bluuebunny/crossref_metadata_2025 huggingface dataset:
TWO SELECTED FIELD EXAMPLES OF EM ANOMALIES IN A CONDUCTIVE ENVIRONMENT
Authors: Lajoie J. J. , West G. F.
Published: October 2002

In a previous paper (Lajoie and West, 1976), the authors described a computer modeling study demonstrating the effects of conductive overburden and host rocks in the interpretation of electromagnetic surveys. In conductive environments, field results may differ substantially from that predicted by free‐air modeling. This is not to say that free‐air modeling data cannot be used in the interpretation process, but that caution must be exercised and that an understanding of the physical processes involved can lead to a more accurate interpretation with resultant savings in exploration expenditures. In this short note, two field examples are presented to illustrate some of the concepts presented in our previous paper.
VES DIPOLE‐DIPOLE FILTER COEFFICIENTS
Authors: Nyman Douglas C. , Landisman Mark
Published: October 2002

The conversion of sampled Schlumberger and dipole‐dipole vertical electrical sounding (VES) apparent resistivity values into raised kernel function values is an important step in the interpretation of these data. This conversion involves the convolution of the sampled values, uniformly spaced on a log distance scale, with a set of filter coefficients. For Schlumberger and dipole‐dipole configurations, these coefficients can be calculated directly from the Schlumberger filter function. Oscillations of the filter coefficients at large indices (distances) can be minimized by proper selection of the sampling interval, as suggested by Koefoed. The choice of an optimal sampling interval has a more pronounced effect on the accuracy of the inverse transform (raised kernel function into apparent resistivity) than on that of the forward transform. The accuracy of the inverse transform can be improved for nonoptimal sampling intervals by “phase perturbation” of the filter function. Both transforms are sensitive to aliasing, which can be severe for certain dipole‐dipole configurations.
ESTIMATION OF DIP AND LATERAL EXTENT OF BEDS WITH BOREHOLE GRAVlMETRY
Authors: Hearst J. R.
Published: October 2002

Simple expressions for the estimation of dip or lateral extent of a bed whose extent is large compared to its thickness have been developed. Let [Formula: see text] be the apparent density contrast between the bed and its surroundings as measured by a borehole gravimeter and [Formula: see text] the true density contrast measured by a gamma‐gamma density log. Then for a dipping bed: [Formula: see text] where α is the dip angle. For a horizontal bed of finite extent; [Formula: see text] where t/w is the ratio of the bed thickness to its horizontal extent. For dipping beds of finite lateral extent the expression is more complicated. Numerical solutions were obtained and graphed.
DIFFRACTION RESPONSE FOR NONZERO SEPARATION OF SOURCE AND RECEIVER
Authors: Berryhill John R.
Published: October 2002

An existing theory based on the Kirchhoff retarded potential method makes distinctive predictions relating the amplitude characteristics of diffraction patterns to the geometry of subsurface reflectors. The application of this theory to seismic stacked sections would offer the geophysicist useful information to be included in subsurface interpretations. However, a possible barrier to such applications arises from the fact that the theory as originally put forth applies only to data recorded with zero source‐receiver separation, whereas stacked sections are produced by averaging data recorded over a wide range of shot‐geophone distances. To deal properly with seismic data us actually recorded, it is desirable to have a theory of diffraction amplitudes formulated for nonzero separation of source and receiver. This paper develops such a theory through an appropriate extension of the Kirchhoff approach. By expressing the problem in a special coordinate system, the Kirchhoff integral solution of the acoustic wave equation is reduced to a time‐domain convolution of the source wavelet with an operator recognized as the impulse response of the subsurface geometry under consideration. Impulse responses are computed explicitly for an infinite reflecting plane and for diffracting edges perpendicular and parallel to the source‐receiver axis. The nonzero‐separation theory is compared to the zero‐separation theory through numerical evaluation of the relevant formulas, and the latter is shown to be a special case of the former, as it should be. More importantly, the unexpected conclusion emerges that diffraction amplitudes at nonzero source‐receiver separation are controlled almost exclusively by the location of the source‐receiver midpoint. Since the data summed together in stacking all share a common shot‐geophone midpoint, the diffraction amplitudes on the stacked trace should behave in good approximation to the zero‐separation theory. Theoretical support is thus obtained for applying the zero‐separation theory to stacked seismic data.
DIRECT AND INVERSE PROBLEMS RELATING REFLECTION COEFFICIENTS AND REFLECTION RESPONSE FOR HORIZONTALLY LAYERED MEDIA
Authors: Koehler Fulton , Taner M. Turhan
Published: October 2002

If a plane wave is normally incident on the boundary of a horizontally layered medium, the reflection and transmission responses can be found in terms of the reflection coefficients at the interfaces of the medium. Recursion formulas for finding these responses have been given by various authors: Goupillaud (1961), Sherwood (1962), Kunetz and d’Erceville (1962), Sherwood and Trorey (1965), and Claerbout (1968). This paper gives explicit solutions of the recursion formulas for the numerator and denominator of the reflection and transmission operators. The solutions are expressed as polynomials in the variable z of the z-transform, with coefficients given as explicit sums of products of reflection coefficients. These formulas are equivalent to those given by Goupillaud. We also give the operator which removes all effects of any given set of layers at the top, the operator being equivalent to one given by Goupillaud. The inverse problem of finding the reflection coefficients from a reflection record or transmission record has been solved in two different, equivalent forms by Kunetz and d’Erceville and Claerbout. This paper gives further results derived along the lines of Claerbout’s paper. Since the solutions are developed for the noise‐free case, the effects of timing errors, scaling errors, and random noise on the reflection coefficient series computation are investigated. Examples are given which show the zones of stabilities. The Appendix gives the general form of the energy conservation equation in elastic wave propagation in strict analogy with the Poynting formulation for electromagnetic wave propagation. The main body of the paper includes a proof that Claerbout’s polynomial identity is an energy conservation equation for the case when no free surface is present.
LEAST‐SQUARES INVERSE FILTERING AND WAVELET DECONVOLUTION
Authors: Berkhout A. J.
Published: October 2002

Detailed comparison between borehole data and seismic data has taught that, in general, conventional seismic inverse filtering is not effective enough to produce desirable deconvolution results, i.e., seismic sections with broad‐band zero‐phase wavelets. Application of conventional seismic reverse filters has the advantage that very little information is needed from the user. However, as is shown in this paper, the phase spectra of these filters may be seriously in error, even if the seismic wavelet has the minimum‐phase property. In wavelet deconvolution the phase spectrum of the filter is correct, provided a good estimate of the seismic wavelet is available. In this paper, wavelet deconvolution is compared with Wiener filtering. The main conclusions are illustrated by examples.
REDUCTION OF MAGNETIC AND GRAVITY DATA ON AN ARBITRARY SURFACE ACQUIRED IN A REGION OF HIGH TOPOGRAPHIC RELIEF
Authors: Bhattacharyya B. K. , Chan K. C.
Published: October 2002

The problem of reduction of magnetic and gravity data, when observed on an arbitrary surface in a region of high topographic relief, is studied with equivalent source representation at the points of observation. It is shown that the analytical relationship between the total magnetic field or the gravity effect and equivalent magnetization or density on an arbitrary observational surface is given by a Fredholm integral equation of the second kind. A rapidly convergent iterative scheme is described for the solution of the integral equation, yielding the surface distribution of magnetization or density. With this distribution, the field at any other surface can be easily computed. Then it has been demonstrated with model examples that the gravity or magnetic field observed on a rough terrain can be accurately reduced to a horizontal plane for processing and interpretation. A new method has been suggested for minimization of terrain‐induced anomalies on a magnetic or gravity map. This method is based on the concept that when the anomalous field observed on an arbitrary surface is continued to a surface parallel to the topography, the terrain effect in the continued field is sharply reduced relative to the field created by bodies of finite extent in the crust. Model examples are presented to show the accuracy and reliability of the method.
TWO‐FOLD RESISTIVITY MAPPING OF LATERAL INHOMOGENEITIES
Authors: Doicin Dariu
Published: October 2002

In mining exploration and engineering studies, resistivity mapping is often used to locate and delineate laterally inhomogeneous structures. When surveying is done with a fixed current source, as in the dipole mapping method (Bibby and Risk, 1973; Keller et al., 1975) the simplicity in its application is often offset by the difficulty of contouring the existing anomalies and in estimating their magnitudes. More recently, Doicin (1976) proposed a bidirectional variation of the dipole mapping method, the quadripole‐quadripole method, which has been found to yield anomaly contours notably less affected by distortion than those obtained with the dipole mapping method. Furthermore, it has been shown that the quadripole apparent resistivity can, to a first approximation, be regarded as independent of the orientation of the source.
INVERSION OF AIRBORNE RADIOMETRIC DATA
Authors: Gunn P. J.
Published: October 2002

The spectral representation describing the gamma ray intensity due to radioactive prismatic sources such as approximate areal distributions of outcropping rocks shows that such intensity fields result from the convolution of factors depending upon the height of the plane of observation, the geometry of the causative bodies, instrumental constants, and the concentrations of radioelements emitting gamma rays. Inverse filters, designed according to the Wiener least‐squares criteria, can deconvolve factors from the observed intensity field to provide direct mappings of the concentrations of radioactive elements.
EXPERIENCES WITH CONCRETE CALIBRATION SOURCES FOR RADIOMETRIC FIELD INSTRUMENTS
Authors: Løvborg Leif , Bøtter‐Jensen Lars , Kirkegaard Peter
Published: October 2002

Four radioactive concrete pads, 3 m in diameter and 0.5 m thick, were constructed to serve as sources of potassium, uranium, and thorium gamma radiation in calibration experiments with portable spectrometers and total‐count scintillometers. One of the pads contains an admixture of high‐grade uranium ore. The radiation output from this pad fluctuates and has a seasonal variation of ±20 percent. This observation indicates that the uranium ore emanates radon into the concrete pores, and that this radon is subject to the same pumping action from the atmosphere as radon in soil. In dry and warm weather the concrete surface has a low content of gamma‐ray emitting radon daughters, because radon is lost from the pad. This situation is reversed when the weather is cold and moist. Therefore a varying, apparent radiometric concentration of uranium must be ascribed to the pad. The apparent uranium concentration correlates the observed radiation output with the stable radiation output produced by the three other pads, and it can at any time be determined from four readings with a total‐count scintillometer operated at an energy threshold of 0.4 MeV.
WAVENUMBER DOMAIN EXPRESSIONS FOR POTENTIAL FIELDS FROM ARBITRARY 2-, 21/2-, AND 3‐DIMENSIONAL BODIES
Authors: Pedersen L. Børsting
Published: October 2002

The spectral representation of potential fields has gained a great deal of importance within the last decade. There have been two main categories of application. The first one relates to the interpretation of single anomalies, which are interpreted in terms of a single homogeneous body by means of certain characteristica of the spectra. Examples have been given by Odegard and Berg (1965), Sharma et al (1970), Sengupta (1974), Bhattacharyya (1966), Bhattacharyya and Leu (1975, 1977), Bhimasankaram et al (1977), Collins et al (1974). The second category relates to the interpretation of a whole collection of anomalies, which are interpreted on a statistical basis by extracting main features from the average spectra. Examples have been given by Spector and Grant (1970), Treitel et al (1971), Cassano and Rocca (1975), and Hahn et al (1976).
Maximum entropy spectral analysis of multiple sinusoids in noise
Authors: Satorius E. H. , Zeidler J. R.
Published: October 2002

An analytical technique based on the method of undetermined coefficients is applied to the problem of computing the theoretical spectral estimate by the maximum entropy method (MEM) when the autocorrelation function of the data is known exactly and corresponds to N sinusoids in additive white noise and to N sinusoids in additive 1-pole, low‐pass noise. For the white noise case, the L prediction filter coefficients are expanded directly in terms of the input sinusoids. This expansion leads to a transformation of the L × L normal equations for the prediction filter coefficients to a set of 2N × 2N equations. The transformed equations are a smaller set of equations to be solved whenever L > 2N and provide a convenient description of the interaction between the various frequency components of the sinusoids which occurs in the MEM estimate. Further, for certain cases where there is little interaction between some of the frequency components of the sinusoids, the solution of the 2N × 2N equations may be approximated (to zeroth order) by the solution of a smaller set of coupled equations. A better approximation to the exact solution of the 2N × 2N equations can then be obtained from a perturbation expansion of the exact solution about the zeroth order approximation. For the case of N sinusoids in l‐pole, low‐pass noise, the L prediction filter coefficients are expanded in terms of the input sinusoids as well as two delta functions which occur at the beginning and end of the filter. This expansion also leads to a set of 2N × 2N equations. For this case the values of the MEM estimate evaluated at the frequencies of the sinusoids are shown to be a function of the frequencies of the sinusoids. This result is reasonable since the signal‐to‐noise ratio per unit bandwidth is also a function of frequency.
Quantitative uranium determinations from gamma‐ray logs by application of digital time series analysis
Authors: Conaway J. G. , Killeen P. G.
Published: October 2002

The desired output from a gamma‐ray logging system in mining applications is an accurate log of the quantitative distribution of radioelements with depth along the borehole. However, the measured log is not the desired output, but it may be visualized as the desired output modified by a series of low‐pass filters of various characteristics. These filters can be related to the distance interval over which each count is made (or the analog ratemeter time constant), the detector length, and the “geologic impulse response” (the response of a point detector to an infinitesimally thin layer of ore of a known grade). In effect, this means that the response due to thin beds is lower than expected, and the resolution will be reduced. An understanding of the nature and properties of these filters will aid in the proper evaluation of the log. To optimize the accuracy and resolution of quantitative uranium (or other radioelement) determinations, especially in thin beds and complex sequences, it is necessary to keep the counting interval short, and to compensate for the effect of the geologic impulse response. Although the effect of the geologic impulse response may be removed in any of several ways, the use of a simple inverse convolution operator as described here allows the data to be processed on‐line, essentially in real time, using a microprocessor or portable minicomputer.
A vector EM system and its field applications
Authors: Hohmann Gerald W. , Vanvoorhis Gerald D. , Nelson Philip H.
Published: October 2002

At Kennecott Exploration Services we built and applied widely a unique vector electromagnetic (VEM) system that measures magnetic field amplitude and phase at four frequencies: 26, 77, 232, and 695 Hz. Stable crystal oscillators enable us to measure phase without a wire link for any transmitter‐receiver configuration. A square‐wave is transmitted into a loop, and the signal is read at two frequencies: the fundamental and the third harmonic. The amplitude readout is logarithmic with switched 10-dB gain increments. Amplitude resolution is 0.1 dB and phase resolution is 0.5 degree. The VEM system has been applied successfully in a wide variety of exploration terranes and in several field geometries. For reconnaissance work, we utilize a large rectangular source loop similar to the Turam geometry, but with only one receiver coil. Amplitude and phase data can be reduced to field strength ratio and phase difference as in Turam or to in‐phase and quadrature components. In‐phase and quadrature are better for deep targets, because the standard Turam reduction discriminates against deep conductors. For investigating narrow zones or for defining a conductor, we use a fixed vertical loop source; in that case, the receiver measures the amplitude and phase of the vertical field, rather than the conventional tilt angle. Borehole EM surveys (surface transmitter‐downhole receiver and downhole transmitter‐surface receiver) are useful both for determining whether an anomaly has been tested by drilling, and for mapping conductors.
Effect of tropical weathering on electrical and electromagnetic measurements
Authors: Palacky G. J. , Kadekaru Kiyoshi
Published: October 2002

Electrical properties of the weathered layer in tropical regions of Brazil were investigated by means of resistivity soundings, airborne, and ground electromagnetic measurements. Five case histories illustrate how changes of climate, lithology, and geomorphology affect geophysical measurements. In humid and subhumid tropical regions (annual rainfall over 650 mm) the weathered layer is between 10 and 80 m thick and moderately conductive. Results from one region (Minas Gerais) indicate that excessive depth of weathering and leaching of massive sulfides, rather than the conductivity of overburden, present the greatest obstacle to effective use of airborne EM methods in mineral exploration. Seasonal variations of precipitation cause changes in soil resistivity, but such changes are not apparent in the underlying weathered layer. In semiarid and temperate regions of Brazil, the weathered layer is 10 to 20 m thick and regional airborne EM surveys are an efficient exploration tool. In all regions, the degree of weathering depends upon lithology and, in several areas, anomaly patterns obtained from airborne EM surveys correlate well with the surface geologic map. However, when comapring electrical properties of similar rock types among regions of the same climatic type, a considerable variation is observed. It seems that also geomorphology plays an important role in weathering. A careful interpretation of airborne EM data is necessary to distinguish anomalies caused by the weathered layer from those due to underlying conductors. Highly conductive, saline alluvia, which cause strong EM anomalies in Australia, were encountered (sporadically) in only one region of Brazil, the semiarid Valley of Curaçá, Bahia.
Numerical evaluation of the transient acoustic waveform due to a point source in a fluid‐filled borehole
Authors: Tsang Leung , Rader Dennis
Published: October 2002

A key measurement employed in oil well wireline logging is the acoustic wave traveltime over a specified formation interval, typically 1 ft. In the traditional measurement, only the compressional head wave is monitored, but for some time it has been obvious that there is significant additional information, such as the shear head wave arrival, in the received waveform. We describe two numerical methods for computing the profile and parameter dependence of the transient waveform based on a model of the acoustic logging problem consisting of a point source on the axis of a fluid‐filled cylindrical borehole. The response to this excitation is determined at a distance from the source, generally on the borehole axis. In the first of the two numerical methods, called “real axis integration”, the complete acoustic waveform is obtained. The second method, called “branch‐cut integration”, evaluates the first compressional and shear‐pseudo‐Rayleigh arrivals individually with much less computation time than the first method. The validity and accuracy of the two methods are demonstrated by their close agreement within appropriate time windows. It is also shown that the results from the ordinary asymptotic method that exist in the literature predict different behavior. The dependence of the transient arrivals on formation parameters is illustrated by various numerical results in both time and frequency domains.
Geophysical Activity in 1978
Authors: Whitmire M. G.
Published: October 2002

This report presents statiscal data on worldwide geophysical activity in 1978. These data were compiled from survey questionaires that were completed by more than 500 companies, contractors, government agencies, and institutions located throughout the free world. We have attempted to gather information from users of geophysical techniques in the areas of petroleum, mining, geothermal, and groundwater exploration; and from those who employ geophysical methods in engineering, oceanography, and research. Information was gathered on airborne, drilled hole, land, and marine geophysical 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 massive statistical averages for the area and technique reported. This process is necessary to avoid distortion of individual statistical items.
Attenuation and dispersion of compressional waves in fluid‐filled porous rocks with partial gas saturation (White model)—Part I: Biot theory
Authors: Dutta N. C. , Odé H.
Published: October 2002

An exact theory of attenuation and dispersion of seismic waves in porous rocks containing spherical gas pockets (White model) is presented using the coupled equations of motion given by Biot. Assumptions made are (1) the acoustic wavelength is long with respect to the distance between gas pockets and their size, and (2) the gas pockets do not interact. Thus, the present theory essentially is quite similar to that proposed by White (1975), but the problem of the radially oscillating gas pocket is solved in a more rigorous manner by means of Biot’s theory (1962). The solid‐fluid coupling is automatically included, and the model is solved as a boundary value problem requiring all radial stresses and displacements to be continuous at the gas‐brine interface. Thus, we do not require any assumed fluid‐pressure discontinuity at the gas‐water contact, such as the one employed by White (1975). We have also presented an analysis of all of the field variables in terms of Biot’s type I (the classical compressional) wave and, type II (the diffusion) wave. Our quantitative results are presented in Dutta and Odé (1979, this issue).
Visible and near‐infrared spectra of rocks from chromium‐rich areas
Authors: Hunt G. R. , Wynn J. C.
Published: October 2002

Bidirectional reflection spectra are presented from 0.35 to 2.5 μm for a suite of ultramafic rocks collected from chromium‐rich areas in southwestern Oregon and northwestern California. Features due to the presence of chromium are entirely absent from these spectra. However, the massive chromite samples are distinguishable from all others on the basis of spectral features produced by absorptions in ferrous ions located in unique tetrahedral sites afforded by the chromite spinel structure. Chromite samples are also distinguishable by their lack of specific absorption near 1.0 μm, normally caused by ferrous ions located in octahedral sites; absorption in the 1.0 μm region is the typical situation found in ultramafic and mafic rocks. The potential usefulness for remote‐sensing purposes of this unusual spectral regime is suggested.
Seismic velocities in transversely isotropic media
Authors: Levin Franklyn K.
Published: October 2002

When a sedimentary earth section is layered on a scale much finer than the wavelength of seismic waves, the waves average the physical properties of the layers; a seismic wave acts as if it were traveling in a single, transversely isotropic solid. We compute the velocities with which P‐waves, SV‐waves, and SH‐waves travel in transversely isotropic solids formed from two‐component solids and find the corresponding moveout velocities from [Formula: see text] plots. The combinations studied are sandstone and shale, shale and limestone, water sand and gas sand, and gypsum and unconsolidated material, one set of typical physical properties being selected for each component of a combination. A reflector at 1524 m and a geophone spread of 0–3048 m are assumed. The moveout velocity for an SH‐wave is always the velocity for a wave traveling in the horizontal direction. The P‐wave moveout velocity found from surface seismic data can be anywhere from the vertical P‐wave velocity to values between those for vertical and horizontal travel; the actual value depends on the elastic parameters and the spread length used for velocity determination. If the two components of the solid have the same Poisson’s ratio, the velocity from surface‐recorded data is the vertical P‐wave velocity. For this case, SH‐wave anisotropy can be computed. SV‐wave data usually do not have hyperbolic time‐distance curves, and the moveout velocity found varies with spread length. Surprisingly, the water sand‐gas sand combination gives a medium with negligible anistropy. A two‐component combination of gypsum in weathered material gives rise to [Formula: see text] plots that seem to explain the unusual behavior of near‐surface SV‐waves seen in field studies reported by Jolly (1956).
Direct inversion procedure for Claerbout’s equations
Authors: Bleistein Norman , Cohen Jack K.
Published: October 2002

Claerbout’s equations admit a closed form solution for the velocity variations in terms of observations at the upper surface. Synthetic examples provide analytical and numerical checks on the result. For example, with a tilted plane, the result exhibits relative error in slope and velocity variation of the order of [Formula: see text] in the tilt angle θ.
A constrained minimum power adaptive beamformer with time‐varying adaptation rate
Authors: Shen Wen‐Wu
Published: October 2002

A linear adaptive algorithm was developed for array beamforming purposes. The design goal for the algorithm is to minimize the squared filter output subject to filter constraints which allow energy propagating from the array steering direction to pass without being distorted. The adaptive filter coefficients were initialized to satisfy the constraints which were preserved during the iterations. The adaptation rate is inversely varied with filter output and total input channel power. Performance of the algorithm was studied using the recorded short‐period array data from the Korean Seismic Research Station. Processed were a high‐amplitude signal from Kamchatka, a medium‐amplitude signal from eastern Kazakh, and a number of low‐amplitude signals from central Eurasia. Results of signal‐to‐noise ratio gain relative to a conventional beamformer among the events tested were consistent and were in the range of 4.5 to 6.5 dB in the wide passband. Much better signal‐to‐noise ratio improvement was obtained in the low‐frequency passband. The adaptive algorithm was programmed in the real‐time mode and can be implemented in a front‐end detection system.
Higher‐order moveout spectra
Authors: May Bruce T. , Straley Donald K.
Published: October 2002

Higher‐order terms in the generalized seismic reflection moveout equation are usually neglected, resulting in the familiar second‐order, or hyperbolic, moveout equation. Modeling studies show that the higher‐order terms are often significant, and their neglect produces sizable traveltime residuals after correction for moveout in such cases as kinked‐ray models. Taner and Koehler (1969) introduced velocity spectra for estimating stacking velocity defined on the basis of second‐order moveout. Through the use of orthogonal polynomials, an iterative procedure is defined that permits computation of fourth‐order moveout spectra while simultaneously upgrading the previously computed, second‐order spectra. Emphasis is placed on the fourth‐order term, but the procedure is general and can be expanded to higher orders. When used with synthetic and field recorded common‐midpoint (CMP) trace data, this technique produces significant improvements in moveout determination affecting three areas: (1) resolution and interpretability of moveout spectra, (2) quality of CMP stacked sections, and (3) computation of velocity and depth for inverse modeling.
Audio‐frequency magnetotelluric sounding—a case history at the Cavendish geophysical test range
Authors: Strangway D. W. , Koziar A.
Published: October 2002

The Cavendish geophysical test site in southern Ontario, a highly conducting massive sulfide zone, has been studied using a variety of different geophysical prospecting methods. We present the results of an audio‐frequency magnetotelluric (AMT) survey which indicate adequate lateral resolution of the conductors. The two main conductive zones are most clearly detected when the electric field sensor is oriented normal to the zones, a feature that is characteristic of narrow buried bodies. When the electric field sensors were parallel to a conductor, the detection of narrow buried zones was limited. There is a clear indication of a broad low‐resistivity zone at depth between the two narrow, highly conducting zones. This interpretation is in general agreement with other survey results in this area.
A new method for shear‐wave logging
Authors: Kitsunezaki Chōrō
Published: October 2002

Reliable evaluation of shear (S) wave characteristics in boreholes may be facilitated by the system proposed here, in a wide variety of geologic conditions and depths. The measurements can be done with a sonde, suspended freely in water contained in a borehole. The main part of the sonde consists of a source, a filter tube, and receivers. In this system, the wave field is treated approximately as that in an infinite homogeneous solid medium, because the wavelength is sufficiently longer than the borehole diameter. The source behaves as a single point force. The direct S‐wave is detected on a line (borehole axis) perpendicular to the force axis, in which pre‐dominant radiation of shear wave is expected. This fact is completely different from some modified systems of sonic log, in which (shear wave) is the converted‐refracted wave propagating as the shear wave along a borehole wall. The proposed source system is the (indirect‐excitation type), wherein the force is applied to a borehole wall indirectly through a pressure distribution of doublet‐type excited in the water. Based on its principle, this source system eliminates generation of dilatational noise waves and also assures operation at greater depths because no work is done against the external pressure, as a whole, at the source. The proposed receiver system is the suspension type, wherein horizontal motion of the borehole wall (ground motion of S‐wave) is detected through corresponding water motion by a detector of neutral buoyancy. The fundamental applicability of this logging system was confirmed by experiments at shallow depths.
A grounded vertical long wire source system for plane wave magnetotelluric analog modeling
Authors: Edwards R. N.
Published: October 2002

A typical electromagnetic (EM) analog modeling apparatus consists of an electrolytic tank with embedded graphite blocks, representing conductivity anomalies. A plane wave magnetotelluric (MT) source is generated by alternating currents in a set of parallel horizontal overhead wires. A uniform horizontal magnetic field is produced over the surface of the electrolyte. A similar uniform field may also be generated by grounded semiinfinite vertical wires. Four such wires, two carrying current upward and two downward, when arranged at the corners of a rectangle of defined dimensions, generate a more uniform field than a corresponding pair of Helmholtz coils. If the size of the rectangle is large compared with a skin depth in the electrolyte, Cagniard’s MT relationships are obeyed both on and beneath the electrolyte. The vertical current source has the advantage over the horizontal current source since it requires no ancillary external connections between the ends of the modeling tank to complete the induced current circuit.
Physical properties of titanomagnetite sands
Authors: Lawton D. C. , Hochstein M. P.
Published: October 2002

Laboratory measurements of density, magnetic susceptibility, remanence, resistivity, and induced‐polarization response of New Zealand titanomagnetite sands (“ironsands”) were made for different volume concentrations [Formula: see text] of titanomagnetite in the range 10 to 100 percent. Both density ρ and magnetic susceptibility κ of these sands increase monotonically with [Formula: see text]; the observed variation of κ as a function of [Formula: see text] can be explained by changes of the resultant relative magnetic permeability μ of a binary mixture of magnetic and nonmagnetic grains. The natural remanence of titanomagnetite sands is small (Koenigsberger ratio Q < 0.2). No significant induced‐polarization response could be observed even for mixtures of almost pure titanomagnetite. When placed in an electric field, the titanomagnetite sands were found to be nonconductive.
Magnetic curve fit for a thin dike: Calculator program (TI 59)
Authors: Ballantyne Edwin J.
Published: October 2002

In the exploration for massive sulfide deposits, many of the sources of magnetic anomalies are long tabular dike‐like bodies which are thin in relation to the depth of burial. That is, the width of the dike is less than the depth of burial. A program is presented to aid in the inverse solution of this interpretation problem. Demagnetization effects are ignored, and the program will not function for symmetrical anomalies. Given field‐observed amplitudes and coordinates of maximum and minimum points on a total‐ or vertical‐field magnetic profile, this program fits the corresponding theoretical profile for a thin dipping magnetic dike. Coordinate and depth of the top of dike, dip of dike, and width‐susceptibility contrast of the dike are calculated. A theoretical anomaly profile may also be calculated for comparison with the original field curve. Successive iterations may be used to refine estimates of the calculated parameters.
The frequency and the time‐domain responses of a buried axial conductor
Authors: Tsubota Koji , Wait James R.
Published: October 2002

Our objective is to analyze the frequency and the time‐domain responses of a two‐dimensional lateral inhomogeneity such as an axial conductor which is buried in a stratified earth. Such inhomogeneities in the earth’s structure give complex transients that cannot be interpreted using just a uniformly layered earth model. To proceed, we determine the fields of a thin axial conductor carrying a filamental current situated in the lower medium of a two‐layered earth. This infinitely long axial conductor is characterized by a specific axial impedance. The impedance condition can be used to determine the total field response when the source is a magnetic dipole. Some results are presented that show the waveform at the induced current in a buried axial conductor excited by an impulsive current in a vertical magnetic dipole source located on the earth’s surface. A feature of the waveform is the reversal of the polarity that would not occur in the absence of the buried cable.
Aeromagnetic survey design
Authors: Reid A. B.
Published: October 2002

Aeromagnetic surveys are flown with a wide variety of terrain clearances, sampling rates, and line spacings. The results are generally presented as contour maps, implying that the survey grid defines the continuous magnetic field sufficiently well to justify interpolation.
Calculation of apparent conductivity for the transient electromagnetic (coincident loop) method using an HP-67 calculator
Authors: Spies B. R. , Raiche A. P.
Published: October 2002

The interpretation of coincident‐loop transient electromagnetic (TEM) surveys is often aided by the transformation of TEM data to apparent conductivity values. Present methods of calculating apparent conductivity have a limited usefulness or require access to a computer. This paper presents an algorithm for calculating apparent conductivity to an accuracy of better than 1 percent for values of the parameter x = σμA/4πt up to 10, where σ = conductivity, μ = magnetic permeability, A = loop area, and t = sample time. This range of x is sufficient for most situations encountered in field work. The algorithm has been programmed for an HP-67 pocket calculator, and the program steps and user instructions are given in standard format. Execution time ranges from 6 to 15 sec. The program should assist the interpretation of TEM surveys in the field.
The linear properties of thin layers, with an application to synthetic seismograms over coal seams
Authors: Koefoed O. , de Voogd N.
Published: October 2002

Thin layers are considered from the point of view of the quasi‐linear relation that exists between their thickness and their reflection response to a seismic signal. The range within which this quasi‐linearity exists is investigated; for a continuous sine wave, this is done by means of the equation for the response given in Rayleigh (1945), and for a seismic wavelet by means of a synthetic seismogram program. For a wavelet, the limiting value of the dominant frequency is found to be smaller than that for a continuous sine wave, the difference being in the order of magnitude of 15 percent. Within the linearity range, a thin layer may be replaced by an equivalent layer which gives the same reflection response but differs in thickness and in acoustic impedance. In the construction of synthetic seismograms over coal seams, this equivalent replacement may be utilized to replace the seams by layers, for which the two‐way traveltime is equal to an integer number of sampling intervals; by this procedure the usual rounding‐off errors are avoided. The method of equivalent replacement is also applicable when the host rock above and below the seam have different velocities.
The development and applications of a wide band electromagnetic sounding system using a pseudo‐noise source
Authors: Duncan P. M. , Hwang A. , Edwards R. N. , Bailey R. C. , Garland G. D.
Published: October 2002

A very wide band (0.03 Hz to 15 kHz) electromagnetic (EM) system has been developed. It was used over the band 1 Hz to 10 kHz to determine the electrical structure of the earth’s crust from depths of a few meters to over 40 km. A direct current of from 1 to 5 A was reversed through a long wire bipole transmitter in a pseudo‐random binary sequence (PRBS). Depending upon the frequencies selected, a sensitive one‐component flux gate magnetometer or an air‐core coil was used to monitor temporal changes in the vertical component of the magnetic field at a recording site. The measured signal was crosscorrelated digitally in real time with an exact copy of the transmitted waveform in order to obtain a good signal‐to‐noise ratio at distances up to 5 times the length of the bipole. The output crosscorrelogram was deconvolved from the system input, the autocorrelogram of the transmitted waveform, using a Wiener least‐squares filter to give the impulse response of the earth. This was then transformed into frequency domain to yield directly the phase and amplitude transfer function, which was inverted in a routine manner with a Marquardt algorithm to find layer conductivities and thicknesses. Two field experiments are described. The first demonstrates the viability of the technique to sound a simple layered section of interbedded shale and limestone. It was a relatively shallow sounding to a depth of about 500 m in the sedimentary basin of southern Ontario. The second experiment was designed to search for a conductive zone in the lower crust. A bipole over 20 km long carried a transmitter current of only 5 A, yet field data were obtained over the range 35 to 70 km. The primary magnetic fields never exceeded 10 mgamma and were often considerably smaller. The interpretation indicates a conductive layer of resistivity less than 270 Ω-m at a depth of about 20 km.
Areal seismic methods for determining the extent of acoustic discontinuities
Authors: McDonald John A. , Gardner G. H. F. , Kotcher J. S.
Published: October 2002

The collection of areal seismic reflection data is becoming fairly routine. It is now generally realized that the solution of three‐dimensional (3-D) structural problems is only possible when the target has been adequately sampled and the data have been correctly migrated to produce an accurate image of the subsurface. However, many of our exploration prospects are associated with lithological changes or stratigraphic features rather than structural features. We show how areal seismic techniques can provide an added dimension in determining the extent of acoustic discontinuities in areas where the strata are generally flat.
Analysis of multisource gravity and magnetic anomaly data sets by moving‐window application of Poisson’s theorem
Authors: Chandler V. W. , Koski J. S. , Hinze W. J. , Braile L. W.
Published: October 2002

Use of Poisson’s theorem to calculate single‐source magnetization‐to‐density ratios may be extended to multisource data by repeatedly applying the theorem within a small, moving data window. In this approach, the moving window traverses the data set in increments of the data interval, and a linear regression is conducted between the first vertical derivative of gravity and the magnetic values reduced to the pole within each window position. Three parameters—correlation coefficient, slope, and intercept—are thereby generated at each data interval that describe the internal correlation existing between gravity and magnetic anomalies and may yield information regarding anomaly source properties. The slope parameter provides an estimate of source magnetization‐to‐density ratio for the anomaly segments within each window position.
On: “Design parameters for aerial gamma‐ray surveys” by J. A. Pitkin and J. S. Duval (GEOPHYSICS, September 1980, p. 1427–1439).
Authors: Gay S. Parker
Published: October 2002

I have noted several major deficiencies in the paper by Pitkin and Duval which I think should be brought to the attention of the readers of Geophysics.
Reexamination of the values used as constants in calculating rock density from borehole gravity data
Authors: Robbins Stephen L.
Published: October 2002

The U.S. Geological Survey has a research program in bore‐hole gravimetry (Mast, 1978, p. 17). As part of this program, I have been examining the accuracy of constants F and k used in the formula for determining in‐situ bulk density from borehole gravity data.
Application of Gauss’s method to magnetic anomalies of dipping dikes
Authors: Won I. J.
Published: October 2002

Quite a few computerized interpretation schemes on the inversion of a magnetic profile across a dike have been reported recently. McGrath and Hood (1970) described an essentially trial‐and‐error method in which each of the dike parameters is varied in turn by an arbitrary amount until a set of best‐fit parameters is found. Rao et al. (1973) described a method of differential correction to improve the initially assumed dike parameters. Similar works may include those by Moo (1965), Hall (1968), Bosum (1968), and others.
1979 SAFETY COMMITTE REPORT
Authors: Lawler Don M.
Published: October 2002

A tabulation of accident statistics for the years 1968 through 1979 and a detailed listing of accidents by category during 1979 are shown below. A number of companies are to be highly commended for responding to SEG’s call for geophysical safety statistics in a responsible and professional manner. It is through their efforts that we are able to obtain data for use in your safety program. Better information will result next year provided more companies will contribute meaningful data on accidents.
The traveltime equation, tau‐p mapping, and inversion of common midpoint data
Authors: Diebold John B. , Stoffa Paul L.
Published: October 2002

Seismic traveltime data can be described in terms of instantaneous slope (dT/dX) and intercept time (τ). With this parameterization, the most general form of the traveltime equation is developed and found to be valid for reflections and refractions, all source‐receiver offsets, and all commonly used experimental geometries. For common source‐receiver experiments, the instantaneous slope of the traveltime data represents the true horizontal ray parameter p, and the intercept time is the sum along the ray of thickness‐vertical slowness products referenced to the fixed surface station. For common midpoint (CMP) experiments, we show that the instantaneous slope is the average slowness of upgoing and downgoing rays at the surface, and the intercept time is again the sum of thickness‐vertical slowness products, but referenced to the common midpoint. This inherent averaging immediately explains the superiority of the CMP experiment in the presence of dip. In addition to providing a general traveltime equation, we find that τ-p parameterization reorganizes traveltime data in a way which greatly facilitates interpretation. For reflections, the τ-p trajectories are sums of ellipses and all turning rays (whether refracted or postcritically reflected) combine to form a well defined and easily identified trajectory. We present a method for deriving a velocity‐depth function from this trajectory which is applicable to both refracted and reflected arrivals.