bluuebunny/crossref_metadata_2025 huggingface dataset:
THREE‐DIMENSIONAL GRAVITY INSTABILITY DERIVED FROM TWO‐DIMENSIONAL SOLUTIONS
Authors: Biot M. A.
Published: October 2002

The theory of three‐dimensional gravity instability of multilayers is developed with particular application to salt structures. It is shown that three‐dimensional solutions are immediately obtained without further numerical work from the solution of the corresponding two‐dimensional problem. Application to a number of typical three‐dimensional structures yields the characteristic distance between peaks and crests and shows that this distance does not differ significantly from the wavelength of the two‐dimensional solution. Various periodic patterns are examined corresponding to rectangular and hexagonal cells. The time history of nonperiodic structures corresponding to initial deviations from perfect horizontality is also derived. The method is applied to the three‐dimensional problem of generation of salt structures when the time‐history of sedimentation is taken into account with variable thickness and compaction of the overburden and establishes the general validity of the geological conclusions derived from the previous two‐dimensional treatment of the same problem (Biot and Odé, 1965). The present method of deriving three‐dimensional solutions, which is developed here in the special context of gravity instability, is valid for a wide variety of problems in theoretical physics.
A RESISTIVITY COMPUTATION METHOD FOR LAYERED EARTH MODELS
Authors: Mooney Harold M. , Orellana Ernesto , Pickett Harry , Tornheim Leonard
Published: October 2002

A procedure is given to compute apparent resistivity and induced‐polarization results for layered earth structures. The method is designed for use with large computers. Results may be obtained for any number of layers, and for any of the commonly used electrode configurations. Specific expressions are given for Schlumberger, Wenner, azimuthal‐dipole, axial‐dipole, and for the potential function. The method consists in expanding a portion of the integrand as a series in [Formula: see text] and integrating analytically term by term. Convergence of the resulting series is established. The required coefficients for each term in the series can be obtained by recursion formulas from preceding coefficients. Accuracy of the results can be estimated and can be preselected. For the Schlumberger electrode arrangement with spacing L, for example, the error produced by truncating the series after M terms will be no greater than [Formula: see text]. A more rigid bound on the error is also given. Accuracy of the method was further checked against published apparent resistivity data and against a numerical integration procedure devised for this purpose. The following characteristics make the method well suited for use with digital computers: (1) The formulation is relatively simple and easily programmed. (2) A single program will handle any number of layers. (3) The computer can be made to generate the required coefficients internally. (4) The computer can be programmed to terminate the computation as soon as any preselected accuracy has been achieved. (5) Complete separation is attained between earth structure and electrode arrangement; thus, a single set of stored coefficients can be used repeatedly for different electrode spacings and different electrode arrangements.
TEMPERATURE DISTRIBUTION IN SALT DOMES AND SURROUNDING SEDIMENTS
Authors: Selig Franz , Wallick George C.
Published: October 2002

A straightforward mathematical procedure allows the calculation of the isogeothermal pattern in and about a salt intrusion. Temperature anomalies are controlled not only by the thermal properties of the media, but also are indicative of the geometry and history of the system. As a simple example, an axially symmetrical structure was chosen to illustrate the kind of inferences one can make from the temperature data, such as discrimination between connected and disconnected domes, and the estimation of the vertical velocity of the intrusion. The numerical results confirm and extend the observations in electrolytic scale models (Hubert Guyod, 1946), and augment W. Heroy’s comments on thermal properties of salt, given at the International Conference on Saline Deposits, Houston, 1962.
FUNDAMENTALS OF ELECTROMAGNETIC METHODOLOGY
Authors: Winkler H. A.
Published: October 2002

The development of electromagnetic prospecting instruments has progressed rapidly in the last decade, particularly in Canada and Sweden. The introduction of miniaturized electronic circuits has permitted the construction of highly portable, sophisticated instruments. Consequently there is a large number of portable and installed EM units in current use all over the world.
MAGNETIZATION OF PRECAMBRIAN SULPHIDE DEPOSITS AND WALL ROCKS FROM THE NORANDA DISTRICT, CANADA
Authors: Schwarz E. J.
Published: October 2002

Forty‐three oriented samples of massive sulphides, sulphide bearing rocks, quartz porphyries, and diabases were collected in the Quemont and Horne mines. The low‐field susceptibility for the sulphides on the average is somewhat higher than that for both types of wall rocks. The average intensity of the natural remanence of the sulphides is appreciably higher than that of the wall rocks. The ratio of the natural remanence and the induced magnetization (H=0.5 oersteds) is of the order of 1.2 and 0.5 for the sulphides and wall rocks, respectively. The directions of the natural remanence of nearly all samples of sulphide and wall rock are closely parallel to the geomagnetic field. Thus, sulphide bodies of the type studied tend to produce positive magnetic anomalies at the earth’s surface. The stability of the natural remanence of all samples was tested by partial demagnetization in alternating fields. The natural remanence contains a very large unstable component.
On: “THE USE OF LINEAR PROGRAMMING TO FILTER DIGITIZED MAP DATA,” BY E. L. DOUGHERTY AND S. T. SMITH (GEOPHYSICS, FEBRUARY 1966, PP. 253–259)
Authors: Neidell Norman S.
Published: October 2002

I wish to comment on the short note by E. L. Dougherty and S. T. Smith titled “The Use of Linear Programming to Filter Digitized Map Data” which appeared in the February, 1966, issue of Geophysics. First, I would like to congratulate the authors for showing explicitly how Taylor’s theory can be used to justify a local polynomial fit. I think, however, that they should have made clear that this same justification is valid for the least squares method. In essence these authors have chosen to make an approximation in the [Formula: see text] Norm instead of the [Formula: see text] or Least Square Norm (see Rice, 1964).
VELOCITY ANISOTROPY MEASUREMENTS IN WELLS
Authors: Stoep D. M. Vander
Published: October 2002

Sedimentary rocks are generally anisotropic to the propagation of seismic waves. Anisotropy can be defined as the difference between propagation time predicted by the simple theory of Snell’s Law and observed propagation time between two points in a layered medium that lie on a line oblique to the layers. This difference can be explained by the more complicated theory of wave propagation in transversely isotropic materials. In the zone about the vertical that is of interest in reflection seismology, the effect of anisotropy usually can be described geometrically by an anisotropy factor A. This simple description is not valid for propagation directions making large angles with the normal to the layers. The anisotropy factor as well as the vertical velocity can vary with depth. A method is given for determining the factor A as a function of depth from a continuous velocity log and a range of oblique shots into a well phone. The method is applied to two field examples. In one of the examples, it is shown by data obtained from the larger shooting distances that the simple A factor description is inadequate for higher angles of propagation direction.
SHEAR‐WAVE DETECTION IN NEAR‐SURFACE SEISMIC REFRACTION STUDlES
Authors: Whitcomb James H.
Published: October 2002

The U. S. Geological Survey, on behalf of the National Aeronautics and Space Administration, has begun investigations designed to relate the data obtained from seismic refraction profiles to the physical properties of lunar analog earth materials. The velocity of shear waves is a significant property, but the recording of these waves presented a serious problem.
GEOLOGICAL SURVEY INVESTIGATIONS OF THE ELECTRICAL PROPERTIES OF THE CRUST AND UPPER MANTLE
Authors: Keller G. V. , Anderson L. A. , Pritchard J. I.
Published: October 2002

The electrical properties of rocks deep in the crust and in the upper part of the mantle may be studied with various electrical methods. The U. S. Geological Survey has conducted such studies using three methods: conventional galvanic resistivity surveys, inductive resistivity surveys using the magnetotelluric field, and the measurement of electrical properties of rock samples at high temperatures. By combined sample measurements, galvanic resistivity measurements and magnetotelluric studies, the resistivity profile through the crust and upper mantle may be defined more closely than has been possible previously. We have recognized the existence of a heterogeneous surface layer consisting of sedimentary rocks and weathered igneous and metamorphic rocks. Within the crust, a marked decrease in resistivity is sometimes observed at depths of about 10 kilometers, perhaps corresponding to the Conrad discontinuity sometimes recognized by seismologists. It has not been possible to recognize an electrical boundary at depths corresponding to those at which the Mohorovičić discontinuity is noted by seismologists. However, a marked decrease in electrical resistivity occurs at somewhat greater depths, between 60 and 100 km.
QUANTITATIVE EVALUATION OF A STABILIZED PLATFORM SHIPBOARD GRAVITY METER
Authors: Lafehr T. R. , Nettleton L. L.
Published: October 2002

A new LaCoste and Romberg surface‐ship gravity meter mounted on a stabilized platform was tested over the known gravity field of the San Luis Pass salt dome, about twenty miles southeast of Freeport, Texas. A 100 ft boat was used in sea conditions considered average for the shallow offshore areas, waves running from 2 to 6 ft high. An average repeatability of 0.7 mgal was determined for 18 line intersections, of which 14 had differences 1.0 mgal or less. A line by line comparison with the known gravity field indicates an average difference between the known field and the surface meter results of 0.5 mgal. Systematic errors of less than 1.0 mgal were recognized. The platform meter was tested simultaneously with a LaCoste and Romberg gimbal‐supported meter, which proved to be more sensitive to the course direction of the boat and could not make adequate horizontal acceleration corrections in rough seas about 15–20 percent of the total time. The meter on the stabilized platform operated at all times in all six directions.
STATISTICAL PROPERTIES OF POTENTIAL FIELDS OVER A RANDOM MEDIUM
Authors: Naidu Prabakar S.
Published: October 2002

Following a study of the statistical properties of a random potential field, which is often encountered as a background noise in observed geophysical potential fields, and particularly a study of conditions under which the random process is homogeneous and gaussian, an approximate analytical method has been developed to evaluate the correlation functions and the power spectrum of the random process when the mass density is locally uniform but varies randomly over an entire semi‐infinite space. The method is particularly convenient for it gives a unified approach to evaluating the statistical properties of gravity as well as magnetic fields. The relationship between the autocorrelation function of the random potential field and the random mass density is quite involved, but by assuming that the mass density in successive cells is completely uncorrelated the relationship can be simplified. It is a matter of considerable interest to estimate, from the autocorrelation function, the size of cell characteristic of the random medium which reflects the geological characteristics of the rocks.
A GEOPHYSICAL HISTORY OF THE LACQ FIELD
Authors: Jenner P. , Dienesch J.
Published: October 2002

Whereas the first success of petroleum exploration in France (the gas deposit in the Saint‐Marcet anticline in the Saint‐Gaudens region) was essentially based on geological surveys, the second (the gas and oil deposits in the Lacq anticline, near Pau) was the result of geophysical techniques. In fact, the Lacq anticline cannot be detected by surface geological survey, because of Tertiary and Quaternary covering.
PREDICTIVE DECOMPOSITION OF TIME SERIES WITH APPLICATION TO SEISMIC EXPLORATION
Authors: Robinson Enders A.
Published: October 2002

In exploration seismology, a charge of dynamite is exploded under controlled conditions, and the resulting vibrations at various points on the surface of the ground are detected by geophones and are recorded as seismic traces on the seismogram. The analysis of such seismic records yields valuable information about the structure of the sedimentary rock layers in potential oil producing areas, and such information is of considerable economic value in increasing the probability of locating new oil fields.
TRAVELING SIGNAL‐TO‐NOISE RATIO AND SIGNAL POWER ESTIMATES
Authors: Simpson S. M.
Published: October 2002

Undesirable seismic noise of a nondeterministic type must be destroyed by making use of its statistical properties. Averaging of one sort or another provides methods for performing this noise removal. Our purpose here is to present a method for direct estimation of signal strength versus seismogram time, with stepout as a parameter. After describing the method and its expected behavior to some extent, we illustrate its application to a set of three noisy records.
CALCULATION OF THE MAGNETIZATION OF UPLIFTS FROM COMBINING TOPOGRAPHIC AND MAGNETIC SURVEYS
Authors: Richards M. L. , Vacquier V. , Van Voorhis G. D.
Published: October 2002

The direction and magnitude of the magnetization of a uniformly magnetized structure can be computed by combining topographic and magnetic surveys. The previously reported method has been extended to include more than one structure, each possessing its particular magnetization. Also, the bottom of the structure need not be a horizontal plane but can be an arbitrary surface. The method was applied to 21 seamounts, one laccolith and two Aleutian volcanoes. Four of the seamounts were found to be reversely magnetized. The virtual paleomagnetic pole positions for 16 Pacific Ocean seamounts, representing three widely separated locations, are significantly different from the present geomagnetic pole position but near Mesozoic virtual pole positions from Australia. For two locations, radiometric age determinations give an average date for their formation in the Cretaceous. The apparent 30 degree shift in geomagnetic latitude of the seamounts is interpreted as the result of large scale movements of the Pacific Ocean floor or, alternatively, as the result of the paleomagnetic equator being north of its present position in the Pacific during the growth of the seamounts.
SOME ASPECTS OF FAN FILTERING
Authors: Treitel Sven , Shanks John L. , Frasier Clint W.
Published: October 2002

The classical fan filtering method can be time‐consuming on the digital computer. This is because one must perform M/2 convolutions on a set of M input traces in order to obtain one output trace, where M is any convenient even integer. It is possible to manipulate the filter equations into a different form such that only one convolution per output trace is required, no matter what the value of M may be. The algorithm found in this manner can easily be programmed for a digital computer. The conventional fan filtering technique allows one to pass only those events whose apparent velocities fall within a certain fan‐shaped region in the (frequency, wavenumber), or (f, k) plane. In some applications it becomes desirable to perform the complementary operation; that is, we wish to reject those events whose apparent velocities fall within this fan‐shaped region in the (f, k) plane. These two operations are called fan pass and fan reject filtering, respectively. We derive the formulas allowing one to compute the operator coefficients for the fan reject filter, and generalize the algorithm obtained for the fan pass case so that it may be used for the fan reject case as well. The fan reject filter is applied to a set of traces from a vertical seismic array, where it becomes desirable to distinguish between upward and downward traveling events.
ANALYTIC AND MODEL STUDIES OF A ROTATABLE FIELD ELECTROMAGNETIC PROSPECTING SYSTEM
Authors: Fraser D. C. , Ward S. H.
Published: October 2002

Two remote orthogonal vertical coils of large dipole moment will produce a fairly uniform, rotatable, linearly polarized field vector suitable for exploration down to depths greater than those achieved with conventional equipment. In this research we have investigated advantages accruing from the use of the direction of primary flux as the relative currents in the two coils are changed. The rotatable field electromagnetic (RFEM) method may permit a quantitative evaluation of the parameters of a conductor, including shape, size, depth, and conductivity. The method especially is suited for conductor discrimination based on shape and strike of the body. Consequently, RFEM techniques may be used to separate out the individual responses of multiple conductors. Model studies verified conclusions reached from theoretical analyses.
SHEAR WAVE VELOCITY DETERMINATION IN SHALLOW SEISMIC STUDIES
Authors: Meidav Tsvi
Published: October 2002

In a recent note in Geophysics, Whitcomb (1966) described a method for the detection and visual amplification of transverse horizontal (SH) waves. The suggested method employs angularly sensitive accelerometers, with a vertical rotational axis. A study of methods of detecting and identifying shear waves on seismic records has also been going on for some time at Huntec Limited, Toronto. Geophysics readers may be interested in our approach to the problem.
SHOOTING FOR DEEP REFRACTION EXPERIMENTS
Authors: Underwood Robert , Elliston John , Mathews Kenneth E.
Published: October 2002

Ammonium Nitrate—Fuel Oil explosive loaded into an abandoned mine shaft can be a convenient energy source for a deep refraction seismic experiment. Two such sites have been shot successfully in Australia.
CHARACTERISTICS OF OPTIMUM MULTICHANNEL STACKING FILTERS
Authors: Galbraith James N. , Wiggins Ralph A.
Published: October 2002

A large class of seismic processing operations involve the linear combination of several traces in such a way that certain “signal” events are emphasized over other “noise” events. If the noise events are randomly distributed between traces, then simple addition of the traces represents the best possible technique for suppressing the noise. However, in many cases a noise event on one trace will also occur at relatively well‐determined points on other nearby traces. For these cases, multichannel linear filters offer better discrimination against organized noise than the straight addition of traces does. Such multichannel filters can be designed with only a knowledge of the relative times of the arrivals for the various signal and noise events but with no knowledge of the actual event shapes. In addition, the filter may be specified to allow for such contingencies as the presence of random noise, variable gain between traces, and random variations of the relative time of arrival between traces. Once these parameters have been specified, the optimum linear filter may be computed. A suite of test computations has provided a set of generalizations which aid in the design of this type of filter and indicate that optimum multichannel linear filters can be far superior to straight addition for a wide class of organized noise.
A SPACE‐TIME FILTER FOR SEISMIC MODELS
Authors: Gangi Anthony F. , Disher David
Published: October 2002

A space‐time filter has been designed and constructed for use with arrays of transducers on seismic models. The filter allows real time, space‐time operations. Adjustments to change the filter parameters are simply performed. The device can be considered an analog implementation of the techniques used in “digital filtering.” The space‐time operator is capable of handling ten spatial (receiving transducer) inputs, and each input can be filtered with up to 20 feedback and feed‐forward filter points. The time operators are composed of lumped constant, electric delay lines tapped every 0.5 μsec with total delays up to 28 μsec available. The outputs from the delay line taps are summed in an operational amplifier summing circuit. The frequency response of the device is from approximately 10 kHz to 500 kHz, which covers the frequency band normally used in two‐dimensional seismology. Some simple experiments have been performed to demonstrate the utility of the device. These experiments are: a) feedforward time operations, b) feedforward/feedback time operations, c) feedforward space operations, and d) space‐time operations.
TERRAIN CORRECTIONS FOR BOREHOLE GRAVIMETRY
Authors: Hearst J. R.
Published: October 2002

The measurement of in‐situ density by borehole gravimetry has now become a commonly accepted, if not commonly used, practice (McCulloh, 1965, 1967; Howell et al., 1966; Hammer 1950). The expression for density as a function of gravity difference at two depths is given in a general form by McCulloh (1967) as [Formula: see text] [Formula: see text] where ρ is the density, F the free air gradient, [Formula: see text] the measured gravity difference between two depths, [Formula: see text] a correction for the effect of sub surface density differences (according to McCulloh, generally negligible), [Formula: see text] the terrain correction, [Formula: see text] a borehole correction, and k the gravitational constant. This equation can be obtained from first principles using Gauss’ law.
AUTOMATIC SEISMIC REFLECTION PICKING
Authors: Paulson Kenneth V. , Merdler Stephen C.
Published: October 2002

A computational system is described that detects seismic reflections recorded digitally on magnetic tape, performs migration if desired, and plots the information in cross‐section form. The detection or “picking” philosophy is based on the selection of the time positions of either all peaks or all troughs in the set of traces which constitutes the seismic section of interest. A lateral continuity search “correlates” these peaks or troughs across the traces in order to define reflection segments. All reflection segments which span at least three consecutive traces are stored in digital format on magnetic tape. They are given a grade which is the product of their spatial span and mean amplitude. A time‐varying grading threshold is employed to limit the number of reflection segments which are output to an automatic plotter or carried for subsequent processing such as computer migration. Several examples of these automatically derived cross sections are shown and compared with the original variable‐area record sections. This system is readily extended to multiline or grid data. Thus it is feasible to generate and display automatically three‐dimensional interpretive aids such as isochrons and isopachs.
REFRACTION DETERMINATION OF WATER TABLE DEPTH AND ALLUVIUM THICKNESS
Authors: Duguid James Otto
Published: October 2002

During an investigation of alluvium using a shallow refraction seismograph, time‐distance curves obtained indicated two refraction interfaces. The shallower of the interfaces is the ground‐water table, whose depth was obtained with an accuracy of ±10 percent. The deeper interface is either the surface of the bedrock or the base of the weathered zone on the bedrock. If this lower interface is the surface of the bedrock, the velocity of seismic waves in the weathered material will be considerably lower than if the interface is the base of the weathered zone. Using this criteria makes it possible to determine whether the seismic wave is refracted from the surface of the bedrock or the base of the weathered zone.
WHAT SOVIET GEOPHYSICISTS SAW IN THE UNITED STATES
Authors: Fedynskiy V. V.
Published: October 2002

Special circumstances occasionally warrant a departure from Geophysics’ policy of not publishing previously published material. Such an exceptional case is the following translation from the Russian of a paper by V. V. Fedynskiy summarizing observations of a group of Soviet geophysicists who recently toured the United States. These remarks should be of interest to a large number of our readers, as an indication of how Soviet geophysicists view their American counterparts. The present text is based on an original translation by V. P. Sokoloff, courtesy of the American Geological Institute.
BASIC EQUATIONS FOR SYNTHETIC SEISMOGRAMS USING THE z‐TRANSFORM APPROACH
Authors: Robinson Enders A.
Published: October 2002

In the past few years several papers have been published on the z‐transform approach to the problem of seismic wave propagation in layered media. Although these papers start with the same fundamental equations, small differences in notation make the study of the synthetic seismograms derived in these papers time consuming. In order to aid the reader, the present note gives the basic equations as well as tables which show the correspondence for the main symbols used in the papers of Goupillaud (1961), Kunetz (1964) Sherwood and Trorey (1965), Treitel and Robinson (1966), Robinson (1967), and Claerbout (1968).
AN EXAMPLE OF LINEAR FILTERING IN AEROMAGNETIC INTERPRETATION
Authors: Naidu Prabakar S.
Published: October 2002

An anomaly east of Great Slave Lake in Northern Canada was selected to test the applicability of linear filtering to aeromagnetic data. Since the anomaly is linear, we have considered two parallel but widely separated profiles across the anomaly. The aeromagnetic anomaly observed along these profiles was filtered using Strakhov’s filter operator. The filtered signal was matched with the computed field of a dike model with an average susceptibility in the range 1.7 to [Formula: see text] CGS Units. Furthermore, from the suppressed noise component, an estimate of the variance of the random magnetization has been made. The estimated variance is in the range 0.61 to [Formula: see text] cgs units. Although there is no available geological information about the value of these parameters, the calculated values seem to be reasonable.
AN AUTOMATED ULTRASONIC PULSE MEASUREMENT SYSTEM
Authors: Thill Richard E. , Bur Thomas R.
Published: October 2002

Many applications of ultrasonic pulse techniques for studying the elastic behavior of rock have already been disclosed by Hughes et al (1950, 1951, 1952, 1957), Birch (1960, 1961), Simmons (1964 a, b, 1965), Christensen (1965, 1966a & b), and others. Although similar ultrasonic pulse measurement systems have been described by Tocher (1957), Fairhurst (1961), and Gregory (1967), the system described in this paper is automatic and permits continuous monitoring of elastic wave traveltime. Once set up, the system can record elastic wave traveltime at a preset rate for nearly any time period without intervention by an operator. This automated system has been used to study effects of environment on longitudinal pulse velocity in several rocks (Podnieks et al, 1968) and to make other preliminary investigations of longitudinal pulse velocity variations in rock under external influences. This report notes also several applications to problems of current interest to investigators of the elastic properties of rock.
INDUCED POLARIZATION OF SYNTHETIC, HIGH‐RESISTIVITY ROCKS CONTAINING DISSEMINATED SULFIDES
Authors: Scott W. J. , West G. F.
Published: October 2002

The induced polarization and resistivity of a suite of 74 synthetic, low porosity, “rock” specimens were measured by the pulse transient method. The specimens contained up to 8 percent by volume of sulfide particles in three different grain sizes. A measuring cycle of 5 sec on and 5 sec off was employed, and discharge potentials were recorded for 4 sec by oscilloscope photography. Charging current density was varied between [Formula: see text] and [Formula: see text]. Chargeability—defined as the time integral, from 15 to 415 ms after cessation of charging, of the instantaneous discharge potential divided by the charging potential—was found to vary from 1 to 30 ms. For current density less than about [Formula: see text], it was independent of charge current density, but decreased about 20 percent/decade with increase in current density. A background chargeability of a few milliseconds, virtually indistinguishable from the sulfide IP, was found in nonmineralized specimens and assumed to be present in all. Chargeability due to metallic minerals was found to be proportional to sulfide content (volume percent) and to decrease with increasing grain diameter, as predicted by the theory developed by Wait. The observed decay forms were not exponential as this theory predicts, but were approximately logarithmic. The metallic mineral‐electrolyte interface impedance determined for the specimens by use of Wait’s theory is consistent with that measured directly by Anderson and Keller for a clean pyrite interface.
VARIATIONS OF VERTICAL GRAVITY GRADIENT IN NEW YORK CITY AND ALPINE, NEW JERSEY
Authors: Kuo John T. , Ottaviani Mario , Singh Shri K.
Published: October 2002

Careful gravity measurements with La Coste‐Romberg geodetic gravimeters were carried out in tall buildings on a floor‐to‐floor basis in New York City and on the Armstrong Tower, Alpine, New Jersey. Corrections for the instrumental drift and tidal gravity variation and for the Bouguer effect, topography, mass of the buildings, and subway and basement excavations have been applied to the gravity data, which are tied to the absolute gravity value of the National Gravity Base Station of Washington, D. C. The observed gravity versus elevation curves are nonlinear, particularly near the surface of the ground; the slope of the observed gravity anomaly versus elevation curves reverses sign at an elevation of about 170 ft for the campus buildings and about 350 ft for the downtown buildings, and is nearly linear without a reversal for the Armstrong Tower. The vertical gradients vary substantially even within short distances. Comparisons of the corrected observed gradients with the theoretical gradients of gravity are made. The anomalous gradient anomalies are positive and are correlated with the positive isostatic surface gravity anomalies. Calibration of gravimeters against the observed vertical gradient of gravity to an accuracy of ±2 μgal is definitely feasible provided the gradient is predetermined to a comparable accuracy by a standard instrument.
ADDENDUM TO GLOSSARY OF TERMS USED IN GEOPHYSICAL EXPLORATION
Authors: Dobrin M. B.
Published: October 2002

All of us concerned with the preparation of the “Glossary of Terms Used in Geophysical Exploration” published in the February, 1968 issue of Geophysics (v. 32, p. 183–228) were highly gratified by the interest it appeared to generate. The number of orders for reprints (4868 until February, 1969) seems to have set a record for sales of SEG special publications. Moreover, the response to our request for comments and corrections was greater than anyone expected.
BANDWIDTH REQUIREMENTS FOR A BINARY GAIN AMPLIFIER SYSTEM
Authors: Távora Carlos J. , Rangel Netto José L. M.
Published: October 2002

In the design of amplifiers with digital gain control, it is possible to specify a maximum sampling error and to derive the minimum bandwidth necessary in each stage of the system to achieve the specified accuracy. The derivation is based on the assumptions that all stages have the same frequency response and synchronized gain controls. The bandwidth may be calculated by an iterative method.
RESISTIVITY SOUNDINGS IN A PETROLEUM PROSPECT IN THE COONAMBLE BASIN, N.S.W., AUSTRALIA
Authors: Henderson R. J. , Emerson D. W.
Published: October 2002

Preliminary investigations at well sites in the Coonamble Basin, New South Wales, have demonstrated that deep resistivity soundings can provide valuable information in petroleum exploration with relative ease. The correct thickness of the unmetamorphosed basin sediments was accurately defined from resistivity data whereas interpretation of seismic reflection and aeromagnetic data had previously yielded large over‐estimates. Field procedures can be conducted in a manner suitable to routine prospecting. The dipole configuration has certain advantages in this respect, not the least of which is its ability to resolve low dips in sedimentary basins. Synthetic sounding curves calculated on a digital computer from digitized well log resistivity data are useful to predict the results to be expected from an actual field sounding and greatly facilitate the interpretation of the results.
THE DETERMINATION OF OPTIMUM GATE LENGTHS FOR TIME‐VARYING WIENER FILTERING
Authors: Wang R. J.
Published: October 2002

The response function of a time‐varying filter changes with the output signal, or observation time. Most existing time‐varying filter techniques involve the empirical division of a seismic trace into a number of gates (or time windows) of given length, and a time‐invariant filter is determined for each such gate. Few treatments have dealt with analytical methods to establish the gate lengths according to some optimum criterion. This paper describes a technique for the determination of optimum gate lengths. It is based on the work of Berndt and Cooper, which is here applied to the calculation of time‐varying Wiener filters. The Berndt and Cooper technique produces an upper bound for the mean‐square error between the true and a given approximated time‐varying correlation function. The minimization of this upper bound leads to a relation which enables one to establish gate lengths directly from the input trace. Thereafter, ordinary time‐invariant Wiener filters can be computed for each gate. The overall filtered trace is obtained in the form of a suitably combined version of the individually filtered gates. Experimentally it is shown that, with the Berndt and Cooper technique to determine optimum gate lengths, time‐varying Wiener filters can be better than a time‐invariant filter.
THE USE OF SCHLUMBERGER AND EQUATORIAL SOUNDINGS IN GROUNDWATER INVESTIGATIONS NEAR EL PASO, TEXAS
Authors: Zohdy Adel A. R.
Published: October 2002

Electrical soundings using the symmetric AMNB Schlumberger and the bipole‐dipole equatorial arrays were made along two profiles near El Paso, Texas, in support of a groundwater exploration program which included seismic refraction and gravity surveys. Electrode spacings ([Formula: see text] or R) reaching 12,000 ft allowed exploration to depths of about 7000 ft. Geoelectrical information on the subsurface materials was augmented by sounding with the bilateral equatorial configuration and by transforming Schlumberger curves into dipole‐polar sounding curves with formulas developed by Al’pin and by Tsekov. The bilateral equatorial sounding curves were found useful for detecting the direction of dip of highly resistive bedrocks whereas transformed sounding curves were used to evaluate the average longitudinal resistivity, and hence the depth, to the “electric basement.” A few of the Schlumberger sounding curves were either clearly or subtly distorted by nonhorizontal geologic structures. The interpretation of these sounding curves illustrates the requirement for careful analysis in processing electrical prospecting data obtained over complex geologic conditions. The interpretation of a combined Schlumberger‐equatorial sounding curve, which did not agree with the preliminary interpretation of seismic refraction data, was confirmed to be correct by data from a test well drilled to a depth of 4363 ft. The application of electrical sounding data in the El Paso area furnished valuable information on the depth to fresh‐water‐salt‐water interfaces and on the depth to highly resistive impervious bedrocks.
QUASI‐STATIC TRANSIENT RESPONSE OF A CONDUCTING PERMEABLE SPHERE
Authors: Wait James R. , Spies Kenneth P.
Published: October 2002

When a conducting body is immersed in a time‐varying magnetic field, eddy currents are induced. These, in turn, produce a secondary magnetic field which may be detected by an observer external to the sphere. It has been demonstrated that a measurement of the external field can be used to estimate the conductivity of the body if certain assumptions are valid. For example, Ward (1953) has shown that the conductivity and permeability of geological core specimens from diamond drill holes may be determined by examining the frequency dependence of the time‐harmonic response of the specimen. In principle, the same information should also be available from the time response of the specimen for a suddenly applied magnetic field (Wait, 1951). In this paper, we wish to discuss the transient solution when a homogeneous conducting sphere is under the influence of a transient magnetic field. Also, as suggested earlier by Wait (1951), the actual time‐domain electromagnetic response of a massive sulfide body should exhibit features akin to this transient solution. Thus, the results should have application in mining geophysics. We consider both a nonpermeable and a permeable sphere, using a different approach for each. Because the former is a special case of the latter, a consistency check is obtained.
SH WAVES FROM EXPLOSIVE SOURCES
Authors: Geyer Robert L. , Martner S. T.
Published: October 2002

Horizontally polarized shear (SH) waves have been reported from nuclear explosions and very small explosions. This paper presents selected examples of SH waves from charges ranging up to 50 lb and detected by downhole three‐component seismometers. The examples include outgoing SH waves and SH reflections from faults and an escarpment. The SH waves usually can be identified wherever three‐component seismometers are used. The SH waves appear to be generated near the point of detonation in earth materials that have no apparent horizontal changes in lithology, but comprise layers with marked vertical velocity contrasts. The observations neither confirm theoretical explanations that require horizontal changes in material surrounding the charge nor contribute to a satisfactory theoretical explanation for the generation of SH waves from explosive sources.
COUPLING OF EXPLOSIVE ENERGY IN THREE‐DIMENSIONAL MODELS
Authors: Bahjat Dhari S. , Kisslinger Carl
Published: October 2002

The coupling of explosive generated P waves to the surrounding medium was investigated in a three‐dimensional laboratory model. For tightly coupled charges the amplitude was found to increase as [Formula: see text], and the pulse width as [Formula: see text], where W is the charge mass. Only a few hundredths of one percent of the energy in the explosion was transmitted in the initial P wave. When charges were fired in air‐filled cavities, the amplitude of the energy in the P wave increased to a maximum and then decreased with increasing cavity radius. The amplitudes from cavity shots were never less than the amplitudes for the tightly coupled shots. As the cavity radius increased, the pulse width of the P wave decreased to a minimum, an indication of a decrease in the size of the equivalent cavity, and then increased with further increase in cavity size. The period minimum is interpreted as corresponding to the transition from nonelastic to elastic response of the cavity wall. The cavity pressure at this transition is about one‐half the nominal tensile strength of the material. Scaling to the Sterling nuclear event is examined, and the conclusion is that the disagreement between field tests of decoupling and our experiments is due to the dominance of short period energy in our experiments. The results cast doubt on the general validity of partial decoupling.
EVALUATION OF OMEGA NAVIGATION SYSTEM
Authors: Kirby R. A.
Published: October 2002

As the exploration and oceanographic effort moves farther and farther offshore, the problem of navigation and accurate position fixing becomes increasingly difficult. Although the present shore‐based, position‐fixing systems such as Raydist, hereinafter called radio frequency location system, Lorac, and Decca are quite satisfactory as far as their inherent accuracy is concerned, their relatively short range makes them inadequate for surveying much of the world’s offshore area. In addition, at their extreme range, their dependable operation is limited to the hours between sunrise and sunset because of nighttime sky wave interference.
A SURFACE INTEGRAL IN POTENTIAL THEORY
Authors: Bodvarsson Gunnar
Published: October 2002

In potential theory it is often convenient to transform volume integrals over homogeneous bodies into surface integrals. The suppression of one dimension will in general simplify the calculation of the integrals. For a long time this technique has been used in magnetic field theory and the transformation is given in standard texts on potential theory (see MacMillan, 1930, section 55). The transformation appears less well known to authors concerned with the gravity method. The purpose of the present short note is to give an example of the application of this technique, a technique which appears to lead to results of practical value in the computation of the acceleration of gravity due to given bodies.
LIMITATIONS OF THE REFLECTION SEISMIC METHOD; LESSONS FROM COMPUTER SIMULATIONS
Authors: Taner M. Turhan , Cook Ernest E. , Neidell Norman S.
Published: October 2002

Piece‐wise local linearity of the subsurface reflectors and uniqueness of primary reflection travel path between particular source‐receiver pairs are two of the fundamental assumptions of multiple ground coverage reflection seismic techniques as currently employed. Some recent developments in seismic exploration which spring from the progress made in velocity determination techniques violate these fundamental assumptions with potentially serious consequences such as spurious geometries after migration and unrealistic interval velocities. In this paper we apply analytic techniques and computer simulation to linear and nonlinear subsurface models in order to obtain better definitions of the limitations of the reflection seismic method. The resulting lessons are of immediate practical value in seismic interpretation and clarify a variety of commonly occurring but puzzling situations such as phantom faults, incomplete contacts and segmented reflectors at unconformities, and apparent reversal of dip. In its basic approach this work has much in common with the generation and study of synthetic seismograms. Examination of the groundwork of the seismic method provides a firm base for extending its use.
INTERPRETATION OF SEISMIC REFRACTION DATA AND THE SOLUTION OF THE HIDDEN LAYER PROBLEM
Authors: Kaila K. L. , Narain Hari
Published: October 2002

A new statistical method is described for the interpretation of seismic refraction data. This method is then applied to the interpretation of a seismic refraction profile 15,220 m long shot by the Oil and Natural Gas Commission of India along the Hoshiarpur‐Tanda road in Punjab State. The 14th iteration least squares straight line fit made to the traveltimes of first refracted arrivals gives for the Hoshiarpur area five layers 144, 322, 726, 769, and 1711 m thick with velocities of 1667, 1906, 2209, 2778, and 3505 m/sec respectively above the basement at a depth of 3672±11 m. The basement velocity is found to be 6514 m/sec. Analysis of later refracted arrivals indicates the existence of a hidden layer with a velocity 4280 m/sec in the Hoshiarpur area. Due to the presence of the hidden layer, the fifth layer with a thickness of 1711 m computed from first arrival analysis is split into two layers with thicknesses of 1160±10 and 752±18 m; the 752‐m‐thick layer is the hidden layer. As a result of the hidden layer, the computed basement depth increases to 3873±21 m. The importance of later refracted arrivals for the solution of hidden layer problems in refraction seismology is duly stressed. An extension of Green’s method (1962) for determining the possible range of a hidden layer thickness beneath a multiple layer overburden is given and applied to the field problem discussed in this paper.
A GRAPHICAL METHOD FOR COMPUTING GEOPHONE GROUP RESPONSE
Authors: Verma Rajni K. , Roy Amalendu
Published: October 2002

The theory for multiple seismometers or shotholes has been dealt with extensively in the literature (see, for instance, Parr and Mayne, 1955; Savit et al, 1958; White, 1958). In deciding upon a particular pattern, a seismic crew, after having determined the relevant noise characteristics of the area, may follow one of the two paths: (1) the crew may decide on a particular response and may then seek analytically, in a rather involved manner, an optimum geophone group whose response satisfactorily approximates the desired response or (2) the crew may compute the responses of a variety of geophone groups to the predominant noise pulse and accept the one that gives the best overall attenuation. Most seismic crews would choose the second alternative as being simpler and more practical, if less elegant than the first one. Besides, a calculated optimum group almost invariably would require fractional weights for the individual elements that are not practically realizable. Our note describes a very simple graphical procedure for fast computation of array responses that is applicable to a standing sine wave or a pulse and to linear, tapered, equally‐spaced, or unequally‐spaced arrays. The method is so obvious that it is probably in use already in some organizations; although no published evidence to that effect seems to be available.
INSTRUMENTAL ARRANGEMENT TO MEASURE GRAVITY WITH GRADIENTS
Authors: Thyssen‐Bornemisza Stephen
Published: October 2002

Every mass generates gravity gradients in addition to the gravitational field itself; this fact suggests that vertical gradients may be determined with a gravity measuring device based on a two‐level observation technique (Hammer, 1938; Thyssen‐Bornemisza, 1944). The inverse process, i.e. measurement of gravity or gravitational acceleration with the help of vertical gradients, apparently has not been investigated. Of course, gravity values can be computed from vertical gradients by integration (Paterson, 1961), but to actually measure gravity with vertical gradients is quite a different problem. The theoretical solution presented here provides the background for possible practical application.
THE DIPPING DIKE CASE: A COMPUTER CURVE‐MATCHING METHOD OF MAGNETIC INTERPRETATION
Authors: McGrath P. H. , Hood Peter J.
Published: October 2002

A self‐adjusting curve‐matching computer program for the interpretation of magnetic anomalies produced by dike‐like bodies is described. The method yields an optimum set of dike parameter values by selecting a best‐fit model anomaly curve for a given set of discrete observed data along a profile across a two‐dimensional anomaly. The criterion used by the computer for the selection of a best‐fit curve may be either the linear or least squares condition. Convergence to either of these conditions requires the solution of a nonlinear problem. In the present case, solution is achieved by automatic trial and error modifications of an initial model. Selection of the initial model is based upon the shape of the observed anomaly; the proper choice of an initial model assures convergence of the error of fit to an absolute rather than to a relative minimum. The repeatability of the convergence of the iterative process can be tested by changing the initial model within the allowable ranges of variation of the dike parameters. Interpretations of finite strike length anomalies are achieved through the use of a correction curve. With this method, best‐depth estimates to the top of dike‐like bodies may be determined and for cases of induced magnetization, the effective susceptibility contrast and the dip of the dike may be calculated. Two practical applications of the method are presented.