bluuebunny/crossref_metadata_2025 huggingface dataset:
SCALE MODEL RESULTS FOR INDUCTIVE LOGGING IN THE REGION OF SINGLE AND MULTIPLE CONDUCTIVE BODIES
Authors: Drinkrow R. L. , Duffin R. H.
Published: October 2002

An analog modeling program has been conducted to provide a suite of anomaly profiles for electromagnetic logging of drill holes passing through and near subsurface conductors. Bodies considered include a small but highly conductive sulfide analog, a larger body of moderate conductivity (representing conductive shale), and a composite of the smaller body embedded in the larger one. Logs were obtained at two frequencies, two body dips, and three background or country rock resistivities. Results indicate that semiquantitative estimates of the extent of intersected conductors or the distance to missed conductors of similar properties to those modeled can be made. The use of two or more field frequencies and the recording of in‐phase and quadrature components are recommended as a guide to differentiating between good and poor conductors. Dip effects are subtle and may not easily be recognized in field data. A conductive host rock will modify the log of a given conductive body.
Seismic radiation from a point source on the surface of a cylindrical cavity
Authors: Greenfield Roy J.
Published: October 2002

The presence of a void or cavity in the vicinity of a seismic source will modify the radiated signal from the classic solution for a point force in an infinite medium. To study this effect, solutions were obtained for the seismic fields from a point force applied to the surface of a cylindrical cavity in an elastic medium. The solutions were evaluated to give P‐ and S‐wave frequency domain radiation patterns. For P wavelengths less than about 3 times the cavity diameter, the cavity acts to decrease the P‐wave amplitude going outward in the direction opposite the source. Data taken in two coal mines show this shielding effect. High‐frequency energy was observed, with surface seismometers, for signals generated by hitting the mine roof, whereas the high‐frequency energy was much smaller on signals generated by hitting the floor. Time domain calculations show that the P‐wave signal is delayed by approximately the time it takes an S‐wave to propagate around the cylinder.
Nonlinear impedance of mineral‐electrolyte interfaces: Part I. Pyrite
Authors: Klein J. D. , Shuey R. T.
Published: October 2002

The impedance of the interface between an acidic electrolyte and polished electrodes of pyrite has been investigated at current densities in the nonlinear range (up to [Formula: see text]). The potential across a single interface relative to a reference electrode was measured in response to a current sinusoid of low frequency (0.002 Hz). Polarization curves, or linear plots of current density versus electrode potential, consisted of distorted Lissajous patterns. At peak current densities the interface impedance is low and is dominated by activation controlled reactions involving pyrite dissolution and hydrogen gas evolution. The polarization curves have a series of step‐like features at intermediate potentials due to current‐limited reactions. These secondary reactions involve solid and/or aqueous reaction products from previous reactions. The high impedance portion of each reaction step corresponds to a limit current caused by either depletion of a particular solid reactant or employment of a current larger than can be carried by diffusion of aqueous reactants. Nonlinear behavior of pyrite was found to be independent of carrier type and conductivity. The potential of the anodic pyrite dissolution reaction was weakly dependent on pH. Potentials of cathodic reactions increased with increasing pH, indicating the involvement of [Formula: see text], as demonstrated by the evolution of hydrogen gas and [Formula: see text] gas.
Nonlinear impedance of mineral‐electrolyte interfaces: Part II. Galena, Chalcopyrite, and Graphite
Authors: Klein J. D. , Shuey R. T.
Published: October 2002

The impedance of the interface between an acidic electrolyte and monomineralic, polished electrodes of galena, graphite, and chalcopyrite has been investigated at current densities in the nonlinear range (up to [Formula: see text]). The potential across a single interface relative to a reference electrode was measured in response to a current sinusoid of low frequency, 0.002 Hz. Polarization curves, or linear plots of current density versus electrode potential, consisted of distorted Lissajous patterns. Onset of a new electrochemical reaction resulted in a decrease in impedance of the interface, and hence increase in slope of the polarization curve. For some reactions, the electrical characteristics were diagnostic of bulk mineral composition. Diagnostic reactions include (1) mineral dissolution and gas evolution reactions at extreme anodic and cathodic potentials, (2) reactions at intermediate potentials which involve reaction products from previous reactions. Response is thus dependent on previous reactions and therefore on sample history. Anodic reactions were generally independent of pH, and consisted primarily of mineral dissolution reactions. Potentials of cathodic reactions increased with increasing pH indicating the involvement of [Formula: see text] as demonstrated by the evolution of hydrogen gas and/or [Formula: see text] gas. The potentials of the main graphite reactions were larger in magnitude than any of the sulfide reaction potentials. Measurements with polymineralic electrodes indicate that current flows mainly through minerals with reactions at less extreme potentials and consequently reactions involving other minerals at higher potential do not occur. Due to its more extreme reaction potentials, graphite does not respond in the presence of sulfide minerals. It appears that nonlinear phenomena could be used for mineral discrimination in drill hole logging.
On K‐factors and gamma log calculations
Authors: Fink James B.
Published: October 2002

In gamma‐ray logging for radioactive source material, the logging system response depends on a variety of internal and external influences, such as the type of material, disequilibrium, source geometry, logging medium, and system components. Of paramount importance is the constant of proportionality that relates the system response in counts per second (cps) to a particular grade of material. This constant has been defined by Broding and Rummerfield (1955) and Scott et al (1961) and is usually called the “K‐factor”. The K‐factor for any particular system cannot be directly calculated from knowledge of the system’s components and assemblage, hence it must be determined empirically from measurements made in a controlled situation.
Magnetotellurics with a remote magnetic reference
Authors: Gamble T. D. , Goubau W. M. , Clarke J.
Published: October 2002

Magnetotelluric measurements were performed simultaneously at two sites 4.8 km apart near Hollister, California. SQUID magnetometers were used to measure fluctuations in two orthogonal horizontal components of the magnetic field. The data obtained at each site were analyzed using the magnetic fields at the other site as a remote reference. In this technique, one multiplies the equations relating the Fourier components of the electric and magnetic fields by a component of magnetic field from the remote reference. By averaging the various crossproducts, estimates of the impedance tensor not biased by noise are obtained, provided there are no correlations between the noises in the remote channels and noises in the local channels. For some data, conventional methods of analysis yielded estimates of apparent resistivities that were biased by noise by as much as two orders of magnitude. Nevertheless, estimates of the apparent resistivity obtained from these same data, using the remote reference technique, were consistent with apparent resistivities calculated from relatively noise‐free data at adjacent periods. The estimated standard deviation for periods shorter than 3 sec was less than 5 percent, and for 87 percent of the data, was less than 2 percent. Where data bands overlapped between periods of 0.33 sec and 1 sec, the average discrepancy between the apparent resistivities was 1.8 percent.
Migration by phase shift—An algorithmic description for array processors
Authors: Dubrulle Augustin A. , Gazdag Jenö
Published: October 2002

The phase shift method (Gazdag, 1978) is based on the solution, in the frequency domain, of an approximation (Claerbout, 1976) to the one‐way wave equation with initial conditions defined by a zero‐offset seismic section. Wave velocity is assumed to be constant within each layer of the section grid and is allowed to vary from layer to layer. Under these conditions, the equation written in the frequency domain reduces to a system of independent ordinary differential equations with initial values that can be solved analytically for each layer. The integration process simply amounts to multiplying the initial values by a complex number of unit modulus. The main advantages of this method are simplicity, stability, and high accuracy, since the precision of the Fourier approximation is limited only by the granularity of the seismic section. From a practical viewpoint of computer implementation, the phase shift method offers a great deal of flexibility. Some accuracy can be traded for speed, as needed, by excluding the waves traveling at an angle that exceeds a specified limit, or by ignoring frequencies at the higher end of the spectrum. The migration of a group of reflections can be excluded as well, since the phase shift angles can be accumulated for application to the frequency representation of the section only within a specified depth interval. The most interesting feature of the method may well reside in the fact that the Fourier coefficients of a seismic section can be treated independently of one another. This is of particular importance if the method is implemented for an array processor. In this case, cross‐sections of the frequency data can be sent to the array processor one‐by‐one (or in groups, as may be suitable) for phase shift operations, at minimal storage cost. A formulation of the method for an array processor is outlined here, which takes advantage of the features mentioned above. It is described in a step‐wise algorithmic fashion, using natural language and standard mathematical notations. Functions for discrete Fourier transformations, evaluation of square roots and circular functions (or fast table look‐up or interpolation), and vector reversal are assumed to exist in the array processor, in addition to elementary vector operations. This formulation can also be used for efficient implementation on conventional computers.
Some alternate vibrator signals
Authors: Cunningham Allen B.
Published: October 2002

The linear swept‐frequency sine wave has been the only seismic vibrator drive signal seriously used in almost two decades of operational vibrator activity. However, there are other drive signals and alterations of this commonly accepted signal which offer improved peak‐to‐side lobe ratios on signal autocorrelations, as well as significant savings in field data acquisition and subsequent data processing times. In fact, one class of input signals allows vibrator signal recordings to have the same time duration as equivalent dynamite signal recordings. Another class provides effective discrimination against periodic interferences such as 60-Hz power‐line interference. Fortunately, these characteristics can be combined in the same input signal. The theory which suggests these improvements is outlined, and experimental seismic data which support the theory are shown. Data recorded at identical locations while using one conventional and three alternate input signals of equal bandwidths and energies are compared to show the advantages of the alternate signals.
Nonlinear least‐squares inversion of traveltime data for a linear velocity‐depth relationship
Authors: Gibson Bruce S. , Odegard Mark E. , Sutton George H.
Published: October 2002

Nonlinear least‐squares inversion of traveltime data is applied to the problem of determining thicknesses, velocities, and velocity gradients in laterally homogeneous, horizontally layered structures. The parametric forms of the traveltime equations are used for the calculations. Results of inversions on randomly inaccurate synthetic data show that the method will not determine the gradient consistently when using reflection traveltimes. Good results are obtained, however, when using traveltimes of energy refracted in a layer by the velocity gradient. Thicknesses and average velocity in the case of reflections, or velocity at the top of the layer in the case of refractions, are also determined. Partial derivatives determined during the course of the least‐squares inversion can be used to place limits on errors in the determined parameters.
Seismic refraction study of the Raft River geothermal area, Idaho
Authors: Ackermann Hans D.
Published: October 2002

The Raft River geothermal system in southeastern Idaho is a convective hot water system, presently being developed to demonstrate the production of electricity from low‐temperature (≊150°C) water. Interpretation of seismic refraction recordings in the area yielded compressional velocities from near the surface to the crystalline basement at a maximum depth of approximately 1600 m. The results show a complex sequence of sediments and volcanic flows overlying basement. Velocities in the sedimentary section vary laterally. Correlation with well data suggests that zones of higher velocities may correspond to zones where sediments are hydrothermally altered. Flowing hot wells occur near the boundary between inferred shallow altered and unaltered rocks. The basement surface does not appear to be displaced by large faults, although there is ample surface evidence of faulting. The deep circulation of hot water necessary for a convective system may be through many small faults and fractures. Fracturing is suggested on the basis of lateral velocity variations within the basement complex.
The self‐potential method in geothermal exploration
Authors: Corwin Robert F. , Hoover Donald B.
Published: October 2002

Laboratory measurements and field data indicate that self‐potential anomalies comparable to those observed in many areas of geothermal activity may be generated by thermoelectric or electrokinetic coupling processes. A study using an analytical technique based on concepts of irreversible thermodynamics indicates that, for a simple spherical source model, potentials generated by electrokinetic coupling may be of greater amplitude than those developed by thermoelectric coupling. Before more quantitative interpretations of potentials generated by geothermal activity can be made, analytical solutions for more realistic geometries must be developed, and values of in‐situ coupling coefficients must be obtained. If the measuring electrodes are not watered, and if telluric currents and changes in electrode polarization are monitored and corrections made for their effects, most self‐potential measurements are reproducible within about ±5 mV. Reproducible short‐wavelength geologic noise of as much as ±10 mV, primarily caused by variation in soil properties, is common in arid areas, with lower values in areas of uniform, moist soil. Because self‐potential variations may be produced by conductive mineral deposits, stray currents from cultural activity, and changes in geologic or geochemical conditions, self‐potential data must be analyzed carefully before a geothermal origin is assigned to observed anomalies. Self‐potential surveys conducted in a variety of geothermal areas show anomalies ranging from about 50 mV to over 2 V in amplitude over distances of about 100 m to 10 km. The polarity and waveform of the observed anomalies vary, with positive, negative, bipolar, and multipolar anomalies having been reported from different areas. Steep potential gradients often are seen over faults which are thought to act as conduits for thermal fluids. In some areas, anomalies several kilometers wide correlate with regions of known elevated thermal gradient or heat flow.
An optimum filter design for discrete integration
Authors: Peacock Kenneth L.
Published: October 2002

An optimum design for convolutional filters that integrate discrete data is presented here. Such integration filters are rarely found in the literature, although counterparts for discrete differentiation are rather common. The subject filter is first defined in the frequency domain as a band‐limited version of the exact integration response. Subsequent inverse Fourier transformation produces an analytical expression in the time domain. Although the resulting digitized impulse response cannot be expressed in closed form, it can be divided into a two‐point recursive filter component and a longer, nonrecursive component. The length of the nonrecursive component must be user‐selected according to the accuracy desired in the frequency domain; that is, a shorter filter acts as an integrator over a narrower frequency band. The included graph of frequency range as a function of nonrecursive component length allows the user to specify the compromise he desires between accuracy and computational effort.
Magnetometric resistivity (MMR) anomalies of two‐dimensional structures
Authors: Trevino E. Gomez , Edwards R. N.
Published: October 2002

An inexpensive, rapid method has been developed for computing all three components of the magnetic field due to galvanic current flow from a point electrode in the vicinity of a conductive subsurface structure of infinite strike‐length and arbitrary cross‐section. For any three‐dimensional (3-D) structure, the magnetic field may be written as a sum of surface integrals over boundaries defining changes in conductivity by a direct modification of the Biot‐Savart law. The integrand of each surface integral includes the components of the electric field tangential to the boundary, which may be evaluated on the boundary using a standard integral equation technique. In the case of a two‐dimensional (2-D) structure, a reformulation of the theory by taking a one‐dimensional Fourier transform along the strike results in the reduction of both the surface integrals necessary to solve the integral equation for the electric field, and the integrals used in computing the magnetic field, to line integrals in wavenumber domain. We evaluate the integrals numerically and solve the integral equation for each of about ten wavenumbers; finally, we obtain the magnetic field in space domain through a concluding one‐dimensional inverse Fourier transform. Type curves and characteristic curves for the simple model of a buried horizontal cylinder beneath a thin layer of conductive overburden are constructed. In the absence of overburden, the half‐width of the anomaly is linearly related to the depth of the cylinder. In the presence of overburden, the form of the anomaly may be predicted in a simple manner from the corresponding anomaly in the absence of overburden, provided the distance from the current source is sufficiently large for most of the available current to have penetrated the overburden.
Error analysis for remote reference magnetotellurics
Authors: Gamble T. D. , Goubau W. M. , Clarke J.
Published: October 2002

Remote reference magnetotellurics (MT) yields estimates [Formula: see text] of the true impedance tensor Z in terms of average crosspowers between a reference field measured at a distant site and the electric and magnetic fields at the sounding location. In contrast to conventional estimates of Z, [Formula: see text] is unbiased by the noise power in any field, provided the reference is uncorrelated with the noise in the electric and magnetic channels. When bias errors are eliminated, the accuracy of an estimate of Z is determined by random errors. The variance in each element of [Formula: see text] can be expressed in terms of known average powers, if it is assumed the noises are independent of the signals, and that the noises are stationary. The variances decrease as the number of measurements N contained in the average powers increases. Hence, they can be made arbitrarily small. For small errors, the variance in any function of Z (for example, apparent resistivity, the phase of Z and the skewness), can be obtained from a Taylor expansion of the function in terms of the errors in [Formula: see text]. By the central limit theorem, the distribution of errors will be Gaussian, independent of the statistics of the signals and noises, provided N is sufficiently large. Thus, confidence limits may be predicted readily. None of the assumptions in the error analysis is particularly restrictive, and the analysis should be applicable to most remote reference MT data. When the analysis is applied to real data, the predicted variances in [Formula: see text] agree well with those determined from the scatter between values of [Formula: see text] calculated from disjoint subsets of the original data.
One‐dimensional inversion of natural source magnetotelluric observations
Authors: Oldenburg Douglas W.
Published: October 2002

Natural source magnetotelluric (MT) data are inverted to find a subsurface resistivity which is isotropic and a continuous function of depth. The inversion consists of two parts. In the first, our concern is to construct a resistivity structure whose responses are acceptably close to the observations, that is, the measured amplitudes and/or phases. This construction proceeds iteratively and is realized through an automated algorithm which is characterized by numerical stability and rapid convergence. In the second part, we attempt to investigate the degree of nonuniqueness inherent in the problem. This is accomplished by considering only spatial averages of the resistivity because all models linearly close to the constructed model will have the same average resistivity. The predictions of this statement are examined quantitatively in an example where the averages from five different models which reproduce the observations are compared. In an example using data from a moderately complex layered model, we have inverted jointly and separately the amplitudes and phases and compared their resolution. Finally, data from the Snake River Plain‐Yellowstone area are inverted and the results compared with those from a parametric inversion.
A model for bottom‐hole temperature stabilization
Authors: Middleton M. F.
Published: October 2002

Bottom‐hole temperature (BHT) stabilization is modeled assuming that the mud temperature in a deep well is uniform for several meters above its base and that the basal portion of the well was formed rapidly so that it may be considered to have formed “instantaneously”. Therefore, knowledge of circulation time of the drilling fluids, which is required in many alternative methods of correction for BHT disturbance, is not necessary. Rectangular coordinates are used to describe the geometrical configuration of the well, which often departs from an ideal cylindrical shape due to caving of poorly consolidated formations. True formation temperature can be found by a simple curve‐matching technique if several time‐sequential BHT measurements are available. This technique is successfully applied to BHT data from a number of wells in the Moomba and Big Lake gas fields of the Cooper Basin, South Australia.
Geophysical Activity in 1979
Authors: Whitmire M. G.
Published: October 2002

This report presents statistical data on geophysical activity in 1979. The data were compiled from survey questionnaires that were completed by more than 500 companies, contractors, government agencies, and institutions throughout the free world. We have gathered 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 surveys, oceanography, and research. Data were gathered on airborne, drill hole, land, and marine geophysical surveys. Statistics were compiled on miles of coverage, drill hole footage, time spent in crew months or man months, and acquisition cost.
Remote sensing method
Authors: Regan R. D.
Published: October 2002

While remote sensing had its simple beginning in the form of conventional aerial photography many years ago, it has advanced over the past few decades to sophisticated digital multispectral data gathering and analysis and is being accepted as a standard exploration technique. In recent years, improvements in instrumentation have permitted the recording of a wide spectrum of electromagnetic radiation, including the following segments: gamma ray, x‐ray, ultraviolet, visible, infrared, microwave, and radio, which have ample transmittance in the atmosphere. The growth of professional interest in remote sensing was greatly advanced by the success of the Landsat satellite system. This has led to magnetic satellite surveys and, more recently, to gravity satellite surveys. Thus, tracking of artificial satellites, combined with various new satellite observational systems and analytical processes, is providing detailed information on global as well as local geophysical anomalies.
Prestack partial migration
Authors: Yilmaz Özdoan , Claerbout Jon F.
Published: October 2002

Conventional seismic data processing can be improved by modifying wide‐offset data so that dipping events stack coherently. A procedure to achieve this improvement is proposed here, which is basically a “partial” migration of common offset sections prior to stack. It has an advantage over full migration before stack in that, in the case of the latter, the final product is a migrated section. However, the prestack partial migration provides the interpreter with a high‐quality common midpoint (CMP) stacked section which can be subsequently migrated. The theory of prestack partial migration is based on the double square‐root equation, which describes seismic imaging with many shots and receivers. The double square‐root operator in midpoint‐offset space can be separated approximately into two terms, one involving only migration effects and the other involving only moveout correction. This separation provides an analysis of conventional processing. Estimation of errors in the separation yields the equation for prestack partial migration. Extension of the theory for separable approximation to incorporate lateral velocity variation yields a significant term proportional to the product of the first powers of offset, dip, and lateral velocity gradient. This term was used to obtain a rough estimate of lateral velocity variation from a field data set.
Attenuation by squirt flow in undersaturated gas sands
Authors: Palmer I. D. , Traviolia M. L.
Published: October 2002

In unconsolidated sand at low saturations, water exists in pendular rings at grain contact points. As the grains are pressed together by a passing wave, the squirt flow will contribute to wave attenuation. In a model which consists of a sphere‐pack framework, fluid flow equations are solved under certain approximations to calculate the viscous losses and, hence, attenuation. Attenuation is negligible for a sphere‐pack composed of equal‐sized spherical grains. The model is extended to include a spectrum of grain contact geometries, in particular a log‐normal spectrum of aspect ratios. The attenuation is substantial and depends markedly upon a lower limit imposed on the aspect ratios. For contacts of Type B (water separating two grains) only, results at 1 Hz are consistent with those of Mavko and Nur (1979) based on two‐dimensional (2-D) cracks. If contacts of Type A (two grains in contact) and Type B are present, the attenuation is greater by 2–3 orders of magnitude. The model with Type B contacts displays a dependence of attenuation on water saturation in the range 1–10 percent; the model with Type A and B contacts does not. When the results were fitted to the measurements of absolute attenuation and saturation dependence at kHz frequencies by Gardner et al. (1964), only the model with Type B contacts with aspect ratios [Formula: see text] could fit the data.
Magnetotelluric response on inhomogeneous layered earth
Authors: Kao Dominic , Rankin David
Published: October 2002

Magnetotelluric response is studied for a three‐layer earth, formed by constant conductivity in the first and the third layer and conductivity varying linearly with depth in the second layer (i.e., the inhomogeneous transition layer). The problem is accounted for by solving an Airy differential equation for the inhomogeneous transition layer; surface impedance and apparent resistivity are then obtained in terms of Airy functions. Apparent resistivities and phases are computed for (1) resistive substratum, (2) conductive substratum, and (3) varying thickness of the transition layer. The results are compared with Cagniard master curves and multi‐layer approximations.
The effect of a finite distance between potential electrodes on Schlumberger resistivity measurements—A simple correction graph
Authors: Mundry E.
Published: October 2002

Resistivity soundings are often carried out using a Schlumberger array. With this configuration, a current I is fed into the ground using two current electrodes A and B placed a distance L apart. At the center of these two electrodes, the voltage U is measured between two potential electrodes M and N placed a distance a apart. The condition a ≪ L is necessary to satisfy the assumption of a Schlumberger sounding curve, which is a log‐log plot of apparent resistivities [Formula: see text]k = geometric factor, Eq. (1) versus half the distance between the current electrodes r = L/2.
Calculation of complete seismograms for an explosive source in a layered medium
Authors: Bouchon Michel
Published: October 2002

We apply the method of discrete wavenumber representation of elastic wave fields of Bouchon and Aki (1977) to the computation of synthetic seismograms for an explosive source in a layered medium. The method is based on the representation of the source radiation by a superposition of plane waves propagating in discrete directions. This discretization is exact and results from a periodic arrangement of sources. The two‐dimensional (2-D) and three‐dimensional (3-D) problems are described, and some examples of calculation are presented. They show that very complex seismograms can be obtained for rather simple geologic structures.
Reconnaissance geophysics of a known geothermal resource area, Weiser, Idaho and Vale, Oregon
Authors: Long C. L. , Kaufmann H. E.
Published: October 2002

Audio‐magnetotelluric (AMT) and telluric current soundings were made in a study of the geothermal potential of the area between Weiser, Idaho and Vale, Oregon, during the spring and fall of 1974. The electrical surveys covered an area on the western edge of the Snake River plain of approximately [Formula: see text] with 89 AMT and 31 telluric current stations at approximately 6-km spacings. The AMT method used the natural electromagnetic (EM) field from 7.5 Hz to 6.7 kHz (10 frequencies) with two VLF radio sources at 10.2 and 18.6 kHz, while the telluric method utilized geomagnetic micropulsations, band‐limited from 0.02 to 0.1 Hz. Maps were compiled using both methods to outline major high‐ and low‐resistivity features. Major high‐resistivity zones appear to trend northwest on the AMT apparent resistivity maps. These zones parallel structural trends between Vale and Weiser. The lowest apparent resistivities are associated with the known geothermal hot springs in the Vale and Weiser areas. The telluric ratio map shows lowest values at the eastern side of the area, and a low trend extending through Vale and to the northeast.
Dielectric relaxation spectrum of ice measured with linear blocking layers
Authors: Gross Gerardo Wolfgang , Hayslip Iris Cox , Hoy Roberta N.
Published: October 2002

A systematic study of the dielectric relaxation spectrum of ionic impurities in ice over a wide range of concentrations and temperatures required the development of methods to compare the spectra. The studied impurities fall roughly into two categories, those that increase the dc conductivity of the ice and attendant space‐charge effects, and those that suppress these effects. The former were measured with blocking layers inserted between the sample and electrodes, the latter with stainless steel guard electrodes. Linearized plots of the Debye expressions were used for separating up to 4 spectral components by an iterative fitting and correction technique. With blocking layers, the Maxwell‐Wagner model of a layered dielectric yields the ice bulk parameters. Analysis suggests that, in general, ice is best described by a small number of discrete components, each characterized by a single relaxation time. Advantages of the blocking‐layer technique are experimental simplicity, reproducible values of electrical parameters obtained exclusively from ac measurements, and the systematic coverage possible. The results are useful for evaluating electrical properties of ice in the environment. The model should be applicable to the investigation of rocks other than ice.
Straightforward derivation of Hubral’s wavefront curvature differential equation in inhomogeneous isotropic media
Authors: Krey Theodor
Published: October 2002

“Wavefront curvatures in three‐dimensional laterally inhomogeneous media with curved interfaces” (Hubral, 1980, this issue) shows a differential equation [formula (4.1)] which describes the alteration of the wavefront curvature matrix along a raypath in the case of an isotropic velocity v which is an arbitrary function of the locus in the three‐dimensional space. Hubral derives his equation by referring to papers of Popov and Pšenčik (1976, 1978) and Hubral (1979).
The calculation of Ricker seismic wavelet functions
Authors: Longman I. M.
Published: October 2002

In a recent publication, Ricker (1977) gave integral representations for his generating, displacement, velocity, and acceleration functions pertaining to the propagation of a seismic pulse in a visco‐elastic medium. These functions are solutions of the Stokes wave equation in one dimension, and were given by Ricker as infinite integrals of oscillatory functions. Here these integrals are transformed into finite integrals of an integrand which is usually one‐signed, and these new integrals are readily evaluated by simple numerical quadrature (e.g., Simpson's rule), and are easily evaluated to high precision. Some numerical results are given and compared with Ricker’s tables.
Variation of density with rock type, depth, and formation in the Western Canada basin from density logs
Authors: Maxant J.
Published: October 2002

Gamma‐gamma logs were used to study the density of sedimentary rocks and formations in the Western Canada basin. The frequency distributions of density of all the main rock types have negative skewness, a characteristic that seems to be a general feature of density distributions of rocks based on well logging. The frequency distributions of density for sandstone and limestone were not found to be different from those of shale and dolomite, respectively, at the 0.01 level of significance. Several density‐depth functions were applied to the shale samples. Athy's (1930) exponential function provided the best fit to observed values. Using this function, the average thickness of sediments removed by denudation in south and central Alberta was calculated to be 4400 or 3600 ft on the assumption that the density of the original surface clay was 1.4 or [Formula: see text], respectively. Maps of average density for six formations in Alberta and Saskatchewan indicate only a partial correlation with lithology and depth. A large scattering of density with depth was found in the case of four Cretaceous formations in which density was observed to be independent of the sandstone/shale ratio. In the case of one formation containing predominantly shale, no obvious dependence on depth was found. Also, no relationship between density and the occurrence of oil and gas fields was apparent.
Borehole logging for uranium by gamma‐ray spectrometry
Authors: Løvborg L. , Nyegaard P. , Christiansen E. M. , Nielsen B. L.
Published: October 2002

The resources in a large syngenetic deposit of low‐grade uranium (U) ore with thorium at Kvanefjeld, South Greenland, were evaluated by spectrometric gamma‐ray logging of 23 boreholes, 46 mm in diameter and 200 m deep. The borehole probe's detector contained [Formula: see text] of sodium‐iodide, and the photomultiplier gain was stabilized electronically using barium‐133 ([Formula: see text]) reference gamma rays. The downhole measurements were carried out at depth intervals of 25 cm, and for each stationary position of the borehole probe, counts were accumulated for 100 sec in four energy windows. The calibration constants of the spectrometer system were determined by calculating the average number of U and thorium (Th) counts per meter of borehole and comparing these with the U‐Th concentrations in 1‐m sections of analyzed drill core. The sensitivity and the background count rate in the uranium window varied appreciably from one hole to another; this variation is believed to be caused by emanation of radon (Rn) from the borehole walls. Block calculations based on individual calibration constants for the boreholes logged made it possible to obtain a reliable estimate of the tonnage of U. This estimate was only slightly different from the tonnage evaluated from a large number of core assays.
Analysis of errors in barometric elevations in northern Nigeria
Authors: Verheijen P. J. T. , Ajakaiye D. E.
Published: October 2002

We present an analysis of errors contributing to the inaccuracy of aneroid altimeter observations under tropical conditions. Instrumental errors were investigated experimentally, while errors due to meterological factors were analyzed using an autocorrelation analysis of actual pressure observations. For an estimated accuracy of ±1.2 to ±2.5 m, it is recommended that a diurnal pressure variation be eliminated from the altimeter field readings and that measurements be restricted to 0500 to 1400 hours LST in the wet season and from 0500 to 1100 hours LST or 1500 to 1830 hours LST in the dry season.
Induction logging for vertical structures in the presence of a borehole fluid
Authors: Howard Allen Q.
Published: October 2002

Mapping hydraulic fractures with borehole instrumentation presents many problems. The resulting fracture eddy currents can be expected to be smaller than the corresponding borehole fluid currents. The transmitting and receiver coils are vertical and are in close proximity to each other. This orientation is necessary to couple strongly to vertical structures and is much more difficult to analyze than the case of horizontal coils. The coils must be physically but not electrically isolated from probable hostile temperature and pressure environment. Analytic expressions are presented which predict the effect of the cavity, misalignment of the coils in the cavity, and an upper bound on the effect of the borehole fluid.
Seismic amplitude anomalies associated with thick First Leo sandstone lenses, eastern Powder River basin, Wyoming
Authors: Balch A. H. , Lee M. W. , Miller John J. , Ryder Robert T.
Published: October 2002

Several new discoveries of oil production in the Leo sandstone, an economic unit in the Pennsylvanian middle member of the Minnelusa formation, eastern Powder River basin, Wyoming‐Nebraska‐South Dakota, have renewed exploration interest in this area. Vertical seismic profiles (VSP) and model studies suggested that a measurable seismic amplitude anomaly is frequently associated with the thick First Leo sandstone lenses. To test this concept, a surface reflection seismic profile was run between two wells about 12 miles apart. The First Leo was present and productive in one well and thin and barren in the other. The surface profile shows the predicted amplitude anomaly at the well where a thick lens is known to exist. Two other First Leo amplitude anomalies also appear on the surface seismic profile between the two wells, which may indicate the presence of additional lenses.
Very low‐frequency electromagnetic interpretation using tilt angle and ellipticity measurements
Authors: Saydam A. Sacit
Published: October 2002

During this study, a plane‐wave computer modeling approach was used to gain more insight into the response of vertical sheet conductors at very low frequency (VLF). The medium which surrounds the conductors is assumed to be a dissipative one. Magnitude and shape of the anomalous field caused by a vertical “thin” conductor at VLF is found to be dependent mainly on (1) conductivity‐thickness product of conductor, (2) resistivity of host medium, (3) depth to the top of conductor, and (4) overlying conductive overburden. Other variables being the same, a shift in frequency in the 17 to 25 kHz range does not produce an appreciable change in the overall response of modeled conductors. The lateral distance between the maximum and minimum on a tilt angle profile is related to conductivity and thickness of conductor, resistivity of the host rock, and depth to the top of the conductor in a complex manner. An interpretation scheme has been proposed to determine the conductivity‐thickness product and depth of vertical sheet conductors from tilt angle and ellipticity measurements at VLF. Resistivity of the host medium had to be incorporated in the scheme due to strong dependence of response on this parameter. Five characteristic diagrams have been constructed corresponding to host rock resistivities of 50, 250, 800, 2500, and 8000 Ω-m, which cover the most commonly encountered resistivities in the field. Conductivity‐thickness product [Formula: see text] and depth (D) of conductor can be read directly from the appropriate diagram when peak‐to‐peak tilt angle and ellipticity values are entered.
Relation of certain geometrical features to the dielectric anomaly of rocks
Authors: Sen P. N.
Published: October 2002

We show that the real part of the dielectric constant ε′ of rocks at low frequencies can be anomalously high due to the presence of a small concentration η of high aspect ratio particles. For oblate spheroidal grains [Formula: see text] with depolarization factor along symmetry [Formula: see text] axis, [Formula: see text] [Formula: see text] the static value of the dielectric constant of rock [Formula: see text] and dc conductivity [Formula: see text] are given for [Formula: see text] by [Formula: see text] [Formula: see text] and for (2) [Formula: see text] [Formula: see text] Here [Formula: see text] is the dielectric constant of the grain; [Formula: see text] is the dc conductivity of the host rock. Case (1) corresponds to the well known Maxwell‐Wagner effect with [Formula: see text] diverging as η → 0, and [Formula: see text] Case (2) gives a novel result that [Formula: see text] may diverge for [Formula: see text], with a nonvanishing σ(0). Case (2) is applied to explain frequency and salinity dependences and the giant values [Formula: see text] of the dielectric constant of conducting sedimentary rocks. For [Formula: see text], [Formula: see text], [Formula: see text], we find [Formula: see text], which is large compared to [Formula: see text] or the dielectric constant of water [Formula: see text].
Interpretation of three‐component borehole magnetometer data
Authors: Silva João B. C. , Hohmann Gerald W.
Published: October 2002

We have made a theoretical study of the problem of locating and defining a magnetized prism in the vicinity of a drillhole. The study shows that a three‐component borehole magnetometer can be very useful in exploration for deep magnetic targets, which are difficult to intersect by drilling. There are nine parameters to be estimated from the data: the x, y, and z positions of the center of the prism; length, width and depth extent; and the three parameters defining the magnetization vector. The parameters are estimated by minimizing the sum of squared differences between the predicted and observed data. Since the problem is nonlinear, it must be solved iteratively. At each iteration a singular value analysis of the sensitivity matrix is performed, and a particular solution is chosen from a set of minimal length solutions; each solution corresponds to a different rank of the sensitivity matrix. The main difficulty is an ambiguity involving the position parameters and the parameters defining the magnetization vector. In order to minimize this obstacle, a parameter scaling matrix is introduced. The effects of noise, wrong model, probe orientation errors, borehole length, and data density are analyzed. We have found that the position and depth extent of the body can be estimated with reasonable accuracy.
Frequency domain least‐squares inversion of thick dike magnetic anomalies using Marquardt algorithm
Authors: Khurana K. K. , Rao S. V. Seshagiri , Pal P. C.
Published: October 2002

The elegance and ease with which the Fourier transformation method can be employed in the interpretation of potential field geophysical data (Dean, 1958; Gudmundsson, 1966) has resulted in the formulation of a number of frequency‐domain approaches for interpreting the magnetic effects of specific source geometries, e.g., Bhattacharyya (1966), Sengupta (1974), Sengupta and Das (1975), Bhattacharyya and Leu (1975, 1977), etc. In these techniques, the parameters of interest are estimated from the characteristics of the amplitude and phase spectra, a process that is often beset with several problems which adversely affect the estimation of parameters. These problems arise partly from the ill‐defined nature of characteristic points in the spectra, either due to spectral noise or due to inadequate digitization spacing, and partly from the fact that the errors in the estimation of certain parameters which are used for subsequent estimation of the other parameters usually cause added spectral distortions. Rao et al. (1978) advocated the use of end corrections to overcome these problems. An effective alternative would be to incorporate the least‐squares method in the interpretational process. This necessitates iteration owing to the nonlinear nature of equations relating the source parameters and the Fourier spectra of corresponding anomalies. While it is tempting to employ the iteration scheme formulated by Gauss in 1821 for this purpose, as has indeed been done in a number of space‐domain approaches (e.g., Hall, 1958; Bosum, 1968; Al‐Chalabi, 1970; Rao et al., 1973; Won, 1981), the convergence in this approach requires the initially assumed values of the parameters to be very close to the final solution. The steepest descent method, an alternative to this, has an extremely slow convergence. It is shown here, with the example of a two‐dimensional (2-D) structure, that Marquardt’s algorithm (Marquardt, 1963), which offers a compromise between these two iteration schemes and has been effectively employed by Johnson (1969) and Pedersen (1977) in their space‐domain interpretation schemes, provides an efficient approach to accomplish the iteration in the case of frequency‐domain magnetic interpretation.