velocity and attenuation functions and demonstrate that these quantities are dependent principally upon frequency, bubble size, and fractional volume of air. Below the bubble resonant frequency and in the frequency range of marine energy sources, acoustic wave velocity is essentially independent of frequency and bubble radius, being well below the velocity in bubble‐free water. In this frequency range, attenuation increases with increasing frequency, decreasing bubble radius, and increasing fractional air volume.
A simple method for calculating apparent resistivity from electromagnetic sounding data
Authors: Wilt Michael , Stark Mitchel
Published: October 2002

Frequency‐domain electromagnetic (EM) sounding is becoming an increasingly useful deep‐exploration tool with recent applications to crustal sounding and geothermal exploration (Tripp et al, 1978; Sternberg, 1979; Duncan et al, 1980; Stark et al, 1980). A major drawback of the method is that most field data are analyzed by computer after the data are returned to the laboratory; no intermediate parameters, such as apparent resistivity, are calculated to provide on‐site information. The following example illustrates this problem.
Deconvolution of seismograms by the autoregressive method
Authors: Nair G. Jayachandran
Published: October 2002

The purpose of deconvolution in seismology is to estimate the basic seismic wavelet and the transfer function of the transmission medium. In reflection seismology, this transfer function refers to the reflectivity function, while in seismograms of earthquakes and explosions it represents the combined effects of the source crust and the receiver crust responses along with the attenuation function. Some of the techniques used for deconvolution of discrete time series data are Wiener inverse filtering (Robinson and Treitel, 1967), homomorphic deconvolution (Ulrych, 1971), and Kalman filtering (Crump, 1974). In the present paper, a method of deconvolution of single‐channel seismic data based on an autoregressive (AR) model of the time series is discussed. With it one can estimate the primary pulse and the deconvolution function simultaneously in an objective manner. Examples are provided to substantiate the applicability of the method using synthetic data simulating single and multiple explosions. The method is also applied to actual data for a presumed underground explosion from Eastern Kazakh.
Normal incidence layered system state‐space models which include absorption effects
Authors: Aminzadeh Fereydoun , Mendel Jerry M.
Published: October 2002

We show how to include absorption effects in a normal incidence state‐space model of a layered media system. Two models are discussed: a convolutional state‐space model and a differential‐delay state‐space model. Extensive results are presented for the former model, whereas only formulative results are presented for the latter. For the convolutional state‐space model we develop absorption impulse responses in closed form for Gaussian and exponential absorption operators. Simulation results demonstrate that this model has the desired low‐pass filtering effect expected of a system with absorption losses. The differential‐delay state‐space model does not appear to have been applied before in the context of seismic modeling. We suggest it as a useful model.
Automated analysis of marine refraction data: A computer algorithm
Authors: Ervin C. Patrick , McGinnis L. D. , Otis R. M. , Hall M. L.
Published: October 2002

A computer algorithm is described that automates the processing of refraction data acquired by conventional, multichannel, seismic reflection profiling, beginning with the determination of the first arrival times and ending with a contoured velocity‐depth section. Traces for several closely spaced shots are first summed to suppress noise. Each stacked trace is convolved with a box operator to convert the first break to a peak that can be detected by the computer. The algorithm calculates intercept times using running averages of slopes. Depths to refractors are plotted and a velocity‐depth profile, interpolated at constant velocity intervals, is printed as a contoured cross‐section. A profile crossing Georges Bank, northwest Atlantic Ocean, is interpreted by both automatic and manual techniques for comparison.
Composite color images of aerial gamma‐ray spectrometric data
Authors: Duval Joseph S.
Published: October 2002

Aerial gamma‐ray data provide estimates of the apparent surface concentrations of potassium (K), equivalent uranium (eU), and equivalent thorium (eTh). These data can be expressed as nine radiometric parameters: K, eU, eTh, eU/eTh, eU/K, eTh/K, eTh/eU, K/eU, and K/eTh. The U.S. Geological Survey (USGS) has developed a technique which combines any three of these parameters to form a composite color image. The color image provides a partial synthesis of the radiometric data that can be used to aid geologic mapping and mineral exploration. The sample data set, from the Freer area in south Texas, illustrates the use of the color images.
A model for the high‐frequency electrical response of wet rocks
Authors: Lysne Peter C.
Published: October 2002

In experiments on the electrical properties of rocks at high frequencies, the measured current has contributions from both conduction and displacement currents, and these are related to the bulk conductivity and dielectric permittivity, respectively. In the present model, the bulk electrical conductivity of a specimen is taken to be a constant given by Archie’s rule, whereas its frequency‐dependent permittivity is taken to be a generalization of Sillars’ model of a composite dielectric. The generalized Sillars’ model treats the pores as being an assemblage of spheroidally shaped inclusions with different orientations and aspect ratios. The conductivity of these spheroids, that is, the conductivity of the pore fluid, influences the frequency dependence of the permittivity in a manner that is in reasonable accord with available data. Furthermore, when applied to the dielectric data obtained in experiments on saturated rocks, the model yields distributions of pore shapes. These distributions are used to estimate the electrical response of oil‐ and water‐wet rocks that are unsaturated.
Recent developments in the use of surface electrical methods for oil and gas exploration in the Soviet Union
Authors: Spies Brian R.
Published: October 2002

A great deal of interest has been expressed in Western countries during recent years on possible applications of surface electrical methods in oil and gas exploration. It has been reported that these methods are widely used in the Soviet Union, but to date few technical details have been available. In late 1979, the author visited the Soviet Union under the auspices of the Australia‐USSR Agreement on Scientific Cooperation, with the purpose of studying recent developments in electrical and electromagnetic (EM) methods. This paper presents a summary of the use and applications of those methods in oil and gas exploration, and includes a number of case histories. The methods can be broadly classified as follows: sounding for structural mapping, sounding to measure the geoelectric properties of the oil‐bearing horizon, and indirect methods which detect the presence of a geochemical plume or halo above the oil deposit. The magnetotelluric (MT) method and transient sounding in the near zone (ZSBZ) are widely used for deep sounding, especially in areas where seismic methods are ineffective. Modern ZSBZ equipment can measure over [Formula: see text] decades of time, from 0.1 msec to 44.8 sec. The transmitter consists of a large (1000 m) square loop carrying a 40 A rectangular waveform, and the receiver is a multiturn coil located at the center. Depths of exploration depend upon geologic conditions, but they are typically of the order of 3 to 4 km. Statistical data, based on logging, of the geoelectric properties of a large number of oil fields have also been collected. There is generally an increase in resistivity in both the oil‐bearing horizon and overlying layers. The resistivity of near‐surface layers appears to be partially dependent upon climatic conditions. Experimental induced‐polarization (IP) surveys have been carried out over oil and gas deposits for about ten years. Both time‐domain and frequency‐domain systems are used, and sometimes an orthogonal electric array is employed to reduce inductive coupling. Either low frequencies (less than 1 Hz) or late times (greater than several seconds) are measured. IP anomalies are thought to be caused by epigenetic pyritization located in a plume over oil fields in a geochemically active environment. The anomalies are either of the form of a wide anomaly centered over the top of the deposit, or the shape of a halo, surrounding the deposit. There is evidence that the behavior of the IP decay at very late times (several minutes) can be used to distinguish IP anomalies inside and outside the boundaries of a hydrocarbon deposit.
Structural interpretation from horizontal seismic sections
Authors: Brown Alistair R.
Published: October 2002

The interpreter of a 3-D survey must utilize a data volume. Horizontal slices through a data volume, called Seiscrop™ sections, have unique properties and structural interpretation from them is fast, convenient, and effective. An event on a Seiscrop section displays local strike, a property which permits direct contouring of a structural surface without any timing and posting. The width of an event on a Seiscrop section is a composition of the frequency of the data and the structural dip. Event terminations indicate faults or other discontinuities when they are transverse to structural strike. Faults parallel to structural strike are much less evident on a single Seiscrop section but become apparent with the relative movement of events from section to section. In practical mapping, the interpreter normally contours one fault block before proceeding to the next with the correlation between them being established from the vertical sections. With dual polarity variable area displays, the interpreter can perceive five amplitude levels and normally picks the edge of a trough. With color amplitude Seiscrop sections, it is possible to pick on the crest of any event. With color phase sections, the interpreter can pick at any arbitrary but consistent point on the seismic waveform. The horizontal perspective often permits the interpreter to recognize subtle structural features which might have gone unnoticed if studying the prospect from vertical sections alone.
Radiation from a downhole air gun source
Authors: Lee Myung W. , Balch Alfred H. , Parrott Kenneth R.
Published: October 2002

This paper describes some characteristics of a downhole air gun as a source of seismic energy for a vertical seismic profiling (VSP) experiment at Salt Valley, Utah. The major emphasis is on primary radiations from the air gun source and secondary radiations from the bottom of the source hole. The observed primary radiation pattern agrees very well with theory developed for a volume displacement source acting on the axis of a fluid-filled hole. The observed secondary radiations, whose amplitudes were much stronger than the primary radiations, can be explained by calculating the wave field resulting from the application of an equivalent point force in an infinite medium, which was caused by the reflection of a tube wave at the bottom of the source hole. The experimental data also indicate that high‐amplitude compressional and shear wave multiples are generated between the bottom of the source hole and the air bubble.
Plane‐wave reflection coefficients for gas sands at nonnormal angles of incidence
Authors: Ostrander W. J.
Published: October 2002

The P-wave reflection coefficient at an interface separating two media is known to vary with angle of incidence. The manner in which it varies is strongly affected by the relative values of Poisson’s ratio in the two media. For moderate angles of incidence, the relative change in reflection coefficient is particularly significant when Poisson’s ratio differs greatly between the two media. Theory and laboratory measurements indicate that high‐porosity gas sands tend to exhibit abnormally low Poisson’s ratios. Embedding these low‐velocity gas sands into sediments having “normal” Poisson’s ratios should result in an increase in reflected P-wave energy with angle of incidence. This phenomenon has been observed on conventional seismic data recorded over known gas sands.
Signal/noise separation and velocity estimation
Authors: Harlan William S. , Claerbout Jon F. , Rocca Fabio
Published: October 2002

A signal/noise separation must recognize the lateral coherence of geologic events and their statistical predictability before extracting those components most useful for a particular process, such as velocity analysis. Events with recognizable coherence we call signal; the rest we term noise. Let us define “focusing” as increasing the statistical independence of samples with some invertible, linear transform L. By the central limit theorem, focused signal must become more non‐Gaussian; the same L must defocus noise and make it more Gaussian. A measure F defined from cross entropy measures non‐Gaussianity from local histograms of an array, and thereby measures focusing. Local histograms of the transformed data and of transformed, artificially incoherent data provide enough information to estimate the amplitude distributions of transformed signal and noise; errors only increase the estimate of noise. These distributions allow the recognition and extraction of samples containing the highest percentage of signal. Estimating signal and noise iteratively improves the extractions of each. After the removal of bed reflections and noise, F will determine the best migration velocity for the remaining diffractions. Slant stacks map lines to points, greatly concentrating continuous reflections. We extract samples containing the highest concentration of this signal, invert, and subtract from the data, leaving diffractions and noise. Next, we migrate with many velocities, extract focused events, and invert. Then we find the least‐squares sum of these events best resembling the diffractions in the original data. Migration of these diffractions maximizes F at the best velocity. We successfully extract diffractions and estimate velocities for a window of data containing a growth fault. A spatially variable least‐squares superposition allows spatially variable velocity estimates. Local slant stacks allow a laterally adaptable extraction of locally linear events. For a stacked section we successfully extract weak signal with highly variable coherency from behind strong Gaussian noise. Unlike normal moveout (NMO), wave‐equation migration of a few common midpoint (CMP) gathers can image the skewed hyperbolas of dipping reflectors correctly. Short local slant stacks along midpoint will extract reflections with different dips. A simple Stolt (1978) (f-k) type algorithm migrates these dipping events with appropriate dispersion relations. This migration may then be used to extract events containing velocity information over offset. Offset truncations become another removable form of noise. One may remove non‐Gaussian noise from shot gathers by first removing the most identifiable signal, then estimating the samples containing the highest percentage of noise. Those samples containing a significant percentage of signal may be zeroed; what remains represents the most identifiable noise and may be subtracted from the original data. With this procedure we successfully remove ground roll and other noise from a shot (field) gather.
Seismic parameters for transversely isotropic media
Authors: Byun Bok S.
Published: October 2002

One of the most important problems in exploration seismology is to relate the surface seismic measurements with the subsurface geologic parameters. The concept of wavefront curvature has been in extensive use for this purpose. Byun (1982) developed relationships between several measurable seismic parameters (e.g., geometrical spreading and normal moveout velocity) and parameters of the media with elliptical velocity dependencies. This paper extends the wavefront curvature concept to more general, transversely isotropic media. After a brief discussion on ray tracing, a procedure is developed to describe the local properties of the ray based on an elliptical surface fit to the actual wave surface. The apparent velocities of the elliptical fit are then used to generalize the seismic parameters developed in Byun (1982). Simple numerical experiments are given to demonstrate the explorational significance of the theory. It is shown that the measurements of the normal moveout velocity are not sufficient to estimate the velocity structure of the transversely isotropic medium. The “side‐slip” effect can lead to significant errors in depth‐mapping dipping reflectors.