bluuebunny/crossref_metadata_2025 huggingface dataset:
PREDICTION FROM LOGS OF RECOVERABLE HYDROCARBON VOLUME, ORDOVICIAN CARBONATES: WILLISTON BASIN
Authors: Pickett G. R. , Artus D. S.
Published: October 2002

The objective of this study was to develop techniques for predicting from well logs the quantity of recoverable oil by primary depletion from the upper Red River formation (Cedar Creek Anticline) in the Williston Basin. Means of providing appropriate rock type zonation from routinely, run logs for geological purposes were also desired. A technique for calculating from logs the ratio k/ϕ where k is permeability to air measured in cores and ϕ is porosity, was determined. The confidence limit for the predicted k/ϕ value from logs is the same as that measured on the cores themselves. In the Red River, the parameter k/ϕ is useful for classifying rocks by pore‐size ranges. A quantitative correlation between primary oil recovery from production decline curves and [Formula: see text] calculated from logs is developed for the Red River formation. In this quantity, [Formula: see text] is oil saturation, h is interval thickness, and the summation is carried out in each well over all zones where [Formula: see text] percent. The method employs the calculated values of k/ϕ to divide zones into appropriate classes of rock quality in terms of pore‐size range for the summation.
QUASI‐STATIC TRANSIENT RESPONSE OF A CONDUCTING PERMEABLE SPHERE IN A DIPOLAR FIELD
Authors: Nabighian Misac N.
Published: October 2002

The paper presents a theoretical and numerical computation of the quasi‐static transient response of the multipoles induced in a conducting permeable sphere by a dipolar field.
TRANSIENT ELECTROMAGNETIC RESPONSE OF AN INHOMOGENEOUS CONDUCTING SPHERE
Authors: Verma Saurabh K. , Singh Rishi Narain
Published: October 2002

Analytic expressions for the quasi‐static electromagnetic response of a sphere in presence of unit‐step and ramp‐type time varying magnetic fields are derived. The conductivity inside the sphere is assumed to vary linearly with radius, i.e. [Formula: see text], where ρ is radial coordinate, [Formula: see text] is a constant and a is the radius of sphere. Curves showing the decay of the magnetic field for both types of fields are presented. In the case of ramp‐type applied magnetic field, the magnitudes of maxima of the induced magnetic field are found to decrease with increase in the rise time of the applied field and, hence, exciting pulses having small values of rise time should be used. It is believed that the analysis will be useful in the geoelectric exploration for highly conducting mineral deposits.
On: “Gravity Interpretation using the Fourier Integral” by J. W. Berg, Jr., and M. E. Odegard (GEOPHYSICS, vol. 30, no. 3, p. 424–438, June 1965)
Authors: Meinardus H. A.
Published: October 2002

The authors do not state explicitly that equations (1) and (12) express gravity anomalies of the two‐dimensional cylinder and fault respectively. These structures have an infinite extension along the strike, and all the gravity profiles perpendicular to the strike are identical. For this reason it is admissible to apply the one‐dimensional Fourier transform to obtain the one‐dimensional amplitude spectra shown in Figures (1) and (5). The situation, however, is different for the sphere, which does not have a one‐dimensional gravity profile in the above sense, but rather a two‐dimensional field with cylindrical symmetry. Instead of using the one‐dimensional Fourier transform as given by (6) along a straight line, we must apply the two‐dimensional Fourier transform over the whole area. Because of the circular symmetry, one radial variable will suffice in place of the two Cartesian coordinates x and y, and the two‐dimensional Fourier transform can be expressed as Hankel transform, so that equation (6) becomes [Formula: see text]
COARSE GRID CALCULATIONS OF WAVES IN INHOMOGENEOUS MEDIA WITH APPLICATION TO DELINEATION OF COMPLICATED SEISMIC STRUCTURE
Authors: Claerbout Jon F.
Published: October 2002

The multidimensional scalar wave equation at a single frequency is split into two equations. One controls the downgoing transmitted wave; the other controls the upcoming reflected wave. The equations are coupled, but in many reflection seismology situations the transmitted wave may be calculated without consideration of the reflected wave. The reflected wave is then calculated from the transmitted wave and the assumed velocity field. The waves are described by a modulation on up‐ or downgoing plane waves. This modulation function is calculated by difference equations on a grid. Despite complicated velocity models (steep faults, buried focus, etc.), the grid may be quite coarse if waves of interest do not propagate at large angles from the vertical. A one‐dimensional grid may be used for a two‐dimensional velocity model. With approximations, a point source emitting waves spreading in three dimensions may be included on the one‐dimensional grid. Calculation time for representative models is a few seconds. Phenomena displayed are interference, spherical spreading, propagation through focus, refraction, and diffraction. Converted waves are neglected. A procedure is suggested for the construction of a depth map of reflectors from observations at the surface. Assuming a velocity model, we may integrate the downgoing wave away from a surface source. Likewise, the upcoming wave may be approximately integrated back down into the earth. Since reflection coefficients are real, the ratio of upcoming to downgoing waves tends to be real at a reflector. An example is given in two dimensions which shows that this ratio over a dipping bed gives the dip correctly independent of source/receiver‐group offset.
MAGNETIC INTERPRETATION OF SEMI‐INFINITE SHEET‐LIKE BODIES
Authors: Rao B. S. R. , Rao T. K. S. Prakasa , Sastri M. V. S. Kutumba
Published: October 2002

Interpretation of magnetic anomalies generated by thin sheets has attracted the attention of several authors and many papers describing diversified interpretation techniques have appeared in geophysical literature. In the present note, the same problem is viewed from a different angle, and three simple and direct methods of determining the depth and direction of magnetization due to thin sheets, are presented. Their relative merits, together with application in two examples for testing their validity, are given.
GRAVITY ANOMALIES OF A FAULT CUTTING A SERIES OF BEDS
Authors: Sharma Bijon , Vyas Mahesh P.
Published: October 2002

The gravity anomaly due to a single horizontal semi‐infinite block terminated by a vertical or dipping fault has been discussed by several authors previously. Geldart, Gill, and Sharma (1966) gave a new and simple expression for calculating the gravity anomaly due to a block cut by an inclined fault at an arbitrary angle and used this expression to obtain the gravity anomaly due to a fault cutting a single bed. In their derivation the effects of both the upthrown and the downthrown blocks were taken into consideration. It is, however, only in the unusual situation that faults cut a single bed of uniform density. More often, faults cut a series of beds of different densities and thicknesses. If the densities and the thicknesses of the various beds differ greatly, an interpretation based upon replacement of the series of beds by a single bed of uniform density may be highly erroneous. Starting from the single block expression given by Geldart et al, an expression can be derived giving the gravity anomaly due to a fault cutting a series of beds having different densities and thicknesses.
QUASI‐STATIC TRANSIENT RESPONSE OF A CONDUCTING PERMEABLE TWO‐LAYER SPHERE IN A DIPOLAR FIELD
Authors: Nabighian Misac N.
Published: October 2002

The paper presents a theoretical and numerical computation of the quasi‐static transient response of the multipoles induced in a two‐layer spherical model by a dipolar field. The sphere and its surrounding shell are of arbitrary conductivities and magnetic permeabilities. The shell, in general, acts as a screen. The early stages of the decay curves are influenced only by the physical and geometrical parameters of the shell. For inner spheres that are good conductors, the latter part of the decay curves are governed predominantly by the physical and geometrical parameters of the core.
PREDICTION ERROR AS A CRITERION FOR OPERATOR LENGTH
Authors: Galbraith James N.
Published: October 2002

Prediction error filtering has been widely used for deconvolution. The mean squared error in prediction is a monotonically nonincreasing function of operator length, and the value of the error is readily available from the Wiener‐Levinson algorithm. In general, the value of this error for the infinitely long operator is not known a priori. It is shown that the final value of the error can be obtained by considering the Kolmogorov spectrum factorization. Simple criteria can then be established for operator effectiveness and length.
AN EFFECTIVE ARRAY SPACING FOR ELECTRICAL SOUNDING CURVES
Authors: Szaraniec Edward
Published: October 2002

The translation of the right asymptotes of electrical sounding curves with respect to the line of the total longitudinal conductance S is considered. The condition is given which the abscissa of sounding curves must satisfy in order to make the asymptote and S line coincide. The coordinate determinated from this condition is an effective array spacing.
DESIGN OF SPATIAL FILTERS AND THEIR APPLICATION TO HIGH‐RESOLUTION AEROMAGNETIC DATA
Authors: Bhattacharyya B. K.
Published: October 2002

Methods for the design of spatial filters are discussed in this paper. For a given response of a one‐dimensional filter, the weighting coefficients are calculated by solving a set of simultaneous equations with a simple matrix inversion procedure. In the case of a two‐dimensional filter, the method for obtaining the coefficients of a double Fourier series representing a set of given values is used to design the spatial operator. The problems connected with the length of the operator and the choice of a suitable decay in the high‐frequency response are discussed in detail. In order to show the usefulness of these methods, the paper presents several examples of operators designed for computing the vertical gradient, the second vertical derivative, and downward continuation of potential field data. A two‐dimensional vertical gradient filter is applied to the total field data obtained during a high‐resolution aeromagnetic survey over an area in the Precambrian Shield of Northeastern Ontario. The calculated gradient maps are compared with maps showing measured gradient values. The quality of the calculated maps in defining trends, patterns, and detailed features of anomalies shows the feasibility of obtaining very accurate vertical gradient maps from observed total field data.
THREE‐DIMENSIONAL SEISMIC METHOD
Authors: Walton G. G.
Published: October 2002

The three‐dimensional seismic method is a different way of gathering and presenting seismic data. Instead of showing the subsurface beneath a profile line, 3-D displays give an, areal picture from the shallowest reflector to the deepest one that can be found seismically. Data are collected in the field with cross‐spreads that provide over 2000 evenly spaced depth points on each reflecting interface. Several variations of the cross‐spread technique give the same subsurface coverage while providing flexibility in data gathering. Because of the dense coverage, the method is best suited for problems requiring great detail, such as production problems. The usual presentation of 3-D data is a visual, moving display of emerging wavefronts covering four sq mi of surface. From this dynamic display, average velocity to each reflector and the dip direction and magnitude can be computed. The method has proved especially useful for the recognition of faults and determination of fault directions.
MARINE SEISMIC REFRACTION STUDIES NEAR KODIAK, ALASKA
Authors: Shor George G. , Von Huene Roland
Published: October 2002

Seismic refraction profiles near Kodiak, Alaska have provided data for construction of a cross‐section showing the crustal structure from the east end of Kodiak Island across the continental terrace and slope to the Aleutian Trench (Figures 1 and 2). Five reversed refraction stations and one unreversed station were available (Table 1); interpretation was aided by access to reflection profiles which cross the refraction lines (Shor, 1972).
MAGNETIC ANOMALIES OF THIN SHEETS AND THEIR INTERPRETATIONS
Authors: Banerjee Buddhadeb
Published: October 2002

A method of quantitative interpretation of the vertical magnetic anomaly (ΔZ) caused by a thin sheet infinite in horizontal extent but limited in depth extent is developed. The determination of the position of the point on the profile above the dike plays an important role in the analysis and is done either by a graphic method or with the help of a rapidly convergent iterative process. When the depth extent of the sheet, either vertical or inclined, becomes infinite, the mathematical steps become simpler, and evaluation of different unknown parameters of the causative body is possible without any prior knowledge of the positions of the cartesian axes. The proposed methods are applied both on theoretical models and on actual field cases.
SEISMIC REFRACTION MODELING BY COMPUTER
Authors: Scott James H.
Published: October 2002

A computer program has been developed by the U.S. Bureau of Mines for designing a two‐dimensional, layered earth model as an aid in interpreting seismic refraction field measurements. The program requires input data consisting of shot‐point and geophone locations, refraction traveltimes, and identification of the refraction layer associated with each traveltime. In the program, a first approximation model is generated by a computer adaptation of the delay‐time method, followed by a series of improved approximations that are made by use of a ray‐tracing procedure. The final result of the program is a model designed to minimize the discrepancy between field‐measured traveltimes and computed traveltimes of rays traced through the model. Test applications indicate that the accuracy of interpretation is improved, and that time and effort of the professional interpreter are greatly reduced by use of the computer technique.
ELECTROMAGNETIC COUPLING IN FREQUENCY AND TIME‐DOMAIN INDUCED‐POLARIZATION SURVEYS OVER A MULTILAYERED EARTH
Authors: Dey Abhijit , Morrison H. Frank
Published: October 2002

Electromagnetic coupling responses in frequency and time‐domain induced‐polarization measurements over a multilayered earth are evaluated. For collinear dipole‐dipole and pole‐dipole configurations over a dissipative layered subsurface, the percent frequency effects of electromagnetic coupling are seen to be as high as 60 percent for large [Formula: see text] values, where L is the length of the receiving dipole, [Formula: see text] is the conductivity of the top layer of the half‐space, and f is the higher frequency of excitation used. In both frequency and time‐domain analyses, the distinctive effects of layering compared to that of a homogeneous half‐space response are shown for different electrode configurations, layer geometry, and electrical parameters of the subsurface. The pole‐dipole configuration of electrodes, in general, exhibits higher coupling compared to the dipole‐dipole configuration. In time‐domain measurements, the late off‐time transient decays reflect almost entirely the normal polarizability of the layered subsurface, in that the coupling responses are significant only during the early off‐time of the transient. The mutual impedance between grounded dipoles of arbitrary length is computed by extension of the complete solution of the boundary‐value problem of a horizontal electric dipole situated over a multilayered half‐space. A number of nomograms are presented for various layered structures to eliminate the electromagnetic coupling response in the induced‐polarization measurements in order to obtain the true polarization effect of the subsurface.
END CORRECTIONS IN MAGNETIC PROFILE INTERPRETATION
Authors: Shuey R. T. , Pasquale A. S.
Published: October 2002

Simple expressions are presented for the vertical and total field magnetic anomalies due to a polygonal body of finite strike length and arbitrary magnetization. These formulas incorporate end corrections into the well‐known Talwani‐Heirtzler (1964) formulas for two‐dimensional polygonal bodies and reduce to the latter for large strike length. Because of their simplicity, the formulas with end corrections lend themselves to rapid use in digital interpretation. Analysis of the formulas shows that interpretation with end corrections gives a body which is deeper and has a larger magnetization and different shape than the body inferred without end corrections.
BOUGUER GRAVITY AND ITS GEOLOGIC EVALUATION IN THE WESTERN PART OF THE BENGAL BASIN AND ADJOINING AREA, INDIA
Authors: Choudhury S. K. , Datta A. N.
Published: October 2002

The Bouguer gravity anomalies in the western part of Bengal Basin and part of eastern Bihar, India, can be explained in terms of basement relief which controls the thickness of the Gondwanas. This relief, however, has no influence on structure within the sedimentary section overlying the Gondwanas. During Gondwana times, the Bengal Basin continued further north at least up to Purnea, but in early Tertiary times, the continuity was interrupted by a basement feature passing through Jangipur and Malda. The main line of connection between peninsular India and the Shillong Plateau may be through the Rajmahal hills and the Darjeeling Himalayas.
QUANTITATIVE INTERPRETATION OF INPUT AEM MEASUREMENTS
Authors: Palacky G. J. , West G. F.
Published: October 2002

Recent improvements of the INPUT airborne electromagnetic system have made possible a more quantitative approach to interpretation. The necessary interpretational aids can be obtained in two ways: either by correlating the system and ground EM measurements, or by devising computational or analog quantitative models. Both approaches have been explored. In the former, the system decay rate can be correlated with the apparent conductivity‐thickness (σt) estimated by ground surveys. In the latter, four quantitative models were investigated, vertical half‐plane, vertical ribbon, dipping half‐plane, and homogeneous half‐space. Nomograms have been constructed which make it possible to determine σt, conductor depth, and dip for sheet‐like conductors, and conductivity for a homogeneous half‐space. Field examples show that this procedure can be used satisfactorily in the routine interpretation of records obtained by this system.
THE USE OF NONLINEAR FUNCTIONAL EXPANSIONS IN CALCULATION OF THE TERRAIN EFFECT IN AIRBORNE AND MARINE GRAVIMETRY AND GRADIOMETRY
Authors: Dorman Leroy M. , Lewis Brian T. R.
Published: October 2002

The terrain corrections for gravity and gravity gradient data are nonlinear functionals of the surrounding topography. We show how to approximate these corrections by use of Volterra‐Wiener functional expansion, which is a sum of linear convolutions using the topography, the square of the topography, etc. The convolution kernels are like Taylor expansion coefficients which depend upon the distance from the source point to the field point. As an example, we compute the field [Formula: see text] for a two‐dimensional ridge by the expansion method and compare the result with the exact result. We then show how the expansion technique can be used to propagate statistical properties through nonlinear functionals. As an example of this, we compute the rms terrain correction for [Formula: see text] as a function of the flight elevation and terrain relief.
MAGNETITE ASSAYS FROM MAGNETIC SUSCEPTIBILITY MEASUREMENTS IN TACONITE PRODUCTION BLAST HOLES, NORTHERN MINNESOTA
Authors: Zablocki C. J.
Published: October 2002

A study was made at U.S. Steel Corp.’s Minntac open‐pit taconite mine in northern Minnesota to assess the applicability of relating magnetic susceptibility measurements made in production blast holes to the magnetite content of the strata penetrated. The measurements were made by continuously recording the changes in self‐inductance of variously constructed air‐core, multiturn coils as they were raised in the holes. These coils were designed so that the magnetic flux which coupled with the wallrock was parallel to the horizontally bedded Biwabik iron‐formation. An analysis of the response for these coils showed that the recorded inductance changes (apparent magnetic susceptibilities) are related linearly to the average weight percent Fe as magnetite within a practical degree of accuracy. In one test hole, 36 assayed intervals, each 18 inches long, fitted a linear line of regression to the corresponding apparent magnetic susceptibilities within a standard error of ±1.2 weight percent Fe. When averaged over three ranges of Fe content, the values fitted a straight line to within ±0.5 weight percent Fe. As susceptibility depends on the mode of occurrence of the magnetite, extrapolation of these results to other areas will require further testing. This method of assay should provide control for the efficient processing of taconite ores. Since the method is rapid and inexpensive, measurements could be made in scores of these accessible holes to delineate cut‐off boundaries between ore and waste. A potential use of these continuous measurements would be found in establishing lithologic contacts from the detailed responses in selected horizons of the iron‐formation.
RESISTIVITY SIGNAL PARTITION IN LAYERED MEDIA
Authors: Roy Amalendu
Published: October 2002

Using a method given by Roy and Apparao (1971), the separate contributions to the measured resistivity sounding curve by the individual layers have been precisely determined for two‐layer and three‐layer [Formula: see text] models. The substratum in the former, and the middle [Formula: see text]-layer in the latter, constitute the targets in the two models. For the two‐layer model, a resistive substratum causes a positive anomaly by contributing higher than normal to the signal measured on the ground surface, while a conducting substratum gives rise to a “negative” anomaly (resistivity low) by contributing less than its normal share. As the two‐layer anomaly develops fully only at infinitely large spacings, it follows that an infinitely resistive basement contributes infinitely although no current flows through it, while a perfectly conducting basement contributes nothing even though it carries the entire current flow. For the three‐layer model, the anomaly peak or trough (as the case may be) is formed at intermediate spacings and is contributed to both by the target and the region external to it. Both these regions contribute more than normal for resistive targets and less than normal for conductive targets. At the anomaly peak for high positive values of [Formula: see text], the contribution from the target dominates and accounts for the bulk of the resistivity high. At the anomaly trough observed for large negative values of k, on the other hand, the fall in contribution from the extra‐target region becomes the main cause for the resistivity low. At each spacing and k, the sum of the contributions from the individual layers, of course, is exactly equal to the observed or measured value.
APPROXIMATION IN GRAVITY INTERPRETATION CALCULATIONS
Authors: Hammer Sigmund
Published: October 2002

Rigorous calculation of gravity effects of geologic bodies for the interpretation of subsurface structure from exploration gravity surveys is time‐consuming and tedious. Resort to electronic computers is convenient but not always necessary. Simple hand calculations can be instructive and are often adequate and useful. This paper reports the errors involved and ranges of validity of various simplifying approximations for commonly used geometric body shapes: (a) equivalent sphere approximation for an infinitely long horizontal cylinder, (b) circular plate approximation for horizontal slabs of finite width, (c) vertical line element and thin horizontal plate approximations for vertical cylinders of variable radii and heights (dome, salt dome, or plug), and (d) thin‐plate approximations for vertical dikes and horizontal fault blocks. The results are evaluated in terms of dimensionless parametric ratios, usually a characteristic dimension of the body versus depth to center of mass. Errors in the approximate calculations are remarkably small for several of the cases considered and the simplified results are valid for given precision over a rather large range of dimensions. For example, the maximum error in the thin‐plate approximation for an horizontal fault plate is less than one percent for a thickness/depth ratio ranging up to 0.75 which represents a very substantial fault throw. This particular example is more than adequate for most practical applications in geologic interpretations of gravity. On the other hand, some examples are shown to be unsuitable for simple approximation calculations.
CROSS‐SPECTRAL ANALYSIS USING MAXIMUM ENTROPY
Authors: Ulrych Tad , Jensen Oliver
Published: October 2002

A great deal of interest has been shown in the maximum entropy method (MEM) of power spectral analysis originally suggested by Burg (1967, 1968). The application of MEM to problems of geophysical and astronomical interest has met with considerable success (Ulrych, 1972; Ulrych et al., 1973; Smylie et al., 1973; Currie, 1973a, b; and Jensen and Ulrych, 1973). We have recently received a number of enquiries concerning the possibility of computing maximum entropy crosspower spectra. The purpose of this note is to present a method of determining the MEM crosspower spectrum from a knowledge of the MEM autopower spectra of the bivariate time series.
THE FREQUENCY RESPONSE OF ELECTROMAGNETICALLY DAMPED DYNAMIC AND RELUCTANCE TYPE SEISMOMETERS
Authors: Silverman Daniel
Published: October 2002

The steady state shaking table has been used to determine the response, as a function of frequency and terminating resistance, of a dynamic and a reluctance seismometer. The observed response of the dynamic seismometer is shown to be identical with that predicted by theory. The response curves for the reluctance seismometer qualitatively substantiate analytical predictions as to the value of terminating resistance for maximum damping, and of the shift, with decreasing resistance, of the peak of response, to the higher frequencies. A short discussion of the electrical equivalent seismometer is given, and a comparison shown between the experimentally determined response of a reluctance type seismometer and that of its equivalent network.
GEOMAGNETIC EXPLORATION IN 1938
Authors: Stearn Noel H.
Published: October 2002

One result of the intense experimental activity in geophysics during the last decade has been the establishment of approximate boundaries to the provinces of applicability of the different geophysical methods. Generally speaking, the oil industry has turned to the seismograph except for certain limited regions and specific problems. The mining industry has turned more to electrical devices except where magnetic methods have demonstrated a specific aptitude.
REFRACTION AND REFLECTION OF SEISMIC WAVES I. FUNDAMENTALS
Authors: Dix C. H.
Published: October 2002

We give in this paper an exposition of the fundamental theory underlying the theory of reflection and refraction seismic waves. It was thought useful to include material not strictly new along with the new material, because the sources are not easily obtained by many readers of this magazine. Knott’s energy equation is given a simple interpretation. Conditions at the interface are discussed together with the limitations of these conditions, as conditions by which our problem is to be solved. It appears that the energy flow must be considered as well as the interface conditions.
TRANSFORMATION OF ELECTROMAGNETIC ANOMALIES BROUGHT ABOUT BY A CONDUCTING HOST ROCK
Authors: Verma O. P. , Gaur V. K.
Published: October 2002

Small scale model experiments have been designed to investigate the electromagnetic anomaly produced by a buried conductor. These experiments show that a partially conducting host rock causes significant changes in the shape of the anomaly profiles as well as a general enhancement of the response. The diagnostic asymmetry of anomaly profiles obtained over inclined sheet models surrounded by a medium of vanishing conductivity was found to be progressively obliterated with increase in the conductivity of the host rock. As the conductivity increased, the sheet models tended to appear almost vertical irrespective of their dip and the coil system used. Both the variation in the apparent dip as well as the enhancement of response were found to be greater in the case of gently dipping sheets and were observed in varying degrees by all of the four different transmitter‐receiver configurations used. The horizontal coplanar system recorded the maximum change and the vertical coplanar the least.
ATMOSPHERIC ABSORPTION OF 2.62 MEV GAMMA‐RAY PHOTONS EMITTED FROM THE GROUND
Authors: Grasty R. L.
Published: October 2002

The actual measured decrease in gamma radiation with altitude, as measured by a gamma‐ray spectrometer, was found to be less than predicted on simple theoretical grounds. The various factors which reduced this apparent absorption were Compton scattering in the ground and in the air, the variation of detector efficiency with angle, the mass of material of the aircraft structure and detector housing below the detectors, the dead‐time of the electronic circuitry, and an increase in ground radioactive concentration with depth. When these effects were incorporated, experimental and theoretical values of the linear absorption coefficient in air for 2.62 Mev photons were found to agree.
SEARCH FOR GEOTHERMAL SEISMIC NOISE IN THE EAST MESA AREA, IMPERIAL VALLEY, CALIFORNIA
Authors: Iyer H. M.
Published: October 2002

A seismic noise experiment was conducted in the East Mesa area of Imperial Valley, California, by the U.S. Geological Survey (USGS) in May 1972. There is a pronounced heat flow anomaly over the area, and between July 1972 and the present five deep test wells have been drilled over the anomaly by the U.S. Bureau of Reclamation (U.S. Bureau of Reclamation, 1974). At the time of our survey, we were aware of results from a preliminary seismic noise survey in East Mesa by Teledyne Geotech (Douze and Sorrells, 1972). A detailed noise survey was conducted by Teledyne Geotech soon after our experiment (Geothermal Staff of Teledyne Geotech, 1972). Both the Teledyne Geotech surveys show noise levels (in the 3.0 to 5.0 hz band) 12–18 db higher over the area where the thermal gradients and heat flow reach maximum values than in the surroundings. Our results, on the other hand, show that the seismic noise field in the area is dominated by cultural noise, and it is impossible to see a noise anomaly that can be related to the geothermal phenomena in East Mesa. We think that it is important to take into account this disagreement between the two results in order to make a critical evaluation of the utility of seismic noise as a geothermal prospecting tool. The purpose of this note is to put our findings on record.
DETERMINATION OF WEATHERING THICKNESS BY A SEISMIC P‐S DELAY TECHNIQUE
Authors: Balachandran K.
Published: October 2002

The proper correction of reflection seismograms to a reference plane requires knowledge of the weathering thickness and the velocity in the weathering layer. When surface sources like Vibroseis are used, it has been the usual practice to ignore weathering correction because the technique provides no means of determining the time through the weathering.
COMPUTER MIGRATION OF OBLIQUE SEISMIC REFLECTION PROFILES
Authors: French William S.
Published: October 2002

A reflecting interface with irregular shape is overlain by a material of constant velocity [Formula: see text]. Multifold reflection data are collected on a plane above the reflector and the reflector is imaged by first stacking then migrating the reflection data. There are three velocity functions encountered in this process: the measured stacking velocity [Formula: see text]; the true overburden velocity [Formula: see text]; and a profile migration velocity [Formula: see text], which is required by present point‐imaging migration programs. Methods of determining [Formula: see text] and, subsequently, [Formula: see text] are well‐known. The determination of [Formula: see text] from [Formula: see text], on the other hand, has not been previously discussed. By considering a line‐imaging migration process we find that [Formula: see text] depends not only on the true section velocity but also on certain geometrical factors which relate the profile direction to the structure. The relation between [Formula: see text] and [Formula: see text] is similar to, but should not be confused with, the known relation between [Formula: see text] and [Formula: see text]. The correct profile migration velocity is always equal to or greater than the true overburden velocity but may be less than, equal to, or greater than the best stacking velocity. When a profile is taken at an angle of (90−θ) degrees to the trend of a two‐dimensional structure, then the appropriate migration velocity is [Formula: see text] and is independent of the magnitude of any dips present. If, in addition, the two‐dimensional structure plunges along the trend at an angle γ, then the correct migration velocity is given by [Formula: see text]. The time axis of the migrated profile for the plunging two‐dimensional case must be rescaled by a factor of [Formula: see text], and structures on the rescaled profile must be projected to the surface along diagonal lines to find their true positions. When three‐dimensional data are collected and automatic three‐dimensional migration is performed, the geometrical factors are inherently incorporated. In that case, the migration velocity is always equal to the true velocity regardless of whether the structure is two‐dimensional, plunging two‐dimensional, or three‐dimensonal. Processed model data support these conclusions. The equations given above are intended for use in conventional migration‐after‐stack. Recently developed schemes combining migration‐before‐stack with velocity analysis give [Formula: see text] directly. In that case, the above equations provide a method of determining [Formula: see text] from [Formula: see text].
THE USE OF THERMAL RESISTIVITY LOGS IN STRATIGRAPHIC CORRELATION
Authors: Beck A. E.
Published: October 2002

Since it has been found that the heat flow along a borehole rarely deviates more than 20 percent from the mean equilibrium value and that formation thermal resistivities may vary by as much as an order of magnitude, the profile of temperature gradient versus depth is equivalent to a log of thermal resistivity (T-log). In this work high precision temperature measurements in cased boreholes were used which yielded temperature gradients as high as 140°C/km. Using the equivalence between thermal resistivity and temperature gradients, it has been found that the T-logs are characteristic of the formations in which they were measured with a general negative correlation between thermal resistivity and electrical resistivity, except in coal (and perhaps gas) bearing formations. In one instance, the character of the resistivity log was used to conclude that a horizon deduced from the electrical resistivity log had been mispicked by nearly 100 m.
INTERPRETATION OF SPLIT‐SPREAD REFRACTION DATA IN TERMS OF PLANE DIPPING LAYERS
Authors: Johnson Stephen H.
Published: October 2002

Several formulations are available for the interpretation of reversed refraction profiles in terms of any number of plane dipping layers. Split‐spread profiles can be interpreted using the same methods. This procedure requires that the intercept times at the origin from data in opposite directions be equal. It can be applied usefully in combination with reversed profiles.
SCATTERING OF ELASTIC WAVES FROM DEPTH‐DEPENDENT INHOMOGENEITIES
Authors: Richards Paul G. , Frasier Clint W.
Published: October 2002

We have studied scattered pulse shapes by modeling inhomogeneities as a sequence of infinitesimally thin homogeneous layers. With oblique incidence of plane P or SV waves, the reflected‐converted‐transmitted waves are obtained by taking the calculus limit for the sum of primary interactions of the incident wave with all layer boundaries. The resulting scattered waves thus present themselves naturally in the time domain. For an incident impulse, the scattered pulse shape is merely an analytic function of the depth from which scatter has taken place within the inhomogeneity. The direct application of this simple method appears to be new, and we have found it remarkably accurate when compared with methods in which higher‐order boundary interactions are also retained (i.e., Haskell methods and an adaptation in the time domain which also keeps all multiples). In specific studies of P-waves incident (up to 30 degrees away from the vertical) upon a 5 km thick crust‐mantle transition, between materials having impedance ratio 1:2.8, we find the scattered pulse shapes are given adequately by our theory, for the passband of short‐period seismometers. Indeed, the theory remains remarkably accurate even for long periods, being in error by only 8 per cent at zero frequency.
APPARENT RESISTIVITY OF A PERFECTLY CONDUCTING SPHERE BURIED IN A HALF‐SPACE
Authors: Singh Shri Krishna , Espindola Juan Manuel
Published: October 2002

Recently, the application of the method of images to obtain apparent resistivity of a perfectly conducting sphere buried in a half‐space has been criticized on the grounds that the number of images needed is too large. Instead, use of the method of graphs, in conjuction with the method of images, has been suggested. Although the number of images needed is large, they are easy to compute and many of them occupy the same position, thereby the amount of necessary computation is drastically reduced. A computer program based on the method of images is used to give normalized total‐field apparent resistivity contours for bipole‐dipole arrays. Such theoretical contours can be used to locate the epicenter of a conductive buried sphere by comparing them with field data.
USE OF DECAY PATTERNS FOR THE CLASSIFICATION OF ANOMALIES IN TIME‐DOMAIN AEM MEASUREMENTS
Authors: Palacky G. J.
Published: October 2002

Interpretation of time‐domain electromagnetic measurements normally comprises visual anomaly selection and determination of body parameters, such as conductance, depth, and dip. A study is made to examine the possibility of in‐flight computer interpretation on the basis of decay patterns. Analysis of system response over conducting loops, vertical and dipping sheets, horizontal strips, and a half‐space indicates that identification of models and some of their parameters by decay patterns is feasible. By the simultaneous use of vertical and horizontal coil receivers, a reliable recognition of models may be achieved. While the secondary magnetic field over a conducting loop decays exponentially, other models show distinctive nonexponential patterns. Decay patterns are affected by conductance and conductor size, but less by depth and dip variations. Field measurements indicate that decay pattern may be used to distinguish between geologic bodies of various types.
Vp/Vs—A POTENTIAL HYDROCARBON INDICATOR
Authors: Tatham Robert H. , Stoffa Paul L.
Published: October 2002

Theoretically and experimentally, the shear‐wave velocity of a porous rock has been shown to be less sensitive to fluid saturants than the compressional wave velocity. Thus, observation of the ratio of the seismic velocities for waves which traverse a changing or laterally varying zone of undersaturation or gas saturation could produce an observable anomaly which is independent of the regional variation in compressional wave velocity. One source of shear‐wave data in reflection seismic prospecting is mode conversion of P waves to shear waves in marine areas of high water bottom P-wave velocity. A relatively simple interpretative technique, based on amplitude variation as a function of the angle of incidence, is a possible discriminant between shear and multiple compressional arrivals, and data for a real case are shown. A normal moveout velocity analysis, carefully coupled with this offset discriminant, leads to the construction of a shear‐wave reflection section which can then be correlated with the usual compressional wave section. One such a section has been constructed, the variation in the ratio of the seismic velocities can be mapped, and potentially anomalous subsurface regions observed.
A METHOD FOR MODELING THE RESISTIVITY AND IP RESPONSE OF TWO‐DIMENSIONAL BODIES
Authors: Snyder Donald D.
Published: October 2002

A method has been developed for the solution of the resistivity and IP modeling problem for one or more two‐dimensional inhomogeneities buried in a space for which the Dirichlet Green’s function is known. The boundary‐value problem reduces to a Fredholm integral equation of the second kind which is parametrically a function of a spatial wavenumber. Using the method of moments, the integral equation is solved for a number of values of the wavenumber. An inverse Fourier transform is then performed in order to obtain the electric potential at any point of interest. The method agrees well with both experimental results and other numerical techniques.
ENHANCEMENT OF SIGNAL‐TO‐NOISE RATIO IN MAGNETOTELLURIC DATA
Authors: Kao Dominic W. , Rankin David
Published: October 2002

A new technique has been developed which can be employed for the enhancement of the signal‐to‐noise ratio of magnetotelluric data and/or for the establishment of a confidence limit as a criterion for final data selection. This technique functions in a consecutively cyclic process which can finally remove the noise from the autopower estimates of MT data if the crosspower estimates are noise free and, consequently, the predicted coherencies can be improved. When noise exists in both autopower and crosspower estimates, the cyclic operation can establish a confidence limit which reveals a direct measure of incoherent noise content and thus is a more reliable criterion than the predicted coherency itself for data selection. The technique is tested using actual MT data and the result shows the adequacy of the technique. It selects useful data from those originally having moderate or low predicted coherencies; thus it appears to be important when the predicted coherency of the majority of the collected data is not high.
GRAVITATIONAL ATTRACTION OF A CIRCULAR DISC
Authors: Singh Shri Krishna
Published: October 2002

The vertical component of gravitational attraction [Formula: see text] of a circular disk is of some interest in geophysics since it can be used to obtain attraction of 3-D bodies whose parallel sections are circular and also since the solid angle Ω subtended by a disc at any point is proportional to [Formula: see text] at the same point (Ramsey, 1940, p. 36). Solid angles may be needed in some diffraction calculations in exploration seismology (see, e.g., Hilterman, 1975). It is clear, however, that in calculation of attraction from 3-D bodies, approximation of the cross‐sections by a polygon (Talwani and Ewing, 1960) has wider application.
INFLUENCE OF ALTITUDE AND DISPLACEMENT CURRENTS ON PLANE‐WAVE EM FIELDS
Authors: Sinha Ajit K.
Published: October 2002

Airborne electromagnetic mapping by wavetilt measurement is especially suitable for highly resistive strata like the permafrost terrains of northern Canada. A theoretical study has been made on the influence of the altitude of the aircraft and displacement currents on the measured wave tilt values over a range of frequency from VLF (very low frequency) to BCB (broadcast band). A computer modeling study has shown that the effects of displacement currents cannot be ignored over resistive ground if the frequency is greater than 100 kHz. The altitude effect is significant beyond 20 m for medium frequencies and resistive ground. The phase of the wavetilt is almost always more sensitive to these factors than the amplitude. When the top layer is thicker than three skin depths in that layer, the effects of lower layers on the wavetilt is negligible. It is also apparent that before attempting to interpret wavetilt data over a layered medium, one should carefully determine the nature of the source field. The results are readily applied to the Barringer E-phase System, an airborne EM technique which measures the quadrature part of the wavetilt at three frequencies from VLF to BCB range. If the effects of altitude and displacement currents are ignored, the calculated apparent resistivities from E-phase data will be much less than the true resistivities, even for moderately resistive ground. A graphical technique is presented for correctly estimating the resistivity values from two‐frequency E-phase measurements when the electrical constants are not frequency dependent.
STRUCTURE MAPPING ON ENHANCED LANDSAT IMAGES OF SOUTHERN BRAZIL: TECTONIC CONTROL OF MINERALIZATION AND SPECULATIONS ON METALLOGENY
Authors: Offield Terry W. , Abbott Elsa A. , Gillespie Alan R. , Loguercio Sabino O.
Published: October 2002

Computer enhancement, particularly contrast‐stretching, reveals a previously unnoticed east‐west structural zone across a Landsat image of the southern Brazilian Precambrian shield. In this zone occur the only known economic or near‐economic deposits of gold, tin, and copper. Such deposits are typically localized by small east‐west structural elements. Non‐economic copper occurrences elsewhere in the region appear to be related to major northeast‐ and northwest‐trending lineaments mapped on Landsat images. Mineral exploration should be primarily directed at the main east‐west lineament, but two other possible east‐west zones might be worthwhile targets also. The major east‐west lineament projects through a break in the continental shelf, across the Atlantic along a large transoceanic fracture zone and into the African continent along a mapped tectonic trend that goes through an area that produces copper, gold, and tin. Global geophysical data suggest that the mapped east‐west trend in South America is a surface reflection of structures that developed in the tectosphere in Precambrian time and that have persisted until the present.
HIGH SENSITIVITY GAMMA‐RAY SPECTROMETRY—STATE OF THE ART AND TRIAL APPLICATION OF FACTOR ANALYSIS
Authors: Duval Joseph S.
Published: October 2002

The remote sensing of terrestrial gamma rays has application in geologic mapping, mineral exploration, reactor site monitoring, location of lost radioactive sources, measurement of the water equivalence of snow, and soil mapping. Although the state of the art is quite good, there is a need to reexamine the use of detectors other than thallium activated sodium iodide detectors (e.g., plastic scintillators) to improve the corrections used for altitude variations and to present the data as apparent concentrations of potassium, uranium, and thorium rather than as counts per unit of time. In an attempt to improve data analysis, the technique known as factor analysis has been applied to airborne gamma‐ray spectrometric data from a survey in South Texas. This analysis technique allows the geologist/geophysicist to perform a coordinate transformation from the four count rates [potassium (K), equivalent uranium (eU), equivalent thorium (eTh), and total count] and the three ratios, (eU/K, eU/eTh, eTh/K) to a system of three independent coordinates. These three coordinates are constrained to reproduce the total variance of the original data, and the data can be separated into groups using the criterion that similar data points have similar coordinates. The distribution of the separated groups can be mapped for comparison with other information such as the mapped geology. This map of the groups represents a synthesis of all of the radiometric data.