bluuebunny/crossref_metadata_2025 huggingface dataset:
SEA‐GRAVIMETER TRIALS ON THE HALIFAX TEST RANGE ABOARD CSS HUDSON, 1972
Authors: Valliant H. D. , Halpenny J. , Beach R. , Cooper R. V.
Published: October 2002

A simultaneous test of a LaCoste and Romberg and a Graf‐Askania sea gravimeter was made over the Halifax Gravity Test Range aboard CSS Hudson in 1972. The test consisted of a total of 33 traverses over precisely located and calibrated test profiles established for this purpose. If errors are defined as the difference between surface and underwater values compared on a common datum, the mean LaCoste gravimeter error observed during a traverse varied from run‐to‐run to form a near normal distribution with mean of 1.8 mgal and standard deviation of 1.0 mgal. The corresponding statistics for the Askania are 2.1 mgal and 3.4 mgal, respectively, with the distribution markedly skew. The data were correlated with three components of accelerations as measured by the LaCoste and Romberg inertial platform. No significant correlation was evident for the LaCoste meter. Some correlation for the Graf‐Askania data, to which crosscoupling corrections are not normally applied, was observed.
COMPUTER‐ASSISTED INTERPRETATION OF ELECTROMAGNETIC SOUNDINGS OVER A PERMAFROST SECTION
Authors: Daniels Jeffrey J. , Keller George V. , Jacobson J. J.
Published: October 2002

In two‐loop electromagnetic sounding, the electromagnetic coupling between two vertical‐axis loops of wire is measured as a function of frequency, for frequencies ranging from 20 Hz to 20 kHz. If the electrical structure of the earth beneath the loops is horizontally stratified, these data may then be interpreted in terms of a sequence of layer resistivities and thicknesses. This interpretation is accomplished by computing a series of curves for various resistivity profiles and comparing them with the field data to determine which matches best. Calculation of the theoretical models is carried out by applying a linear filter to solve the appropriate integral expression. Interpretation is aided by using an interactive nonlinear least‐squares algorithm iteratively to adjust the model parameters. This procedure was used to interpret two‐loop induction soundings made along the Arctic Slope of Alaska during 1969 to determine permafrost thickness and character. The results indicate that two‐loop induction sounding is an effective method for mapping permafrost characteristics.
EFFECT OF BRINE‐GAS MIXTURE ON VELOCITY IN AN UNCONSOLIDATED SAND RESERVOIR
Authors: Domenico S. N.
Published: October 2002

Gas in an unconsolidated sand reservoir encased in shale often results in a dramatic increase in amplitude of the seismic reflection from the shale/gas‐sand interface. Unfortunately, reflection amplitude appears not to vary linearly with water (brine) saturation, and thus cannot be used to estimate gas quantity. Previously presented theoretical velocity computations, for a Tertiary sedimentary section, which demonstrate that compressional‐wave velocity in an unconsolidated gas sand varies nonlinearly with brine saturation, qualitatively agree with laboratory velocity measurements on a sand specimen composed of pure quartz grains. However, significant departure of measured and theoretical velocities at high brine saturation indicates that the technique for partially saturating the sand specimen by flowing a gas‐brine mixture through the specimen does not provide a sufficiently uniform distribution. The gas preferentially seeks larger pores. In a subsequent experiment on a specimen composed of spherical glass beads of nearly uniform size, the previous, as well as a modified, fluid injection technique was used. For the latter, brine only was injected into the pore space previously filled with a mixture of gas and brine in nearly equal proportions. This resulted in a more uniform distribution of the gas‐brine mixture. For approximately equal brine saturations, this modified technique resulted in a measured compressional‐wave velocity approximately one‐half of the velocity measured for the previously used fluid injection technique. This result implies that if the gas‐brine mixture is uniformly distributed in a reservoir, the fluid compressibility is the weighted‐by‐volume average of the constituent compressibilities.
RESIDUAL STATICS ANALYSIS AS A GENERAL LINEAR INVERSE PROBLEM
Authors: Wiggins Ralph A. , Larner Kenneth L. , Wisecup Robert D.
Published: October 2002

To estimate near‐surface time anomalies, it is commonly assumed that apparent seismic reflection times are comprised of the sum of “surface‐consistent” source and receiver static terms, “subsurface‐consistent” structure and residual normal moveout (RNMO) terms, and indeterminate noise. The model parameters (statics, RNMO, and structural terms) that, in a least‐squares sense, best satisfy traveltime observations in multifold seismic data are solutions to a set of linear simultaneous equations. Because these equations are ill conditioned and their solutions are known to be nonunique, conventional direct methods of solution are not applicable. Problems of this type which have both overdetermined and underconstrained aspects can be analyzed using the general linear inverse methodology. In this approach, observed time deviations are decomposed into linear combinations of orthogonal eigenvectors, each of which determines a related linear combination of model parameters. A property of this decomposition is that the uncertainty (standard deviation) in a model‐parameter eigenvector is functionally related to the uncertainty in its associated observation eigenvector. In particular, statics corrections having spatial wavelengths much shorter than a cable length have smaller uncertainties than do the observations themselves, whereas long‐wavelength corrections have much larger standard deviations and are thus poorly determined. In practice, iterative methods are commonly used to solve the large number of equations encountered for typical seismic profiles. Using the Gauss‐Seidel iterative formalism, we can know in advance how many iterations are required to obtain a given reduction of the original error for any wavelength contribution. Errors in shorter‐wavelength corrections converge rapidly to zero while a heavier price is exacted to compute longer‐wavelength corrections. However, because those longer‐wavelength corrections can be estimated only with large uncertainty, it is desirable to exclude them from the statics solution through judicious choice of the number of iteration cycles.
LONG WAVELENGTH STATIC ESTIMATION
Authors: Booker Aaron H. , Linville A. Frank , Wason Cameron B.
Published: October 2002

Estimation and removal of near‐surface effects in common‐depth‐point (CDP) data have been frequently discussed in the literature. A common problem with many automated statics techniques is their inability to extract statics whose spatial wavelengths are longer than a spread length. This, of course, can result in false structural anomalies. This paper describes an approach which extends the useful static estimation bandwidth to wavelengths of the order of 4 to 8 spread lengths. Traveltimes from one or more reflecting horizons are picked at each depth point and CDP offset. The time profiles are then decomposed into source static, receiver static, structure, and residual normal moveout (RNMO) estimates, and the process is iterated if required. A suite of analytical displays provides the user with direct QC measures of the traveltime picking performance. The technique will be demonstrated on model data to illustrate the theoretical performance over slowly changing near‐surface weathering anomalies. In addition, field examples will be shown from the Mackenzie Delta where permafrost variability in the near‐surface can create large traveltime anomalies.
MICROEARTHQUAKE LOCATION USING A MAXIMUM LIKELIHOOD PROCESSOR
Authors: Leslie H. D. , Porter R. P. , Spindel R. C.
Published: October 2002

Sonobuoy arrays located over the Mid‐Atlantic Ridge, during the French‐American Mid‐Ocean Undersea Study (FAMOUS), have been used to measure the spatial coherence of sound pressure radiated from sea quakes and provide refined estimates of epicenter locations. These measurements were made with array elements separated by two km and tracked to an accuracy of 30 m relative to a bottom‐moored transponder. Spatial coherence, the normalized cross‐spectral density, has been found to be as high as 0.7 at 13 Hz and to be typically about 0.5 between 16 and 60 Hz. Short‐range events are significantly more coherent than long‐range events whose onset is smeared in time. A maximum‐likelihood array processor has been used to obtain accurate estimates of horizontal epicenter locations. The high‐resolution processor yields about three times the resolving power of the conventional delay and sum processor. The resolving power obtained with these arrays is consistent with measured spatial coherence.
SEISMIC REFLECTION TECHNIQUES FOR CRUSTAL STRUCTURE STUDIES
Authors: Mair J. A. , Lyons J. A.
Published: October 2002

A wide‐angle reflection/refraction survey, conducted near Yellowknife, N.W.T., Canada has revealed a complex crust in contrast to the simple model indicated by a large‐scale refraction survey. A near‐vertical reflection survey, conducted in north‐central British Columbia, using dynamite detonated in a shallow lake, recorded coherent energy at two‐way times of up to 11 sec. A reflector at a depth of 30–35 km is thus indicated which corresponds to a refraction‐determined Moho depth for the area. CRP stacking attempts have been unsuccessful for both of these surveys due mainly to the complex overburden encountered but possibly due in part to the nature of the deep reflectors themselves. A reflector comprised of a series of thin layers of alternating high and low velocities will produce a composite reflected pulse, whose amplitude, frequency content, and apparent arrival time will change as the reflecting angle changes. Successful application of the CRP method would require restricting its use to a much narrower range of incident angles than we have employed. A Vibroseis survey, duplicating the coverage of the B.C. reflection profile, indicated the same reflector at 11 sec. The Vibroseis section remains superior despite exhaustive attempts at enhancement of the dynamite‐produced data.
RECONNAISSANCE GRAVITY SURVEY OF AUSTRALIA
Authors: Fraser A. R. , Moss F. J. , Turpie A.
Published: October 2002

The Australian Bureau of Mineral Resources, Geology and Geophysics, completed a 15 year systematic reconnaissance gravity survey of Australia in 1974. Using helicopters, gravity stations were established at 11 km spacing over most of the continent, and at 7 km spacing in South Australia and Tasmania. Station elevations were measured barometrically and station positions were marked on aerial photographs and transferred to 1:250,000 photocenter base maps. Gravity and elevation controls were maintained by ties to specially established control networks. A Bouguer anomaly map of Australia has been compiled from the reconnaissance data, and from information from other gravity surveys by state governments, petroleum exploration companies, and academic institutions which together cover about 15 percent of the total area. The compilation uses a total of about 170,000 gravity observations. The accuracy of Bouguer anomaly values, taking account of errors in the gravity, elevation, and position measurements, is estimated to be better than ± 2.0 mgal. The Bouguer anomaly fields over Australia are divided into nine regional gravity divisions, in each of which the gravity contour pattern has some degree of uniformity, or is such as to imply tectonic affinities between the sources of individual gravity features. The gravity divisions can be correlated with various metamorphic complexes and orogenic provinces, ranging in age from Archaean to Paleozoic, which also form the basements to extensive sedimentary platform covers. The reconnaissance gravity results have been made public as the survey has progressed to assist in regional geologic studies and the search for petroleum and minerals. They have proved to be of considerable benefit in delineating regional structures and in providing leads for more detailed geophysical investigations. Predictions of the structures of sedimentary basins have been made and possible extensions to mineral provinces have been indicated. The results have also been used in deep crustal and upper mantle studies and in geodesy.
APPLICATION OF THE GENERALIZED LINEAR INVERSE TO THE INVERSION OF STATIC POTENTIAL DATA
Authors: Cribb J.
Published: October 2002

The inversion of static potential geophysical data by means of the generalized linear inverse of a matrix is discussed. The method provides the unique distribution of anomalous permeability or density, as the case may be, which gives rise to the observed data and which has a minimum total sum of squares of variation. It is shown that the same results can be achieved by the application of Fourier transform theory which leads to a better understanding of the process and reduces the amount of computation. In the case of magnetic data due to a vertical earth’s field, it is shown that the method is the same as upward continuation.
INTERPRETATION OF GRAVITY ANOMALIES DUE TO FINITE INCLINED DIKES USING FOURIER TRANSFORMATION
Authors: Bhimasankaram V. L. S. , Nagendra R. , Rao S. V. Seshagiri
Published: October 2002

The Fourier transform of the gravity field due to a finite dipping dike is derived and its real and imaginary components are separated. Analysis of these two functions in a certain high‐frequency range yields simple relations that can be used to estimate the unknown parameters of the dike. The theoretical considerations are tested on synthetic data after performing the discrete Fourier transform (DFT), and the validity of the method of interpretation is established from a comparison of the actual and estimated parameters.
DISCRIMINATION OF HYDROTHERMALLY ALTERED AND UNALTERED ROCKS IN VISIBLE AND NEAR INFRARED MULTISPECTRAL IMAGES
Authors: Rowan Lawrence C. , Goetz Alexander F. H. , Ashley Roger P.
Published: October 2002

Mineralogical differences between altered rocks and most unaltered rocks in south‐central Nevada cause visible and near‐infrared (0.45 to 2.4 μm) spectral‐reflectance differences which can be used to discriminate them broad categories of rocks in multispectral images. The most important mineralogical differences are the increased abundance of goethite, hematite, and jarosite, and the presence of alunite, montmorillonite, and kaolinite in the altered rocks. Analysis of reflectance spectra recorded in the field showed that the altered rock spectra are characterized by broad absorption bands in the 0.45–0.50 μm and 0.85–0.95 μm regions which are due to electronic processes in the iron ions, and a band near 2.2 μm which is due to vibrational processes in the OH ions. These features are absent or weak in most of the unaltered rock spectra. Therefore, the shapes of the 0.45–2.4 μm spectra for these altered and unaltered rocks are conspicuously different. However, because of the wavelength positions and widths of the Landsat Multispectral Scanner (MSS) bands, these spectral differences are not apparent in individual or color‐infrared composite MSS images. The technique developed to enhance these subtle spectral differences combines ratioing of the MSS bands and contrast stretching. The stretched ratio values are used to produce black‐and‐white images which depict materials according to spectral reflectance; ratioing minimizes the influence of topography and overall albedo on the grouping of spectrally similar materials. Color compositing of two or more stretched ratio images to form color‐ratio composites provides additional enhancement. The most effective color‐ratio composite for discriminating between the altered and unaltered areas, as well as among many of the unaltered rocks in south‐central Nevada, was prepared using the following diazo color and stretched ratio image combinations: blue for MSS 4/5, yellow for MSS 5/6, and magenta for MSS 6/7. Altered areas appear green and brown in this combination. Field evaluation of this color‐ratio composite shows that excluding alluvial areas, approximately 80 percent of the green and brown color patterns are related to hydrothermal alteration. The remaining 20 percent consists mainly of pink hematitic crystallized tuff, a result of vapor‐phase crystallization, and of tan and red ferruginous shale and siltstone. Discrimination of this unaltered tuff from the altered rocks may be possible in the 2.2 μm region because this absorption band is absent in the tuff spectra. However, because the shale and siltstone are mineralogically and spectrally similar to the altered rocks, there appears to be little prospect of distinguishing these rocks from altered rocks in visible and near‐infrared multispectral images.
URANIUM EXPLORATION WITH COMPUTER‐PROCESSED LANDSAT DATA
Authors: Vincent Robert K.
Published: October 2002

A spectral signature has been constructed for an oxidized topsoil found in the vicinity of near‐surface uranium deposits in porous, arkosic sandstones of the Wind River Basin, Wyoming. A new type of contour map, created from Landsat computer compatible tapes, designed to connect regions of equal percentage of ground area covered by a specified target of interest, was applied to a Landsat frame covering the entire basin. The resulting maps showed relatively high percentages of ground cover by this particular topsoil in regions adjacent to open pit uranium mines in the Gas Hills region, as well as on or near known uranium prospects (as yet undisturbed) in the basin interior. A 10,000-pixel test area west of Lander, Wyoming was found to contain only one pixel (a 0.01 percent recognition rate) identified as the topsoil of interest. However, a whole‐frame recognition map produced a much higher (0.58 percent) recognition rate, indicating that the false alarm rate for this signature is still significantly high, although better than what can be expected from photointerpretation of single ratio images or color composite ratio images. This “signature” has been applied to Landsat frames in other geographical areas with known uranium mines in porous sandstone, and oxidized topsoil has been recognized near these mines.
ACOUSTIC WAVES IN OCEAN SEDIMENTS
Authors: Stoll Robert D.
Published: October 2002

An acoustic model for unconsolidated sediments is used to study velocity, attenuation, and reflection in ocean sediments. The model predicts attenuation and wave velocity on the basis of physical parameters such as porosity, grain size, permeability, and effective stress. Two mechanisms for energy loss are included in the model; one accounts for intergranular losses in the skeletal frame and the other for viscous losses in the porewater as it moves relative to the frame. As a result, in certain sediments such as sands and silts, attenuation is found to vary in a manner quite different from the usual dependency on the first power of frequency that is almost universally assumed. Furthermore, the amplitudes of reflected and refracted waves at boundaries between water and sediment or between sediment layers become frequency dependent. In the immediate vicinity of such boundaries, a significant amount of energy may be lost owing to the generation of a second kind of dilatational wave with extremely high attenuation. The model is able to handle variations in a variety of different physical parameters such as overburden stress, fluid compressibility, and stiffness of the sediment frame owing to lithification. For this reason it is well suited for use in predicting changes in velocity and attenuation with depth in real sediments where nonhomogeneous changing conditions are the rule and simple extrapolation of experimental data is not possible.
ADAPTIVE DECONVOLUTION: A NEW TECHNIQUE FOR PROCESSING TIME‐VARYING SEISMIC DATA
Authors: Griffiths L. J. , Smolka F. R. , Trembly L. D.
Published: October 2002

A time‐varying deconvolution method has been developed which is based upon adaptive linear filtering techniques. This adaptive deconvolution is applicable for use in processing reflection seismic data which contain multiples with periods that vary with traveltime. Filter coefficients are designed for each sample of the input trace using an adaptive algorithm. Convergence properties of the adaptive processor are discussed and compared to conventional deconvolution techniques. The adaptive deconvolution method is illustrated using both synthetic and field reflection seismic data. By proper selection of parameters and by procesing the data in both time‐reverse and time‐forward directions, adaptive deconvolution removes multiples with varying periods while leaving primary reflections relatively undistorted.
EVALUATION TECHNIQUE TO DETERMINE RELATIVE ACCURACIES OF CONTOUR MAPS
Authors: Kalkani E. C.
Published: October 2002

From the computer programs developed to contour geophysical data, the most appropriate is the one that reproduces the observation points with the minimum error. The evaluation method for a set of data points consists of determining the difference between the piecewise cubic spline approximation functions representing the original and contour map data along any given profile. This difference is evaluated along principal map directions in order to generate a measure of the contouring accuracy of these programs.
THE LAW OF THE SEA CONFERENCE: DRAFTING A CONSTITUTION FOR THE OCEANS OF THE WORLD
Authors: Pollock Howard W.
Published: October 2002

While a number of concepts date back to Roman Law, the origins of the modern law of the sea might very well be said to have occurred early in the 16th Century. Charles V, King of Spain and Emperor of the Holy Roman Empire, decreed that the seas were to be divided between the Portuguese and the Spanish. His first challenge came from his cousin, Francis I, King of France, who had organized and financed an expedition to explore the New World. Charles sent an Ambassador to Francis reminding him of the imperial decree forbidding all but the Portuguese and Spanish to navigate to the New World. Francis’ answer to the envoy was very straightforward: “Tell my good cousin Charles that if he will show me where in Adam’s will the sea was bequeathed to the Spanish and Portuguese, then I will obey.” Accordingly, Francis’ expedition, led by an Italian, Verrazano, sailed and discovered what is now New York harbor.
On: “Negative Induced‐Polarization Effects from Layered Media” by Misac N. Nabighian and Charles L. Elliot (GEOPHYSICS, December 1976, p. 1236–1255).
Authors: Ward S. H. , Pelton W. H.
Published: October 2002

Nabighian and Elliot (1976) have stated that “it is imperative to know the complete resistivity section in order either to plan an effective induced polarization survey program or to be able to interpret quantitatively the resulting IP data.”
DIFFRACTIONS FOR ARBITRARY SOURCE‐RECEIVER LOCATIONS
Authors: Trorey A. W.
Published: October 2002

The author’s earlier diffraction paper (Trorey, 1970) for coincident source and receiver locations is extended to the case of arbitrary source and receiver locations. The extension requires no additional assumptions and, as in the earlier paper, is in closed form suitable for calculations. The new results permit modeling of normal‐moveout effects and can thus, for example, be used to study migration before and after common‐depth‐point stacking as well as to study common‐depth‐point stacking itself. In general, the point source/receiver response of an arbitrary subsurface with arbitrary source/receiver locations can be calculated.
WEIGHTING FACTORS IN THE CONSTRUCTION AND RECONSTRUCTION OF ACOUSTICAL WAVE FIELDS
Authors: Kuhn Milos J. , Alhilali Khalid A.
Published: October 2002

The numerical solution of the Helmholtz equation is examined for a separated source and receiver over a model having a single planar interface. Expressions describing the construction and reconstruction of acoustical wave fields are derived in terms of Green’s functions. Their relation to the Fourier transform is briefly discussed. Three simple Green’s functions—free space, free surface, and rigid surface—are used to test the relative accuracy of the respective weighting factors by comparing the numerically calculated field for a simple model to a field obtained analytically by application of rigorous diffraction theory. The main purpose of this paper is to study the behavior of the total response (amplitude and phase) for models in which the aperture is not sufficiently sampled (e.g., close to half the wavelength). The degree of distortion in the response due to spatial undersampling is unacceptable for all three Green’s functions. A modified weighting factor relative to the free‐space Green’s function is introduced, which effectively reduces the degree of distortion in the total response under the same sampling condition. The importance of this finding to exploration geophysics in the construction of the synthetic seismograms by application of the Huygen’s principle and in seismic migration will be demonstrated.
BEAM FORMING WITH VIBRATOR ARRAYS
Authors: Arnold M. E.
Published: October 2002

Seismic field tests conducted near Tulsa, Oklahoma, and the Handy area in north‐central Texas used linear arrays of vibrators to concentrate sound waves into a beam, which is directed vertically when all vibrators operate in‐phase or at an angle from the vertical when relative time delays are introduced to each vibrator. The sound wave directivity was verified in the Tulsa area by recordings from subsurface seismometers, and at the same time reflection enhancement by wave beaming was exhibited from surface seismometers. Visual inspection and statistical analysis of reflection continuity indicated that it makes no difference in the Handy area whether beam‐forming techniques are used in the field or are applied later in processing. This result was anticipated, since previous seismic work in the Handy area indicated that the random noise level was low enough to minimize the theoretical advantage of field summing.
GRAVITY APPROXIMATION AND INTERPRETATION OF A TWO‐DIMENSIONAL VERTICAL FAULT
Authors: Saha Jay Gopal
Published: October 2002

The error in depth determination due to the thin plate approximation of a two‐dimensional vertical fault is investigated by the maximum slope method and the traditional [Formula: see text] method. We show here that although the gravity curves for thin and thick faults are not greatly different, faults with large throws have significant percentage errors in the depth to the central plane which warrant attention in precision interpretation. When the ratio of the throw of the fault to its mid‐depth varies from 1.5 to 1.9, the percentage error in depth increases from 10 to more than 16 percent. The maximum slope method for determination of depth gives larger percentage errors compared to [Formula: see text] measures. When the fault throw is large, a gradient method may be used to find the principal parameters of a two‐dimensional vertical fault. The method is theoretically free from any ambiguity and has other advantages which may facilitate precise interpretation. The thin plate approximation should be avoided in such cases.
THEORY OF THE BOUGUER ANOMALY
Authors: Ervin C. Patrick
Published: October 2002

Although the Bouguer anomaly is the form of the gravitational field most commonly utilized for the investigation of geologic problems on land, the true significance of the combined free‐air and Bouguer correction is frequently misunderstood by geologists, geophysical students, and many senior geophysicists with specializations in areas other than potential fields. This confusion arises from the established practice of describing the Bouguer anomaly as having been reduced to a common datum, suggesting that the reduced gravity values are those that would be observed if the measurements could be made on the datum plane (e.g., Dobrin, 1976, p. 417).
MULTIPLE SCATTERING BETWEEN A BURIED LINE CONDUCTOR AND THE EARTH’s SURFACE
Authors: Wait James R.
Published: October 2002

By way of a simple example we show how the currents excited or induced in a subsurface conductor can be deduced directly. The results agree with a previous approach that accounted iteratively for the interaction of the induced currents and the air‐earth interface. The method is applicable to thin cylindrical conductors.
RESISTIVITY MAPPING WITH AN AIRBORNE MULTICOIL ELECTROMAGNETIC SYSTEM
Authors: Fraser Douglas C.
Published: October 2002

Dighem is a helicopter‐borne 900 Hz multicoil electromagnetic survey system. The EM device consists of a 30-ft towed bird containing a transmitter coil in the front and three mutually orthogonal receiver coils in the rear. Resistivity contour maps can be prepared from the EM data using any of several half‐space models. In this paper, two such models are selected and field examples of apparent resistivity derived from them are shown. The multicoil system has encountered areas of widespread conductivity while surveying for metallic minerals. In such areas, EM anomalies can be generated by changes of less than 10 m in survey altitude. EM anomalies of apparent significance, therefore, can reflect decreases in survey altitude as well as increases in conductivity of the earth. Under such conditions, apparent resistivity contour maps can aid the interpretation of the airborne data. The advantage of the contour maps is that anomalies caused by altitude changes are substantially reduced, and the contours reflect mainly the conductive anomalies. Resistivity contour maps improve the interpreter's ability to differentiate between conductive trends in the bedrock and those patterns typical of conductive overburden. Airborne resistivity mapping can be applied to a number of engineering problems. The multicoil system has been used for permafrost delineation and gravel detection. To be useful, the geologic units being mapped should have a resistivity less than 1000 Ω-m (for the 900 Hz frequency) and a surface extent of several acres.
THE REFLECTION, REFRACTION, AND DIFFRACTION OF WAVES IN MEDIA WITH AN ELLIPTICAL VELOCITY DEPENDENCE
Authors: Levin Franklyn K.
Published: October 2002

Assuming media having a velocity dependence on angle which is an ellipse, we have confirmed previously reported time‐distance relations for reflections from single interfaces, for reflections from sections of beds separated by horizontal interfaces, for refraction arrivals, and added the expression for diffractions. We also have derived expressions for plane‐wave reflection and transmission coefficients at an interface separating two transversely isotropic media. None of the properties differs greatly from those for isotropic media. However, velocities found from seismic surface reflections or refractions are horizontal components. There seems to be no way of obtaining vertical components of velocity from surface measurements alone and hence no way to compute depths from surface data.
THE INTERPRETATION OF DIRECT CURRENT RESISTIVITY MEASUREMENTS
Authors: Oldenburg D. W.
Published: October 2002

The linearized inverse theory of Backus and Gilbert has been used to invert potential difference measurements obtained from direct current resistivity soundings. The resistivity is assumed to be a continuous function of depth, hence many of the difficulties encountered when assuming that the earth is a layered half‐space are avoided. An iterative technique is used to construct a resistivity model whose calculated responses agree with the observations, and the model is then appraised to find those features which are uniquely determined by the surface observations. Also, the existence of the Fréchet kernels allows direct comparisons of the resolution provided by various electrode geometries and thus the design of electrode arrays to enhance resolution becomes more feasible.
SPECTRAL ANALYSIS OF TOTAL MAGNETIC ANOMALIES OF STEP MODEL
Authors: Ramanaiah Chowdary M. V.
Published: October 2002

A great deal of interest has been shown in the frequency analysis of gravity and magnetic data originally suggested by Dean (1958). The application of this method for potential field problems has met with considerable success. The purpose of this note is to show that the interpretation of total magnetic anomalies due to a sloping step model, which represents a contact between zones having different magnetic properties in terms of model parameters, is less complicated in the frequency domain than in the spatial domain.
ELECTROMAGNETIC FIELDS ABOUT A HORIZONTAL ELECTRIC WIRE SOURCE OF ARBITRARY LENGTH
Authors: Kauahikaua James
Published: October 2002

The electromagnetic fields about an electric wire source of an arbitrary finite length are usually calculated by numerical integration of the dipole field equations along the length of the wire (Scriba, 1974; Anderson, 1974). However, many of the required integrals for the fields over a multilayered half‐space can be solved analytically. The integrations may also be extended to solve for the fields about an infinitely long wire source. In so doing, it becomes clear which portions of each field component are produced by the wire and which are produced by its grounded ends. Although the horizontal electric fields of a finite wire source have been derived previously (Sunde, 1949), the complete field equations for a finite‐length wire appear here for the first time in explicit form.
A MULTIRATE DIGITAL FILTERING APPROACH TO INTERPOLATION: APPLICATION TO COMMON‐DEPTH‐POINT STACKING
Authors: Lu Chyi H. , Gupta Someshwar C.
Published: October 2002

An efficient FFT interpolation scheme, using the concept of multirate digital filtering, is presented in this paper. The method relies on a block representation and employs a parallel implementation. Timing comparisons show that this method is faster than the conventional FFT technique demonstrated by Gold and Rader. In addition, it permits a shorter word length, smaller memory size, and is well suited to seismic data processing. Application of this method to common‐depth‐point trace summation (“stacking”) is illustrated.
INTERPRETATION OF BURIED ELECTRODE RESISTIVITY DATA USING A LAYERED EARTH MODEL
Authors: Daniels Jeffrey J.
Published: October 2002

The layered earth model is a fundamental interpretation aid for direct current resistivity data. This paper presents a solution for the layered earth problem for a buried current source and a buried receiver. The model is developed for source and receiver electrodes buried anywhere within a horizontally stratified layered earth containing an arbitrary number of resistivity layers. Model results for the normal well‐logging array indicate that large departures between true and apparent resistivity can be caused by thin beds or highly resistant layers. A true resistivity distribution from well logs can be established by modeling when the effects from borehole rugosity and fluid resistivity are negligible. The equations derived for resistivity well logs can be used to interpret hole‐to‐hole, hole‐to‐surface, and conventional surface array data. A field example demonstrates that deviations between hole‐to‐hole field data and model results, based on well logs in the receiver hole, can be accounted for by combining the resistivity logging models in the receiver holes with information from geologic logs. Differences between the field data and the layered‐model results are attributed to lateral changes between or near the source and receiver holes.
Synthetic seismic sections of typical petroleum traps
Authors: May Bruce T. , Hron Franta
Published: October 2002

Interpretation of seismic reflections remains a key problem in seismic exploration because reflections have complex behavior, especially near geologic structures. One method to gain an understanding of this complex behavior is to study synthetic seismic sections of models of typical petroleum traps as computed by zero‐offset ray tracing for primary P‐waves only. These synthetic sections have features of significant interpretative value to the practicing geophysicist, such as variations in reflection amplitudes and complexities in reflection‐time geometries. Asymptotic ray theory was applied to calculate reflection amplitudes, accounting for mode conversion and three‐dimensional geometric divergence of ray tubes in the presence of curvilinear interfaces. This suite of synthetic seismic sections illustrates the difficulties in making correct seismic interpretations of geologic structures and suggests three conclusions: (1) The customary assumption that seismic sections are simple images of geologic cross‐sections is an oversimplification and can lead to erroneous interpretations. (2) Variations in overlying strata produce marked disturbances in reflection amplitude and traveltime. (3) Critical reflections that provide key structural information are often difficult to recognize and are apt to be ignored, or misidentified, especially if trace processing has been cursory. A fundamental principle of interpretation is underscored by this study: CDP stacked seismic sections must be interpreted so as to be consistent with structural depth models because stacked sections are not simple images of geologic structures.
Reflection and transmission coefficients for seismic waves in ellipsoidally anisotropic media
Authors: Daley P. F. , Hron F.
Published: October 2002

The deficiency of an isotropic model of the earth in the explanation of observed traveltime phenomena has led to the mathematical investigation of elastic wave propagation in anisotropic media. A type of anisotropy dealt with in the literature (Potsma, 1955; Cerveny and Psencik, 1972; and Vlaar, 1968) is uniaxial anisotropy or transverse isotropy. A special case of transverse isotropy which assumes the wavefronts to be ellipsoids of revolution has been used by Cholet and Richard (1954) and Richards (1960) in accounting for the observed traveltimes at Berraine in the Sahara and in the foothills of Western Canada. The kinematics of this problem have been treated in a number of papers, the most notable being Gassmann (1964). However, to appreciate fully the effect of anisotropy, the dynamics of the problem must be explored. Assuming a model of the earth consisting of plane transversely isotropic layers with the axes of anisotropy perpendicular to the interfaces, a prime requisite for obtaining amplitude distance curves or synthetic seismograms is the calculation of reflection and transmission coefficients at the interfaces. In this paper the special case of ellipsoidal anisotropy will be considered. That the quasi‐shear SV wavefront is forced to be spherical by this assumption is unfortunate, but it is instructive to investigate this simple anisotropic model as it incorporates many features inherent to wave propagation in a more general anisotropic medium. A brief outline of the theory for wave propagation in an ellipsoidally anisotropic medium is given and the analytic expressions for the reflection and transmission coefficients are listed. A complete derivation of reflection and transmission coefficients in transversely isotropic media can be found in Daley and Hron (1977). Finally, all 24 possible reflection and transmission coefficients and surface conversion coefficients are displayed for varying degrees of anisotropy.
Detectability of Intermediate conductive and resistive layers by time‐domain electromagnetic sounding
Authors: Verma Rajni K. , Mallick Kumarendra
Published: October 2002

An analysis in the time domain has been made of the detectability of an intermediate layer in a three‐layer earth model by the horizontal coplanar loops (system I) and loop‐wire element (system V) electromagnetic (EM) sounding systems for a train of half‐sinusoidal and square waveforms of alternating polarity. The studies involve conversion into the time domain by a Fourier series summation of the matched complex mutual coupling ratios, computed by the digital linear filter method, of the layered‐earth models. The three‐layer earth models considered here have the following resistivity distribution: [Formula: see text], [Formula: see text] for the conductive case, and [Formula: see text], [Formula: see text] for the resistive case (subscripts 1, 2, and 3 represent the first, second, and third layer in the three‐layer sequence; ρ is the resistivity). The intermediate‐layer thickness varies over a wide range. The responses of the three‐layer earth models have been compared with that of a homogeneous earth with the resistivity of the top layer in the three‐layer sequence. The measurement error is assumed to be of the order of 3 percent, and an rms difference of 10 percent between the responses for the three‐layer and the homogenous earth is defined as the detectability level. On the basis of this definition, it is observed that the horizontal coplanar loops system (system I) is better than the loop‐wire element system (system V) in detecting the thin intermediate layer, which may be either conductive or resistive. For a transmitter‐receiver separation (R) of 1000 m by square‐pulse excitation, a conductive intermediate layer as thin as 1/14 of the top layer can be detected by system I and as thin as 1/6 by system V. For the resistive intermediate layer, the corresponding thickness ratios are 0.6 for system I and 1.25 for system V. The detectability is lower in the case of half‐sinusoidal pulse excitation. Instead of normalizing the mutual coupling of the layered earth to the free‐space coupling, the detectability is enhanced markedly if the normalization is done to the coupling over a homogeneous ground. For system V, it is observed that an intermediate layer as thin as 1/100 in the conductive case and 1/4 in the resistive case of the top layer can be detected easily by this approach. Some direct comparisons between the time‐domain and the frequency‐domain results also are given.
Relationship of the self‐potential Green’s function to solutions of controlled source direct‐current potential problems
Authors: Fitterman David V.
Published: October 2002

This note presents a simple relationship between the self‐potential (SP) Green’s function and the solution of the controlled‐source direct‐current (dc) potential problem which allows a simplified means of determining the SP Green’s function. An example of its application to the vertical contact problem will be presented. The case of a streaming potential source mechanism will be considered, although any of the SP source mechanisms described by Nourbehecht (1963) could be substituted.
Statistics of semblance
Authors: Douze E. J. , Laster S. J.
Published: October 2002

Stacking velocities are often used in the processing of seismic data by applying normal moveout (NMO) corrections for a fixed velocity or velocity function (“stacking” velocity), then computing semblance for the set of corrected traces. A large value of semblance is taken to imply the presence of a signal associated with the corresponding stacking velocity. The question arises as to what is a “large” value of semblance. The theory developed here allows one to attach quantitative levels of statistical significance to the value of semblance.
On: “A summary of the geology, geochemistry, and geophysics of the Roosevelt Hot Springs thermal area, Utah,” by S. H. Ward, W. T. Parry, W. P. Nash, W. R. Sill, K. L. Cook, R. B. Smith, D. S. Chapman, F. H. Brown, J. A. Whelan, and J. R. Bowman (GEOPHYSICS, December 1978, p. 1515–1542).
Authors: Kellogg William C.
Published: October 2002

I heard the initial presentation of the subject paper during the SEG Annual Meeting at Calgary in 1977 and discussed some of my following remarks with the principal author. With its appearance in Geophysics, I believe it worthwhile to call attention to certain aspects of this paper which may not be recognized by some readers. I continue to be disturbed at the tendency of the recent computer‐oriented generation of geophysicists to concentrate on their remarkable accomplishments in computation using the digital computer, while at the same time displaying a tendency to ignore accurate portrayal of the results of such work. It seems that the means now justify the end regardless of the quality of the end. No question that the means are remarkable in the power and speed which they bring to handling otherwise intractable geophysical calculations. Nevertheless, the results of such work, in many cases in the past few years, leave much to be desired in terms of faithful reproduction of data related to natural phenomena. Furthermore, the willingness of otherwise very competent authors and geophysicists to ignore the deficiencies of the published results and maps continues to puzzle me.
Calculations of self‐potential anomalies near vertical contacts
Authors: Fitterman David V.
Published: October 2002

The self‐potential anomalies due to streaming potential effects in the vicinity of a vertical contact are analyzed. This approach is different from most previous studies in that the source is tied to a specific physical mechanism instead of arbitrarily selected charge distributions or current sources. The analysis is valid for any source mechanism that can be thought of in terms of crosscoupled flows, e.g., the thermoelectric effect or chemical potential gradients. The anomalies tend to be antisymmetric across the contact with the magnitude of the anomaly being larger on the more resistive side of the contact. An analytic expression for the case of a constant intensity, rectangular source is derived from the general solution. The anomalies for this simple case are computable with a handheld calculator and can be used to estimate the location, extent, and magnitude of the anomaly source region. With this information it is possible to determine the most probable crosscoupling source mechanism.
Compressional velocities from multichannel refraction arrivals on Georges Bank—northwest Atlantic Ocean
Authors: McGinnis L. D. , Otis R. M.
Published: October 2002

Velocities were obtained from unreversed, refracted arrivals on analog records from a 48‐channel, 3.6-km hydrophone cable (3.89 km from the airgun array to the last hydrophone array). Approximately 200 records were analyzed along 1500 km of ship track on Georges Bank, northwest Atlantic Ocean, to obtain regional sediment velocity distribution to a depth of 1.4 km below sea level. This technique provides nearly continuous coverage of refraction velocities and vertical velocity gradients. Because of the length of the hydrophone cable and the vertical velocity gradients, the technique is applicable only to the Continental Shelf and the shallower parts of the Continental Slope in water depths less than 300 m. Sediment diagenesis, the influence of overburden pressure on compaction, lithology, density, and porosity are inferred from these data. Velocities of the sediment near the water‐sediment interface range from less than 1500 m/sec on the north edge of Georges Bank to 1830 m/sec for glacial deposits in the northcentral part of the bank. Velocity gradients in the upper 400 m range from [Formula: see text] on the south edge of the bank to [Formula: see text] on the north. Minimum gradients of [Formula: see text] were observed south of Nantucket Island. Velocities and velocity gradients are explained in relation to physical properties of the Cretaceous, Tertiary, and Pleistocene sediments. Isovelocity contours at 100-m/sec intervals are nearly horizontal in the upper 400 m. Isovelocity contours at greater depths show a greater difference from a mean depth because of the greater structural and lithological variation. Bottom densities inferred from the velocities range from 1.7 to [Formula: see text] and porosities range from 48 to 62 percent. The most significant factor controlling velocity distribution on Georges Bank is overburden pressure and resulting compaction. From the velocity data we conclude that Georges Bank has been partially overridden by a continental ice sheet.