bluuebunny/crossref_metadata_2025 huggingface dataset:
RADIATION OF RAYLEIGH WAVES FROM A VERTICAL DIRECTIONAL CHARGE
Authors: Crosson Robert S. , Chao Chi‐Chang
Published: October 2002

Continuous linear directional charges have been used in exploration seismology in attempts to improve signal‐to‐noise ratio. The directivity of body‐wave energy from directional charges has been demonstrated to reduce appreciably ghost‐type secondary reflections. It is of interest to examine whether the directional charge has advantages over conventional sources in reducing surface‐wave amplitude. In this paper we investigate a theoretical model of a vertical directional charge, focusing attention on the surface‐wave response. We obtain the surface‐wave displacement for a directional charge in closed form, suitable for rapid numerical evaluation. The directional‐charge solution is compared to an appropriately scaled point‐source solution, revealing that the directional charge does not reduce the amplitude of surface waves relative to the point source. Numerical examples of the directional charge displacement reponse also reveal a surprising insensivity of amplitude to reversal of detonation direction. This is true even relatively near the source (i.e., about eight times the depth of source center), although in this region the pulse shape is altered by direction reversal. For constant total source strength the amplitude is an increasing function of detonation velocity. Production of primary conical wavefronts by the source, due to detonation velocities greater than the compressional velocity in the medium, apparently does not modify the surface wave response.
FOREWORD AND INTRODUCTION
Authors: Ward Stanley H.
Published: October 2002

The motivation for preparing a special issue of Geophysics devoted to electromagnetic scattering lies largely with a sudden and dramatic improvement in our ability to model the earth in theoretical analyses of the electromagnetic exploration problem. This improvement was in turn generated by a need for better interpretation of electromagnetic data, a need for better design of electromagnetic systems, and a need to develop better criteria for selection of electromagnetic systems suited to a given prospecting problem.
THEORETICAL MAGNETOTELLURIC AND TURAM RESPONSE FROM TWO‐DIMENSIONAL INHOMOGENEITIES
Authors: Swift Charles M.
Published: October 2002

The “network solution” technique for obtaining numerical solutions to Maxwell’s equations in two‐dimensional inhomogeneous media, originally developed by Madden, is presented in detail in this paper. By using plane‐wave and infinite line‐current sources respectively, theoretical magnetotelluric and Turam responses can be calculated above and within an arbitrarily complex two‐dimensional earth conductivity structure. Analysis of a few representative model results emphasizes the importance of considering all the electromagnetic field components, particularly those of the anomalous field. In magnetotellurics, the vertical magnetic field is useful in determining strike direction, and only the E parallel apparent resistivities are representative of deep structure in the presence of near‐surface conductivity inhomogeneities. In Turam, a finite resistivity background causes buried conductors to appear deeper and more conductive than they would appear if the background had infinite resistivity; and strong conductors can affect the total field sufficiently to cause screening.
A SEAMOUNT WITH A NONMAGNETIC TOP
Authors: Harrison C. G. A.
Published: October 2002

The bathymetry and magnetic anomaly of a seamount in the Gulf of Guinea have been surveyed. The seamount is regular in shape and rises about 2000 m to a depth of 2650 m at the peak. The magnetic anomaly is well developed and consists of a low to the north and a high to the south of the peak, the field changing by 700 gammas between these points. In an attempt to duplicate the observed anomaly by model studies, it was found that despite the regularity of the observed anomaly, only a poor fit could be obtained when the seamount was modeled with a body equivalent to the topographic shape. The best fitting calculated anomaly was of shorter wavelength than the observed anomaly, suggesting that the average depth to the magnetic material is deeper than the topography indicates. Further model studies have shown that the top portion of the seamount is relatively much less magnetic than the rest and that there is magnetic material below the level of the surrounding ocean floor. The nonmagnetic top of the seamount is explained by the hypothesis of Bonatti and Nayudu, calling for intense alteration of basalt to palagonite and smectites, which have a very low magnetization. This alteration would be more intense at the top of a seamount and would thus cause a relatively nonmagnetic top. Other published seamount surveys contain many examples where the same thing appears to occur. The changes in direction of magnetization produced by the various models for the seamounts are very small; hence, the original paleomagnetic pole positions should not be in error by more than a few degrees. The paleomagnetic significance of results from seamounts is not affected.
DATA ADAPTIVE SPECTRAL ANALYSIS METHODS
Authors: Lacoss R. T.
Published: October 2002

Two new methods (Maximum Likelihood Method or MLM, and Maximum Entropy Method or MEM) for power spectral density estimation have been experimentally investigated. Both methods, unlike conventional methods, adapt to the actual characteristics of the noise process under study. The new techniques are particularly valuable if the process contains one or more narrow peaks in frequency which are to be resolved. In this case, the output peaks from MEM are proportional to the square of the power in the narrow peaks but the area is equal to power. The peak values of the MLM reflect power directly. Both methods yield the true spectrum if the spectrum changes slowly enough with frequency. Neither of the new methods appears to be unduly sensitive to small statistical fluctuations of the estimated correlation function used to obtain spectral estimates.
NUMERICAL SIMULATION OF SEISMIC SOURCES
Authors: Aboudi Jacob
Published: October 2002

A numerical treatment of body forces which are equivalent to seismic sources in elastic media is given. This treatment allows the formulation of sources of different orders and with different spatial and temporal dependence. It includes internal sources having different extensions in different directions in space; the internal sources can be located arbitrarily close to a free surface or an interface. The method also handles distributed surface forces acting on a finite area. Complete solutions, calculated for impulsive sources of first, second, and third order, buried in an elastic half‐space and extending over a finite region, and for a distributed surface loading, are illustrated.
LINEAR FILTERS TO SUPPRESS TERRAIN EFFECTS ON GEOPHYSICAL MAPS
Authors: Clarke Garry K. C.
Published: October 2002

Frequently, on maps of magnetic or electromagnetic data, strong interference due to surface relief can obscure anomalies which are unrelated to terrain. By the simultaneous use of topographic and geophysical maps, mining geophysicists attempt intuitively to remove terrain effects. In previous work (Richards et al, 1967), topographic and magnetic maps were used together to obtain direction and polarization of marine magnetic anomalies. In this paper, a simple linear filter to reduce terrain “noise” will be developed. The filter has two inputs: the geophysical and topographic maps. These are combined to form the output geophysical map.
ISOCHRONS FOR A THREE‐DIMENSIONAL SEISMIC SYSTEM
Authors: Dunkin J. W. , Levin F. K.
Published: October 2002

The three‐dimensional seismic system discussed by Walton (1970–1971) explores seismically a column of earth extending from the surface to deep reflectors. Interpretation of data from the system is done directly from displays on a grid of the detected signals. In this paper we derive expressions for the patterns expected on three‐dimensional system displays when energy returning to the detectors consists of reflections from planes, diffractions from faults or point scatterers, reflections from reflectors of arbitrary shape, refractions and reflected‐refractions from planes, or direct or surface waves. Examples of typical patterns are shown. Our derivations are limited to ray geometry and shed no light on amplitude variations. Also, an average velocity from the surface to a reflector, refractor, or diffractor is assumed.
A GEOPHYSICAL CASE HISTORY OF THE LAKESHORE ORE BODY
Authors: Hallof Philip G. , Winniski Emil
Published: October 2002

The Lakeshore ore body is in Pinal County, Arizona about 30 miles south of Casa Grande. In February, 1969 when the latest figures were published, the ore reserves were reported at 241 million tons of disseminated sulfide ore (0.7 percent copper) and 24 million tons of concentrated metallic ore (1.69 percent copper). Sulfide copper ore was first intersected in July, 1967 in Hole P‐3. The magnetite‐pyrite‐chalco‐pyrite mineralization occurred in a banded tactite at a depth of 1147 ft. Hole P‐3 was the fourth of several holes that were drilled to determine the source of an induced polarization anomaly that had been outlined, at depth, to the west of the old Lakeshore pit. The successful conclusion of this exploration program by El Paso Natural Gas Company is an excellent example of an integrated exploration approach. The application of regional geological planning, geophysical methods, and detailed geological reasoning resulted in the discovery of a major copper ore body. Due to the depth of the ore zone and the disseminated character of most of the ore, the only geophysical technique that was useful in the direct detection of the ore mineralization was the induced polarization method. Field measurements were made sporadically between August, 1966 and July, 1968. Variable‐frequency induced‐polarization measurements, made using the dipole‐dipole electrode configuration and electrode intervals from 300 ft to 1000 ft, successfully indicated the presence of the metallic mineralization at depth and gave some indication of its extent. Comparisons of the induced polarization data and the appropriate geological sections give information concerning the usefulness of the method.
GASBUGGY SEISMIC SOURCE MEASUREMENTS
Authors: Perret William R.
Published: October 2002

Gasbuggy, a 29‐kt nuclear experiment, was detonated December 10, 1967, at a depth of 4240 ft in the San Juan Basin in New Mexico. Its purpose was to develop techniques for stimulation of natural gas production from tight gas‐bearing formations. Data from four subsurface instrument stations in a boring 1500 ft from Gasbuggy indicated formation of a spherical cavity of 88 ft radius and a microfracture radius of about 480 ft. The mean peak value of reduced displacement potentials which defined the seismic source was [Formula: see text]. Calculations indicate that about 2 percent of the energy released remained in the seismic source at 1500 ft. Data from surface motion instruments distributed between surface zero and 8400 ft were similar to those observed above any other contained nuclear explosions. Spalling was indicated at all surface stations. Transient vertical displacements indicate a mound about 6.7 inches high near surface zero and extending through the most remote station where uplift was 0.3 inch.
SOME CHARACTERISTICS OF MARINE SPARKER SEISMIC DATA
Authors: Allen F. T.
Published: October 2002

The marine seismic survey technique of frequent recordings with a single detector group can provide intricate details valuable to relatively shallow investigations. Velocities may be computed from time anomalies, under certain circumstances. The extent of multiple energy response is an indication that the 100,000-joule source is strong enough for the purpose. Recognizable minor details in primary reflections are important clues in identifying related multiple reflections. Bounces off the underside of the water layer are rarely found. The pattern and character of reflections are influenced by recording conditions; thus, the relationship between recorded events and sedimentary beds is not simple. Seismic time profiles frequently give wrong impressions of structural attitudes because of the horizontal‐to‐vertical exaggeration, time anomalies, and multiple reflections, as well as the usual effects of velocity differences. The interpreted cross‐section gives a reasonably correct (even if velocities are assumed) impression of structure; the profile often does not.
UNDERGROUND WASTE MANAGEMENT AND ENVIRONMENTAL IMPLICATIONS
Authors: Nettleton L. L.
Published: October 2002

The following is a brief report of the symposium on “Underground Waste Management and Environmental Implications” made by the official representative of the Society of Exploration Geophysicists. The symposium was held at the Astro World Hotel in Houston, December 6–9, 1971. The approximate final registration was 561. A volume of proceedings will be published by the AAPG at a prepublication price of $18. The complete program for the meeting was published in the AAPG Bulletin, vol. 55, no. 11 (November 1971), p. 2078–2092, which includes abstracts of the papers.
On: “Geomagnetic Effects of Sloping. and Shelving Discontinuities of Earth Conductivity,” by F. Walter Jones and Albert T. Price (GEOPHYSICS, February 1971 p. 58–66)
Authors: Hermance John F.
Published: October 2002

In this recent paper, Jones and Price describe the application of finite difference methods to determining the electromagnetic fields near a vertical or inclined contact between materials having different resistivities. In the case of H polarization (the only case discussed in the following comments) they observed that as one approaches the contact from its conductive side the apparent resistivity decreases, which is to say that E horizontal decreases since H horizontal is constant. This behavior has previously been pointed out by d’Erceville and Kunetz (1962) for an analytical calculation. Jones and Price interpret this phenomenon as the effect of surface charges concentrating at the interface due to the impinging current.
On: “An Automatic Method of Compilation and Mapping of High‐Resolution Aeromagnetic Data,” by B. K. Bhattacharyya (GEOPHYSICS, August 1971, p. 695–716)
Authors: Reford M. S.
Published: October 2002

The use of high‐resolution aeromagnetic surveys for detailed geologic mapping and mineral exploration is not yet common, nor are the flying and compilation techniques as standardized as those of conventional or lowsensitivity aeromagnetic surveys. Dr. Bhattacharyya has made a valuable contribution by presenting particularly interesting results and describing the techniques in some detail. But there are some points in his comparisons between high‐resolution and conventional surveys which could be misleading.
On: “Reduction of Unevenly Spaced Potential Field Data to a Horizontal Plane by Means of Finite Harmonic Series” by Roland G. Henderson and Lindrith Cordell (GEOPHYSICS, October 1971, p. 856–866)
Authors: Herring Alan T.
Published: October 2002

The authors recommend on page 864, that their technique for correcting for anomalous vertical gradients in the gravity field be applied only in “cases where high topographic relief produces large corrections to the station Bouguer anomaly.” The correction applied by the authors amounts to the continuation of the observed gravity field onto a single‐elevation plane from the varying elevations of the observation points. The authors should therefore caution the reader against choosing the datum plane such that the gravity field is continued through sources of interest lying above the datum plane—an easily made mistake in areas of high topographic relief.
EXPERIMENTAL COMPARISON OF NaI(Tl) AND SOLID ORGANIC SCINTILLATION DETECTORS FOR USE IN REMOTE SENSING OF TERRESTRIAL GAMMA RAYS
Authors: Duval Joe S. , Worden J. M. , Clark Ronald B. , Adams J. A. S.
Published: October 2002

An experimental comparison of NaI(Tl) and solid organic scintillation detectors for use in remote sensing of terrestrial gamma rays demonstrates that organic detectors can be used for this purpose with the advantages of being less expensive and more rugged.
On: “Computer Mapping of Seismic and Velocity Data in Australia,” by Harry A. Shetrone (GEOPHYSICS, April, 1972, p. 313–324)
Authors: Steedman R. K.
Published: October 2002

The application of Acheson’s velocity‐depth equation to the regional velocity problems of the Great Artesian Basin (central Australia) was initiated by Hosking (1966). Shetrone has not acknowledged this earlier work.
ELECTROMAGNETIC FIELDS DUE TO DIPOLE ANTENNAS OVER STRATIFIED ANISOTROPIC MEDIA
Authors: Kong J. A.
Published: October 2002

Solutions to the problem of radiation of dipole antennas in the presence of a stratified anisotropic media are facilitated by decomposing a general wave field into transverse magnetic (TM) and transverse electric (TE) modes. Employing the propagation matrices, wave amplitudes in any region are related to those in any other regions. The reflection coefficients, which embed all the information about the geometrical configuration and the physical constituents of the medium, are obtained in closed form. In view of the general formulation, various special cases are discussed.
THREE‐DIMENSIONAL MODEL—GULF OF MEXICO
Authors: Meek John W.
Published: October 2002

The 1969 U. S. Geological Survey Sparker Survey of the Gulf of Mexico was used to construct a three‐dimensional model. The result is a comprehensive presentation which quickly conveys an understanding of the main features of the Gulf of Mexico basin. The time and equipment needed to construct this model were minimal, and the method is adaptable for any area. The model indicates that the geologic composition of the Gulf of Mexico is varied. The profiles show sedimentary dips and no‐reflection areas that can be interpreted as homogeneous masses such as salt. The massive salt off the Texas‐Louisiana coast is truncated and does not overflow the Abyssal Plain sediments. The salt trends inland between Tampico and Veracruz. Massive salt reenters the Gulf from the Isthmian Tertiary salt basin of Mexico furnishing the salt for the Sigsbee knolls. If salt exists under the Florida scarp, it forms a ridge rather than massive salt. Igneous plugs are present between Cuba and Florida. In the Abyssal Plain the sediments are a minimum of 10,000 to 20,000 ft thick. The thickest sediments are adjacent to the Sigsbee scarp. The structures in the Abyssal Plain are many and varied. This study indicates that the entire Gulf of Mexico could be a potential hydrocarbon‐bearing province. The sedimentary thickness in both the shelf and Abyssal Plain area is sufficient for hydrocarbon accumulations.
THE STRANGULATION OF EXPLORATION GEOPHYSICS
Authors: Savit Carl H.
Published: October 2002

Exploration geophysics is a profession whose sole activity is to explore the solid earth by physical means. The aim or purpose of exploration geophysical activity is to obtain information about the earth which may be economically or scientifically useful. During the first few decades of the application of geophysics, constraints on geophysical professional activity were of a type we can call “natural”; that is, our profession was limited in its accomplishments by our own level of knowledge, by the general level of associated sciences and technologies, and by the cost and perceived value of the information we produced. All of these natural constraints have the common property that they can be modified by improvements within our profession. Each time we tried something new we observed the results and adopted those new ideas and techniques which produced beneficial effects. Variations which resulted in poorer results or results not worth the extra cost were dropped from the repertoire.
VIBROSEIS APPLICATIONS TO ENGINEERING WORK IN AN URBAN AREA
Authors: Mossman R. W. , Heim George E. , Dalton Frank E.
Published: October 2002

In 1967 and 1968, seismic exploration using the Vibroseis system was conducted in support of the Deep Tunnel Project of The Metropolitan Sanitary District of Greater Chicago to map certain shallow rock strata in which extensive tunneling was planned for relief of storm water overflow in the combined sewer system of the Chicago area. The capabilities of this seismic system for operating under extreme conditions of ambient noise without disturbance to the urban environment made the method ideal for providing detailed density of control at a much lower cost than any other means. Results were relatively precise and have been largely confirmed by drilling and construction of tunnels in the area.
RADIO INTERFEROMETRY DEPTH SOUNDING: PART II—EXPERIMENTAL RESULTS
Authors: Rossiter James R. , LaTorraca Gerald A. , Annan A. Peter , Strangway David W. , Simmons Gene
Published: October 2002

In such highly resistive geologic environments as ice sheets, salt layers, and the moon’s surface, radio waves penetrate with little attenuation. The field strengths about a transmitting antenna placed on the surface of such an environment exhibit interference maxima and minima which are indicative of the in‐situ electrical properties and the presence of subsurface layering. Experimental results from an analog scale model and from field tests on two glaciers are interpreted on the basis of the theoretical results of Part I. If the upper layer is thick, the pattern is very simple and the dielectric constant of the layer can be easily determined. An upper bound on the loss tangent can be estimated. For thin layers, the depth can be determined if the loss tangent is less than about 0.10, and a crude estimate of scattering can be made.
EFFECT OF NORMAL MOVEOUT ON A SEISMIC PULSE
Authors: Dunkin J. W. , Levin F. K.
Published: October 2002

Using a synthetic seismogram as input, normal moveout correction stretches a reflection pulse in such a way that the spectrum of the pulse is a linearly compressed version of the uncorrected pulse spectrum. The amount of compression depends on [Formula: see text], the source‐detector separation, velocity, and the rate at which velocity varies with [Formula: see text]. The amplitude of the spectrum is increased by the same factor that expresses the spectral compression. As a result, the summed pulse from a CDP stack is richer in low frequencies than one might anticipate and has a smaller signal‐to‐noise ratio than the square root of the number of traces in the stack.
GROSS PROPERTIES OF TIME‐DISTANCE CURVES
Authors: Shah Pravin M. , Levin F. K.
Published: October 2002

As the spread length increases, rms velocities determined from seismic data differ from those given by the Dix relation. We have examined the nature of time‐distance curves and the velocities found from them, using as our model of the subsurface beds separated by plane horizontal interfaces or a subsurface with an interval velocity that is a continuous function of depth. Our conclusions are as follows: (1) For a given reflection, the slope of the [Formula: see text] time‐distance curve at any spread distance x is equal to [Formula: see text] where [Formula: see text] is the rms velocity computed along an appropriate raypath. [Formula: see text] is the velocity given by the Dix relation. As x increases, [Formula: see text] approaches a limiting value equal to the maximum value of the interval velocity in the section of the earth transversed by the raypath. (2) The [Formula: see text] curve is such that [Formula: see text] is a monotonically increasing function of x. Conclusions (1) and (2) permit us to place bounds on the moveout velocity. (3) If the interval velocity is assumed to increase exponentially with depth, the Taylor series expansion for [Formula: see text] about x=0 converges for all valid values of x. To illustrate the effect of the [Formula: see text] term on moveout velocity, subsurfaces were generated from several continuous velocity logs and moveout velocities computed with the usual Dix relation and with a three‐term [Formula: see text] expansion. In most cases, the errors made by dropping the [Formula: see text] term were less than 2 percent.
A COMPARISON OF IP ELECTRODE ARRAYS
Authors: Coggon J. H.
Published: October 2002

The responses of dipole‐dipole, pole‐dipole, and gradient arrays to a set of ten structures have been computed using the finite element method. Comparison of the responses indicates that: (1) The dipole‐dipole array usually gives the largest anomalies, but the anomalies rarely give information on dip of a structure and are strongly influenced by overburden irregularities. This array gives best overall resolution. (2) Pole‐dipole anomalies are almost as large as dipole‐dipole ones and have the same general form but show less resolution. The lower resolution and the asymmetry of this array make results more difficult to interpret. (3) The gradient array provides dip information and good horizontal resolution. Responses to thin vertical structures are weak, anomalies are strongly affected by overburden irregularities, and there is no effective depth discrimination.
ON AXIALLY SYMMETRIC ELECTRIC CURRENT INDUCED IN A CYLINDER UNDER A LINE SOURCE
Authors: Singh Shri Krishna
Published: October 2002

An infinite conducting cylinder excited by an infinite line current located outside the cylinder is a useful model in the interpretation of electromagnetic prospecting data. Several authors, with geophysical applications in mind, have considered the problem of the source being parallel to the axis of the cylinder (Wait, 1952; Negi et al., 1972). In the latter paper, the cylinder is surrounded by a shell; conductivity of both the cylinder and the shell is a function of radius. The secondary fields are written in the form of an infinite series of cylindrical wave functions. This solution is then specialized to the quasi‐static case. For reasons not explained, the authors neglect the n = 0 term. In this short note, computed results are presented which show that the contribution from the n = 0 term (corresponding to an axially symmetric electric current induced in the cylinder causing a transverse secondary magnetic field outside) is significant and must be taken into account for the two‐dimensional problem.
PLANE WAVES AT SMALL ARRAYS
Authors: Woods John W. , Lintz Paul R.
Published: October 2002

The resolving power of a seismic array is defined in terms of the array response function and via the classical uncertainty principle. Using the theory of maximum likelihood wavenumber spectra (Capon, 1969), we show for the case of two correlated plane waves that arbitrarily high resolution is achievable in the limit as the background white noise tends to zero. This extends Barnard’s (1969) result to the case of correlated plane waves. The increased resolution arises from the additional assumption that the data are plane waves over all space, and not zero off the array as the classical result assumes. It is found that a sample rate (in time) large compared to the Nyquist rate, is needed in the case of a short time gate at a small array. Cross‐power spectral matrices are estimated at 4 hz from 1 sec of computer generated data consisting of two correlated plane waves in white noise. These spectral matrices are then used to generate maximum likelihood wavenumber spectra. The two plane waves are resolved at various signal‐to‐noise ratios and at correlations up to ρ=0.8. The need for using a high sampling rate is demonstrated. Results are compared with conventional wavenumber spectra, where the classical resolution results hold. The use of a 1‐sec window provides improved resolution of the wavenumber structure as it changes in time, resulting in better separation of any time‐overlapping phases and multipathed waves that arise from one event.
RESISTIVITY INVERSION
Authors: Inman Joseph R. , Ryu Jisoo , Ward Stanley H.
Published: October 2002

The problem of direct interpretation of apparent resistivity curves from horizontally layered models is accomplished by using the generalized linear inverse theory. The method permits the resolution of the model parameters to be determined. The method also indicates which data points contain relatively important information necessary to resolve the model parameters. Two models were chosen to test the inversion scheme. One model has increasing resistivity with depth, and the other model possesses an intermediate resistive layer. Both models were resolved with a very high degree of accuracy from noise‐free data. When noise was added to the data, the values of the parameters oscillated about a mean value. The noise had little effect on the well‐resolved parameters but the poorly resolved parameters were in error by as much as 15 percent. The importance of each data point relative to the model was analyzed. The effect of certain data points on specific parameters was also determined. The generalized inverse method requires that the eigenvalues and eigenvectors of the system matrix be found. A comparison of the eigenvalues indicates those parameters that are well‐resolved and those that are poorly resolved from a given set of data.
TOPOGRAPHIC AND TERRAIN CORRECTION FOR AIRBORNE GRAVITY
Authors: Hammer Sigmund
Published: October 2002

A simple, convenient procedure is outlined for evaluating topographic and terrain effects in airborne gravimetry. The method assumes that continuous terrain data (flight elevation and terrain clearance) are available along a traverse. The terrain is taken to be uniform to infinity in both directions perpendicular to the flight line. The correction is easily calculated from a simple product series. The error tolerances are not serious. Nonperpendicular, irregular topography cannot be evaluated by this simple procedure, but order of magnitude considerations demonstrate that this problem ordinarily is not prohibitively important. In flight, real‐time correction for topographic and terrain effects is not a major obstacle in the development of airborne gravity exploration.
A REVIEW OF PROGRESS IN GEOPHYSICAL INSTRUMENTS FOR THE YEAR 1938
Authors: Krasnow Shelley
Published: October 2002

Any account of progress in a rapidly changing field will necessarily be out of date. This holds with special emphasis in geophysics. Equipment that will be described as new might have been in successful use for perhaps two or three years. The date of its release rather than of its appearance, however, must be used. Similarly, there have been improvements during the past year which show exceptional promise. Discussion of these however, must be deferred until their formal release.
SEISMIC PATHS, ASSUMING A PARABOLIC INCREASE OF VELOCITY WITH DEPTH
Authors: Houston Charles E.
Published: October 2002

This paper discusses the paths of seismic waves, assuming that the velocity increases parabolically with depth. A graph of the reflection time‐distance curves, in dimensionless coordinates, is included.
A CRUSTAL GEOELECTRICAL MODEL FOR SOUTH AFRICAN PRECAMBRIAN GRANITIC TERRAINS BASED ON DEEP SCHLUMBERGER SOUNDINGS
Authors: Van Zijl J. S. V. , Joubert S. J.
Published: October 2002

Three crustal Schlumberger soundings on widely separated granitic basement areas were recently completed. One sounding was carried out on the 1000 my old metamorphic complex of Namaqualand, a second on the 2600 my old Rhodesian Craton, and the third on the Cape‐Vaal Craton, also of 2600 my age. The fact that the sounding curves are very similar in shape suggests that the sounding data reflect vertical changes in resistivity at crustal depths. This contention is substantiated by an analysis of the results from several measuring stations for one of the soundings. Interpretation of the field measurements leads to a model which has the following characteristics: The crust has a maximum resistivity not exceeding [Formula: see text] ohm‐m at a depth of less than 10 km. Its transverse resistance varies from 5 to [Formula: see text]. The resistivity then decreases progressively to reach a value of 700–1300 ohm‐m at a depth of 16–20 km, after which the resistivity does not decrease appreciably until a depth of at least 200 km is reached. The finding that resistivities as low as 700 ohm‐m exist at a depth of 16–20 km is at variance with Brace’s 1971 laboratory model for the stable crustal area of the eastern U.S. However, this zone of low resistivities coincides with low velocity layers reported by Bloch et al. (1969), and it is suggested by the authors that hydrated rocks near the base of the sialic layer are the cause of the anomalous zone.
SYNTHETIC SEISMOGRAMS: A FINITE ‐DIFFERENCE APPROACH
Authors: Kelly K. R. , Ward R. W. , Treitel Sven , Alford R. M.
Published: October 2002

Recent interest in the extraction of fine detail from field seismograms has stimulated the search for numerical modeling procedures which can produce synthetic seismograms for complex subsurface geometries and for arbitrary source‐receiver separations. Among the various techniques available for this purpose, the replacement of the two‐dimensional wave equation by a suitable finite‐difference representation offers distinct advantages. This approach is simple and may be readily implemented. It automatically accounts for the proper relative amplitudes of the various arrivals and includes the contributions of converted waves, Rayleigh waves, diffractions from faulted zones, and head waves. Two computational schemes have been examined. For the so‐called “homogeneous formulation,” the standard boundary conditions between media of different elastic properties must be satisfied explicitly. In the case of the alternate “heterogeneous formulation”, these elastic properties may be specified at each grid point of a finite‐difference mesh, and the boundary conditions are satisfied implicitly. The proper simulation of the source requires special treatment for both cases. Synthetic seismograms computed for several models of exploration interest serve to illustrate how the technique may help the interpreter. The examples also illustrate various implementational aspects of the finite‐difference approach, which involve such phenomena as grid dispersion, artificial reflections from the edge of the model, and choice of spatial and temporal sampling increments.
FINITE‐DIFFERENCE RESISTIVITY MODELING FOR ARBITRARILY SHAPED TWO‐DIMENSIONAL STRUCTURES
Authors: Mufti Irshad R.
Published: October 2002

Resistivity surveying is commonly done by using a point‐source dipole. Consequently, a finite‐difference evaluation of apparent resistivity curves implies the use of three‐dimensional simulation models which necessitate prohibitive computer costs. However, if we assume variation of resistivity only in two dimensions and use a line‐source dipole for setting up the finite‐difference model of a given structure, the potential field can be evaluated easily. A discrete version of the resistivity problem in two dimensions, which takes into account nonuniform grid spacing, is presented as a system of self‐adjoint difference equations. Since the iterative solution of such a system does not require grid spacing to be less than a certain critical value, it was successfully used for the development of fast‐convergence finite‐difference models. By examining in detail the characteristics of the matrix associated with the evaluation of the potential field, it is demonstrated that the proposed modeling procedure will remain stable for all conceivable geometries and resistivity distributions. It was used for the investigation of certain models for which the corresponding results could also be computed analytically. A direct superposition of results obtained in the two cases shows that they are virtually identical. By making use of the reciprocity theorem, a computational short‐cut, which provides the evaluation of vertical sounding curves for a line‐source dipole in a single step, is put forward. Special problems related to the optimization of acceleration parameters as well as the estimation of the potential function along the subsurface boundaries of the model are discussed. It is concluded that by surrounding the model by a termination strip of very large effective width, either Neumann‐ or Dirichlet‐type boundary conditions can be used for simulating a semiinfinite medium without introducing signficant errors in the results.
INTERPRETATION OF MAGNETOTELLURIC RESISTIVITY AND PHASE SOUNDINGS OVER HORIZONTAL LAYERS
Authors: Patella D.
Published: October 2002

The present paper deals with a new inverse method for quantitatively interpreting magnetotelluric apparent resistivity and phase‐lag sounding curves over horizontally stratified earth sections. The recurrent character of the general formula relating the wave impedance of an (n−1)‐layered medium to that of an n‐layered medium suggests the use of the method of reduction to a lower boundary plane, as originally termed by Koefoed in the case of dc resistivity soundings. The layering parameters are so directly derived by a simple iterative procedure. The method is applicable for any number of layers but only when both apparent resistivity and phase‐lag sounding curves are jointly available. Moreover no sophisticated algorithm is required: a simple desk electronic calculator together with a sheet of two‐layer apparent resistivity and phase‐lag master curves are sufficient to reproduce earth sections which, in the range of equivalence, are all consistent with field data.
DEVELOPMENTS IN EXPLORATION GEOPHYSICS, 1969–74
Authors: Levin F. K. , Bayhi J. F. , Dunkin J. W. , Lea J. D. , Moore D. B. , Warren R. K. , Webster G. M.
Published: October 2002

Talks on developments in exploration geophysics traditionally have been given at SEG meetings every two or three years. That tradition has not been honored recently. Except for a specialized paper on seismic data processing and analysis by W. A. Schneider (1971), there has been no review since the 1968 Denver meeting. In Denver, P. C. Wuenschel and his coauthors discussed geophysical research and progress in exploration from 1965 through 1968. Their account was published in the April, 1969 issue of Geophysics.
AN IMPROVED PULSE TRANSIENT AIRBORNE ELECTROMAGNETIC SYSTEM FOR LOCATING GOOD CONDUCTORS
Authors: Sarma D. Gupta , Maru V. M. , Varadarajan G.
Published: October 2002

Measuring the transient field values (H), rather than their time derivatives (dH/dt), with an inductive, pulse‐excited electromagnetic prospecting system makes the device capable of reducing much of the geologic noise due to poor superficial conductors and enhancing the response from good conductors. The factor of improvement in signal‐to‐geologic noise ratio in H measurement may be as much as [Formula: see text], where [Formula: see text], and [Formula: see text] are the expected values of decay time constant of signal and noise components, respectively. Moreover, signal‐to‐noise ratio increases for later sample channels, thus allowing equal gate timings for all sample channels. Field results of a comparative study with H and dH/dt measurements are shown. It is predicted that both depth of exploration and reliability of interpretation of data in quantitative terms may improve somewhat with such a system.
TOPOGRAHIC CORRECTIONS FOR GRADIENTS IN AIRBORNE GRAVIMETRY
Authors: Hammer Sigmund
Published: October 2002

Progress in airborne gradiometer instrumentation has advanced to the point that procedures for determining corrections for the variable effects of topography merit consideration. An approximate practical method, based on real‐time recording of flight elevation and variable terrain clearance as a function of position along the flight line, appears to be appropriate for the purpose. The procedure is similar to that for determing topographic corrections for airborne gravity published previously by the author in Geophysics (Hammer, 1974a).
PRESSURE AMPLITUDES OF SEISMIC SIGNALS IN OFFSHORE LOUISIANA
Authors: Arnold M. E.
Published: October 2002

Pressure amplitudes were determined for various kinds of seismic signals observed on special test records obtained during field tests conducted along a 14,000-ft seismic lines in Eugene Island Block 184, offshore Louisiana. Vibrators attached to a Seismograph Service Corp. (SSC) boat generated swept‐frequency and monofrequency signals. Signals from detectors on a streamer cable towed by the boat were recorded by an SSC recording system. Signals from a vertical spread of detectors were recorded by a DFS/9000 recorder on the Transco 184 platform centrally located in the test area. Location of the boat was determined by analysis of time relations of signals from responders located at established positions some distance from the test area. Clock times from manually referenced timing code generators were recorded by both the SSC and DFS recorders to permit synchronization between separately recorded signals. The signals analyzed were separated into three classes: [Formula: see text] includes direct and refracted waves; [Formula: see text] consists of primary reflections; and [Formula: see text] includes signals diffracted from scatterers. The average level of first‐arrival signal [Formula: see text] and reflected signal [Formula: see text] for frequency sets 25, 40, 42.2, 50, and 70.4 Hz in the range of 1414 and 2143 ft, which encompasses streamer cable single‐detector groups, is 337 and 29.6 microbars, respectively. The amplitude of signals [Formula: see text], believed to be diffracted from the contact between key reflectors and a salt dome, ranges from 13 to 20 microbars and is 10 to 100 times the amplitudes of towing and ambient noise, respectively. The observed decay of first‐arrival signal amplitude is approximately proportional to the square root of range distance, or about 2 dB/1000 ft. The observed decay of reflected signal amplitude with range distance is approximately 1 dB/1000 ft.
SPECTRAL ANALYSIS OF GRAVITY AND MAGNETIC ANOMALIES DUE TO RECTANGULAR PRISMATIC BODIES
Authors: Bhattacharyya B. K. , Leu Lei‐Kuang
Published: October 2002

The spectra of gravity and magnetic anomalies due to a prismatic body can be expressed as sums of exponentials. The complex exponents of these exponentials are functions of frequency and locations of the corners of the body. An exponential approximation method is used for the analysis of the radial spectra of an anomaly and its first order moments for obtaining accurate estimates of the depths to the top and bottom of the body. A method has also been developed for determining approximately the location of the centroid of the body. When the location of the centroid and the depths to the top and bottom are known for the causative body, it is possible to calculate the horizontal dimensions with the help of the spectrum of the anomaly.
METHOD FOR DETERMINING TERRESTRIAL HEAT FLOW IN OIL FIELDS
Authors: Da Silva Carvalho Humberto , Vacquier Victor
Published: October 2002

A method of determining terrestrial heat flow in oil fields from bottom‐hole temperatures, electric logs, and thermal conductivity of core samples has been tried in six Reco⁁ncavo Basin oil fields in Brazil. The average heat‐flow value so determined for the Reco⁁ncavo Basin is [Formula: see text]. The technique can be used for calculating heat flow in continental areas elsewhere. A more significant outcome of our experiment is that it demonstrates an inexpensive method of obtaining terrestrial heat‐flow values in the sedimentary basins of the world.
Discussion on “Digital Convolution of Computing Gravity and Magnetic Anomalies Due to Arbitrary Bodies,” by B. K. Bhattacharyya and M. E. Navolio (GEOPHYSICS, December 1975, p. 981–992)
Authors: Qureshi I. R.
Published: October 2002

I congratulate the authors on their elegant mathematical approach to the computation of gravity and magnetic anomalies due to arbitrary bodies. But I consider their concluding remark, “In this approach, it is not necessary to perform integration of the dipolar magnetic field or the gravitational field due to a point mass,” to be inaccurate and misleading. The method proposed by the authors represents, in effect, the division of arbitrary bodies into cubes of a small side and numerical integration [their equations (20) and (25)] of the effects of “equivalent” dipoles or point masses located at the centers of these cubes. Hence, the smaller the side of the cube, the better the accuracy of the method.
ETHICS AND ECONOMICS OF TECHNOLOGY TRANSFER IN CONTRACTOR‐CLIENT RELATIONSHIPS
Authors: Sheriff Robert E.
Published: October 2002

EDITOR’S NOTE: The author is uniquely qualified to comment on his subject. He has recently retired from a major oil company and has had an opportunity to view the contractor‐client relationship from both sides of the fence. It is my hope that his editorial will stimulate comment from interested readers on all sides of this issue. Exploration geophysics is high technology business. A seismic crew or data processing center with state‐of‐the‐art equipment, processes, and techniques of a few years ago would have difficulty finding work today (even though they might be adequate to the problems). Technological obsolescence of equipment involves large depreciation costs. A rapid, steady flow of new ideas support the high technology. These ideas are created by both contractor and client companies. Generating these ideas involves large costs but the costs are not always shared by the benefactors. The interchange of ideas throughout exploration geophysics is here called technology transfer and it involves economic and ethical considerations. Examination of these is the subject of this editorial.
FIRST RESULTS FROM ELECTRICAL AND SEISMIC STUDIES OF LOW‐RESISTIVITY, LOW‐VELOCITY MATERIAL BENEATH EASTERN COLORADO
Authors: Landisman M. , Chaipayungpun W.
Published: October 2002

First results are reported for magnetotelluric (MT) observed apparent resistivities and phases, corresponding to both off‐diagonal components of the 2 by 2 electromagnetic impedance tensor, derived from E and H field recordings near Anton, in the High Plains of eastern Colorado. The data were recorded by a gain ranged, multichannel, multirate digital field system. The results, which span a frequency range of roughly [Formula: see text], extend from about 0.05 sec (20 Hz) to several thousand seconds. Corresponding sets of tightly clustered, practically continuous MT apparent resistivities and phases are virtually identical, constituting strong evidence for lateral homogeneity and isotropy of the electrical conductivity near this site. The derived one‐dimensional resistivity‐depth model is concordant with the depth and thickness of the local water table, with resistivities and depths for the Pierre shale derived from nearby well logs, with reconnaissance audiomagnetotelluric (AMT) data, and with ultra‐long period apparent resistivities derived from regional Deep Geomagnetic Sounding observations. The derived resistivity depth distribution also compares well with the depth to the crystalline basement from the neighboring U.S.G.S. Lamar‐Sterling, Colorado, seismic refraction profile. The concordance, between depths interpreted for a mid‐crustal seismic low‐velocity zone and a corresponding low‐resistivity zone at this site, parallels similar results previously reported for west Texas‐eastern New Mexico, and various sites in the U.S.S.R.
GEOPHYSICAL ACTIVITY IN 1976
Authors: Espey H. R.
Published: October 2002

This report provides statistics on worldwide use of geophysical methods in 1976. Data were obtained primarily through a survey questionnaire which was mailed out to more than 500 companies, government agencies, and universities that use geophysical techniques for petroleum exploration, oceanography, engineering, mining, geothermal exploration, and groundwater exploration. Response to the survey was excellent, providing detailed information on all types of airborne, land, and marine surveys. Computer processing was utilized in tabulating the statistics to provide detailed information on methods used, line‐miles covered, and average costs. Data not supplied on the questionnaire for costs or line‐mileage were estimated on the basis of worldwide averages to produce a more comprehensive report.