bluuebunny/crossref_metadata_2025 huggingface dataset:
TEMPERATURE DEPENDENCE OF COMPRESSIONAL AND SHEAR WAVE VELOCITIES IN ROCKS
Authors: Timur Aytekin
Published: October 2002

Concurrent measurements of compressional and shear wave velocities were conducted as functions of temperature on two sandstone and seven carbonate samples subjected to simulated overburden and pore fluid pressures. On the average, the compressional wave velocities decreased by 1.7 percent and the shear wave velocities by 0.9 percent for a 100°C increase in temperature.
RIDGE REGRESSION INVERSION APPLIED TO CRUSTAL RESISTIVITY SOUNDING DATA FROM SOUTH AFRICA
Authors: Petrick W. R. , Pelton W. H. , Ward S. H.
Published: October 2002

Ridge regression inversion has been used to test the applicability of various one‐dimensional crustal models to the interpretation of deep Schlumberger sounding data from southern Africa (Van Zijl and Joubert, 1975). Four main models were investigated: a simple three‐layered earth, a layered earth with a transition zone exhibiting a linear decrease in log resistivity with depth, a similar earth with the transition zone determined by cubic splines, and a model having exponential resistivity behavior at depth. The last model corresponds to temperature‐dependent semiconduction through solid mineral grains (Brace, 1971). It was found that all of these models are capable of fitting the sounding data from southwestern Africa, while all except the semiconduction model fit the data from southeastern Africa. One is, thereby, immediately alerted to the problem of lack of resolution in Schlumberger sounding data where geologic control is not available. A major with the inversion of Schlumberger data alone is that accurate information is obtainable only for the resistivity‐thickness product of the resistive portion of the crust. On the other hand, magnetotelluric data, when available, tends to provide information on the thickness, but very little information on the true resistivity of the section. In order to resolve both resistivity and thickness it is possible to invert simultaneously Schlumberger and magnetotelluric (MT) data. Results obtained from the combined inversion of the African resistivity data and hypothetical MT data show that a considerable improvement in model resolution can be achieved using MT amplitude data even of poor accuracy from a relatively limited frequency range (0.1 to 100 Hz), whereas inclusion of MT phase information is of negligible additional benefit. Unfortunately, no significant test can be made, from data available at the time of our analysis, of the applicability of one‐dimensional inversion in a geologic circumstance which probably demands more dimension.
HOMOMORPHIC DECONVOLUTION BY LOG SPECTRAL AVERAGING
Authors: Otis Robert M. , Smith Robert B.
Published: October 2002

A homomorphic system is used to map the convolution of a source function with the impulse response of the earth into the sum of the log spectra of the source function and the earth’s response. If the source function is considered stationary and the earth’s response spatially nonstationary, by averaging the log spectra of several reflection records, the log spectrum of the source function will be enhanced and the log spectrum of the earth’s response will average out. An inverse homomorphic system maps the average log spectrum to an estimate of the source function. The source function is then deconvolved from reflection records. Application of log spectral averaging as a method of deconvolution to a seismic reflection profile obtained in a shallow water‐geologically complex area indicates the homomorphic technique to be an effective method for suppressing the air gun bubble pulse oscillation.
SOME APPLICATIONS OF LINEAR PROGRAMMING TO THE INVERSE GRAVITY PROBLEM
Authors: Safon Claude , Vasseur Guy , Cuer Michel
Published: October 2002

An approach is presented for solving the inverse gravity problem in the presence of various constraints such as bounds on density. This approach takes into account the nonuniqueness of the solution: for a finite set of measurements, the region studied is divided into a great number of rectangular prisms of unknown density. The set of all solutions of this undetermined problem may be described through various convex diagrams of moments; plots of these moments give bounds on some physical parameters such as the partial and total mass or the position of the center of mass. Numerical solutions are obtained using linear programming algorithms. Also, particular solutions such as the so‐called ideal body may readily be obtained using this technique. Only two‐dimensional cylindrical structures are considered, but application of this technique to three‐dimensional bodies is straight‐forward.
BAUBEE LAKE TEST SITE, ONTARIO, CANADA
Authors: Anderson Chris D.
Published: October 2002

The author, in conjunction with the University of Manitoba, has established a geophysics test site in the Canadian Shield at Baubee Lake, Ontario. The location of the site is shown in Figures 1 and 2. The availability of the site to industry and universities was announced in the Canadian literature last year. This report is a result of the interest in such sites expressed in the report on “The Frontiers of Mining Geophysics” (Geophysics, June 1977, p. 878–886).
STRUCTURE OF THE SEG POINT DATA EXCHANGE AND FIELD FORMATS (A PROPOSED SEG DIGITAL RECORDING STANDARD)—SPECIFICATION
Authors: Dampney C. N. G. , Funkhouser Don , Alexander Michael
Published: October 2002

Editor's Note: This is an interim report from the Potential Fields and Positioning Subcommittee. Comment is invited. Technological progress in all fields of endeavor depends upon three things—namely, methodology, metrology, and standardization. The aim of this paper is to contribute a proposed standard in the belief that it will reduce a great deal of tedious effort that authors are all too familiar with and make a small contribution to the geophysical state of the art.
MAPPING EARTH CONDUCTIVITIES USING A MULTIFREQUENCY AIRBORNE ELECTROMAGNETIC SYSTEM
Authors: Seigel H. O. , Pitcher D. H.
Published: October 2002

The Tridem vertical coplanar airborne electromagnetic system provides simultaneous in‐phase and quadrature information at frequencies of 500, 2000 and 8000 Hz. The system can map a broad range of earth conductors of simple geometry and provide quantitative estimates of their conductivities and dimensions. Computer programs have been developed to automatically interpret the six channels of Tridem data, plus the output of an accurate radar altimeter, to determine the depth of burial, conductivity and thickness of a near‐surface, flat‐lying conducting horizon. In limiting cases, the interpretation provides the conductance (conductivity‐thickness product) of a thin sheet (ranging from 100 mmhos to 100 mhos) or the conductivity of a homogeneous earth (ranging from 1 mmhos/m to 10 mhos/m). Two actual field examples are presented from Ontario, Canada; one relating to the mapping of overburden conditions (sand, clay and rock, etc) and the other to the mapping of the distribution of a buried lignite deposit. Other areas of potential application of the system to surficial materials would include groundwater mapping, permafrost investigations, and civil engineering studies for roads and pipelines.
GRAVITY SURVEYS IN GLACIER‐COVERED REGIONS
Authors: Watts Raymond D. , Isherwood William
Published: October 2002

Prior to 1974, glaciologists used gravity surveys as a means of determining ice thickness, but the density variations in the underlying rocks were inherent sources of errors in their measurements. Because of these errors and because of the poor resolution of the gravity techniques, better geophysical tools for obtaining ice depths were sought. In polar regions, where the ice is below its melting temperature throughout most of the ice mass, radio echo sounders were used successfully starting about 1960. Until 1974, however, radio echo sounder experiments on temperate glaciers were unsuccessful. Temperate glaciers are found throughout the mountains of Washington, Western Canada, and Alaska. In many of these areas, terrain is so rugged that a helicopter‐borne gravity crew can find very few landing sites which are not on glaciers (Figure 1).
On: “Reduction of magnetic and gravity data on an arbitrary surface acquired in a region of high topographic relief” by B. K. Bhattacharyya and K. C. Chan (GEOPHYSICS, December 1977, p. 1411–1430)
Authors: Courtillot V. , Ducruix J. , Le Mouël J. L.
Published: October 2002

In their paper, Bhattacharryya and Chan address the problem of reduction of magnetic and gravity data on an arbitrary surface acquired in a region of high topographic relief. In their work, the authors are kind enough to mention our contribution to the solution of the problem of finding the sources responsible for an observed magnetic or gravity anomaly, using the general formalism of inverse problems (Courtillot et al, 1974). Unfortunately, however, the authors seem to be unaware of our other publications which are far more relevant to their subject. Courtillot et al (1973) solved the problem of continuation of a potential field measured on an uneven profile, using the Backus and Gilbert approach. Another reference relevant to this problem (solved by Bhattacharryya and Chan on p. 1424) is Parker and Klitgord (1972), who used the Schwartz‐Christoffel transformation. The work was extended to the case of three‐dimensional potential fields measured on an uneven surface by Ducruix et al (1974). Indeed, the development of our paper is strikingly similar to that of Bhattacharryya and Chan, although the method is quite different. In our paper, we give many illustrations of both theoretical and real cases, in which our method is seen to perform very well. We leave it to the reader to compare the results provided by both methods and to compare the methods themselves. In a third paper (Le Mouël et al, 1975), we generalized the method and showed how one could obtain excellent approximate analytic solutions of the Dirichlet and Neumann problems in the two‐dimensional case for a contour with any arbitrary shape. Finally, let us take the opportunity of this discussion to mention a review of the subject which appears in French in Courtillot (1977) and in English, much expanded, in Courtillot et al (1978). In this last paper, which should be of interest in solving a variety of geophysical problems, we show how our method allows one to continue a potential field measured on an entirely arbitrary set of data points in any number of dimensions for the various coordinate systems in which the Laplace and Helmholtz equations are separable. We also establish the relationship between our method and a generalization of the theory of generalized inverse matrices. One other relevant reference on that subject is Parker (1977). In the case of spherical coordinates, an application can be the continuation of satellite data, a problem studied by Bhattacharryya (1977).
GEOPHYSICAL ACTIVITY IN 1977
Authors: Whitmire M. G.
Published: October 2002

This report provides worldwide statistical data on geophysical activity in 1977. The primary source of information was a survey questionnaire that was mailed to more than 500 companies, contractors, government agencies, and universities. We have attempted to gather data from all users of geophysical methods in the areas of petroleum, minerals, geothermal, and groundwater exploration; and from those who employ geophysical techniques in engineering, oceanography, and research. In general, response to the survey was good. Detailed information was collected on airborne, drill hole, land, and marine surveys. Statistics were compiled on miles of coverage and acquisition cost. Data not supplied on the questionnaires for cost and line miles were estimated on the basis of worldwide averages to minimize distortion of individual statistical items.
Suppression of long‐delay multiple reflections by predictive deconvolution
Authors: Sinton John B. , Ward R. W. , Watkins Joel S.
Published: October 2002

Multiple coverage seismograms acquired in the western Gulf of Mexico during 1975 by the University of Texas Marine Science Institute contain prominent, long‐delay multiple reflections, which obscure primary reflections from deep within the earth’s crust. Predictive deconvolution operators, containing gaps of zeros with a duration of 4 to 5 sec, proved effective in suppressing the long‐delay multiple reflections in these seismograms. There is a correspondence between the complexity of the long‐delay multiple reflection and the length of the optimal prediction filter, with the more complex multiple reflection requiring the longer prediction filter. Overall, the deconvolved reflection profiles provide an extremely detailed picture of the reflector characteristics producing both the sedimentary and deep crustal reflections. The top of oceanic layer 3, at a two‐way reflection travel of 10.1 sec in the area of this study, exhibits minor localized structure in three of the four profiles that were deconvolved. A possible Moho reflection was identified in one profile at a two‐way reflection traveltime of 11.6 sec, showing little apparent dip across the entire seismic section.
On: “Wavefront Curvature in a Layered Medium” by Bjørn Ursin (GEOPHYSICS, August 1978, p. 10011–1013)
Authors: Hubral Peter
Published: October 2002

It is interesting that not only traveltime, but also wavefront radius and geometrical spreading (and possibly also amplitude and thereby the entire synthetic point‐source seismogram) can be expanded by the ray method in terms of offset X and seismic parameters related to the vertical zero‐offset ray. Such expansions provide a fast computation of these quantities if they are to be obtained for many shot‐receiver offsets.
Magnetic anomaly of a circular lamina
Authors: Singh S. K. , Castro E. R. , Guzman S. M.
Published: October 2002

Closed form expressions for the gravity anomaly of a circular lamina and the gravity and magnetic anomalies due to a vertical right circular cylinder have been obtained previously (Singh, 1977a; Singh, 1977b; Singh and Sabina, 1978) by a method which avoids complicated integrations commonly used in deriving such solutions (e.g., Nabighian, 1962; Rao and Radhakrishnamurty, 1966). The method involves use of the Fourier‐Hankel transform of Poisson’s equation. The final expressions are obtained in closed form by employing certain tabulated integrals.
The relationship between maximum entropy and maximum likelihood spectra
Authors: Pendrel J. V. , Smylie D. E.
Published: October 2002

Burg (1972) established an analytical relationship between maximum entropy and maximum likelihood spectral density estimates. If [Formula: see text], j = 0, 1,… M − 1 and M successive maximum entropy spectral density estimates at frequency f, then the M‐length maximum likelihood spectral density estimate [Formula: see text] is given by [Formula: see text]. (1) The maximum entropy estimates are made via Burg's well‐known formula,[Formula: see text], (2) where [Formula: see text], k = 0, 1,…,N are the coefficients of the N‐length prediction‐error filter, [Formula: see text] is its error power, and Δt is the sample interval. These estimates can be recursively calculated as shown by Burg (Smylie et al, 1973).
On: “Elastic properties of unconsolidated porous sand reservoirs,” by S. N. Domenico (GEOPHYSICS, December 1977, p. 1339–1368).
Authors: Brown R. J. S. , Korringa J.
Published: October 2002

Domenico gives a valuable set of data on acoustic velocities in a sand pack and a glass bead pack with various brine saturations and various pressures. There are, however, two difficulties with the extensive discussion of the data in terms of the fluid‐solid coupling factor k in Biot’s equations for acoustic waves in fluid‐saturated porous media, and there is given a wrong relationship among some compressibilities. We are addressing these points and not reviewing the paper as a whole.
Geophone sensitivities for Chebyshev optimized arrays
Authors: Rietsch Eike
Published: October 2002

In 1963 Holzman published a theoretical analysis on so‐called Chebyshev optimized geophone arrays, i.e., geophone patterns whose reject band response has lobes of equal amplitude. For equidistant geophones in a linear array, this reject band response can be realized only if the geophone sensitivities are suitably weighted. One of the results of Holzman’s paper is a method for computing weights (relative sensitivities) for the geophones to achieve this desired reject band behavior (here, for simplicity, the term “geophone” is used in a generic sense for any seismic source and receiver).
In‐situ induced‐polarization and magnetic susceptibility measurements—Yerington mine
Authors: Ware G. Hunter
Published: October 2002

The Yerington Mine is an open pit mine of “porphyry‐copper” type. Primary mineralization (quartz veins, Cu‐Fe sulfides, magnetite, and silicate alteration) is closely associated with a complex sequence of quartz monzonite porphyry intrusives. This primary mineralization is disrupted by subsequent alteration and faulting. The fault block containing the deposit has been rotated almost 90 degrees to the west; plan maps represent pretilt vertical sections. Almost one mile of pretilt vertical extent is exposed in the pit. Geologic observations may be taken only upon mining advances at discrete intervals. Detailed in‐situ induced‐polarization response and magnetic susceptibility surveys, both taken upon the pit floor, have been used to supplement geologic mapping. These in‐situ measurements reveal the distribution patterns of sulfides and magnetite and are useful in interpolation or correlation of contacts, faults, and mineralization patterns between mapped advances. In‐situ induced‐polarization (IP) response was measured using a Scintrex IPR-7 receiver. Observations were made on a 10 × 10-ft grid using a Wenner electrode array with 3 1/3-ft a‐spacing. Susceptibility measurements were taken with a Bison model 3101-A bridge and an in‐situ coil arrangement 5 ft in diameter. A composite area approximately 3000 by 500 ft, representing 15,000 data points, has been surveyed. Contoured maps of in‐situ IP response and magnetic susceptibility reveal the mineralization pattern in detail. These maps correlate well with the geologically mapped distribution of porphyry intrusives and also indicate zones of sulfide remobilization and/or magnetite destruction, including fault zones. IP response and magnetic susceptibility correlate well with elemental assays, mineral abundances, ratios, and textures determined from bench mapping and pit floor samples. The spatial distribution of physical properties is extremely complex, which demonstrates the need for caution in the use of simple interpretative models. The abundance of magnetite locally present in this particular porphyry‐copper environment is noteworthy.
On: “A propagating algorithm for determining nth‐order polynomial, least‐squares fits,” by A. F. Gangi and J. N. Shapiro (GEOPHYSICS, October 1977, p. 1265–1276).
Authors: Ohta Yoich , Saito Masanori
Published: October 2002

Gangi and Shapiro (1977) proposed a recursive algorithm for determining coefficients of least‐squares polynomials. The algorithm is simpler and more efficient than Trench’s (1965) algorithm or Phillips’ (1971) triangular decomposition algorithm and has an advantage that by monitoring the mean‐square errors at each iteration we can find an optimum order of polynomial fit. We have tried their algorithm and encountered a difficulty. It may be worth recording the source of the difficulty.
On: “Enhancement of Signal‐to‐noise Ratios in Magnetotelluric Data” by Dominic W. Kao and David Rankin (GEOPHYSICS, February 1977, p. 103–110).
Authors: Hernandez W. , Jacobs J.
Published: October 2002

In their paper, Kao and Rankin present a technique designed to enhance the signal‐to‐noise ratio of magnetotelluric (MT) data. In particular, they claim it can remove the noise from the autopower estimates of MT data if the cross‐power estimates are noise free and, hence, increase the coherency of the data. We present a hypothetical, but realistic, example which we feel raises serious questions related to the general validity of the technique.
Topographic effects in resistivity and induced‐polarization surveys
Authors: Fox Richard C. , Hohmann Gerald W. , Killpack Terry J. , Rijo Luiz
Published: October 2002

We have made a systematic study of dipole‐dipole apparent resistivity anomalies due to topography and of the effect of irregular terrain on induced‐polarization (IP) anomalies, using a two‐dimensional (2-D), finite‐element computer program. A valley produces a central apparent resistivity low in the resistivity pseudosection, flanked by zones of higher apparent resistivity. A ridge produces just the opposite anomaly pattern—a central high flanked by lows. A slope generates an apparent resistivity low at its base and a high at its top. Topographic effects are important for slope angles of 10 degrees or more and for slope lengths of one dipole‐length or greater. The IP response of a homogeneous earth is not affected by topography. However, irregular terrain does affect the observed IP response of a polarizable body due to variations in the distance between the electrodes and the body. These terrain‐induced anomalies can lead to erroneous interpretations unless topography is included in numerical modeling. A field case demonstrates the importance of including topography, where it is significant, in interpretation models. A technique for correcting apparent resistivity for topographic effects uses the finite‐element program to compute correction factors.
A sum autoregressive formula for the reflection response
Authors: Hubral Peter , Treitel Sven , Gutowski Paul R.
Published: October 2002

The normal incidence unit impulse reflection response of a perfectly stratified medium is expressible as an autoregressive‐moving average (ARMA) model. In this representation, the autoregressive (AR) component describes the multiple patterns generated within the medium. The moving average (MA) component, on the other hand, bears a simple relation to the sequence of reflection coefficients (i.e., primaries only) of the layered structure. An alternate representation of the reflection response can be formulated in terms of a superposition of purely AR time‐varying minimum‐delay wavelets. Each successive addition of a deeper interface to the layered system gives rise to an AR wavelet whose leading term is equal to the magnitude of the primary reflection originating at this interface. We accordingly call these wavelets “generalized primaries.” The AR component of every generalized primary contains only those multiple reflections that arise from the addition of its particular interface to the layered medium.
Terrain effects of cultural features upon shallow borehole gravity data
Authors: Schmoker James W.
Published: October 2002

Borehole gravity surveys of oil and gas reservoirs are usually run at depths of thousands of feet, and such surveys are not likely to be significantly affected by the ordinary works of man. However, borehole gravity surveys for engineering applications and the evaluation of mineral deposits and aquifers may have maximum depths of only a few hundred feet and, therefore, are more likely to be affected by the gravity fields of man‐made surface features such as basement excavations, quarries, gravel pits, sewage plants, road fills and cuts, storage piles, mine dumps, tank farms, etc.
A comparison of seismic trace summing techniques
Authors: Gimlin D. R. , Smith J. W.
Published: October 2002

The results of an analysis and comparison of mantissa only, sign bit, true amplitude, automatic volume control, and diversity stack signal trace summations are presented. The results show that under conditions frequently encountered for low‐level sources, all these summation methods preserve the fidelity of input signals to within a time variant scale factor. The value of the signal‐to‐noise (S/N) average power ratio of the true amplitude summer is shown to be much more sensitive to intertrace noise variance differences than the other summing techniques. Excessive signal distortion may occur for mantissa only or sign bit summers when instantaneous input signal magnitude is large in comparison with the input noise standard deviations of the summed data. There will be no significant differences in the performance of mantissa only and sign bit summers under conditions frequently encountered for low‐level sources. The effects of signal summation methods are not directly dependent upon any instantaneous gain ranging characteristics of the amplifier used to create the input data.
On: “A collocation formulation of wave equation migration” by K. Pann, E. Eisner, and Y. Shin (GEOPHYSICS, April 1979, p. 712–721).
Authors: Laura Patricio A. A.
Published: October 2002

The writer wishes to congratulate the authors for their very interesting and useful approach. Their methodology is certainly applicable in other fields of applied science and engineering: stress analysis, diffusion problems, electromagnetic theory, structural dynamics, etc. A very minor criticism which I wish to make concerns the statement made by the authors: “In the generalized Galerkin’s method, if we select a set of Dirac delta functions for [Formula: see text], we arrive at the collocation method.”
On: “Effect of tropical weathering on electrical and electromagnetic measurements” by G. J. Palacky and K. Kadekaru (GEOPHYSICS, January 1979, p. 69–88).
Authors: Bhattacharya B. B. , Hazra P. K.
Published: October 2002

In this paper, the authors showed a series of electromagnetic (EM) profiles (Figure 20, p. 86) obtained from the ground follow‐up using a slingram system with a frequency of 1600 Hz and coil separation of 120 m. Measurements along all the profiles (A to E) show considerable in‐phase and quadrature anomaly. The host rock in this case is amphibolite. The vertical electrical soundings over the amphibolite (Figure 16, p. 82) indicate that the weathered amphibolite has mean thickness and resistivity of 8.5 ± 2.1 m and 29.7 ± 6.5 Ω-m, respectively. It would, therefore, be reasonable to assume that the in‐phase and quadrature anomalies have been influenced due to the presence of conducting overburden, and the response of the target in such a situation can be altogether different compared to the anomaly of the target in the presence of an insulating or highly resistive overburden.
Position measurements with the LaCoste and Romberg air/sea gravimeter
Authors: Valliant H. D. , Cooper R. V.
Published: October 2002

The LaCoste and Romberg air/sea gravimeter with its three‐axis inertial platform may be operated in the same mode as the new inertial survey systems (ISS) to obtain relative positions, elevations, and gravity. In this mode of operation, the instrument is stopped at approximately 5‐minute intervals to obtain a zero‐velocity update to permit compensation for sensor drift and the effects of anomalous gravity. Two tests were made to determine the gravimeter’s ability to measure positions when used in this mode. The first test, carried out at Ottawa, Canada, indicated that although gravity errors as large as 6.0 mgal were observed due to the effects of platform damping and power source (motor‐generator) instability, elevations were obtained with a mean and standard error of 0.07 and 0.9 m, respectively, for two‐way traverses. No attempt was made to determine horizontal positions.
Data‐adaptive polarization filters for multichannel geophysical data
Authors: Samson J. C. , Olson J. V.
Published: October 2002

The design of data‐adaptive filters requires that the noise be defined, statistically or otherwise, by parameters which allow some means of separating the noise from the signal. We consider here multichannel data in which one knows only that the noise is less polarized than the signal in a unitary space. This description of the noise is not sufficient for designing filters which are optimum in any sense; consequently, the filters may require a number of changes in the parameters before a satisfactory design can be found. Once this design has been achieved, the filters can be used to enhance waveforms of arbitrary shape, requiring little prior knowledge of the spectral content or temporal features of the signal. In contrast to many other data‐adaptive filters which give a scalar time‐series output, the filters we describe here with vector time series input have an equal number of input and output channels. A number of examples of filtered magnetic and seismic data are given in order to emphasize the wide range of uses for the filters. Some suggestions for application of the filters to multichannel seismic data are given.
An experimental test of a large‐volume plastic detector for use in aerial gamma‐ray spectroscopy
Authors: Duval Joseph S. , Pitkin James A.
Published: October 2002

A large‐volume plastic detector for use in aerial gamma‐ray spectroscopy has been evaluated by the U. S. Geological Survey. The purpose of the evaluation was to compare the plastic polyvinyl toluene (PVT) detector with thallium‐activated sodium iodide [NaI(T1)] detectors in ability to define the surface distribution of gamma radiation from the decay of natural thorium, uranium, and potassium. Good performance from PVT detectors will allow aerial gamma‐ray detection to be less expensive and much less susceptible to physical or thermal shock compared to the use of NaI(T1) detectors. The experimental results indicate that PVT detectors can be used to measure the surface distribution of potassium (K), equivalent uranium (eU), and equivalent thorium (eTh). However, because of the relatively low‐energy resolution of the PVT detector and the subsequent increased interference among the energy windows used to measure K, eU, and eTh, the results of data reduction are statistically less accurate than those obtained using NaI(T1).
The physical foundation of formation lithology logging with gamma rays
Authors: Bertozzi W. , Ellis D. V. , Wahl J. S.
Published: October 2002

We develop a theory for the gamma‐ray spectrum in a scattering and absorbing medium. Expressions are derived for the spectrum when sources are uniformly distributed in an infinite medium. We express the view that the formation of the spectrum at a point is a local phenomenon, originating from the Compton degradation of high‐energy photons which are transported from the source to the neighborhood of the point of interest. This allows one to apply the theory to a point source in an infinite medium, as well as to a geometry approriate for well logging. Confirming evidence via Monte Carlo results and experiment is presented. We show an application to a well logging device for measurement of gamma‐ray absorption via the photoelectric effect, a parameter which is sensitive to lithotogy.
On: “Shear zones in North Egypt interpreted from gravity data” by S. Riad (GEOPHYSICS, October 1977, p. 1207–1214).
Authors: Bayoumi A. I. , Abdelrahman E. M.
Published: October 2002

Riad has indicated that the juxtaposition of relatively high gravity anomalies against low‐anomaly features and vice versa, particularly along zones of high gravity gradients, “can only occur when shear movements are acting on previously faulted or folded formations.” He cited two models to illustrate this criterion; the first is a sheared step‐like faulting and the second is a sheared folding system constituting anticlines and synclines.
Relationships among the slopes of lines derived from various data analysis techniques and the associated correlation coefficient
Authors: Cohen Steven C.
Published: October 2002

This note points out some simple and insightful features associated with fitting a straight line to data which may be useful to many members of the scientific community. Specifically, there exist relations connecting the correlation coefficient and slopes of the lines derived using alternative data analysis criteria. In the following discussion, [Formula: see text] and [Formula: see text] are to be regarded as observations, neither of which can be preferentially regarded as an independent or dependent variable. The analyses use the data along with error minimization criteria to derive the intercept a and slope b in the assumed relationship between X and Y, namely Y = a + bX.
A new torsional shear‐wave generator
Authors: Won I. J. , Clough J. W.
Published: October 2002

A simple new device for generating shear waves on the ground surface has been developed and tested. The device is designed for determining elastic moduli of earth materials at shallow depths, in connection with engineering applications. However, with proper modifications, it may be developed into a larger unit for deeper exploration.
Deconvolution of gamma‐ray logs in the case of dipping radioactive zones
Authors: Conaway John G.
Published: October 2002

It has been shown both theoretically and experimentally that the grade‐thickness product of a zone of uranium or other radioelement, computed on the basis of a gamma‐ray log, will vary inversely with the cosine of the angle between the radioactive zone and the normal to the borehole. I present experimental studies in model boreholes containing dipping radioactive zones, extending the theory of deconvolution of gamma‐ray logs to the case of dipping zones. The experimental data were found to agree well with a simple numerical model which takes into account borehole diameter, detector length, and thickness of the radioactive zone as well as dip angle.
Microwave properties of drilling fluids
Authors: Lange James N. , Shope Steven S.
Published: October 2002

The application of electromagnetic (EM) techniques to well logging is initiated in an environment dominated by the properties of the drilling fluids. An impulse technique using nanosecond pulses is applied to a coaxial waveguide containing drilling fluids to measure the velocity (dielectric constant ε) and absorption (attenuation coefficient α) of EM impulses. It is the large difference in dielectric constants of water and oil which makes EM propagation techniques attractive for logging. Dielectric properties of some nondispersed drilling fluids (bentonite and attapulgite clays) are found to be largely dependent upon the volume of water present. Both bentonite and attapulgite clays exhibit the same range of dielectric constants (ε = 81 → 75) when the weight percent of clay is increased to 10 percent. In contrast, the microwave attenuations of these two clays are quite different, with that of the bentonite increasing at about 4 times the rate of the attapulgite suspensions. Microwave attenuation measured for a variety of commercial drilling fluids varies over a wide range, with the lignosulfonates the largest (91 dB/m) and oil inverts the smallest (3 dB/m). The oil inverts also have a small dielectric constant (ε = 3 → 6). Temperature dependence of the attenuation for these same drilling fluids is determined in the range from 23 °C to 45 °C to indicate their behavior under in situ conditions.
The Continuing Challenge and Opportunity for Exploration Geophysics
Authors: Backus Milo M.
Published: October 2002

You have heard about the energy crisis. “The easy oil has been found. We are about to run out of oil. Oil discovered per new field wildcat has been decreasing exponentially.” Does this mean there is no future for exploration geophysics? Quite the contrary. If it were easy to find new oil, there would be no need for exploration geophysics. If we weren’t concerned about running out of oil, we would logically postpone the search.
A hybrid three‐dimensional electromagnetic modeling scheme
Authors: Lee K. H. , Pridmore D. F. , Morrison H. F.
Published: October 2002

We present an efficient numerical method for computing electromagnetic (EM) scattering of arbitrary three‐dimensional (3-D) local inhomogeneities buried in a uniform or two‐layered earth. In this scheme the inhomogeneity is enclosed by a volume whose conductivity is discretized by a finite‐element mesh and whose boundary is only a slight distance away from the inhomogeneity. The scheme uses two sets of independent equations. The first is a set of finite‐element equations derived from a variational integral, and the second is a mathematical expression for the fields at the boundany in terms of electric fields inside the boundary. The Green’s function is used to derive the second set of equations. An iterative algorithm has been developed to solve these two sets of equations. The solutions are the electric fields at nodes inside the finite‐element mesh. The scattered fields anywhere may then be obtained by performing volume integrations over the inhomogeneous region. The scheme is used for modeling 3-D inhomogeneities with plane‐wave and magnetic dipole sources. The results agree with earlier model analyses using the finite‐element technique.
The electromagnetic fields of a horizontal dipole over a vertically inhomogeneous and anisotropic earth
Authors: Chlamtac M. , Abramovici F.
Published: October 2002

Electromagnetic (EM) field components are expressed as a set of improper integrals, the integrands of which depend upon two impedance functions that satisfy nonlinear differential equations. The components are normalized with factors representing the solution for a homogeneous half‐space having the same conductivity characteristics as the first layer. The normalized components of the magnetic field depend only upon the longitudinal conductivity [Formula: see text]. Their variation with normalized distance [Formula: see text] (actual distance divided by first‐layer skin depth computed using [Formula: see text]) reflects the change of resistivity with depth. The normalized components of the electrical field depend upon both [Formula: see text] and transversal conductivity [Formula: see text] and thus reflect changes with depth of both [Formula: see text] and [Formula: see text]. The examples considered show that anisotropy is felt for small normalized distances. Transition layers result in moderate variation of the field components with [Formula: see text] as compared to stacks of homogeneous layers presenting abrupt changes in electrical properties of the structure.
Experimental uses of short pulse radar in coal seams
Authors: Coon J. B. , Fowler J. C. , Schafers C. J.
Published: October 2002

We discuss here a series of radar tests conducted to determine the penetration distances and reflection ranges obtainable in coal with currently available short‐pulse radar. The various field tests were conducted in 1977 and 1978. Three different types of tests were presented. The first series of tests examined the penetration distances, velocity of propagation, and attenuation versus frequency in a large coal pillar. The velocity of propagation was approximately half that in the air. The tests showed that it was possible to transmit electromagnetic (EM) energy in the range of 20 to 500 MHz through 50 ft of coal. The tests also showed the definite polarization effects that had previously been measured in coal. The second series of tests was designed to show the maximum reflection distances for radar with a center frequency of 100 MHz. Using common‐depth‐point (CDP) reflection techniques, a tenfold CDP line obtained reflections for distances greater than 50 ft. The final series of tests was conducted to locate a six‐inch borehole within a coal pillar. The radar system located the hole quickly and easily even though it was 20 ft into the pillar. These tests have shown that it is possible to use short‐pulse radar to probe coal seams before mining.
A new distributed charge
Authors: Arnold M. E. , Haylett J. W.
Published: October 2002

A new type of distributed charge intended to be a seismic source in geophysical prospecting is described; it uses a number of lump charges, located in a shothole at uniformly spaced separations, connected by a time‐delay fuse. The time‐delay fuse consists of appropriate lengths of linear detonating explosive (Primaline®) in contact with a nondetonating linear deflagrator (Nonel®). Nonel is a nonelectrical delay detonator invented by Nitro Nobel AB of Sweden and licensed to Ensign Bickford for manufacturing and marketing Nonel‐based products. Field tests performed near Tulsa, Oklahoma, determined that (1) Nonel deflagration was both consistent and reliable for pressures from atmospheric to 150 psi, (2) Nonel did not cross‐detonate even when “bundled,” and (3) distributed charges using Nonel as an element in the timing fuse detonated 26 days after being loaded in a shothole. Field tests of distributed charges using Nonel performed in central Louisiana showed that (1) distributed charges increase the signal‐to‐noise (S/N) ratio relative to that yielded by concentrated charges by reducing shot‐generated surface noise, (2) distributed charges increase the usable frequency content over that yielded by concentrated charges, and (3) a reliable and efficient distributed charge can be constructed with Nonel Primadet delay cord.
Density and compressibility profiles of a layered acoustic medium from precritical incidence data
Authors: Coen Shimon
Published: October 2002

The density and compressibility profiles of a layered fluid are obtained from the reflection coefficient due to plane waves at two precritical angles of incidence and all the frequencies. The inverse scattering problem for a layered fluid, at oblique incidence, is transformed to an equivalent inverse scattering problem for a layered refractive index profile, at normal incidence. The latter inverse scattering problem is transformed to an inverse scattering problem in quantum mechanics whose solution is obtained by the Gelfand‐Levitan theory.
An analytic method for the determination of distant terrain corrections
Authors: Danés Z. F.
Published: October 2002

Terrain corrections of gravity data are by far the most awkward and time‐consuming reductions needed in exploration geophysics. The terrain must be subdivided into small prisms; the average elevation of each prism must be found on a topographic map; the gravity field of each prism must be determined by analytical methods; and the contributions of all the prisms must be added up.
The full acoustic wave train in a laboratory model of a borehole
Authors: Chen S. T.
Published: October 2002

We studied the characteristics of acoustic wave propagation in a fluid-filled borehole using as a laboratory model a concrete cylinder 2 ft high and 2 ft in diameter with a 1/4-inch diameter borehole along its axis. The model represents sonic logging in the field reduced by a factor of 40. We recorded the full wave train consisting of a refracted compressional P wave, a refracted shear S wave, and guided waves including a number of normal modes and a Stoneley wave. Exploiting the dispersive properties of a modal wave and the source-receiver frequency characteristics, we were able to isolate the S–wave, which contains much valuable information about the formation rock, but which has not been widely used since it is difficult to extract from the full wave train. The observed Stoneley wave had a very high amplitude at low frequency and showed little dispersion. Stoneley-wave velocity is closely related to S–wave velocity and formation density, and can be measured very accurately because the Stoneley wave generally has high amplitude and low attenuation. It can therefore be used indirectly to obtain the S–wave velocity, even when the S–wave cannot be measured directly. In general, the observed characteristics of each component wave agreed with our theoretical calculations but their relative amplitudes did not. We believe these discrepancies were caused, in part, by the fact that rock attenuation and the latitudinal angular dependence of the source radiation were not taken into account in the theoretical calculations.
Electrical properties of artificial clay‐bearing sandstone
Authors: Klein James D. , Sill William R.
Published: October 2002

Rocks which contain clay minerals often display electrical properties which cannot be predicted by bulk electrical properties of the constituents (Cohen, 1981). Interactions between clay minerals and groundwater can produce polarization phenomena, decreases in resistivity, and supposed nonlinear behavior.
Bright spots, milligals, and gammas
Authors: Ramo Alan O. , Bradley James W.
Published: October 2002

Spatially discontinuous high‐amplitude seismic reflections were encountered in seismic data acquired in the early 1970s in northeast Louisiana and southwest Arkansas. Large acoustic impedance contrasts are known to result from gaseous hydrocarbon accumulations. However, amplitude anomalies may also result from large density and velocity contrasts which are geologically unrelated to hydrocarbon entrapment. A well drilled on the northeast Louisiana amplitude anomaly encountered 300 ft of rhyolite at a depth of 6170 ft. Subsequent gravity and total field magnetic profiles across the feature revealed the presence of 0.2 mgal and 17 gamma anomalies, respectively. The measured magnetic susceptibility of the rhyolite was 0.0035 emu and the measured density contrast was [Formula: see text]. Model studies based on the seismically determined areal extent of the anomaly and the measured thickness of rhyolite accounted for the observed gravity and magnetic anomalies. The southwest Arkansas amplitude anomaly was a sheet‐like reflection which terminated to the north and west within the survey area. Two north‐south gravity profiles exhibited a negative character over the sheet‐like reflector but did not exhibit a clear spatial correlation with the north limit of the seismic anomaly. Two north‐south magnetic profiles exhibited tenuous 4 gamma anomalies which appeared to be spatially correlated with the interpreted north edge of the seismic anomaly. A subsequent wildcat well encountered no igneous material but did penetrate 200 ft of salt at about 7500 ft. Reassessment of the gravity and magnetic data indicated that this seismic amplitude anomaly is not attributable to an intrasedimentary igneous source; it suggested a salt‐related 0.2 to 0.3 mgal minimum coextensive with the observed seismic amplitude anomaly. Present amplitude analysis technology would treat these seismic data with suspicion. However, gravity and magnetic data acquisition can provide a relatively inexpensive means for evaluation and verification of amplitude anomalies and thus should be an adjunct for land seismic exploration utilizing amplitude analysis.