bluuebunny/crossref_metadata_2025 huggingface dataset:
On: “Convergence and Divergence in Downward Continuation” by Janardan G. Negi (GEOPHYSICS, October 1967, p. 867–871)
Authors: Paul P. A.
Published: October 2002

In the paper under discussion, Dr. Negi has investigated the boundaries of convergence of three power series and has considered the behavior of the series on their respective boundaries of convergence. For convenience he reduces the series to their standard forms. By consulting Bromwich (1964), he finds the boundaries of convergence and with the help of a theorem (Morse and Feshbach, 1953), derives three relations, one for each series, for testing the presence of singularities on the boundaries of convergence. Dr. Negi (1968) states that the relations predict all the three series would always be divergent on their respective boundaries of convergence. In what follows it is shown that (1) the relations found by Dr. Negi tell us that the singular points are ordinary and many ordinary points on the boundaries are singular, (2) the relations do not predict the behavior of the series, and (3) the last two series do not diverge on their respective boundaries of convergence.
On: “The anomalous vertical gradient of gravity” by Sigmund Hammer (GEOPHYSICS, February 1970, p. 153–157).
Authors: Thyssen‐Bornemisza Stephen
Published: October 2002

The interesting analysis by Hammer seems to be founded partly on the approach of Heiskanen and Moritz (1967), in which a real gravity and a vertical gravity gradient are correlated according to the relation [Formula: see text] Here Δg denotes the free‐air anomalies on a horizontal plane passing through observation point p and [Formula: see text] is the anomaly at p, the latter representing the center of the (x, y) coordinates.
GEOMAGNETIC EFFECTS OF SLOPING AND SHELVING DISCONTINUITIES OF EARTH CONDUCTIVITY
Authors: Jones F. Walter , Price Albert T.
Published: October 2002

The surface effects on electric and magnetic variations arising from interfaces in three two‐dimensional conductivity models are compared in detail for four frequencies. It is found that the horizontal extent of the surface effects greatly depends on the dimensions of the different structures relative to the skin depths at the frequencies used. Also, a fundamentally important difference is revealed between H‐polarization and E‐polarization problems in that the apparent resistivity computed on the surface near the contact is discontinuous and changes abruptly in the H‐polarization case, whereas for E‐polarization the apparent resistivity is continuous and changes gradually across the contact.
A FILTERING TECHNIQUE FOR INTERPRETING THE GRAVITY ANOMALY GENERATED BY A TWO‐DIMENSIONAL FAULT
Authors: Davis Thomas M.
Published: October 2002

In order to combine the advantages of a direct quantitative interpretation with an efficient regional‐residual separation, an interpretation technique based on characteristic curves derived through digital bandpass filtering has been developed for the two‐dimensional gravity fault model. Although the two‐dimensional fault model suffers from the lack of characteristic parameters, reasonably accurate direct interpretations are possible even in the presence of regional fields whose long wavelength spectral components overlap those of the anomaly. The characteristic curves given here were designed for use on relatively large deep structures with depths and thicknesses ranging from 0.8 to 8 km; however, the slight modifications necessary to accommodate any range are presented in detail.
AN AEROMAGNETIC AND AERORADIOACTIVITY SURVEY OF LIBERIA, WEST AFRICA
Authors: Behrendt John C. , Wotorson Cletus S.
Published: October 2002

A 140,000 km aeromagnetic and total‐count gamma radiometric survey was made over Liberia in 1967–68 along north‐south lines spaced 0.8 km over land and 4 km over the continental shelf. The data approximately delineate the boundary between the Liberian (ca. 2700 m.y.) age province in the northwestern two‐thirds of the country, and the Pan‐African (ca. 550 m.y.) age province in the coastal area of the northwestern two‐thirds of the country, as well as a boundary marking the northwest extent of the isoclinally folded paragneisses and migmatites deformed within the Eburnean (ca. 2000 m.y.) age province in the southeast one‐third. A zone of diabase dikes about 90 km inland can be traced, parallel to the coast from Sierra Leone to Ivory Coast on the basis of the magnetic data. Another zone of diabase dikes about 185 m.y. old is located along the coastal area and beneath the continental shelf parallel to the coast northwest of Greenville. Intrusion of these dikes probably coincides with the separation of Africa from North and South America. The magnetic data suggest basins of sedimentary rocks possibly 5 km thick on the continental shelf. The map indicates high‐amplitude magnetic anomalies greater than 600 gammas; some reach amplitudes as great as 18,000 gammas over iron formation and about 1800 gammas over mafic and ultramafic intrusive bodies. The radioactivity data have a background level less than 100 counts per second (cps) over mafic granulite‐facies rocks and unmetamorphosed sedimentary rocks in the coastal area. Granitic rocks have the greatest variation. The central area of the country has the highest background radiation level with large areas above 250 cps; the level in the eastern one‐third of the country is low. These data are proving quite useful in reconnaissance geologic mapping. All anomalies over 500 cps are shown; some reach amplitudes over 750 cps. Total‐count radiation levels have a significant correlation with percent [Formula: see text] in bedrock analyses, but anomalous amounts of Th and U must be present to account for the highest amplitude anomalies. A few specific anomalies have been correlated with concentrations of monazite and zircon in bedrock as well as in beach deposits.
THE USE OF POISSON’S RELATION FOR THE EXTRACTION OF PSEUDOTOTAL MAGNETIC FIELD INTENSITY FROM GRAVITY OBSERVATIONS
Authors: Robinson Edwin S.
Published: October 2002

Investigation of geological structure by gravimetric and magnetic field surveys requires consideration of relationships between gravity anomaly and magnetic anomaly generating sources. The possibility of using Poisson’s Relation to examine magnetic and gravity fields related to a common source is intriguing. This relation is expressed as follows: [Formula: see text] (1) where A (x, y, z) is the magnetic field potential and U (x, y, z) is the gravity field potential at a point in space due to a source of uniform density ρ and uniform magnetization I in the direction α. This expression has been used to derive magnetic anomalies over idealized forms (Nettleton, 1940) and, by Baranov (1957), to extract pseudogravity fields from magnetic field data. The purpose of this paper is to develop an expression for extracting a pseudomagnetic field from gravity field data and to examine the practical applications of this expression.
AN AUTOMATIC METHOD OF COMPILATION AND MAPPING OF HIGH‐RESOLUTION AEROMAGNETIC DATA
Authors: Bhattacharyya B. K.
Published: October 2002

An automatic method has been developed for compilation of digital aeromagnetic data. This method has been applied to the data obtained during a high‐sensitivity aeromagnetic survey over an area in the Precambrian shield of northeastern Ontario in Canada. With this method, all points of intersection between traverse and base lines are determined automatically and adjusted within the limits of positional error for minimizing differences in magnetic values at the intersections. Then the data are corrected for diurnal variation and leveled to tie the magnetic measurements together. Next, the resulting total field values are contoured with a machine method at a scale of 1:25,000. For such a scale, the minimum contour interval that can be used in the present area is two gammas. However, because of the accuracy of the method of compilation, with a larger scale, it is possible to trace one‐gamma contours. The maps thus compiled have been compared with published aeromagnetic maps of data obtained with conventional flux‐gate and proton‐precession magnetometers. The new maps are vastly superior to the old ones for delineating trends, patterns, and fine features of available detailed geological maps. This superiority is mainly due to the excellent definition of small amplitude anomalies, some of only a few gammas in magnitude, on the high‐resolution magnetic maps.
THE BOUGUER CORRECTION FOR THE SPHERICAL EARTH
Authors: Karl John H.
Published: October 2002

It is common practice for authors of geophysics textbooks to correct for the Bouguer effect by using the gravitational attraction due to an infinite horizontal slab of material (Dobrin, 1960, p. 189; Griffiths and King, 1969, p. 142; Heiland, 1940, p. 137; and Howell, 1959, p. 221). Such a slab of thickness h and density ρ will give rise to an acceleration [Formula: see text] where γ is the universal gravitational constant.
DPM‐1: A LONG‐WIRE DIP‐ANGLE ELECTROMAGNETIC METHOD
Authors: Anderson Chris D. , Sutherland D. B.
Published: October 2002

The source for the DPM‐1 electromagnetic exploration method is a long, grounded wire (5000 to 25,000 ft in length). The transmitter operates at a frequency of approximately 500 hz and has a power output of 20 watts. The dip angle is measured in a plane perpendicular to the wire at distances up to 12,000 ft from the wire. A conductor is characterized by a marked flexure in the dip‐angle curve, with the conductor axis lying approximately beneath the inflection point of the curve. Case histories from the Manitoba nickel belt and the Uchi Lake mine, Ontario show good responses for conductors from near surface to 150 ft depth.
GEOPHYSICAL CASE HISTORIES OF DISSEMINATED SULFIDE DEPOSITS IN BRITISH COLUMBIA
Authors: Fountain David K.
Published: October 2002

The growth of the mining industry in British Columbia in recent years has been mainly due to the development of large low‐grade disseminated sulfide deposits. The two problems faced by the geophysicist in the exploration for deposits of this nature are the difficulty of, one, detecting large volumes of rock containing a low‐percentage content of total sulfide mineralization and, two, obtaining some idea of the economic significance of the mineralization. Although the standard geophysical methods used in the search for disseminated sulfide deposits have been successfully applied, there are some aspects of the British Columbia deposits which create additional problems for the geophysicist. As illustrated by the Brenda, Valley Copper, and Lornex deposits, the lack of appreciable pyrite associated with the economic mineralization results in a low total sulfide content; this in turn makes it difficult to obtain a diagnostic anomalous geophysical response. Because of the rugged terrain and generally heavy forest growth, the more powerful geophysical methods, such as induced polarization, are high‐cost techniques. However, alteration and fracturing associated with the disseminated mineralization (Babine Lake area) may lower the overall resistivity of a deposit sufficiently to allow detection with electromagnetic (EM) methods. Although limited in application, the lower cost of EM methods in many situations renders their use practical, especially if airborne techniques can be employed.
THE SEISMOVAC MONOPULSE PNEUMATIC REBOUND SEISMIC SOURCE
Authors: Goldberg Seymour
Published: October 2002

This note describes a new seismic source developed for offshore exploration. The source develops an impulse output and has the desirable feature of eliminating the secondary bubble, or oscillation, pulses characteristic of impulsive sources, permitting improved resolution of geologic features in the subbottom. The output spectrum is normally contained entirely in the useful seismic band below 100 Hz, but can be tailored by the operating parameters to yield lower or higher cutoff frequencies if desired. Optimum spectrum generation, along with the conservation of the energy normally wasted in secondary oscillations, results in high source efficiency.
QUASI‐STATIC TIME‐DOMAIN ELECTROMAGNETIC RESPONSE OF A HOMOGENEOUS CONDUCTING INFINITE CYLINDER
Authors: Verma Saurabh K.
Published: October 2002

Under some simplifying assumptions, the transient electromagnetic response of a homogeneous conducting infinite cylinder is derived. Two types of primary disturbances, represented by step‐function and ramp‐function pulses, are considered. Based on the theoretical results, response and decay constant curves are plotted and their utility in time‐domain electromagnetic surveys for conductive elongated ore deposits is described.
A NONLINEAR LEAST‐SQUARES METHOD FOR SEISMIC REFRACTION MAPPING—PART II: MODEL STUDIES AND PERFORMANCE OF REFRAMAP METHOD
Authors: Ocola Leonidas C.
Published: October 2002

The Reframap Method performance is tested along with Profile and Time‐Term Methods through numerical models which simulate problems that frequently arise in crustal seismic refraction studies and partially investigate configurations poorly resolved by standard methods. The Reframap Method is found as accurate as the Profile Method for depth and velocity estimation and as flexible as the Time‐Term Method in the layout of sources and detectors. This method does not require the constraints of nearly planar interfaces, in‐line elements, and reversed traveltimes of the Profile Method. The Reframap Method suppresses the small slope and curvature restrictions imposed upon the refracting interfaces by the Time‐Term Method and eliminates the undetermined constant of the time terms (delay times).
ELECTROMAGNETIC DEPTH SOUNDING EXPERIMENT ACROSS SANTA CLARA VALLEY
Authors: Ryu Jisoo , Morrison H. Frank , Ward Stanley H.
Published: October 2002

An electromagnetic depth sounding experiment with a horizontal loop carrying an oscillating current was carried out in Santa Clara Valley, California. The field data are interpreted in terms of the polarization parameters of the magnetic polarization ellipse. The chosen parameters are tilt‐angle, ellipticity, and the modulus of wavetilt. The electrical discontinuities deduced are in general agreement with well data and with a geologic section based on resistivity soundings. The results clearly reveal an intermediate, highly resistive layer, which is a permeable stratum for groundwater recharge. It is concluded that a portable electromagnetic sounding system, measuring only tilt‐angle and ellipticity, should easily locate highly resistive gravel deposits bounded by conductive clay beds. It is theoretically obvious that the system is useful in those regions where it is extremely difficult to inject current into the ground.
COMPRESSIONAL‐WAVE ATTENUATION IN MARINE SEDIMENTS
Authors: Hamilton Edwin L.
Published: October 2002

In‐situ measurements of compressional (sound) velocity and attenuation were made in the sea floor off San Diego in water depths between 4 and 1100 m; frequencies were between 3.5 and 100 khz. Sediment types ranged from coarse sand to clayey silt. These measurements, and others from the literature, allowed analyses of the relationships between attenuation and frequency and other physical properties. This permitted the study of appropriate viscoelastic models which can be applied to saturated sediments. Some conclusions are: (1) attenuation in db/unit length is approximately dependent on the first power of frequency, (2) velocity dispersion is negligible, or absent, in water‐saturated sediments, (3) intergrain friction appears to be, by far, the dominant cause of wave‐energy damping in marine sediments; viscous losses due to relative movement of pore water and mineral structure are probably negligible, (4) a particular viscoelastic model (and concomitant equations) is recommended; the model appears to apply to both water‐saturated rocks and sediments, and (5) a method is derived which allows prediction of compressional‐wave attenuation, given sediment‐mean‐grain size or porosity.
On: “An Aeromagnetic and Aero‐radioactivity Survey of Liberia, West Africa,” by John C. Behrendt and Cletus S. Wotorson (GEOPHYSICS June 1971, p. 590–604)
Authors: Brod R. Jerry
Published: October 2002

Perhaps the most significant criticism of this paper is that it does not establish the priorities that determined the equipment used and the manner in which the radiation and magnetic data were to be collected. According to Dr. Behrendt, considerations for the magnetic survey were dominant and the radiation capability was added as a subordinate system. The desirability of using a system with spectral capability was recognized, but cost considerations dictated the final choice. Operationally, the prime consideration was for the factors influencing the collection of magnetic data so that radiation data were sometimes collected at less than ideal times.
On: “Depth of Investigation in Direct Current Methods,” by A. Roy and A. Apparao (GEOPHYSICS, October 1971, p. 943–959)
Authors: Koefoed O.
Published: October 2002

In this very interesting paper, the authors ascribe the potential difference that is measured in resistivity methods to electrical‐current polarization of the subsurface. This description enables them to compare the relative contributions of different portions of the subsurface to the measured potential difference.
On: “The Use of Linear Filtering in Gravity Problems,” by C. C. Ku, W. M. Telford, and S. H. Lim (GEOPHYSICS, December 1971, p. 1174–1203)
Authors: Hibler William D.
Published: October 2002

In a subsection of their paper entitled “Convolution versus Multiplication,” the authors state that when using finite samples of equally spaced data, multiplication of the filter function in the frequency domain and the equivalent convolution in the space domain will yield different outputs. This is normally true but may be circumvented by using a carefully chosen frequency space‐filter function. Such a technique is normally referred to as the aperiodic fast Fourier transform convolution technique (Stockham, 1966), whereas normally multiplication in frequency space yields a periodic convolution with spurious boundary effects.
CONTINUOUS VELOCITY ESTIMATION AND SEISMIC WAVELET PROCESSING
Authors: Sherwood J. W. C. , Poe P. H.
Published: October 2002

An economic computer program can stack the data from several adjoining common depth points over a wide range of both dip and normal moveout. We can extract from this a set of seismic wavelets, each possessing a determined dip and normal moveout, which represent the original seismic data in an approximate and compressed form. The seismic wavelets resulting from the processing of a complete seismic line are stored for a variety of subsequent uses, such as the following: 1) Superimpose the wavelets, or a subset of them, to form a record section analogous to a conventional common‐depth‐point stacked section. This facilitates the construction of record sections consisting dominantly of either multiple or primary reflections. Other benefits can arise from improved signal‐to‐random‐noise ratio, the concurrent display of overlapping primary wavelets with widely different normal moveouts, and the elimination of the waveform stretching that occurs on the long offset traces with conventional normal moveout removal. 2) By displaying each picked wavelet as a short dip‐bar located at the correct time and spatial position and annotated with the estimated rms velocity, we can exhibit essentially continuous rms‐velocity data along each reflection. This information can be utilized for the estimation of interval and average velocities. For comparative purposes this velocity‐annotated dip‐bar display is normally formed on the same scale as the conventional common‐depth‐point stack section.
GEOTHERMAL GROUND‐NOISE SURVEYS
Authors: Douze E. J. , Sorrells G. G.
Published: October 2002

In recent years there has been increasing interest in the role of geothermal steam as a source of energy. Only recently has geophysical exploration begun to play a major role in the exploration for geothermal resources; heat flow, gravity, and resistivity measurements have been the techniques most used. One recent development is the discovery that high surface‐noise levels are associated with the presence of geothermal reservoirs below the surface. Field surveys using short‐period seismographs have been conducted in the Imperial Valley of California in areas where heatflow measurements or drilling have indicated the presence of a geothermal deposit. In all three surveys abnormally high noise levels were found above the reservoir in the frequency range of 0.5 to 5.0 hz. The ground‐noise anomalies show a more complex pattern than the associated heat flow and gravity measurements. A theoretical model has been developed in which small, random pressure variations in a convecting geothermal reservoir are suggested as the source of the noise. Using this model, the noise level above one of the anomalies was duplicated, using a reasonably sized reservoir with pressure variations of less than 1 millibar.
On: “Reduction of Unevenly Spaced Potential Field Data to a Horizontal Plane by Means of Finite Harmonic Series,” by R. G. Henderson and L. Cordell (GEOPHYSICS, October 1971, p. 856–866)
Authors: Khattri K. N. , Datta A. N.
Published: October 2002

We have read the interesting article by Henderson and Cordell (197l). The authors transform the observed data using the finite Fourier series and compensate for the distortion introduced in the observed potential field due to observations taken on uneven topography.
INTERPRETATION OF VELOCITY SPECTRA THROUGH AN ADAPTIVE MODELING STRATEGY
Authors: Davis James M.
Published: October 2002

The value of the velocity spectra ultimately rests upon the capability of properly identifying primary reflections. Usually, this identification involves a significant expenditure of time by an experienced geophysicist. Because of increasing quantities of data requiring processing and the variance in the skill between individual interpreters, it is desirable to perform a substantial amount of the interpretational work automatically on a digital computer. The procedure presented in this paper uses an adaptive modeling strategy. Interval velocity and layer thickness are systematically adjusted by an iterative parameter optimization algorithm until the velocity spectra (semblance function surface) generated by the model match the spectra generated from the field data in a given optimal sense. In an example, comparisons are made between the spectra derived from the field data and the model. The interval velocity, as a function of depth, calculated from the model compares favorably with the sonic log of a nearby well.
HIGH‐FREQUENCY ELECTROMAGNETIC COUPLING BETWEEN SMALL COPLANAR LOOPS OVER AN INHOMOGENEOUS GROUND
Authors: Fuller James A. , Wait James R.
Published: October 2002

An integral formulation is given for the fields of a loop current source which is located over a horizontally stratified half‐space and has a vertical axis. The electrical properties of the half‐space vary exponentially with the depth into the earth. An asymptotic solution is developed for the case of source and observer on the interface but separated by a large numerical distance. The approximate solution is then used to determine the mutual impedance between two small loops and between the loop and a horizontal electric dipole, when the antennas are on the interface. It is found that the effect of stratification on the mutual impedance is represented approximately by a single multiplicative factor.
ELECTRICAL PARAMETERS OF ROCKS IN DEVELOPING GEOPHYSICAL TECHNIQUES
Authors: Collett L. S. , Katsube T. J.
Published: October 2002

An electrical rock measurement program has been initiated at the Geological Survey of Canada. Although there has been a considerable amount of work done in the past on electrical properties of rocks, the basic concept of the electrical theory applied to rocks and the definition of electrical parameters seem to differ between each group of scientists. This paper reviews the theory of the parameters that describe the complex electrical characteristics (conductivity, permittivity, and loss tangent) of rocks. A review is made of the measuring systems and classified by technique over the frequency range from [Formula: see text] to [Formula: see text] hz. For measurements on dry rocks and massive sulfide minerals, a 2‐electrode configuration is used. Measurement accuracy and reproducibility are within ±2 percent. The accuracy is checked against international standard samples. The research aspect of the laboratory is concerned with problems encountered in field surveys. Besides measurements on dry rocks, investigations on ultramafic rocks that exhibit high permittivity and large loss tangent and nonlinear behavior in dry and moist rocks and minerals, have been singled out for study, especially for developing measuring techniques. Experience with these techniques is useful for studying the electrical characteristics of moist rocks, including anisotropic effects, conductive overburden materials, the separation of copper‐rich sulfides from iron‐rich sulfides, and graphite, coal, frozen soils, and ice in permafrost. A better understanding of the complex behavior of electrical currents in rocks and sulfides will lead to improvements in equipment design, techniques, and interpretation in exploration geophysics.
MEASURING TECHNIQUES FOR ROCKS WITH HIGH PERMITTIVITY AND HIGH LOSS
Authors: Katsube T. J. , Collett L. S.
Published: October 2002

In this paper, perhaps for the first time, more realistic trends for the electrical characteristics of rocks with high loss and high permittivity are demonstrated. With decrease in frequency, below [Formula: see text] hz, the dissipation factor of these rocks generally increases, while the real relative permittivity generally levels off at about l0 to 30. These results were obtained by use of the airgap and several correction techniques at the electrode‐specimen contacts. The rocks used in these measurements contain a certain amount of moisture since they were dried in room atmosphere. When these rock samples are measured by the conventional electrode contact method, the real relative permittivity rises with decrease in frequency, while the dissipation factor levels off at a value of about 1.0 or below. These results are contrary to those obtained by measurements with the airgap technique and by applying several correction factors. For a serpentinite specimen, real relative permittivity measured by the conventional contact method rises 10 times above that measured by the airgap technique. Most results published in the past on electrical characteristics of rocks containing moisture show trends similar to those made by the electrode contact method. Therefore, these results published in the past perhaps contain errors due to insufficient consideration of the electrode‐specimen contacts. Since the rocks used in this study are not saturated with moisture, no conclusion can yet be drawn. However, the results in this paper indicate some important considerations which should be taken into account in the measurements on moist rocks that exhibit high permittivity and loss characteristics.
COMPUTER SYSTEMS FOR REAL‐TIME MARINE EXPLORATION
Authors: Darby E. K. , Mercado E. J. , Zoll R. M. , Emanuel J. R.
Published: October 2002

The goals of the Gulfrex are to perform marine exploration and to conduct research and development in the various facets of marine exploration. The Gulfrex is equipped with various geologic and geophysical data‐gathering instruments along with a highly sophisticated navigational package. Computerized control systems were designed to output data in real time so that preliminary interpretations could be made concurrently with data collection. One system, based on an EMR‐6130 computer, handles multichannel seismic data. This system includes routines for real‐time demultiplexing, normal moveout, stacking, and deconvolution. Output of demultiplexed data is to magnetic tape and optionally to paper records. Output of moveout‐corrected, stacked data is to a visual monitor and to magnetic tape via a PDP‐8 computer. Deconvolution may be applied to either the single‐channel traces or the stacked traces. A moveout‐corrected, CDP group is output to a paper record every 24 shots so that estimates of average velocities can be made for a normal‐moveout correction. Another system, designed for a PDP‐8 computer, is used to collect data from devices interfaced to it. These include single‐channel seismic data, gravity and magnetic measurements, and navigational measurements such as course, speed, ship position, and direction. Deconvolved single‐channel seismic data are plotted in real time on a visual monitor along with a corrected gravity profile, magnetic profile, speed, course, and time of day. A map of the ship’s course is plotted in real time on a drum plotter Inquiries may be made of the system for current position in latitude and longitude.
THE DETERMINATION OF DIGITAL WIENER FILTERS BY MEANS OF GRADIENT METHODS
Authors: Wang R. J. , Treitel S.
Published: October 2002

The normal equations for the discrete Wiener filter are conventionally solved with Levinson’s algorithm. The resultant solutions are exact except for numerical roundoff. In many instances, approximate rather than exact solutions satisfy seismologists’ requirements. The so‐called “gradient” or “steepest descent” iteration techniques can be used to produce approximate filters at computing speeds significantly higher than those achievable with Levinson’s method. Moreover, gradient schemes are well suited for implementation on a digital computer provided with a floating‐point array processor (i.e., a high‐speed peripheral device designed to carry out a specific set of multiply‐and‐add operations). Levinson’s method (1947) cannot be programmed efficiently for such special‐purpose hardware, and this consideration renders the use of gradient schemes even more attractive. It is, of course, advisable to utilize a gradient algorithm which generally provides rapid convergence to the true solution. The “conjugate‐gradient” method of Hestenes (1956) is one of a family of algorithms having this property. Experimental calculations performed with real seismic data indicate that adequate filter approximations are obtainable at a fraction of the computer cost required for use of Levinson’s algorithm.
TECHNIQUE FOR IMPROVED CONVERGENCE IN ITERATIVE ANALYSIS OF GRAVITY ANOMALY PROFILES
Authors: Fournier K. P. , Krupicka S. F.
Published: October 2002

Narrow two‐dimensional gravity anomalies are difficult to interpret iteratively by the relatively simple flat‐plate method suggested by Bott (1960) and employed by others. In this report Bott’s formula is modified empirically after a number of iterations have proceeded. The process is repeated until satisfactory convergence is obtained. The results can be applied to other profiles across the anomaly of interest and thus save substantial computer time by requiring fewer iterations. The technique is applied to two Bouguer anomaly profiles. Analysis of adjoining anomaly profile data with similar amplitudes and widths can be used to speed the convergence in the iterative modeling process.
HEAT FLOW IN A “BLIND” GEOTHERMAL AREA NEAR MARYSVILLE, MONTANA
Authors: Blackwell David D. , Baag Czang‐Go
Published: October 2002

A unique geothermal area has been discovered during the course of a regional heat‐flow study using holes drilled for mineral exploration. There are no surface manifestations of abnormal subsurface temperature in spite of the fact that at one locality a temperature of 58°C was measured at a depth of only 220 m. The area of anomalous heat flow straddles the Continental Divide near the old gold mining camp of Marysville, Montana about 30 km northwest of Helena. Measured values of conducted heat flow range from 3.2 to [Formula: see text] sec. The immediate source of the high heat flow is either an unexposed reservoir of thermal fluids or a very shallow still‐cooling magma chamber. At the present time the magma chamber model is preferred. The presence of additional similar areas in the western U.S. is suggested by the data from regional heat‐flow studies. However, in most of the other areas only single anomalous heat‐flow value is available, whereas at Marysville a region of several tens of [Formula: see text] is known to have abnormal heat‐flow values. It is suggested that temperature measurements should be made in available drill holes deeper than 30 m in the high heat‐flow regions of the western U.S. as an inexpensive way to explore for other “blind” geothermal reservoirs.
THE COVARIANCE OF EXTERNAL SHOT NOISE AT A SENSOR ARRAY
Authors: Edrington T. S.
Published: October 2002

Noise (of seismic and other origins) is often modeled as a shot process, i.e., as a random combination of wavelets. Specifically, if X is the random noise process and g is the deterministic waveform of the wavelet, then [Formula: see text]where the [Formula: see text] are Poisson points in time. One useful result (derived by Backus et al., 1964) under this model is that if the wavelets are propagating across a sensor array, and if the wavelet origins are uniformly distributed in azimuth, then the cross‐power spectrum for a pair of sensors is the product of the power spectrum at a single sensor and a zero‐order Bessel function. In the notation of Backus et al., [Formula: see text]
REMARKS ON WAVENUMBER FILTERING IN THE FIELD
Authors: Krey Th. , Toth F.
Published: October 2002

For many years exploration seismologists have endeavored to carry out wavenumber filtering by using a multiplicity of geophones per trace in order to attenuate undesired waves and to intensify the signal. In a similar manner, wavenumber filtering is carried out very often at the place of origin of seismic energy by shooting from several shotholes or by combining several elements of a nondynamite source in a suitable manner. The following consideration refers to receivers (geophones or hydrophones) as well as to emitters (shots and other sources of energy); for simplicity, in the following, the term “geophones” stands for all kinds of receivers and sources of energy.
SEISMIC SIGNAL DETECTION USING SIGN BITS
Authors: Cochran M. D.
Published: October 2002

By casting the problem of seismic signal detection as one of statistical detection theory, one can develop a myriad of detection statistics or detectors. Of these, one of the most promising appears to be sign‐bit semblance. This nonparametric detector makes use of only the sign bits of the seismic data and, hence, requires less storage and is faster to compute than other detection statistics. In addition, it is independent of the noise statistics, as are all nonparametric detectors. An automatic velocity analysis and interpretation program has been developed using sign‐bit semblance as the detection statistic. The statistical properties of sign‐bit semblance were such that this system could do a velocity analysis and an interpretation with no human intervention. In this mode of operation it yielded state‐of‐the‐art accuracy at greatly increased speed and with greatly reduced storage requirements. These results indicate that sign‐bit semblance can be used to advantage for certain other seismic‐data processing problems.
THE INVERSION OF VERTICAL MAGNETIC DIPOLE SOUNDING DATA
Authors: Glenn W. E. , Ryu Jisoo , Ward S. H. , Peeples W. J. , Phillips R. J.
Published: October 2002

It is demonstrated that the generalized linear inverse theory may be applied to vertical magnetic dipole sounding problems. An analysis of inversion of theoretical data for a two‐layer model illustrates the method and indicates certain features not inherent in the commonly practiced curve‐matching method of interpretation. In particular, the standard deviations of the layered model parameters may be estimated. Also the data may contain varying degrees of information about individual model parameters. Indeed, the information density matrix may be used to optimize the data information distribution by choosing only data that contributes information above some minimal level. The relative importance of the information distribution to the determination of individual model parameters may be assessed using both the structure of the information density matrix and the size of the estimated parameter standard deviations. Data may be removed until the estimated standard deviations of the parameters exceed some critical values. This process may be viewed as a method of experimental design such that information/cost ratios may be maximized. Also, if the economy of the interpretation is a serious consideration, then the same process could be used to eliminate those data that have minimal information and whose exclusion does not significantly effect the parameter resolution. This process would tend to maximize interpretation/cost ratios. Inversion analyses of four sets of data previously interpreted by the curve‐matching method illustrate the inherent features of the inverse method. Results of the inverse method of interpretation may be used to make a statistical evaluation of both the fit between observed and predicted data and the resolution of the model parameters.
A KALMAN FILTER APPROACH TO THE DECONVOLUTION OF SEISMIC SIGNALS
Authors: Crump Norman D.
Published: October 2002

It is common practice to model a reflection seismogram as a convolution of the reflectivity function of the earth and an energy waveform referred to as the seismic wavelet. The objective of the deconvolution technique described here is to extract the reflectivity function from the reflection seismogram. The most common approach to deconvolution has been the design of inverse filters based on Wiener filter theory. Some of the disadvantages of the inverse filter approach may be overcome by using a state variable representation of the earth’s reflectivity function and the seismic signal generating process. The problem is formulated in discrete state variable form to facilitate digital computer processing of digitized seismic signals. The discrete form of the Kalman filter is then used to generate an estimate of the reflectivity function. The principal advantages of this technique are its capability for handling continually time‐varying models, its adaptability to a large class of models, its suitability for either single or multi‐channel processing, and its potentially high‐resolution capabilities. Examples based on both synthetic and field seismic data illustrate the feasibility of the method.
AN UNDERGROUND ELECTROMAGNETIC SOUNDER EXPERIMENT
Authors: Dolphin Lambert T. , Bollen Robert L. , Oetzel George N.
Published: October 2002

An electromagnetic sounder developed for an archaeological application in Egypt has been successfully tested in a California dolomite mine. Chambers in the mine 100 to 130 ft from the surface gave intense, well‐defined echoes consistent with an average attenuation coefficient of 0.6 db/m and a relative dielectric constant of 11. By moving the transmitter and receiver units on the hillside above the underground chambers, various characteristics of the propagation could be observed such as dispersion, chamber aspect sensitivity, and cross‐section. The transmitted pulse was one to one‐and‐a‐half radio‐frequency cycles long at a peak power of 0.2 Mw. Frequencies employed were 16 to 50 Mhz. The light weight, highly portable, battery‐powered equipment is potentially suited to other underground sounding applications.
MAGNETIZATION ESTIMATION FOR NONUNIFORMLY MAGNETIZED SEAMOUNTS
Authors: Emilia David A. , Massey Robert L.
Published: October 2002

Commonly used methods for estimation of a seamount’s magnetization parameters directly from its observed magnetic anomaly assume uniform magnetization amplitude and direction throughout the seamount. We approximate seamount bathymetry by a set of stacked rectangular prisms. Definition of the magnetization vector in terms of its amplitude and direction, instead of its three Cartesian components, is then used to separately estimate the magnetization amplitude of each component prism and their common magnetization direction by the iterative algorithm of Marquardt. Therefore, the magnetization properties of a seamount can be estimated under the assumptions of uniform magnetization direction and nonuniform magnetization amplitude within the seamount. A theoretical example shows that the method can have good resolution in determining magnetization direction. A field example, already extensively studied by Harrison under the assumption of a uniform magnetization vector, shows that the method can be successfully applied to observed data.
THE APPLICATION OF HOMOMORPHIC DECONVOLUTION TO SHALLOW‐WATER MARINE SEISMOLOGY—PART II: REAL DATA
Authors: Buhl Peter , Stoffa Paul L. , Bryan George M.
Published: October 2002

The application of the techniques of Part I to seismic reflection data acquired on the Argentine continental shelf yields results which appear superior to time‐domain, minimum‐phase inverse filtering via the auto‐correlation function. This is in part because of a narrow‐band, maximum‐phase source component. Minimum‐phase deconvolution disperses this component rather than compressing it. Very slight exponential weighting [Formula: see text] appears to make the reflector series minimum phase. This weighting in conjunction with quadrupling the trace length by extending it with zeros virtually eliminates aliasing in the complex cepstrum. Simple zeroing of the complex‐cepstrum terms works well as a deconvolution technique even though for exactness their harmonics at longer cepstrum periods should also be removed.
STACKING OF REFLECTIONS FROM COMPLEX STRUCTURES
Authors: Miller Max K.
Published: October 2002

Common‐depth‐point seismic reflection data were generated on a computer using simple ray tracing and analyzed with processing techniques currently used on actual field recordings. Constant velocity layers with curved interfaces were used to simulate complex geologic shapes. Two models were chosen to illustrate problems caused by curved geologic interfaces, i.e., interfaces at depths which vary laterally in a nonlinear fashion and produce large spatial variations in the apparent stacking velocity. A three‐layer model with a deep structure and no weathering was used as a control model. For comparison, a low velocity weathering layer also of variable thickness was inserted near the surface of the control model. The low velocity layer was thicker than the ordinary thin weathering layers where state‐of‐the‐art static correction methods work well. Traveltime, moveout, apparent rms velocities, and interval velocities were calculated for both models. The weathering introduces errors into the rms velocities and traveltimes. A method is described to compensate for these errors. A static correction applied to the traveltimes reduced the fluctuation of apparent rms velocities. Values for the thick weathering layer model were “over corrected” so that synclines (anticlines) replaced false anticlines (synclines) for both near‐surface and deep zones. It is concluded that computer modeling is a useful tool for analyzing specific problems of processing CDP seismic data such as errors in velocity estimates produced by large lateral variations in overburden.
BEST BOUNDS ON DENSITY AND DEPTH FROM GRAVITY DATA
Authors: Parker Robert L.
Published: October 2002

Gravity data cannot usually be inverted to yield unique structures from incomplete data; however, there is a smallest density compatible with the data or, if the density is known, a deepest depth of burial. A general theory is derived which gives the greatest lower bound on density or the least upper bound on depth. These bounds are discovered by consideration of a class of “ideal” bodies which achieve the extreme values of depth or density. The theory is illustrated with several examples which are solved by analytic methods. New maximum depth rules derived by this theory are, unlike some earlier rules of this type, optimal for the data they treat.
INVERSE FILTER APPLIED TO THE OUTPUT OF AN ASKANIA Gss-2 SEA GRAVIMETER
Authors: Haworth R. T. , Loncarevic B. D.
Published: October 2002

The internal damping of the Askania gravimeter introduces both phase lag and attenuation into its measurements. Without adequate signal restoration, correlation between data collected on adjacent survey lines is misleading. A simple weighting function of either five or six terms, depending upon the data‐recording method used, can adequately restore both amplitude and phase and also recover data apparently lost while the gravimeter settles down after course or speed changes or meter adjustments.
On: “Resistivity Inversion,” by Joseph R. Inman, Jr., Jisoo Ryu, and Stanley H. Ward (GEOPHYSICS, December 1973, p. 1088–1108)
Authors: Szaraniec Edward
Published: October 2002

In their paper, the authors have presented a very interesting method of direct interpretation of apparent resistivity curves.
A NOTE ON THE RELATION OF SUDDENLY APPLIED DC EARTH TRANSIENTS TO PULSE RESPONSE TRANSIENTS
Authors: White G. E.
Published: October 2002

From some work by Carson, the relation between the earth response to a Heaviside function voltage and the response to an impulse is pointed out. A method of obtaining all other electrical measurements from the impulse response is indicated. It is suggested that a new experimental method might yield more accurate measurements of the electrical earth responses than can be had from suddenly applied DC transients, or any of the steady state measurements.
DETERMINATION OF VOLCANIC THICKNESS AND UNDERLYING STRUCTURES FROM AEROMAGNETIC MAPS IN THE SILET AREA OF ALGERIA
Authors: Curtis C. E. , Jain S.
Published: October 2002

In the course of the interpretation of an aero‐magnetic survey of Algeria, techniques based on spectral analysis were found to be quite useful in the determination of volcanic thicknesses and the resolution of underlying structural trends in a mineralized region near Silet. One‐dimensional analysis, applied to a series, of traverses crossing the area, indicates the average thickness of volcanics to be about 100 m. Two‐dimensional spectra were used to design appropriate filters for attenuating anomalies from shallow sources thereby enhancing the responses of the underlying regional structures.
COMPUTED SEISMIC SPEEDS AND ATTENUATION IN ROCKS WITH PARTIAL GAS SATURATION
Authors: White J. E.
Published: October 2002

Calculations for an unconsolidated sand with partial gas saturation show a 20 percent increase in compressional wave velocity between 1 and 100 hz and attenuation of 27 db/1000 ft at 31 hz and 82 db/1000 ft at 123 hz. Shear velocity and attenuation are not affected. Fluid‐flow waves are shown to be responsible for the dispersion and attenuation at low frequencies; relations are derived by extending Gassmann’s viewpoint to include coupling between fluid‐flow waves and seismic body waves. This appears to be an important loss mechanism for heterogeneous porous rocks.
TESTS OF CRYOGENIC SQUID FOR GEOMAGNETIC FIELD MEASUREMENTS
Authors: Zimmerman J. E. , Campbell W. H.
Published: October 2002

A type of cryogenic SQUID (superconducting quantum interference device) magnetometer was designed for geomagnetic measurements. Field tests of the instrument including comparisons to observatory variometers, a rubidium magnetometer, an induction loop, and a fluxgate magnetometer showed the new cryogenic systems to be reliable, accurate, portable, and simple to operate. Directional measurements of natural magnetic field fluctuations as small as 0.0001 gamma with periods from 0.5 sec to several hours were demonstrated.