bluuebunny/crossref_metadata_2025 huggingface dataset:
MOVING‐BASE GRAVITY GRADIOMETER SURVEYS AND INTERPRETATION
Authors: Jordan Stanley K.
Published: October 2002

Moving‐base gravity gradiometers are currently under development at Bell Aerospace Co., Charles Stark Draper Laboratory, and Hughes Aircraft Co. In principle, these instruments can be mounted on stable platforms in aircraft or marine survey ships. Unlike a conventional gravimeter, the gradiometer is insensitive to vertical (heave) accelerations of the vehicle and does not require an Eötvös correction. Gradiometers thereby offer an overwhelming improvement in the speed and accuracy of gravity surveys. In addition, a gradiometer provides more information than a gravimeter because a tensor quantity is measured, rather than a scalar. How should the gradient data be used? The simplest approach is to convert the gradients into gravity anomalies by integrating along the vehicle path, and then apply conventional interpretation models. This approach is sound if the survey tracks are closely spaced because continuous two‐dimensional gravity anomalies completely define the gravity field outside the earth, according to potential theory. On the other hand, one can widen the survey tracks considerably if a more sophisticated approach is adopted that exploits the extra information in the gradients. In this paper, we compare the “simple” approach and a more elaborate (“optimal”) one that uses the entire gradient tensor. The comparison is based on information theory concepts: How much information is in the gradient data? How much is lost if the gradients are converted to gravity anomalies? Statistical models are fitted to Bouguer residual gravity anomalies from a salt dome field in the Louisiana Gulf Coast, and the interpretation rms errors are evaluated for each approach based on optimal estimation theory. The results show that a gradiometric survey with 18-km spacing contains the same information as a gravimetric survey with 8-km spacing. The sensitivities of the results to the following survey parameters are determined: track spacing, gradiometer noise, vehicle speed, and aircraft altitude.
THEORETICAL TRANSFORMS OF THE GRAVITY ANOMALIES OF TWO IDEALIZED BODIES
Authors: Regan Robert D. , Hinze William J.
Published: October 2002

Odegard and Berg (1965) have shown that the interpretational process can be simplified for several idealized bodies by utilizing the Fourier transform of the resultant theoretical gravity anomalies. Additional studies relating similar conclusions for other idealized bodies have been reported by Gladkii (1963), Roy (1967), Sharma et al (1970), Davis (1971), Eby (1972), and Saha (1975), and a summary of the spatial and frequency domain equations is given in Regan and Hinze (1976, Table 1); however, the transforms of the three‐dimensional prism and vertical line elements, often utilized in interpretation, have not been previously examined in this manner. Although Bhattacharyya and Chen (1977) have developed and utilized the transform of the 3-D prism in their method for determining the distribution of magnetization in a localized region, it is still of value to present the interpretive advantages of the transform equation itself.
TOTAL FIELD RESISTIVITY MAPPING AND SOUNDING OVER HORIZONTALLY LAYERED MEDIA
Authors: Zohdy Adel A. R.
Published: October 2002

The bipole‐dipole resistivity mapping method has been used extensively in reconnaissance surveys of geothermal areas. Although the method has been used primarily for delineating lateral changes in resistivity, it is shown that significant anomalies can be produced by horizontal layers. Formulas are derived for the calculation of five types of bipole‐dipole apparent resistivities, and a distinction is made between what is defined as: primary field, simple total field, and total field apparent resistivities. It is shown that if the Schlumberger sounding curve for a given horizontally layered medium is known, then the magnitude and direction of the bipole electric field, as well as all types of bipole‐dipole apparent resistivity maps, can be calculated using simple algebraic equations (without having to solve the boundary value problem). Conversely, it is also shown that for each total electric field measurement, two Schlumberger apparent resistivities can be calculated easily, except along the polar axis of the current bipole. In addition, a useful formula is given for extending a symmetric Schlumberger sounding curve using measurements that are made with an asymmetric array. This is illustrated by field examples. Theoretical examples of perpendicular, parallel, primary field, total field, and simple total field apparent resistivity maps (as well as Schlumberger AB profiles and rectangle of resistivity maps) are calculated for a four‐layer earth model to illustrate the type of anomalies that can be obtained over layered media. The characteristic peanut‐shape of simple total field apparent resistivity maps, over horizontally layered media, is illustrated by four field examples from three different geothermal areas. The calculation of residual or “percent‐lateral‐effect” maps or profiles is recommended for the interpretation of all types of bipole-dipole apparent resistivity data, and is also illustrated by a field example.
INVERSION OF TWO‐DIMENSIONAL RESISTIVITY AND INDUCED‐POLARIZATION DATA
Authors: Pelton W. H. , Rijo L. , Swift, C. M.
Published: October 2002

A fast ridge regression inversion technique has been devised for the interpretation of simple two‐dimensional resistivity and induced‐polarization data. The program will determine the rectangular source under a single layer of overburden which best fits the observed data. Several advantages are derived from using the ridge regression method; they include convergence from very poor initial guesses, stability in the presence of high‐frequency geologic noise, readily obtained estimates of parameter statistics, and the ability for simultaneous inversion of multiple data sets. Unfortunately, each ridge regression inversion requires a great many forward problem evaluations; thus in order to achieve speed and reasonable cost, it is essential to reduce the calculation time for the forward problem to an absolute minimum. One method of achieving this is to store in the computer a data bank containing solutions for the entire range of expected parameter combinations. The forward problem then reduces to numerical interpolation between these precalculated data sets. For compilation of the data bank of forward solutions, two main numerical methods were investigated: the finite element and transmission‐surface algorithms. Although these algorithms are conceptually quite different, the resulting matrix equations are very similar. The efficiency of either method depends mainly on the scheme chosen for solving the resultant large system of linear equations. Once the data bank has been created, it is possible to obtain inverse solutions for less cost than the computation of one finite element or transmission‐surface forward problem. Tests on theoretical data and field data show the inversion technique to be reasonably accurate, stable, and fast. The statistics estimated by the inversion program provide additional useful information on the uncertainty in the parameters of the derived model and on high correlations between parameters. The most highly correlated parameters are, as might be anticipated, the resistivity and the width of thin conductive bodies. Two practical methods for carrying out inversion in spite of highly correlated parameters are, preferably, to add extra data sets which provide more information on some of the parameters or, alternatively, to fix some of the parameters at geologically reasonable values and invert to a more restricted model.
A STATISTICAL ANALYSIS OF POTENTIAL FIELDS USING A VERTICAL CIRCULAR CYLINDER AND A DIKE
Authors: Pedersen L. Børsting
Published: October 2002

Analytical expressions for the Fourier transforms of potential fields caused by inclined dikes and vertical circular cylinders are evaluated. Their spectra are used for a statistical analysis along the lines proposed by Spector and Grant (1970). Special attention is paid to the term which is related to the effect of the lateral extent of the sources. This term is found to decay via an inverse power relationship. Thus it decays fairly slowly, and it may become quite important to correct the spectrum prior to interpretation. Numerical calculations of the term for the cylinder and the prism indicate that an inverse third power relationship is appropriate for correction of practical power spectra.
Reconnaissance geophysical studies of the geothermal system in southern Raft River Valley, Idaho
Authors: Mabey Don R. , Hoover Donald B. , O’Donnell James E. , Wilson Carol W.
Published: October 2002

Gravity, aeromagnetic, and telluric current surveys in the southern Raft River have been used to infer the structure and the general lithology underlying the valley. The gravity data indicate the approximate thickness of the Cenozoic rocks and location of the larger normal faults, and the aeromagnetic data indicate the extent of the major Cenozoic volcanic units. The relative ellipse area contour map compiled from the telluric current survey generally conforms to the gravity map except for lower values in the area of the geothermal system. An area of low apparent resistivity values defined by the audiomagnetotelluric (AMT) survey appears to outline the extent of the geothermal reservoir even though the reservoir is deeper than the penetration of the survey. Self‐potential anomalies relate to near surface hydrology. The Raft River Valley is underlain by about 2 km of Cenozoic rock, most of which is Tertiary sediments of the Salt Lake formation. On the west side of the valley the Tertiary rocks appear to be separated from the underlying Precambrian basement by a low angle fault along which the Tertiary rocks have slid off a buried basement dome. Subsequent normal faults displace both the basement surface and the Tertiary beds. The geothermal system occurs where these north‐trending faults intersect a poorly understood northeast‐trending zone that may be a basement shear zone. Apparently deep circulating water at a temperature of about 150°C rises in the area of this intersection and then spreads laterally in porous zones near the base of the Tertiary rocks to form the geothermal reservoir. Upward leakage from the reservoir produces shallower effects that were measured by the AMT survey.
Relative precision of vertical and horizontal gravity gradients measured by gravimeter
Authors: Hammer Sigmund
Published: October 2002

Several recent publications advocate the use of the vertical gradient of gravity from gravimeter measurements at two elevations in a portable tower (Thyssen‐Bornemisza, 1976; Fajklewicz, 1976; Mortimer, 1977). Contrary opinions have also been expressed (Hammer and Anzoleaga, 1975; Stanley and Green, 1976; Thysen‐Bornemisza, 1977; Arzi, 1977). The disagreement revolves around the question of practically attainable precision of the vertical gradient tower method. Although it is possible to calculate both horizontal and vertical gradients from conventional gravity survey data by use of the Hilbert transform (Stanley and Green, 1976), it should be noted that highly precise gravity data are required. Also the need for connected elevation and location surveys, the major cost in gravity surveying, is not avoided. This is a significant advantage of the gradient methods. The purpose here is to present a brief consideration of the relative precision of the horizontal and vertical gradients, as measured in the field by special gravimeter observations.
Hybrid‐ray approximation in electromagnetic sounding
Authors: Kan Tze‐Kong , Clay C. S.
Published: October 2002

Our purpose is to develop electromagnetic (EM) transient sounding methods that can handle resistivity structures having dipping interfaces. The method is an approximation that uses wave theory for part of the calculation and ray theory for the rest. The approximation is a refinement of Yost’s (1952) image method in which the source on a half‐space becomes the image beneath the reflecting interface. This approach shows excellent agreement with experiment and promises simple applications. To test the approximation, we compare numerical integration of the fields over a two‐layer half‐space with approximate values. We separate the EM integrals into integrals for the surface wave, primary, and multiple reflections. For a current dipole along the x‐axis, the electric field [Formula: see text] is the sum of the transverse electric and transverse magnetic components. For skin depths >1, the main contributions to the integrals come from directions near the specular direction, and by moving the reflection coefficients at the lower interface outside of the integral, we obtain the hybrid‐ray approximation. The transmission coefficients from the source into the earth remain inside because the source is near the interface. Computations for a two‐layer model that includes the geometry, and system functions for deep dipole‐dipole soundings in the Precambrian shield of northern Wisconsin, give time‐domain signals that closely approximate the measurements. The theoretical model consists of a layer 18 to 23 km thick with a conductivity of [Formula: see text] over a half‐space with a conductivity of [Formula: see text].
A new high‐resolution or deep penetration airgun array
Authors: Brandsaæter H. , Farestveit A. , Ursin B.
Published: October 2002

A new airgun array has been designed to provide a source for deep penetration or high‐resolution seismic surveys. The airgun array consists of 21 airguns with a total volume of [Formula: see text] operated at a pressure of 2000 psi. The far‐field signatures with the source array at depths of 5 and 10 m were recorded at a depth of 100 m, with a calibrated hydrophone located directly beneath the energy source. The signature was recorded digitally with a 1-msec sample interval, using a presampling filter with a high‐frequency cutoff at 124 Hz and an attenuation rate of 72 dB/octave. The peak‐to‐peak amplitude of the recorded signature is 49.5 bar‐m; the primary‐to‐bubble peak‐to‐peak amplitude ratio is 16.7. The signal frequency band (measured between the 6 dB points) ranges from 9.5 to 90 Hz. For several streamer depths, the effect of the surface reflection at the hydrophone streamer is derived by subtracting the recorded far‐field signature, delayed by the two‐way traveltime from the streamer to the surface, from itself. By studying the derived far‐field signatures and their amplitude spectra, it is seen that for a high‐resolution seismic survey, the airgun array and the hydrophone streamer should both be placed at shallow depths (5–8 m). For a deep penetration seismic survey, where the signal energy in the low‐frequency band is most useful, the airgun array and hydrophone streamer should be placed at medium depths (8–12 m). Far‐field signature tests show that adding a large airgun [Formula: see text] to the airgun array increases the signal energy in the frequency band 0–90 Hz and improves the primary‐to‐bubble peak amplitude ratio from 11.8 to 16.7. Two comparison seismic sections demonstrate that the large airgun improves both resolution and penetration.
The prediction of porosity in the Permian (Zechstein 2) carbonate of eastern Netherlands using seismic data
Authors: Maureau G. T. F. R. , van Wijhe D. H.
Published: October 2002

The depositional model of the Zechstein 2 carbonate section is of such a complex nature that it is not possible to predict high‐porosity zones solely from geologic data. We describe a two‐phase study into determination and prediction of porosities in the Zechstein 2 carbonate in the eastern Netherlands. Phase 1 was the basic feasibility study and consisted of two parts, namely, a model study followed by a well‐log study. In both parts, acoustic impedance logs were filtered to test the feasibility of resolving porosity changes within the frequency range of seismic data. Based on our comprehensive model of the environment of deposition of the Zechstein evaporites, a number of acoustic impedance logs were constructed, each log representing a particular environment. These logs then were filtered with a suite of band‐pass filters. It was our observation that the gross changes in porosity were easily recognized. The model study was followed by a study of acoustic impedance logs from actual downhole measurements taken over intervals with depositional environments similar to those represented in the model study. In spite of geologic variations, the changes in porosity were easily observable. Phase 2 involved the prediction and verification of porosity. An exploration well, planned to penetrate the Zechstein 2 carbonate, was connected to a previously drilled carbonate test by a good quality seismic section. In the previous well, porous limestone approximately 60 m in thickness was penetrated. From the combined well and seismic data, we predicted that a similar development of porosity could be expected in the new location. Subsequent drilling proved the presence of porous carbonate approximately 50 m in thickness developed in an environment of deposition similar to that encountered in the other well. This technique has allowed us to predict reservoir porosity and will be used to develop new structural/stratigraphic prospects.
Theoretical studies of in‐situ rock density determinations using underground cosmic‐ray muon intensity measurements with application in mining geophysics
Authors: Malmqvist L. , Jönsson G. , Kristiansson K. , Jacobsson L.
Published: October 2002

The feasibility of in‐situ rock density determinations by means of subsurface cosmic‐ray muon intensity measurements is based on theoretical calculations for two hypothetical scintillation counter telescopes: one is intended for registration in a gallery and the other is intended for use in narrow boreholes. It is shown that it is possible to measure the mean density of the rock traversed by the muons by measuring the muon intensity. The sensitivity of the method is favorable—a 1 percent change in mean rock density corresponds to a change of about 3 percent in the counting rate. A possible use of cosmic‐ray muon technique is the localization of an anomalous density distribution in overlying rock. A characteristic minimum registration time to detect a certain density anomaly varies from a few hours to about 10 days, depending on the geologic situation and the depth and design of the detector. The device is found to be most applicable for massive sulfide and iron exploration. This tecnique provides some new possibilities. A certain spatial resolution can be achieved at the expense of the registration time, and the overlying rock can, to some extent, be investigated in different directions from one point of observation. The method seems to be useful down to depths of approximately 600 m for the gallery application and 400 m for the borehole application. However, these limits are a consequence of the size of the detector, the size and density contrast of the target, and the maximum registration time accepted for each observation.
Modeling resistivity anomalies from localized voids under irregular terrain
Authors: Spiegel R. J. , Sturdivant V. R. , Owen T. E.
Published: October 2002

In applying earth resistivity methods to the problem of locating and delineating subsurface structures, surface elevation variations along the surveyed terrain introduce distortions in the soundings. The analysis presented here is aimed at characterizing such terrain variations in the detection of relatively small subsurface targets such as caves, sinks, and tunnels in otherwise homogeneous earth materials. The analytical approach involves, first, the development of a suitable earth resistivity model for localized three‐dimensional subsurface anomalies in a homogeneous flat half‐space. Next, in order to apply the half‐space resistivity model to irregular terrain, a Schwarz‐Christoffel transformation is utilized to map the terrain surface variations into an equivalent flat half‐space. The technique is illustrated by calculating the resistivity response of three tunnels located below a hill with 40-m valleys on either side.
Probabilistic evaluation model of an electromagnetic target with a given gravity anomaly
Authors: St‐Amant M. , Gaucher E.
Published: October 2002

An exploration strategy for base metals (sulfides) frequently involves electromagnetic (EM) surveys followed by detailed gravity surveys over the areas of EM anomalies to define clearly drilling targets. Since drilling costs are high, an objective and accurate quantitative target evaluation is necessary. The expected sulfide content of a target is the only evaluation parameter available from geophysics which may have some correlation with the target economic value. The depth, dip angle, and strike‐length of a dipping lenticular electrically conductive body can be evaluated from EM data. Then, the expected sulfide content is evaluated from the gravity data by excess mass calculation. However, there is always some geologic noise in the gravity data which tends to cause an overestimation of the sulfide content. We find that statistical properties of the gravity noise and sulfide content statistical histograms of EM anomalies must be taken into account to make an accurate evaluation of the expected sulfide content of EM anomalies.
Seismic velocities and electrical resistivity of recent volcanics and their dependence on porosity, temperature, and water saturation
Authors: Ibrahim A. W. , Keller George V.
Published: October 2002

Variation of P‐wave velocities and electrical resistivities of several suites of water‐saturated recent volcanics was investigated. Both P‐velocities and resistivities exhibited strong dependence on porosity. Resistivity was also dependent upon degree of water saturation and temperature. P‐wave velocities, while showing a strong dependence on porosity, appear to be independent of water saturation and temperature. Volcanics, in general, exhibit higher resistivities compared to other igneous rocks and sediments. Electric resistivity of fine‐grained basalts is anomalously low, probably due to higher content of disseminated iron. Pyroclastics and volcanic breccia, on the other hand, exhibit higher resistivities in relation to fine‐grained basalts.
Magnetite mapping with a multicoil airborne electromagnetic system
Authors: Fraser Douglas C.
Published: October 2002

The information content of data from an in‐phase quadrature electromagnetic (EM) system consists of a combination of conductive eddy current response and magnetic polarization response. The secondary field resulting from conductive eddy current flow is frequency‐dependent and consists of both in‐phase and quadrature components of positive sign. Conversely, the field resulting from magnetic polarization is commonly frequency‐independent and consists of only an in‐phase component of negative sign. A magnetite mapping technique was developed for the horizontal coplanar coils of a closely coupled multicoil airborne EM system. The technique yields contours of apparent weight percent magnetite when using a homogeneous half‐space model. The method can be complementary to magnetometer mapping in certain cases. Compared to magnetometry, it is far less sensitive but is more able to resolve closely spaced magnetite zones. The method is also independent of remanent magnetism and magnetic latitude. It is sensitive to .25 percent magnetite by weight when the sensor is at a height of 30 m above a magnetitic half‐space. It can individually resolve steeply dipping narrow magnetite‐rich bands which are separated by 60 m.
Continuation of heat flow data: A method to construct isotherms in geothermal areas
Authors: Brott Charles A. , Blackwell David D. , Morgan Paul
Published: October 2002

A continuation technique for conductive heat flow in a homogeneous isotropic medium is presented which utilizes observe surface heat flow data. The technique uses equivalent point sources and is developed for transient or steady‐state conductive heat flow problems for a homogeneons half‐space with plane surface and a surface with topographic relief. The technique is demonstrated by comparison with a steady‐state fault model and the terrain correction problem; it is also compared to observed heat flow data in two geothermal areas (Marysville, Montana, and East Mesa, Imperial Valley, California). Calculated subsurface temperature distributions are compared to analytical models and the results of geophysical studies in deep drillholes in geothermal systems. Even in geothermal systems, where convection is involved in the heat transfer, the boundaries of the “reservoir” associated with the convective system can be treated as a boundary condition and the depth and shape of this boundary can be calculated, since many geothermal systems are controlled by permeability barriers. These barriers may either be due to the natural development of a trap or to self‐sealing. Continuation of surface heat flow data is a useful technique in the initial evaluation of geothermal resources as well as an additional tool in the interpretation of regional heat‐flow data.
American Mining Geophysics Delegation to the People’s Republic of China
Authors: Breiner S. , Corbett J. D. , Daniels J. J. , Hansen D. A. , Horsnail R. F. , Morrison H. F.
Published: October 2002

An American mining geophysics delegation visited the People’s Republic of China in 1979, at the invitation of the Ministry of Geology, to lecture and visit various technical institutions. Places visited included a geophysical computer center, air‐borne geophysical brigade, geophysical instrument factories in Beijing and Shanghai, Changchun Geological College in northeast China, the Geological Bureaus of Anhui and Jiangxi Province, Dexing copper mine in Jiangxi Province, the Uranium Geology Research Institute, and the State Seismological Bureau. A thorough understanding was obtained of the overlapping and sometimes competing exploration organizations in China. The delegation heard descriptions of mining geophysical and geochemical exploration techniques and case histories and saw a variety of geophysical instruments and geophysical and geological maps of various ore deposits and oil fields. Largely closed to outside contact for the past 30 years, China appears to practice the science of geophysical exploration as is done in the West but, as expected, does not yet possess state‐of‐the‐art exploration technology such as modern computers and instrumentation. The large numbers of trained professional persons in exploration, the variety of adequate instrumentation, and the many ore deposits discovered over the past 30 years is nevertheless impressive.
On: “Variation of density with rock type, depth, and formation in the Western Canada basin from density logs” by J. Maxant (GEOPHYSICS, June 1980, p. 1061–1076).
Authors: Brown Alan R.
Published: October 2002

This is to call attention to some misleading statements in the paper. In the abstract of that paper, the author states “Also, no relationship between density and the occurrence of oil and gas fields was apparent.” He then reenforces that statement later in the paper. To the casual reader, this would appear to imply that rock bulk density is unrelated to the occurrence of hydrocarbons. I believe this is misleading for two reasons.
The influence of currents induced in the host rock on electromagnetic response of a spheroid directly beneath a loop
Authors: Kaufman Alexander A.
Published: October 2002

A variety of time‐domain and frequency‐domain electromagnetic (EM) methods has come into use in minerals exploration for detection of conductive ore bodies. Because the responses of these various systems differ markedly from one another, the question arises as to which is the most effective for use in discovering a buried, conductive ore body. The question can be posed as follows: What type of exploration system provides the best signal‐to‐noise (S/N) ratio, when signal is defined as the anomalous EM field caused by the presence of a target body and noise is defined as EM responses from the surrounding medium? Analytic solution of the problem is tedious and has not yet been reported in the literature. I describe some results for a special case which reduces the complexity of the problem somewhat. The case treated is that of a conducting spheroid situated directly beneath a source loop. The EM responses caused by currents in the spheroid and in the surrounding medium were computed in both the frequency domain and time domain, using the integral equation approach, supplemented with evaluations of asymptotic expression for various field components. Results show that the transient method provides the best S/N ratio of the methods considered.
Multifrequency Turam measurements over a sulfide deposit
Authors: Poddar M.
Published: October 2002

A six‐frequency electromagnetic (EM) system has been developed and used for a Turam‐type survey over the sulfide deposit of Mailaram copper belt in Andhra Pradesh, India. Turam field strength ratio (FSR) and phase difference (PD) are measured in the range 84 to 2688 Hz. Sulfides in this belt occur as thin concentrated lodes which grade into disseminations. Soil covers a major part of the area. Interpretation of the results is carried out using a thin sheet model in free space. However, an attempt has been made to account for finite resistivity of the normal ground by computing Turam response of a conducting half‐space appropriate for the area and subtracting the same from the observed response. The results of the survey confirm the known advantage of the Turam method, viz., its greater depth of investigation compared to a method like Slingram. The normal ground at Mailaram is only mildly conducting so that the free‐air approximation is valid at low frequencies. A correction for the finite resistivity of the ground is required only at high frequencies. However, the limitation of a single‐body interpretation scheme for the bedrock conductor is well brought out by the results.
Geophysical Activity in 1980
Authors: Senti Russell J.
Published: October 2002

Statistical data on geophysical activity in 1980 are gathered from two survey questionnaires mailed to more than 500 companies, contractors, government agencies, and institutions throughout the free world. One questionnaire was the original form which was designed primarily for reporting petroleum activities, and the other, an alternate form, was designed to facilitate reporting of nonpetroleum geophysical activities. We have attempted to gather data from all 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. In general, response to the survey was good. However, we feel that there are new users of geophysical methods we are not reaching. If you did not receive our 1980 questionnaires, please let me know so we can include you on our mailing list for 1981. (As always, data submitted by individual organizations are held strictly confidential.)
Controlled‐source audiomagnetotellurics in geothermal exploration
Authors: Sandberg Stewart K. , Hohmann Gerald W.
Published: October 2002

Theoretical and field tests indicate that the controlled‐source audiomagnetotelluric (CSAMT) method provides an efficient means of delineating the shallow resistivity pattern above a hydrothermal system. Utilizing a transmitter overcomes the main limitation of conventional audiomagnetotellurics—variable and unreliable natural source fields. Reliable CSAMT measurements can be made with a simple scalar receiver. Our calculations for a half‐space show that the plane‐wave assumption is valid when the transmitter is more than 3 skin depths away in the broadside configuration and more than 5 skin depths away in the collinear configuration. Three‐dimensional (3-D) numerical modeling results for a bipole source 5 skin depths away compare well with those for a plane‐wave source, showing that the method is valid. A CSAMT survey at the Roosevelt Hot Springs geothermal area in Utah produced apparent resistivity contour maps at four frequencies: 32, 98, 977, and 5208 Hz. These maps show the same features as those of a dipole‐dipole resistivity map. We also collected detailed CSAMT data at 10 frequencies on two profiles. Two‐dimensional (2-D) plane‐wave modeling (transverse magnetic mode) of the resulting pseudo‐sections yields models similar to those derived by modeling the dipole‐dipole resistivity data. However, CSAMT resolved details not shown by the resistivity modeling. Thus, high resolution along with an efficient field procedure make CSAMT an attractive tool for geothermal exploration.
Relationship between electrical and thermal resistivities for differing grades of coal
Authors: Kayal J. R. , Christoffel D. A.
Published: October 2002

Borehole geophysical logging has been successfully used for identification and evaluation of coal deposits and was recorded by many authors, e.g., Tixier and Alger (1970), Bond et al (1971), Reeves (1971), Lavers and Smits (1976), Kayal (1979), and Kayal and Das (1981). They have mainly discussed the application of electrical, gamma‐ray, density, caliper, neutron, and sonic logging for analyzing coal deposits.
Some applications of the underground tower gravity vertical gradient
Authors: Fajklewicz Zbigniew , Glinski Andrzej , Sliz Janusz
Published: October 2002

Underground tower gravity vertical gradient (UTGVG) measurements made in mine workings have been applied to the detection of caverns, old mine workings, dislocations, and erosive or karstic forms. Since UTGVG measurements are subject to large errors, Kotelnikov’s criterion can be used to determine the probability of isolation of the useful anomalies. The properties of the UTGVG reduce the ambiguity of interpretation of underground gravity surveys. Simultaneous Δg and UTGVG measurements carried out in the same underground profile frequently replace a three‐dimensional or multilevel gravity survey. The paper presents some applications of the UTGVG.
Array length magnification through the use of multiple sources and receiving arrays
Authors: Buhl P. , Diebold J. B. , Stoffa P. L.
Published: October 2002

For wide aperture seismic experiments, very long effective arrays can be synthesized by using multiple sources and receiving arrays. In marine seismic work where a source is coincident with the end of each array, the total length of the arrays can be multiplied by a factor between [Formula: see text] and n, where n is the number of sources. Three ships, each with a source and 3.6-km array, can be positioned so that the offset range 0 to 25.2 km can be continuously sampled with duplication only in the 0 to 3.6-km range. With 100-m group spacing these data can be gathered into 252-fold common‐depth‐point (CDP) data with a CDP spacing of 150 m. While it is not likely that we will ever see five or six ships steaming in line, two and three ships will be used in the near future for detailed sedimentary and deep crustal studies.
The use of vertical seismic profiles in seismic investigations of the earth
Authors: Balch A. H. , Lee M. W. , Miller J. J. , Ryder Robert T.
Published: October 2002

During the past 8 years, the U.S. Geological Survey has conducted an extensive investigation on the use of vertical seismic profiles (VSP) in a variety of seismic exploration applications. Seismic sources used were surface air guns, vibrators, explosives, marine air guns, and downhole air guns. Source offsets have ranged from 100 to 7800 ft. Well depths have been from 1200 to over 10,000 ft. We have found three specific ways in which VSPs can be applied to seismic exploration. First, seismic events observed at the surface of the ground can be traced, level by level, to their point of origin within the earth. Thus, one can tie a surface profile to a well log with an extraordinarily high degree of confidence. Second, one can establish the detectability of a target horizon, such as a porous zone. One can determine (either before or after surface profiling) whether or not a given horizon or layered sequence returns a detectable reflection to the surface. The amplitude and character of the reflection can also be observed. Third, acoustic properties of a stratigraphic sequence can be measured and sometimes correlated to important exploration parameters. For example, sometimes a relationship between apparent attenuation and sand percentage can be established. The technique shows additional promise of aiding surface exploration indirectly through studies of the evolution of the seismic pulse, studies of ghosts and multiples, and studies of seismic trace inversion techniques. Nearly all current seismic data‐processing techniques are adaptable to the processing of VSP data, such as normal moveout (NMO) corrections, stacking, single‐and multiple‐channel filtering, deconvolution, and wavelet shaping.
Very low‐frequency electromagnetic response of a perfectly conducting half‐plane in a layered half‐space
Authors: Poddar M.
Published: October 2002

Following Dmitriev (1961), a rigorous theoretical solution for the problem of scattering by a perfectly conducting inclined half‐plane in a layered half‐space in a plane‐wave field has been obtained. The solution is in the form of a Fredholm integral equation of the second kind, where the unknown is the Laplace transform of scattering current in the half‐plane. The integral equation is solved numerically by the method of successive approximations. The scattered fields at the surface of the half‐space are found by integrating the half‐space Green’s function over the transform of the scattering current. The effects of depth of burial and inclination of the half‐plane, conductivity contrast between the overburden and the substratum, and thickness of the overburden are studied in some detail. As expected, the tangent of the tilt angle and the ellipticity of the ellipse of magnetic polarization decrease rapidly with increasing depth of burial, conductivity contrast, and thickness. Inclination introduces asymmetry in the anomaly profile besides affecting its magnitude. Depth of exploration is greater for the ellipticity than for the tilt angle. A target depth equal to half of the skin depth in the substratum is the limiting depth of detection in the very low‐frequency, electromagnetic (VLF-EM) method. An interpretation scheme for VLF-EM field data is presented, based on peak‐to‐peak separation and difference between peaks of the two polarization parameters.
Desired seismic characteristics of an air gun source
Authors: Larner Ken , Hale Dave , Zinkham Sharon Misener , Hewlitt Charles
Published: October 2002

Marine seismic data are generally contaminated with both “bubble pulses” and “tow noise.” Air gun sources are deployed in arrays designed to reduce the effective level of the bubble pulses. Because the signal from a source array is profoundly altered by the filter characteristics of the earth and because the received signal is subjected to noise‐generating computer processes such as deconvolution, array designs should be optimized to obtain the minimum aggregate noise, and hence the greatest reflection stand‐out, in output traces. For a fixed air‐compressor capacity, a trade‐off in array design exists between maximizing source strength and the fine tuning required to maximize the first‐pulse‐to‐bubble ratio. Except for shallow, high‐resolution surveys where the deconvolution step can be bypassed, optimum suppression of total noise in the output can often be obtained using the available air capacity to increase the source strength of a moderately tuned array, rather than to achieve fine tuning of the array. Processing noise produced by deconvolution will prevent detection of a weak reflection closely following a strong one if the ratio of the two is more than about 21 dB, no matter how finely tuned the source array may be.
A rectangular loop source of current on multilayered earth
Authors: Poddar M.
Published: October 2002

The idea of using electromagnetic (EM) fields to delineate layered‐earth structure is not new. In fact, the theory of EM depth sounding is very well developed mainly because all EM problems in stratified media in which stratification is horizontal are amenable to elegant mathematical solutions. A recent book on EM sounding by Patra and Mallick (1980) lists important earlier publications on the subject. With the addition of Wait (1962) and Ward (1967), this is a fairly comprehensive list and is recommended for interested readers.
Propagation of dispersed compressional and Rayleigh waves on the Texas coastal plain
Authors: Ebeniro Joseph , Wilson Clark R. , Dorman James
Published: October 2002

During a refraction profile on the Texas coastal plain, a strong surface wave with predominant frequencies between 2 and 10 Hz and group velocities near 1.9 km/sec was observed to ranges as great as 64 km. This dispersed wave, with velocities near the compressional wave speed of near‐surface sediments, corresponds to the “leaky” compressional or PL wave. PL dispersion can be predicted from the theory of the normal modes of a layered liquid medium. Efficient propagation of the PL wave is related to the high Poisson ratio of the unconsolidated sediments in the shallow subsurface, and additional examples from the published literature show that the PL wave is commonly excited by shallow sources both on land and offshore. In addition to the PL waves, dispersed waves with group velocities between 0.3 and 0.7 km/sec were observed at ranges less than 10 km. These are identified as Rayleigh waves (LR). Smoothly varying P and S velocity structures for the upper 1 km are obtained by fitting theoretical dispersion curves to the observed PL and LR data.
Self‐potential modeling from primary flows
Authors: Sill William R.
Published: October 2002

This paper discusses a new method for the investigation of self potentials (SP) based on induced current sources. The induced current sources are due to divergences of the convection current which is driven, in turn, by a primary flow, either heat or fluid. As a result of using this approach there is a shift in emphasis toward the vector flow field and its interaction with current cross‐coupling structure when compared with the total potential approach of Nourbehecht (1963) which emphasized the primary flow potential and the voltage cross‐coupling. This shift in emphasis is advantageous because it is analogous to the actual physical processes. For example, fluid flow in the ground gives rise to drag (convection) currents, and the interaction of the convection currents with the electrical structure gives rise to the electrical potentials (SP). This simple physical picture should aid in developing a better intuitive understanding of the generation of SP effects. The convective current approach is easily adapted to numerical modeling techniques, as illustrated by its implementation using a two‐dimensional (2-D) transmission surface algorithm. When the primary flow is driven by the gradient of a potential, joint modeling of the primary flow and the resultant SP is possible with this algorithm. Examples of the SP effects generated by point sources of the primary flow in the presence of simple geometrical structures show the diversity of the possible responses. The various responses can be understood in terms of the distributions of the induced current sources caused by the primary flow. The results from field studies at Red Hill Hot Springs, Utah, are used in an example of the joint modeling of thermal and SP data.
Synthetic seismograms in attenuating media
Authors: Bourbié Thierry , Gonzalez‐Serrano Alfonso
Published: October 2002

Synthetic seismograms are computed in a two‐dimensional medium that contains contrasts both in the elastic and in the attenuation (Q) properties with depth. The case of a liquid‐solid interface, with and without a Q contrast between the two media, is examined in detail. We show the results when the S velocity in the solid is greater than the P velocity in the liquid. The synthetic seismograms show that the effect of the attenuation contrast is not noticeable on precritical reflections when the change in elastic properties is large even in the case of large Q contrast. When this situation occurs, there is a relative increase in the amplitude of the postcritical reflections and a decrease in amplitude of the head waves. It is interesting to notice that when two head waves are generated, the P head wave always has a very low amplitude with respect to the S head wave. This result agrees with what has been observed in borehole data. We also show the results obtained at a solid‐solid interface with and without Q contrast. The synthetic seismogram shows the same type of results as in the previous case but less pronounced due to the smaller portion of the total path occurring in attenuating media. It also shows that the converted waves (S — P and P — S) are of opposite polarities and so nearly cancel each other. The synthetic seismograms are calculated in the horizontal wavenumber‐frequency [Formula: see text] domain, and Fourier transformed back into offset‐time (x, t) domain. The choice of the [Formula: see text] domain as a working space is motivated by the fact that it allows us to include easily the effects of attenuation by specifying complex frequency‐dependent elastic moduli. The reflection and transmission coefficients in the ([Formula: see text], ω) domain are derived from a plane wave theory. Accurate geometric divergence is represented by using the exact Green’s functions in a two‐dimensional medium. The constant Q model for attenuation, derived by Kjartansson, is used.
Spatial filtering of marine seismic data
Authors: Ursin Bjørn
Published: October 2002

Extended source and receiver arrays have proved to be an effective tool for improving the data quality in marine seismic exploration. The extended arrays may be implemented in the field, or in a computer by the summation of traces with a common receiver coordinate or a common source coordinate, respectively. A tilted source or receiver array may be used to enhance reflectors with a specific dip. A tilted source array can be implemented in the field by delaying the pulses at the source subarrays, or in the computer by time‐shifting the traces before implementing the long source array. A tilted receiver array can approximately be implemented in the computer by time‐shifting the traces before implementing the long receiver array. In areas with complex geologic structure, the data can be corrected for normal moveout prior to implementing the extended arrays. The theoretical response of reflected waves from dipping reflectors for different extended array filters is given. Vertical and horizontal stacking constitute a spatial filter which is similar to an extended array filter. Vertical stacking with linear time shifts between the traces can be used to enhance reflectors with a specific dip. The theoretical response of reflected waves from dipping reflectors for different vertical and horizontal stacking filter is given. In order to discriminate against coherent noise travelling in the cross‐line direction, areal arrays must be used. The theoretical responses of three‐dimensional spatial filters are derived in the appendices. These responses are based on quadratic traveltime approximations for reflections in inhomogeneous layered media. A data example is presented which demonstrates the practical use of extended array filters, both implemented in the field and in the computer. From this example and others have come the following conclusions. In areas with strong coherent noise, a field‐implemented extended source array gives a signal‐to‐coherent noise improvement which cannot be obtained in data processing. In other areas, computer implementation of the extended arrays gives signal‐to‐coherent noise improvement as effectively as a field‐implemented extended source array. In such cases, the extended array filters should be implemented in the computer due to greater flexibility in testing on data and to the possibility of producing different stacked sections. Noise reduction is done more effectively by extended array filtering than by vertical and horizontal stacking prior to CDP stacking (weighted or unweighted full‐fold horizontal stacking).