bluuebunny/crossref_metadata_2025 huggingface dataset:
EM RESPONSE OF AN ARBITRARY SOURCE ON A LAYERED EARTH: A NEW COMPUTATIONAL APPROACH
Authors: Lajoie J. , Alfonso‐Roche J. , West G. F.
Published: October 2002

The electromagnetic response of a layered earth is expressed in rectangular coordinates and the solution is obtained as a double inverse Fourier transform. This simplilfies the theoretical development for any EM source, grounded or not, and the fast Fourier transform algorithm may be used to evaluate the fields. In some cases where complicated source geometries are used and where the fields are required over a large area or volume, the procedure may lead to a substantial reduction in computation costs. The standard problems of the numerical fast Fourier transform algorithm may be overcome by using correction techniques such as pre‐aliasing. An analysis of the ground‐wire EM source shows that the infinite line is a poor approximation to the long grounded line due to the strong component of galvanic current in the ground.
RECENT ADVANCES AND APPLICATIONS IN COMPLEX RESISTIVITY MEASUREMENTS
Authors: Zonge Kenneth L. , Wynn Jeffrey C.
Published: October 2002

Several years of accumulating complex resistivity spectral measurements have indicated that there are still many unexplored areas in induced polarization surveying that need to be investigated for a more complete understanding of the polarization process. In addition to providing mineral discrimination capabilities, complex resistivity spectra can be used to differentiate between various barren host rock responses, to facilitate the complete removal of electromagnetic coupling, and to identify pipeline, fence, and various other cultural coupling effects. Results of field measurements are presented in an effort to demonstrate the utility of and necessity for making complete spectral measurements for serious geophysical interpretation. Correlation of field measurements with laboratory measurements on core samples from the same area demonstrates that strong electromagnetic coupling can be accurately removed from complete spectra without removing the important rock response. Recent field and laboratory measurements indicate that most, if not all, induced polarization responses attributed to magnetite are not really due to this mineral but can be traced to a host rock alteration response. Also, characteristic host rock signatures for sulfide environments appear to vary according to locality and type of deposits making it impossible to provide a universal set of signatures for sulfide deposits.
SEEING THROUGH ROCK SALT WITH RADAR
Authors: Stewart Robert D. , Unterberger Robert R.
Published: October 2002

A VHF radar has been used in a salt mine in the Cote Blanche Salt Dome to probe horizontally for the dome flank and vertically for the top of the salt. Discontinuities in the salt's complex electric permitivity caused by anhydrite stringers, shale, sandstone, water, or fractures will caused radar reflections, A narrow beam, ±9 degrees in salt, helps to determine the direction to the discontinuity. The time of the radar reflection determines the range to the discontinuity. The maximum,one‐way penetration was 2040 ft. The radar speed in Cote Blanche salt was measured by timing radar reflections through known distances of salt. The measured radar speed was 188ft/μsec slightly less than the radar speed in pure salt (203 ft/μsec. ). A shear zone in the salt was found to have many discontinuities or radar targets. A discontinuity was detected in a discontinuity about 400 ft above the mine level, but still inside the salt, was also found. The top of the salt dome was detected from several locations in the mine.
AN IMPROVED METHOD OF COMPUTING THE THERMAL CONDUCTIVITY OF FLUID‐FILLED SEDIMENTARY ROCKS
Authors: Beck A. E.
Published: October 2002

In the absence of suitable material on which to make thermal conductivity measurements, it is still possible to compute a value on the basis of an idealized model. The Maxwell model for the thermal conductivity of a two‐component material, a model which has frequently been successfully used for relatively small values of porosity (<0.1) and moderate conductivity ratios (<10), may be extended to much wider ranges of these parameters (0.3 and 300, respectively) by the use of appropriate correction factors obtainable from data already in the literature. Significant errors due to overlooking a thermal contact resistance have been detected in some experimental work. The errors can be as high as 50 percent of the measured value for an air‐filled sandstone of 0.2 porosity. Unless corrected, such errors can contribute a great deal to the lack of agreement between computed and experimental values. Correction for these errors also leads to much better consistency among published experimental values measured with various techniques. Conductivity values accurate to better than 15 percent may therefore be predicted for specimens saturated with low conductivity fluids such as air, steam, or oil.
FINITE‐ELEMENT COMPUTATION OF SEISMIC ANOMALIES FOR BODIES OF ARBITRARY SHAPE
Authors: Bolt Bruce A. , Smith Warwick D.
Published: October 2002

A method which uses observed frequency spectral ratios of seismic plane waves for exploration of ore bodies is now available. The new method is based on the numerical solution of the response of a two‐dimensional shallow structural anomaly to an upward‐moving seismic wave from a distant earthquake or explosion. Finite‐element analysis is used for both P- and S-waves. Solutions to the direct problem for bodies of arbitrary shape have not previously been available. Results in the time and frequency domains are discussed here for a salt ridge and for a massive sulfide body. For the inverse problem, interpretation using contours of spectral ratios along a surface profile is suggested.
THE EFFECT OF FINITE DATA LENGTH IN THE SPECTRAL ANALYSIS OF IDEAL GRAVITY ANOMALIES
Authors: Regan Robert D. , Hinze William J.
Published: October 2002

The interpretation of gravity data which has been transformed from the space domain to the frequency domain is subject to limitations imposed by the transformation process and the nature of the data. One potential source of error in the spectral analysis of gravity anomalies is the incomplete definition of the anomaly due to insufficient data length. Comparison of theoretical Fourier transforms and the transforms of finite‐length segments of gravity anomalies due to two‐dimensional vertical slabs and horizontal cylinders indicates that the interpretation error can be held to less than ten percent if the profile length is at least six times the maximum depth to the source of the anomaly. This result was achieved with a rectangular data window in the transformation process. Greater error resulted from the use of the Bartlett, Parzen, and Tukey data windows.
GRAVITY TERRAIN CORRECTIONS USING MULTIQUADRIC EQUATIONS
Authors: Krohn Douglas H.
Published: October 2002

A digital computer method of making gravity station terrain corrections has been developed that uses a linear system of multiquadric equations. This system is fitted to the points defined by square topographic compartments and the point defined by the station itself to give a mathematically described surface. The surface is a better model of the actual topography than the digital terrain model, especially near the station. Terrain correction of this surface is calculated using a simple and fast numerical integration. A theoretical example shows that the multiquadric equation method is potentially more accurate than a hand chart method for near‐station terrain corrections. Field examples in an area of rugged topography show that the method can be successfully used for actual gravity stations.
To: “Search for Geothermal Seismic Noise in the East Mesa Area, Imperial Valley, California,” (v. 40, December 1975, p. 1066–1072).
Authors: Iyer H. M.
Published: October 2002

In the article, “Search for Geothermal Seismic Noise in the East Mesa Area, Imperial Valley, California,” by H. M. Iyer (v. 40, December 1975, p. 1066–1072), the following corrections should be made:
STRUCTURE ELUCIDATION WITH BOREHOLE GRAVIMETRY
Authors: Hearst Joseph R. , McKague H. L.
Published: October 2002

The observed densities, assuming infinite homogeneous‐horizontal beds, computed from borehole gravimetry were as much as 15 percent greater than those derived from gamma‐gamma density logs in the lower portion of several clustered boreholes at the U.S. Energy Research & Development Administration (USERDA) Nevada Test Site. A model having a single high‐density buried scarp to the west of the holes was constructed, the gravimetric density calculated, and the model varied in an attempt to improve agreement between measurement and calculation. Only slight improvement was obtained. Newly acquired geologic information made it possible to construct a more complex multiscarp model, providing much better agreement between measurement and calculation. In turn, this more complete model was refined with the help of the calculations. This method permits one to choose between qualitatively different models and, given a qualitatively correct model, to improve it quantitatively.
On: “Search for Geothermal Siesmic Noise in the East Mesa Area, Imperial Valley, California,” by H. M. Iyer (GEOPHYSICS, December 1975, p. 1066–1072)
Authors: Katz Lewis J.
Published: October 2002

Dr. Iyer rightfully points out that cultural noise sources (e.g., traffic, canals) tend to mask smaller amplitude geothermally generated microtremors. These problems have been recognized by other investigators as well. However, we take exception to his conclusions questioning the validity of groundnoise surveys as a geothermal exploration tool based solely on this study or his other survey in Long Valley, California where he had 1000 head of cattle nibbling on his geophones. Areas adjacent to cultural noise generators are not meant to be surveyed by this technique, as he points out. But what of the hundreds of other areas that are culturally silent? Results from these areas have yet to be weighed. Preliminary results from suitably applicable areas appear to have favorable indications. One prospect drilled on a groundnoise anomaly has been confirmed as a major geothermal find. Crustal inversion techniques applied to groundnoise spectra have been used to interpret geological structure. Gravity and resistivity profiles, and well log information over the same areas, have verified the groundnoise interpretations. As with many other geophysical methods, groundnoise should be used as a reconnaissance tool or in conjunction with other surveys.
PETROLEUM EXPLORATION IN CANADA
Authors: Lindseth R. O.
Published: October 2002

Canada is a country rich in petroleum. Not only has it been, for many years, the only country in the western world totally self‐sufficient in production, it has also been able to supply a healthy export market. For many years discoveries of new oil exceeded consumption, and substantial proven reserves were accumulated. In spite of this rather considerable development of production, only one major basin has been explored to any substantial degree, and only a fraction of the total oil believed to lie in place has been discovered to date. Provinces with oil reserves have prospered under stable, equitable legislation that fostered substantial expenditures of funds for exploration (Figure 1 ).
EXACT WAVE‐SHAPING WITH A TIME‐DOMAIN DIGITAL FILTER OF FINITE LENGTH
Authors: Mereu R. F.
Published: October 2002

When the signs of alternate terms of a symmetric discrete time series are reversed and the newly created series is then convolved with the original series, the resultant time‐series will have alternate values equal to zero. This property of symmetric functions may be exploited to design a new deconvolution and wave‐shaping time‐domain filter which is capable of transforming a given wavelet into an output made up of a sequence of spikes separated by zeros, or a sequence of wavelets, whose shapes are identical to that of any desired wavelet. In its design, no Z-transform polynomials are factored or divided and no equations are solved. The weights are derived entirely in the time domain from a series of successively derived subfilters ([Formula: see text], [Formula: see text], [Formula: see text] ⋯ [Formula: see text]) which, when convolved with the original wavelet, creates the spike sequence output. These subfilters may be conveniently grouped into a symmetric component which is derived from the autocorrelation function, a component which depends upon the characteristics of the original wavelet and a component which depends upon the desired wavelet. The number of zeros separating the spike outputs may be controlled by increasing the number of sub‐filters N according to the formula [Formula: see text]. The Wiener filter is an optimum filter in the least‐squares sense but its errors occur across the output. The new filter is an optimum filter in an “error‐distribution” sense. Its errors are in reality the noncentral spikes of the spike sequence. By choosing the length properly, the errors may be moved away from the region of interest leaving that region effectively “error‐free”. A limitation to this procedure is the computational round‐off error which increases as the filter length is increased. In a series of experiments with various types of wavelets it was found that the spike position always occurs at the center of the filter, with the anticipation and memory components automatically falling into place. A very important property of the filter is the fact that the input parameters required for its design are identical to those needed for the normal equations of the Wiener filter. Initial tests with a noisy time‐series showed that the new filter could be effectively employed using the statistical properties of the noise in the same manner that the Wiener filter is applied.
RECURSION FILTERS FOR DIGITAL PROCESSING OF POTENTIAL FIELD DATA
Authors: Bhattacharyya B. K.
Published: October 2002

Zero‐phase two‐dimensional recursive filters, with a specified frequency domain response, have been designed for processing potential field data. In the case of second vertical derivative filters, it is possible to use the rational approximation of symmetrical functions of a single variable for the design of a short recursive filter. The filter so designed has an excellent response in the frequency domain. For vertical gradient and continuation filters, a method is developed for obtaining, by the least‐squares method, a rational expression for a two‐dimensional symmetrical function. In order to ensure the stability of the recursive filter, the denominator of the rational expression is approximated by a product of two factors, each being a function of a single variable. Finally, to keep the error of the filter response as small as possible, an iterative procedure is used for adjusting the zeros of the denominator and then determining the coefficients of the numerator of the rational expression.
DIAGRAMS FOR MAGNETOTELLURIC DATA
Authors: Lilley F. E. M.
Published: October 2002

Observed magnetotelluric data are often transformed to the frequency domain and expressed as the relationship [Formula: see text]where [Formula: see text] [Formula: see text] and [Formula: see text] [Formula: see text] represent electric and magnetic components measured along two orthogonal axes (in this paper, for simplicity, to be north and east, respectively). The elements [Formula: see text] comprise the magnetotelluric impedance tensor, and they are generally complex due to phase differences between the electric and magnetic fields. All quantities in equation (1) are frequency dependent. For the special case of “two‐dimensional” geology (where structure can be described as having a certain strike direction along which it does not vary), [Formula: see text] with [Formula: see text]. For the special case of “one‐dimensional” geology (where structure varies with depth only, as if horizontally layered), [Formula: see text] and [Formula: see text].
On: “Computer migration of oblique seismic reflection profiles,” by William S. French (GEOPHYSICS, December 1975, p. 961–980)
Authors: Schneider William A.
Published: October 2002

In his excellent paper, French cites a caveat for 2-D and 3-D imaging which I repeat for completeness: “The data collection area must contain the specular raypath from a subsurface reflection point if that point is to be seen in the results of migration.” As a general rule of thumb for planning a seismic survey, I have no dispute with the above statement. However, I feel the author overstates the case for specular reflection from curvilinear surfaces. As is well known, complex surfaces reradiate incident energy by both specular reflection and diffraction. The relative importance of the diffraction term depends upon the wavelength of the impinging pulse and the radius of curvature of the structure.
On: “Inversion of gravity profiles by use of a Backus‐Gilbert approach” by William R. Green (GEOPHYSICS, October 1975, p. 763–772).
Authors: Chander Ramesh
Published: October 2002

An important possible constraint on a density model obtained from inversion of gravity data has been overlooked in the seminal paper by Green. The computed density model should be such that the corresponding total mass excess or deficit per unit length in a two‐dimensional case, or total mass excess or deficit in a three‐dimensional case, should be comparable to the value obtained by applying Gauss’s theorem to the observed gravity anomaly data (Grant and West, 1965, p. 227–28 and p. 232).
MODERN STANDARDS FOR GRAVITY SURVEYS
Authors: Morelli Carlo
Published: October 2002

The modern gravity meter is capable of determining the difference in gravity between measurement sites with very high accuracy. However, the gravity values established by one survey can be related to those established by another survey only when both are tied to gravity base stations whose gravity values are in a uniform system.
THE CORRECTIVE GRADIENT PROJECTION METHOD AND RELATED ALGORITHMS APPLIED TO SEISMIC ARRAY PROCESSING
Authors: Gangi Anthony F. , Byun Bok S.
Published: October 2002

A general iterative method is described for the problem of optimizing a nonlinear function of multiple parameters subject to a system of linear constraint equations. This scheme, derived by Frost (1972), is called the “corrective gradient projection (CGP)” method. It is obtained by modifying Rosen’s (1960) “gradient projection (GP)” method to correct for any deviation of the parameters from the constraint equations. The CGP method can be applied to the problem of multichannel tapped‐delay‐line (TDL) sensor array processing as used in the fields of seismic, radar and acoustic communications. A variety of algorithms associated with signal processing problems—such as those developed by Frost (1972), Rosen (1960), Booker and Ong (1971), Widrow et al. (1967), and Griffiths (1969)—can be derived from or related to the CGP method. They are obtained by making some basic assumptions and statistical approximations. Comparative simulation experiments have been made using a digital computer to examine the performance of some of the algorithms for real seismic data. The algorithms examined are the constrained (unbiased) adaptive algorithm of Frost (1972), the unconstrained adaptive algorithm of Widrow et al. (1967) and the constrained (unbiased) optimum processor (Claerbout, 1968; Frost, 1972). The adaptive algorithms are capable of discriminating against noise signals in real time. The Frost algorithm strictly maintains a specified constrained frequency response for the array, while the Widrow algorithm alters the equivalent array response during the adaptations. For the Frost algorithm, the adaptive processor approaches the optimum one as the number of adaptations increases because it progressively learns the noise statistics.
REFLECTED AND TRANSMITTED FILTER FUNCTIONS FOR SIMPLE SUBSURFACE GEOMETRIES
Authors: Schoenberger M. , Levin F. K.
Published: October 2002

A zone of sands embedded in shale acts as a filter, both in reflecting energy back to the surface and in transmitting energy to reflectors below them. For a single layer of sand, the reflection filter is periodic—reflecting no energy at some frequencies and more than either of the two individual interfaces at other frequencies. Separating the sand zone into two parts by inserting a thin layer of shale results in reflection filters which differ greatly from one another. The particular filter curve generated depends upon the location of the shale layer. A sand zone filters reflections from interfaces below the zone in a manner complementary to the reflection filter. Where the most energy is reflected, the least is transmitted; conversely, where the least energy is reflected, the most is transmitted. The models considered in this report could easily give rise to high‐amplitude reflections; but, unless the amplitudes were very high, there would be little filtering of deeper reflections. However, for very high‐amplitude reflections and narrow‐band data, little energy would be transmitted and a shadow zone would result. For very high‐amplitude shallow reflections and broad‐band data, a low‐frequency shallow reflection would cause high‐frequency deep reflections; a high‐frequency shallow reflection would cause low‐frequency deep reflections.
A DUAL‐WAVELENGTH THERMAL INFRARED SCANNER AS A POTENTIAL AIRBORNE GEOPHYSICAL EXPLORATION TOOL
Authors: LeSchack Leonard A. , Del Grande Nancy Kerr
Published: October 2002

We are investigating a new airborne method for measuring surface temperatures that may be useful for identifying thermal anomalies of geologic origin. From Planck’s equation we derive the valuable approximation that, for small temperature variations, the radiant emittance is proportional to the emissivity times the absolute temperature to the power of (50/wavelength in μm). From this, expressions are obtained for the emitted infrared (ir) radiation measured simultaneously in the 5 and 10 μm bands. Ratios of these expressions are shown to have the following useful properties at 288 K: (a) they are insensitive to surface emissivity variations for vegetated terrain, (b) they vary nearly as the 5th power of the surface temperature, and (c) they distinguish emissivity‐related from temperature‐related effects. We have made preliminary tests of this methodology at a field site in Scipio Center, New York. We have characterized the observed surface temperature variations, the significant effects of soil moisture, and separated out the purely emissivity‐related features of vegetated terrain. Cluster analysis served to divide the ir data into groups that behave similarly as a function of the measured soil moisture. Two such distinct terrain groups were identified at the field site. The ir data were corrected for: (a) natural surface emissivity variations, (b) the intervening atmospheric path, and (c) the reflected sky radiation. The corrected surface temperature data were compared with calculated values computed from a model that simulates the surface temperature, using meteorological, hydrological, topographical, and soil thermal input parameters. The simulated mean surface temperatures, 291.9 K (group 1) and 291.6 K (group 2), differed only by, respectively, 0.0 K and 0.1 K from the measured mean surface temperatures. Our preliminary results suggest the potential for developing a new airborne geophysical method for isolating abnormal heat flows. Weak heat flows, about 10–20 times the terrestrial average, have the effect of raising the surface temperature about 0.1–0.2 K. These temperature anomalies would, with the methodology suggested, appear as a residual difference between the measured (corrected) surface temperature and the simulated surface temperature. Such surface temperature differences appear, from our research, to be measurable by airborne ir scanners when data over surface areas of [Formula: see text] or larger are averaged. Accordingly, our research appears to support the conclusion that surface temperature enhancements of geophysical origin between 0.1 and 0.2 K can be identified using airborne infrared methods.
THE MAGNETIC EQUIVALENCE OF AN ANTICLINAL THIN BED AND A THIN DIPPING DIKE
Authors: Shalley M. J.
Published: October 2002

The nonuniqueness of inverse solutions of a magnetic profile is well known. The example described here provides a useful insight into the interpretation of anticlinal structures which contain a continuous and uniform magnetic horizon. It also highlights the necessity of applying geologic control in the interpretation of such situations. It must be made clear from the outset that the mathematical reasoning developed below applies to a geometric approximation to an anticlinal bed. The approximation is very good for a thin bed contained in a tight fold. It is less successful for a thick bed and for more gentle folding. Hence, only the thin bed is considered in any detail. Complications involving remanent magnetization, demagnetization, and large magnetic susceptibilities are ignored.
GRATICULE SPACING VERSUS DEPTH DISCRIMINATION IN GRAVITY INTERPRETATION
Authors: Hammer Sigmund
Published: October 2002

Very serious distortions in both magnitude and extension of local gravity anomalies result from the still widely used 9-point “residual” and 17-point “second derivative” graticules. Although these types of residual maps are very useful for recognizing and pinpointing the existence of interesting local anomalies, the distorted results cannot be used to derive geologic interpretations of significant reliability. A practical procedure based on changes in anomaly magnitudes from two (or more) different grid spacings, effectively overcomes shortcomings of previous interpretation methods.
COMPUTER‐ASSISTED ANALYSIS TECHNIQUES FOR REMOTE SENSING DATA INTERPRETATION
Authors: Anuta Paul E.
Published: October 2002

The development of airborne and satellite multispectral scanning radiometers has created widespread interest in the application of such sensors to mapping of earth resources. The energy sensed in each band can be used as a parameter in a computer‐based, multidimensional‐pattern‐recognition process to aid in the interpretation of the nature of elements in the scene. Images from each band can also be interpreted visually. Visual interpretation of 5 or 10 multispectral images simultaneously becomes impractical, especially as the area studied increases; hence, great emphasis has been placed on machine (computer‐assisted) techniques in the interpretation process. A number of other data sets have recently been studied and integrated by digital registration with the multispectral reflectance and radiance phenomena. Topographic data, which have been registered with four‐band Landsat multispectral scanner (MSS) data, are being studied to determine relationships between spectral and topographic variables. Geophysical variables. including gamma‐ray and magnetic data, have also been registered and studied using the multivariate analysis approach.
ON SHALLOW‐HOLE TEMPERATURE MEASUREMENTS—A TEST STUDY IN THE SALTON SEA GEOTHERMAL FIELD
Authors: Lee Tien‐Chang
Published: October 2002

Shallow‐hole (<13 m) temperature measurements made at various depths and/or times may yield reliable values of geothermal gradient and thermal diffusivity if the groundwater table is shallow (a few meters) such that the effect of time‐dependent moisture content and physical properties is negligible. Two numerical methods based on nonlinear least‐squares curve fitting are derived to remove the effect of annual temperature wave at the ground surface. One method can provide information on the gradient and diffusivity as a function of depth while the other gives average value over the depth interval measured. Experiments were carried in six test holes cased with 2 cm OD PVC pipes in the Salton Sea geothermal field. A set of 5 to 7 thermistors was permanently buried inside the individual pipes with dry sand. Consistent gradient determinations have been obtained with both numerical methods from six monthly observations. By linearly extrapolating the depths to the 100°C and 200°C isotherms from the calculated gradients and mean ground temperatures, we have found good agreement with the nearby deep‐well data for four holes. Discrepancy is found for two holes, one of which is located near the field of [Formula: see text] mud volcanoes and the other near the volcanic Red Hill, reflecting complicated local hydrologic conditions.
1974 TEST OF LACOSTE AND ROMBERG INERTIAL NAVIGATION SYSTEM
Authors: LaCoste Lucien J. B.
Published: October 2002

In mid-1974 a test was made in the Gulf of Mexico of a LaCoste and Romberg inertial system for the measurement of the Eötvös correction for shipboard gravity meters. Since the system is designed for operation with satellite navigation, inertial velocities were updated at 2- to 3-hour intervals, using Lorac information because no satellite information was available. Two types of comparison were made between the inertial and Lorac data. One comparison was the rms difference between results from the two methods—0.46 knot, which corresponds to 3.0 mgal at a latitude of 30 degrees. The other comparison related to noise which could be mistaken for anomalies of interest in oil exploration. The comparison indicated that such noise in the inertial data was only about one third that in the Lorac data.
COMPUTATION OF GRAVITY AND MAGNETIC ANOMALIES DUE TO INHOMOGENEOUS DISTRIBUTION OF MAGNETIZATION AND DENSITY IN A LOCALIZED REGION
Authors: Bhattacharyya B. K. , Chan K. C.
Published: October 2002

Analytical expressions in the form of a convolution of two functions have been derived for the gravity and magnetic anomalies due to inhomogeneous distribution of magnetization and density in a localized region. These expressions show explicitly that the anomalies are completely determined by the divergence of magnetization and the first vertical derivative of density. It is thus analytically established that when a body with uniform magnetization or density is considered, the impulsive change in the physical property of the rock mass at the boundary of the body is responsible for generating an anomalous magnetic or gravity field. These properties are utilized to derive efficient algorithms for computing gravity and magnetic anomalies. An example is given to demonstrate the usefulness of the derived algorithm in computing the anomaly due to magnetization of an irregularly shaped topography.
A PARAMETRIC STUDY OF INDUCED‐POLARIZATION MODELS
Authors: Dodds A. R. , Raiche A. P. , Vozoff K.
Published: October 2002

The transmission surface or network technique was used to study the induced‐polarization response of a model having infinite strike length. The model consisted of a horizontal cylinder (the target) in a uniform half‐space overlain by an overburden. Responses to the dipole‐dipole, pole‐dipole, and gradient electrode arrays were compared as model parameters were systematically varied. Target conductivity, target depth, and overburden thickness and conductivity were each changed in turn. For different electrode spacings and locations, the peak response amplitudes and positions were plotted. It was found that the peak response amplitude did not decrease when large electrode spacings were used. The maximum percent frequency effect (PFE) response remains relatively constant for mildly conductive to very conductive targets. At the same time, the electrode separation required to achieve this maximum response increases by about a factor of two. The position of the peak PFE and the amplitude of the peak metal factor (MF) response were insensitive to overburden conductivity. There is a sharp drop of both PFE and MF when a thin overburden is introduced, and a gradual further reduction as its thickness increases. Responses to the pole‐dipole and dipole‐dipole arrays were comparable in magnitude, whereas results for the gradient arrays were at best equivalent and sometimes much smaller.
WORKSHOP ON ELECTRICAL METHODS IN GEOTHERMAL EXPLORATION
Authors: Ward S. H.
Published: October 2002

A workshop on Evaluation of Electrical Methods in the Geothermal Environment was held at Snowbird, Utah, November 4 through 7, 1976. The workshop was financed by the U.S. Geological Survey under a grant to the University of Utah. S. H. Ward, Chairman of the Department of Geology and Geophysics, University of Utah, aided by a steering committee consisting of William Dolan (Amax), Frank Frischknecht (ex‐officio, U.S.G.S.), George Keller (Group 7), Tsvi Meidav (Geonomics), Frank Morrison (University of California, Berkeley) and Sulhi Yungul (Chevron) arranged to invite 50 participants while the U.S.G.S. arranged the attendance of four federal program managers as observers. Balanced representation of industry, government, and academia was stressed during the invitation process. Will Forsberg and Mary DeWitt of the University of Utah arranged accommodations and provided workshop management. R. L. Christiansen, F. C. Frischknecht, and D. W. Klick of the U.S. Geological Survey were very generous with their time in helping me organize and edit this report.
THE INTERPOLATION ERROR OPERATOR: TIME SERIES ERROR DETECTION AND CORRECTION
Authors: Nyman Douglas
Published: October 2002

The interpolation error operator is an effective tool for the detection of digital time series data errors characterized by discontinuities in the function itself or its low order derivatives. The generalized interpolation operator of multiplicity n simultaneously interpolates n consecutive terms. This operator is effective in correcting short lengths of erroneous data.
INDUCED POLARIZATION: A PRELIMINARY STUDY OF ITS CHEMICAL BASIS
Authors: Angoran Yed , Madden Theodore R.
Published: October 2002

An investigation of the effect that the in‐situ solution chemistry would have on the induced‐polarization phenomena was carried out by means of laboratory studies of the electrode impedances of metallic and sulfide minerals. Reaction rate theory shows that this effect is largely due to the impedance associated with the diffusion of the ions involved in the charge transfer reaction to and from the reaction sites. This impedance is inversely proportional to the concentration of the reacting ions and inversely proportional to the square root of the frequency. Impedance measurements were made with different concentrations of a variety of ions in order to identify the active ion species. The ion concentrations were controlled by doping and by excursions in Eh and pH. These measurements showed that cupric and sulfide ions are active in the charge transfer reaction and control the electrode impedances whenever their concentrations exceed [Formula: see text]. None of the other common anions and cations tested appeared active. A background impedance level was found which was relatively insensitive to the solution chemistries tested. These impedances varied somewhat with the type of mineral tested, but generally ran between 2000 and [Formula: see text] at 1 radian/sec. The identity of these background reactions is still an open question.
On: “Gravity vertical gradient measurements for the detection of small geologic and anthropogenic forms” by Z. J. Fajklewicz (GEOPHYSICS, October 1976, p. 1016–1030)
Authors: Thyssen‐Bornemisza Stephen
Published: October 2002

In his paper, Fajklewicz discusses the improvement of vertical gravity gradient measurements arising from a very stable tower apparently not affected by wind gust vibration and climatic changes. Further, the lower plate where the gravity meter is resting can be changed in position to avoid possible disturbances from surface and near‐surface variation, and new methods for correcting and interpreting observed gradients over the vertical interval of about 3 m are presented. Some 1000 field stations were observed, including research work and industrial application.
On: “Gravitational Attraction of a Circular Disc,” by Shri Krishna Singh (GEOPHYSICS, Feb. 1977, p. 111–113)
Authors: Lafehr T. R.
Published: October 2002

It is unfortunate that in the Short Note no mention whatever is made of Nettleton’s classic paper on the subject: “Gravity and Magnetic Calculations”. (Geophysics, 1942, p. 308). Not only did Nettleton’s paper precede Singh by 35 years but it provides a more practical approach (stacked discs) to solving a difficult three‐dimensional problem. Nettleton’s method endured long and well until modern computers rendered it less efficient several years ago; his technique has been applied countless times to thousands of salt dome and mineral resource problems.
On: “Gravity vertical gradient measurements for the detection of small geologic and anthropogenic forms” by Zbigniew J. Fajklewicz (GEOPHYSICS, October 1976, p. 1016–1030)
Authors: Arzi Avner A.
Published: October 2002

Microgravimetry is indeed a powerful method for the detection of many small geologic and anthropogenic bodies (Arzi, 1975). Fajklewicz essentially advocates a particular procedure for microgravimetric surveys. Whereas gravity measurements are usually performed at the ground surface, Fajklewicz simultaneously performs measurements also at a surface situated 3 m above ground, using a portable tower. His data are presented as a map of the difference between the bottom and the top measurements. This difference (which may be measured in gradient units after being divided by the tower height) is called the Gravity Tower Vertical Gradient (GTVG). Similar procedures have been occasionally employed for various purposes (Thyssen‐Bornemisza, 1976). My main comments on the paper by Fajklewicz are as follows.
SIMPLIFIED GRAVITY INTERPRETATION BY GRADIENTS—THE GEOLOGICAL CONTACT
Authors: Stanley John M.
Published: October 2002

A simple method is presented for the on‐site interpretation of gravity anomalies arising from a geological contact. The method is very easy and rapid to apply; it is unambiguous and does not necessarily require computing facilities but may be automated if desired. The method employs the second horizontal derivative of the gravitational anomaly which results in a very high rejection of neighboring disturbances at the expense of reduced tolerance to instrument reading errors. As the interpretation requires only a relatively short length of traverse, it is economic to overcome measurement errors by increasing the number of readings within the traverse and smoothing the curve. The gradient method has the advantages of high immunity to regional and topographic interference and is independent of latitude.
FINE‐SCALE CORRELATION BETWEEN TEMPERATURE GRADIENT LOGS AND LITHOLOGY
Authors: Conaway John G. , Beck A. E.
Published: October 2002

Two high‐precision continuous temperature gradient logs have been produced in a 600-m partially cased, water‐filled borehole for which a nearly complete core is available. The agreement between these gradient logs, obtained using a logging speed of 8 m/minute, is good; there is virtually no dc offset (less than 0.03°C/km), and the mean absolute error is approximately 0.5°C/km. A comparison of these profiles with detailed geologic logs of the core shows excellent precision and resolution. In situations where the thermal resistivity contrast is 50–100 percent, isolated strata as thin as 0.5 m are clearly indicated on the temperature gradient log. More subtle lithologic changes extending over larger depth intervals are also detectable. In several instances gradient contrasts on the order of 1°C/km extending over several meters have been clearly resolved. Comparison of the gradient logs with a thermal resistivity profile from laboratory measurements on the core material indicates that the steel well casing has no observable effect on the temperature gradient profile.
REDUCTION OF TERRAIN‐INDUCED AEROMAGNETIC ANOMALIES BY PARALLEL‐SURFACE CONTINUATION; A CASE HISTORY FROM THE SOUTHERN SAN JUAN MOUNTAINS OF COLORADO
Authors: Wynn J. C. , Bhattacharyya B. K.
Published: October 2002

A method for reduction of terrain‐induced anomalies in aeromagnetic data collected at constant or variable elevation has recently been developed by Bhattacharyya and Chan (1977a). The method utilizes an equivalent‐source approach to continue the anomalous field data to a reference surface parallel to the terrain, thereby attenuating the topographic effect. This approach to the reduction of terrain effects requires no assumption about physical properties or distribution of causative bodies. We present a test of the method in the Chama‐southern San Juan Mountains wilderness study area of southwestern Colorado, a region of substantial topographic relief (exceeding 1600 m) and extensive volcanic cover. This study shows that in many places in this area terrain effects contribute appreciably to the inaccuracy in geologic interpretation of the original aeromagnetic data. Comparison of interpretations based on original and revised data shows that some anomalies are shifted with respect to the original data by as much as 6 km, while others are not. The process now permits the interpreter to easily separate terrain‐induced lows from lows caused by hydrothermal alteration. In one example, a northern extension of a magnetic low thought to be caused by topography was shifted and aligned with several small zones of hydrothermal alteration. Another magnetic low incursion previously thought to be alteration‐caused was removed entirely. Several terrain‐induced highs were removed or combined, and several lows associated with valleys became relatively more prominent, leading to significant reevaluation of earlier interpretations. Several of these revisions have possible economic consequences. We show that this technique cannot be duplicated by filtering or upward continuation, and in regions of significant topographic relief the value of this new method to exploration geophysicists may be substantial.
FOURIER TRANSFORMS OF FINITE LENGTH THEORETICAL GRAVITY ANOMALIES
Authors: Regan Robert D. , Hinze William J.
Published: October 2002

The mathematical structure of the Fourier transformations of theoretical gravity anomalies of several geometrically simple bodies appears to have distinct advantages in the interpretation of these anomalies. However, the practical application of this technique is dependent upon the transformation of an observed gravity anomaly of finite length. Ideally, interpretation methods similar to those for the transformations of the theoretical gravity anomalies should be developed for anomalies of a finite length. However, the mathematical complexity of the convolution integrals in the transform calculations of theoretical anomaly segments indicate that no general closed analytical solution useful for interpretation is available. Thus, in order to utilize the Fourier transform interpretation method, the data must be of sufficient length for the finite transform to closely approximate the theoretical transforms.
A GENERAL EXPRESSION FOR THE FOURIER TRANSFORM OF THE GRAVITY ANOMALY DUE TO A FAULT
Authors: Sharma Bijon , Bose T. K.
Published: October 2002

The application of the method of the Fourier transform in interpreting gravity anomalies of faults has so far been based upon the Fourier transform of the gravity anomaly due to a single semi‐infinite block cut by a vertical fault. A general expression for the Fourier transform of the fault anomaly is here derived which is valid for an arbitrary angle of inclination of the fault plane. For deriving the general expression, the gravity anomaly of the fault is first separated into a constant and a variable term. The transforms of the two terms are calculated separately and then added to give the general expression for the Fourier transform of the fault anomaly.
THE GRAVIMETRIC DENSITY FORMULA FOR A SPHERICAL SHELL
Authors: Hearst J. R. , Carlson R. C.
Published: October 2002

The formula for the gravimetric density (ρ) of material, between two stations a vertical distance ΔZ apart in a borehole, is (Hammer, 1950): [Formula: see text]where k is the gravitational constant, F the free air gradient (the derivative of gravity with respect to depth in the absence of matter), and Δg the change in gravity. This formula is generally derived by integrating the vertical component of gravity caused, at a measuring point, by an infinite horizontal plate of thickness t, and then doubling it to account for one measuring point above and below the plate. This results in (Heiskanen and Vening‐Meinesz, 1958), [Formula: see text]In fact, however, the earth is not made up of infinite horizontal plates; it can perhaps more reasonably be regarded as a series of spherical shells. It is therefore instructive to derive the expression for the change in gravity associated with passing through one of these spherical shells.
On: “Gravity Vertical Gradient Measurements for the Detection of Small Geologic and Anthropogenic Forms” by Z. J. Fajklewicz (GEOPHYSICS, October 1976, p. 1016–1030)
Authors: Mortimer Zofia
Published: October 2002

In his paper Fajklewicz discusses the use of the gravity tower vertical gradient [Formula: see text] for the detection of small, shallow bodies that show a density contrast with the surroundings (geologic structures, caverns and old mine workings). The reasons given by the author for such application of [Formula: see text] are the properties of the gravity vertical gradient and the vast experience of many years.
CONVERSION OF WIDEBAND EM FREQUENCY RESPONSE TO TRANSIENT RESPONSE USING SEGMENTED TRANSFORMATION
Authors: Asten M. W. , Verma S. K.
Published: October 2002

Electromagnetic (EM) methods measure the distortions of a primary field which are caused by a sub‐surface conductor. The resultant field is recorded as a function of frequency or of time, depending upon the harmonic or transient nature of the primary field. The two different types of measurements thus recognized are frequency domain (or continuous‐wave) and time domain (transient or TEM) methods. Interpretation of EM data is possible by comparing field response with the analytic or experimental response of a heuristic model. Most of the interpretive developments have been done for the frequency domain technique, which is mathematically more tractable than the TEM technique when we consider generalized models possessing a conductive halo, over‐burden, or host rock. For such models, TEM response is more easily obtained from analog model experiments (e.g., Velekin and Bulgakov, 1967; Palacky, 1975). Response curves thus obtained, however, are dependent upon the shape of the excitation pulse which varies among the different transient EM systems available; e.g., the Input system (Barringer, 1962) uses a 1.1 msec half‐sine pulse, the Crone pulse EM system (Crone, 1975) uses a linear ramp pulse with 1.4 msec rise time, while the Russian MPP01 system (Velekin and Bulgakov, 1967) uses a 15 msec square step pulse.
On: “Application of the generalized inverse to the inversion of static potential data” by J. Cribb (GEOPHYSICS, December 1976, p. 1365–1369)
Authors: Shuey R. T. , Wannamaker P. E.
Published: October 2002

In his paper, Cribb showed that the generalized inverse of gravity or magnetic fields could be found in closed form. At first we thought that the density or susceptibility distribution thus computed might have interpretive value, but after some study we decided this was not so. The purpose of this note is to share our experience.
A COMPUTER PROGRAM TO OBTAIN THE WEIGHTS OF A TIME‐DOMAIN WAVE‐SHAPING FILTER WHICH IS OPTIMUM IN AN ERROR‐DISTRIBUTION SENSE
Authors: Mereu R. F.
Published: October 2002

The program presented in this paper (Appendix A) computes the weights of a time‐domain wave‐shaping filter F which will transform a given input signal into a desired output signal in an optimum “error‐distribution” sense (see Mereu, 1976).