bluuebunny/crossref_metadata_2025 huggingface dataset:
MAXIMUM LIKELIHOOD FILTERING OF REFLECTION SEISMOGRAPH DATA
Authors: Mercado Edward
Published: October 2002

Two types of adaptive multichannel filters applied to exploration reflection seismograph data are the maximum likelihood filter and the Wiener filter. Derived equations for the two types of filters demonstrate that the maximum likelihood filter equations are identical in form to the Wiener equations and are solved by the same matrix inversion techniques. Application of the maximum likelihood filter to common‐depth‐point (CDP) data demonstrates that the output reflection quality is sensitive to reflection alignment on input traces. Presently, sufficient coherency of reflections on input traces can be achieved only on CDP traces with a time window in which the reflection signal is dominant or on vertical array data for which reflection alignment is straight‐forward.
ON POWER ESTIMATION IN MAXIMUM ENTROPY SPECTRAL ANALYSIS
Authors: Johnsen S. J. , Andersen N.
Published: October 2002

A conceptually simple method for power estimation in maximum entropy spectral analysis, based on evaluation of complex residues of the spectral density estimator, is suggested. Numerical integration of the peaks of the power density function is thus avoided. The agreement in simple cases with conventional estimates is demonstrated, and the explicit performance is analyzed in detail in a series of examples. The close connection between the residue power estimate and the estimate proposed recently by Pisarenko is pointed out. The method is particularly suitable for spectral decomposition of low noise time series with several harmonic components, because it allows a direct listing of frequency and power estimates, provides an indication of the purity of the obtained harmonic components and enhances the resolution of the maximum entropy spectral density estimator. Computing facilities with modern program libraries are required for efficient use of the method.
FREQUENCY AND TRANSIENT RESPONSES OF ELECTROMAGNETIC FIELDS CREATED BY CURRENTS IN CONFINED CONDUCTORS
Authors: Kaufman A.
Published: October 2002

This paper considers the behavior of the frequency and transient responses of the magnetic field created by currents in conducting bodies. It is assumed that the surrounding medium is an insulator. The relationship between the low‐frequency part of the frequency spectrum and the late stage of the transient response depends on the type of conductor. These responses and the distribution of the spectrum poles differ for conductors having finite and infinite dimensions. The dependence of various components of the field on conductivity differs, which is important for understanding the resolving capabilities of the inductive methods.
ULTRASONIC PROPAGATION IN SOLENHOFEN LIMESTONE AT HIGH PRESSURES
Authors: Fritz I. J.
Published: October 2002

The measurement of ultrasonic velocities at high pressure in minerals and rocks provides information pertinent to a variety of geophysical and engineering problems such as those of determining the state of matter in the earth's interior, understanding the propagation of seismic waves, and characterizing mechanical behavior of materials that are important in mining technology. In recent years there have been a number of reported high pressure sound velocity measurements in various kinds of limestone. (A concise review of this work can be found in a recent paper by Singh and Kennedy, 1974.) Such measurements continue to be of interest because of the relationship to shock‐wave propagation properties (Grady et al, 1977). From the previous measurements it has been found that the phase transitions in calcite, which is the main constituent of limestone, strongly influence the sound velocities. The phase transitions in pure calcite occur at 14.5 kbar (calcite I–II) and 17.4 kbar (calcite II–III) (Singh and Kennedy, 1974); however, because the transitions may be shifted in pressure and spread out over a range of pressures in a rock, it is necessary to make measurements to pressures in excess of 20 kbar in order to characterize the effects of the transitions. To date there has been only one experimental study of the effect of the II–III transition on sound propagation, namely the longitudinal wave measurements in Oak Hall limestone made by Wang and Meltzer (1973). In order to further characterize the effect of the II–III transition on sound propagation in limestone, we have made measurements to 25 kbar on Solenhofen limestone. We were able to measure both longitudinal and transverse velocities over the full pressure range; thus, our measurements represent the first study of the effect of the II–III transition on shear wave propagation under conditions of hydro static pressure.
GRAVITATIONAL CORRECTIONS AND THE BOUGUER FORMULA OF 1749
Authors: THYSSEN‐BORNEMISZA S.
Published: October 2002

Over 200 years ago the French geodesist Bouguer (1749) published a formula about the relation of densities referring to a spherical earth and its crust. From that formula, free‐air, Bouguer and elevation corrections can be derived. This procedure may be of some interest in geophysics and geodesy. Generally, textbooks have not considered this interesting perspective, and for this reason a brief analysis of the 1749 formula is presented here.
FIELDS OF A VERTICAL MAGNETIC DIPOLE OVER A VERTICALLY INHOMOGENEOUS EARTH
Authors: Abramovici F. , Chlamtac M.
Published: October 2002

The electromagnetic field components are expressed in terms of Sommerfeld‐type improper integrals, the integrands of which depend upon the solution of a nonlinear differential equation. Combining a collection of Runge‐Kutta numerical integration methods with the extrapolation to the limit, a desired accuracy in the final result is obtained by varying the step‐size along the structure according to the local error. Actual calculations were performed for: (1) A homogeneous half‐space having on top several layers with linearly varying conductivity sandwiching a very conductive or a very resistive layer. (2) A stack of homogeneous layers over a resistive or a conductive half‐space. (3) A transition layer in a stack of homogeneous layers. The ratios of the field components to corresponding components for a half‐space as functions of “numerical distance” are strongly correlated to the variation of resistivity with depth. (The main portions of the graphs of the absolute values and phases for all the ratios show a striking similarity for a given model.) This characteristic curve, called loosely “model line” starts with a maximum for a resistive half‐space and a minimum for a conductive one. Further maxima and minima reflect the conductivity of the various layers. A similar behavior occurs when transition layers are present, although the model lines show lower peaks and are smoother.
ANALYSIS OF AIRBORNE SPECTRORADIOMETRIC DATA AND THE USE OF LANDSAT DATA FOR MAPPING HYDROTHERMAL ALTERATION
Authors: Collins William
Published: October 2002

An airborne spectroradiometer system has been developed to take 500 channel ground target measurements simultaneously in the spectral region between 400 and 1100 nm. Survey flights with the instrument over an exposed hydrothermal alteration zone in Goldfield, Nevada provide the high‐spectral resolution and spatially correlated data necessary to establish a computerized technique for spectral discrimination of limonitic zones that could indicate mineralization. The data generated by the airborne system are used, in particular, to determine the spectral properties of alteration materials as they appear in integrated measurements over extended field areas, to determine which spectral properties are unique under field conditions and will remain unique in the low‐spectral resolution Landsat data, and to determine accurately the nature and magnitude of the relative spectral differences among geologic targets under the broadband configuration. Field measurements from the aircraft are spatially integrated over contiguous 18 m square fields‐of‐view along traverses flown to cover both background and altered rock assemblages. A small spectral signal unique to zones enriched in ferric iron minerals is recoverable in the aircraft data. Based on a differential spectral discriminant, a computer‐compatible method has been devised to extract the ferric iron signal from atmospheric and background terrain and geologically induced variations in Landsat data. The discrimination technique, adapted to satellite spectral data, was applied to the Goldfield region, including the area of known alteration and metallic mineralization. Field reconnaissance and comparison with published maps for this region has affirmed that limonitic alteration is reliably delineated by the computer analysis technique. Assessment of current satellite instrumentation based on the aircraft data analysis indicates that inclusion of more appropriate band‐pass regions in future sensors could increase spectral contrast among geologic targets by 100 percent. Reducing the field‐of‐view can also increase spectral contrast, and can help reduce spectral ambiguities among extended targets.
Boundary conditions for the numerical solution of wave propagation problems
Authors: Reynolds Albert C.
Published: October 2002

Many finite difference models in use for generating synthetic seismograms produce unwanted reflections from the edges of the model due to the use of Dirichlet or Neumann boundary conditions. In this paper we develop boundary conditions which greatly reduce this edge reflection. A reflection coefficient analysis is given which indicates that, for the specified boundary conditions, smaller reflection coefficients than those obtained for Dirichlet or Neumann boundary conditions are obtained. Numerical calculations support this conclusion.
The calculation of magnetotelluric quantities for three‐dimensional conductivity inhomogeneities
Authors: Jones F. W. , Vozoff K.
Published: October 2002

Three‐dimensional numerical modeling is applied to magnetotelluric principal axis and tipper analysis. Calculations of the impedance, rotation angles, and tipper azimuths are made at the surface of a conducting half‐space which contains perturbed two‐dimensional models. The object is to determine which of the directions is less disturbed by the perturbations, and hence is the more reliable guide to probable strike direction. The results indicate that the principal axis direction is a marginally more stable indicator of strike direction than is the tipper azimuth.
Electromagnetic and Schlumberger resistivity sounding in the Roosevelt Hot Springs KGRA
Authors: Tripp A. C. , Ward S. H. , Sill W. R. , Swift, C. M. , Petrick W. R.
Published: October 2002

One‐ and two‐dimensional modeling of the Schlumberger soundings at the Roosevelt Hot Springs KGRA have indicated a low‐resistivity zone of approximately 5 Ω-m paralleling the dome fault. The low resistivity of this zone is probably due to intensely fractured and altered water‐saturated rock. A zone of resistivity 12 Ω-m extending to the west of the dome fault is probably due to leakage of brine away from the geothermal system through alluvium or moderately altered rock. A resistive basement underlies the conductive zones and is believed to be essentially nonporous and unaltered rock. A major problem in the application of one‐dimensional (1-D) modeling of Schlumberger data in the Roosevelt Hot Springs KGRA is poor resolution of the 1-D parameters. The joint inversion of Schlumberger and electromagnetic sounding data gives a least‐squares 1-D conductivity model in which parameters are much better resolved than are the model parameters estimated by the inversion of Schlumberger data alone. One‐dimensional modeling of Schlumberger soundings along a traverse does indicate the presence of a 2-D inhomegeneity but it gives no hint of the possible complexity of that inhomogeneity even though the parameters of the models fitting each sounding have acceptable standard deviations when constrained by electromagnetic sounding data. Since the model parameter standard deviations are model dependent, good resolution of 1-D model parameters does not indicate that the assumption of a 1-D model is valid. On the other hand, the possible complexity of structure is brought out by 2-D modeling of the same data, but since the degrees of freedom for complex 2-D models is large, a thorough study of the resolution of such models is prohibited costly at present. In these circumstances, we must constrain the 2-D models with independent geologic or geophysical data and then accept the subsequent best‐fit model as semiquantitative.
Some results from audiomagnetotelluric investigations in geothermal areas
Authors: Hoover D. B. , Long C. L. , Senterfit R. M.
Published: October 2002

Audiomagnetotelluric (AMT) surveys have been made in more than 40 geothermal areas in the western U.S. The majority of the measurements was made in the Basin and Range and Columbia Plateau physiographic provinces. Complete spectral analyses made in conjunction with the field surveys illustrate the impulsive nature of the natural signals which appear to overshadow the Schumann resonances at the low frequencies. The spectrum below 200 Hz is fairly flat and the signal strength is sufficient for operation throughout the year. Above 200 Hz and centered near 2 kHz, an absorptive band limits the signal strength making data acquisition difficult if not impossible in the mid‐band region, particularly during the winter months. The spectra also show that power line harmonics may be very strong to about 1000 Hz and can be 30 to 50 dB above the natural signal level. Histograms of the measured apparent resistivities at 7.5 and 27 Hz for all measurement locations within the Basin and Range and Columbia Plateau physiographic provinces indicate a narrow distribution for the Basin and Range, while a broader distribution characterizes the Columbia Plateau. Comparisons between the average resistivities and Known Geothermal Resource Areas (KGRA) show that low average resistivities appear to correlate with regions containing a number of large volume geothermal systems and not with high heat flow.
A summary of the geology, geochemistry, and geophysics of the Roosevelt Hot Springs thermal area, Utah
Authors: Ward S. H. , Parry W. T. , Nash W. P. , Sill W. R. , Cook K. L. , Smith R. B. , Chapman D. S. , Brown F. H. , Whelan J. A. , Bowman J. R.
Published: October 2002

The Roosevelt Hot Springs thermal area is a newly discovered geothermal power prospect in Utah. Seven production wells have been drilled with a maximum per well flow capability averaging [Formula: see text] of combined vapor and liquid per hour at a shut‐in bottom hole temperature near 260°C. The thermal area is located on the western margin of the Mineral Mountains, which consist dominantly of a Tertiary granitic pluton 32 km long by 8 km wide. Rhyolitic tuffs, flows, and domes cover about [Formula: see text] of the crest and west side of the Mineral Mountains within 5 km of the thermal area. The rhyolitic volcanism occurred between 0.8 and 0.5 m.y. ago and constitutes a major Pleistocene thermal event believed to be significant to the evaluation of the Roosevelt Hot Springs thermal area. Thermal waters of the (now) dry spring, a seep, and the deep reservoir are dilute (ionic strength 0.1 to 0.2) sodium chloride brines.
The National Geothermal Exploration Technology Program
Authors: Ball L. , Salisbury J. W. , Kintzinger P. R. , Veneruso A. F. , Ward S. H.
Published: October 2002

In response to the Geothermal Energy Research, Development, and Demonstration Act of 1974, a federal geothermal program has been established with the objective of stimulating the commercial development of geothermal resources. The program goal is to increase the annual rate of energy utilization from the present 0.04 quads (500 MWe) to 0.3–0.5 quads in the near term (about 1985), 4.0–9.0 quads in the mid‐term (1985–2000), and 16.0–28.0 quads in the long term (by about 2020). The realization of these goals depends upon the discovery and exploitation of many new geothermal resource areas. The Department of Energy program for geothermal exploration and assessment has been structured to address technological barriers presently hindering the economical discovery and delineation of geothermal resources. We describe the program elements—exploration technology, reservoir assessment, reservoir confirmation, and reservoir engineering—in light of the need to evaluate some 1500 new prospects in order to meet the federal midterm electric power goal of 20,000 MWe on‐line by the year 2000. We illustrate the program elements with suggested sequences for exploration, assessment, and confirmation of a 200-MWe resource in the eastern Basin and Range physiographic province. The estimated costs for these sequences are $385,000, $565,000, and $3,190,000, respectively. Deep drilling constitutes the major element in the confirmation costs. An economical exploration strategy requires use of cost‐effective techniques; thus, we have initiated a number of technology assessment studies. Pursuant to these studies, we present a summary of our initial findings and discuss the status and needs for topics such as system modeling, thermal methods, rock and fluid properties, seismic, and electrical methods. Accurate reservoir assessment requires new developments in logging instrumentation for high‐temperature, hostile environment boreholes and improved means for interpreting acquired data. The logging instrumentation program is aimed at upgrading logging systems for operation at 275°C and 48.3 MPa in the near term (1982) and 350°C and 138 MPa by 1986. Existing hardware is being upgraded, components and materials are being developed, and critically needed prototype tools for temperature, flow, and pressure measurements will be evaluated. A program addressing log interpretation problems uses industrial expertise to analyze specific shortcomings in our ability to infer critical reservoir parameters from acquired data. The program will also establish and maintain test and calibration wells and support research logging and petrophysical studies.
The effect of subsurface sampling on one‐dimensional synthetic seismograms
Authors: Schoenberger M. , Levin F. K.
Published: October 2002

In generating one‐dimensional (1-D) synthetic seismograms from sonic and density logs, the actual subsurface is replaced by a sequence of beds so constructed that the time required to traverse each bed is the same. Since Wuenschel (1960) devised the method, geophysicists have assumed that as long as the beds were thin, i.e., subsurface time sampling was fine enough to avoid aliasing in the source waveform, the synthetic seismograms would be independent of the sampling rate. We have found this assumption to be unjustified. Even when the subsurface is sampled more than twice per period at the maximum source frequency, the traces generated by a synthetic seismogram program may change as the sampling changes. For the wells that were analyzed, the reflections were delayed by an increasing amount as the sampling became finer. The delay appeared to approach a constant value; but for the most cyclic of the logs, even a 0.5-msec sample interval apparently was not small enough. However, when instead of computing the velocity for a sample interval by simply summing transit times we turned to a method suggested by Sherwood (1962), the total reflection time delay became less dependent on the sampling rate. Attenuation over the seismic band increased as the sampling became finer, regardless of the method used to obtain velocity from transit times. Although the time delays found in this study are greatly reduced when the method of computing velocity from sonic log transit times is modified from that of simply summing sample values, it is not clear that this more complicated procedure is free of problems due to correction of sonic log integrated times to check‐shots times. Part of the difficulty in tying check‐shot traveltimes to seismic reflection times may originate in the phenomena discussed here.
Studies on the interaction of low‐frequency acoustic signals with the ocean bottom
Authors: Santaniello Salvatore R. , DiNapoli Frederick R. , Dullea Robert K. , Herstein Peter D.
Published: October 2002

Understanding the mechanisms by which the ocean sediment redirects impinging sound back into the ocean is necessary in developing propagation models for sonar performance prediction. The Naval Underwater Systems Center (NUSC) has (1) conducted controlled, self‐calibrating acoustic measurements where the ocean bottom interacted signal is isolated in time for analysis, (2) developed deconvolution processing techniques to aid in describing the impulse response of the ocean sediment, and (3) performed modeling to study the interaction of acoustic waves at the ocean bottom. This paper presents a synopsis of studies showing the necessity of considering the refraction of sound by the ocean sediment when predicting low‐frequency propagation loss. Constructive interference between nonplanar wave sediment refracted sound and sound reflected by the ocean‐sediment interface and subbottom layering can cause negative values of bottom loss when using plane‐wave models to interpret measured data. These models cannot account for all possible acoustic arrivals at a receiver. In addition, for a given frequency and constant ocean bottom grazing angle, bottom loss can be dependent upon both processing bandwidth and source/receiver depth. Deconvolution has aided in time resolution of signals that make up the bottom‐interacted signals. Resolution of these signals aids in interpreting results. A modeling effort utilizing the Fast Field Program (a computer technique for evaluating the field integral by the fast Fourier transform) provides quantitative evidence for the necessity of accounting for the refraction of sound by subocean sediments to interpret properly low‐frequency propagation loss measurements.
Integrated radon monitoring by the diffusional barrier technique
Authors: Fleischer Robert L. , Likes Richard S.
Published: October 2002

In many searches for uranium in the earth by mapping of [Formula: see text] it is difficult or inconvenient to drill conventional holes approximately 60 cm in depth. A simpler and more economical method is to use a mat to create a diffusional barrier to radon at the surface and monitor radon beneath the mat. Theory gives the effective depth to which radon is sampled as a function of mat radius, the relation being linear for radii that are small relative to the mean diffusion distance of the radon. Calculations based on this theory indicate what barrier properties are needed. For solubilities of radon that are below 100, a polyethylene mat thickness of at least 50 μm gives a barrier efficiency of more than 90 percent. Field experiments at two sites demonstrate that the predicted variation with mat radius occurs and that measurements are reproducible.
Treatment of singularities in the integral equation approach
Authors: Okabe Masayuki
Published: October 2002

Boundary integral equation techniques are becoming increasingly popular in geophysics as well as in other engineering fields. Excellent contributions have been made in electrical prospecting (Alfano, 1959; Dieter et al., 1969; Barnett, 1972; Snyder, 1976). Through the formulation of the integral equations, we always encounter some singularities. This note presents a method of treating such singularities in the integral equation approach governed by Poisson’s equation. The problems encountered in the numerical integration in each element level also are discussed by reconsideration of previous formulations.
Wave attenuation in partially saturated rocks
Authors: Mavko Gerald M. , Nur Amos
Published: October 2002

A model is presented to describe the attenuation of seismic waves in rocks with partially liquid‐saturated flat cracks or pores. The presence of at least a small fraction of a free gaseous phase permits the fluid to flow freely when the pore is compressed under wave excitation. The resulting attenuation is much higher than with complete saturation as treated by Biot. In general, the attenuation increases with increasing liquid concentration, but is much more sensitive to the aspect ratios of the pores and the liquid droplets occupying the pores, with flatter pores resulting in higher attenuation. Details of pore shape other than aspect ratio appear to have little effect on the general behavior provided the crack width is slowly varying over the length of the liquid drop.
Onshore and offshore measurements of temperature gradients in the Salton Sea geothermal area, California
Authors: Lee Tien‐Chang , Cohen Lewis H.
Published: October 2002

By removing the effect of the annual temperature cycle with an iterative least‐squares method, geothermal gradients at 44 sites were determined from onshore and offshore temperature measurements at depths less than 16 and 2 m, respectively, around the southeastern end of the Salton Sea (water depth <20 m). Onshore temperatures were measured at 1-m intervals in drill holes cased with 0.25‐inch PVC pipes and offshore temperatures were obtained with a 2-m lance‐like thermal probe (0.50 inch OD) which contains 7 equally spaced thermistor sensors. The gradients range from 0.02° to 1.7°C/m and from 0.72° to 4.3°C/m in the onshore and offshore areas, respectively. Heat fluxes, calculated from an assumed value of heat capacity, and in‐situ determined thermal diffusivities and gradients, decrease away from a chain of four Quaternary volcanic domes, near the present shoreline. Superimposed on this general trend are several sites with high heat flux, reflecting local hydrological influence. Corrections for sedimentation, for presence of the lake, or for disturbance around the thermal probe may individually exceed 0.030°C/m (normal geothermal gradient of the upper crust); however, the net effects cannot account for the disparity between the onshore and offshore data sets. Even if the offshore gradients are divided by a factor of 3 to make the two sets of data compatible, these new data (together with data from 22 onshore sites measured previously) indicate the offshore anomaly covers an area at least the size of the onshore anomaly, defined somewhat arbitrarily as an area with heat flux greater than [Formula: see text].
Technological Innovation: Lifeblood of our profession
Authors: Northwood E. John
Published: October 2002

Today, I am not going to talk to you about government regulation and legislation. I am not going to talk to you about the intransigence of some who call themselves environmentalists. I am not going to talk to you about the sluggish pace of offshore leasing in federal coastal waters. I am not going to talk to you about land withdrawals, some the size of California, for parks and wilderness areas. I am not going to talk to you about government price policies on natural gas and crude oil. Nor am I going to talk to you about the newly enacted national energy policy — once described by the President as the moral equivalent of war. What I am going to talk to you about today is Technological Innovation, The Lifeblood of Our Profession.
Resistivity modeling for arbitrarily shaped three‐dimensional structures
Authors: Dey A. , Morrison H. F.
Published: October 2002

A numerical technique has been developed to solve the three‐dimensional (3-D) potential distribution about a point source of current located in or on the surface of a half‐space containing an arbitrary 3-D conductivity distribution. Self‐adjoint difference equations are obtained for Poisson’s equation using finite‐difference approximations in conjunction with an elemental volume discretization of the lower half‐space. Potential distribution at all points in the set defining the subsurface are simultaneously solved for multiple point sources of current. Accurate and stable solutions are obtained using full, banded, Cholesky decomposition of the capacitance matrix as well as the recently developed incomplete Cholesky‐conjugate gradient iterative method. A comparison of the 2-D and 3-D simple block‐shaped models, for the collinear dipole‐dipole array, indicates substantially lower anomaly indices for inhomogeneities of finite strike‐extent. In general, the strike‐extents of inhomogeneities have to be approximately 10 times the dipole lengths before the response becomes 2-D. The saturation effect with increasing conductivity contrasts appears sooner for the 3-D conductive inhomogeneities than for corresponding models with infinite strike‐lengths. A downhole‐to‐surface configuration of electrodes produces diagnostic total field apparent resistivity maps for 3-D buried inhomogeneities. Experiments with various lateral and depth locations of the current pole indicate that mise‐à‐la‐masse surveys give the largest anomaly if a current pole is located asymmetrically and, preferably, near the top surface of the burried conductor.
Reevaluation of the effects of horizontal accelerations on the LaCoste and Romberg gravimeter platform
Authors: Valliant H. D.
Published: October 2002

The effect of sinusoidal horizontal accelerations on the LaCoste and Romberg gravimeter platform was first formulated in detail using a flat earth approximation (LaCoste, 1967) for platform periods not exceeding 6 minutes. In response to user requirements for ever‐increasing accuracy, the platform period was increased gradually from 6 minutes in 1967 to 18 minutes by 1972 and, finally, to 84 minutes in 1977. Further motivation for incorporating long platform periods came from several aerial tests, both rotarywing (Gumert and Iverson, 1974) and fixed‐wing (Valliant, 1976) for which Schuler tuning appears particularly desirable. The introduction of azimuth stabilization in 1972 made the use of long periods possible without introducing transients after vehicle maneuvers. Current gravimeters are also equipped with switched damping to provide a user choice of no damping or damping set at 1/[Formula: see text]2 times critical damping. The preferred procedure is to operate the platform at the 84‐minute tuning with damping and to switch off the damping during turns. The effect of horizontal accelerations requires reevaluation in the light of these new conditions.
U.S. National Magnetic Anomaly Survey Specifications Workshop Report
Authors: Hinze William J.
Published: October 2002

The National Magnetic Anomaly Map (NMAM) Committee, which was formed as an ad hoc committee in 1975 by the SEG Executive Committee, has recommended a multistage program for improving the aeromagnetic data of the U.S. On behalf of the geoscience community, the committee and the U.S. Geological Survey are currently engaged in the preparation of a photo‐composite map of the U.S. derived from publicly available data plus regional magnetic data contributed from the private sector. This map is at present being compiled and is to be pulished by the U.S.G.S. at a scale of [Formula: see text]. Eventually, utilizing this map, the committee plans to assist in the preparation of a North American magnetic anomaly map to supplement the North American tectonic map and the proposed continental gravity anomaly map.
Harmonic and transient fields on the surface of a two‐layer medium
Authors: Kaufman Alexander A.
Published: October 2002

The article deals with the behavior of the quasi‐static electromagnetic (EM) fields created by currents in a two‐layer medium when the source is a vertical axis circular loop. This analysis is of use for determining the depth of investigation of induction methods in mining prospecting. The relationship of the active and reactive components of the field with the geoelectrical parameters is different, depending on the range of frequencies. Unlike the case of a confined body, the low‐frequency part of the spectrum is presented as a series made up of integral and fractional powers of frequency ω and also logarithmic terms of ω. The late stage of a transient process is described with a sum of terms, proportional to the inverse power of time t. These representations are useful for determining the host rock effect against which the signal from an ore body of finite extent must be detected.
COCORP southern Appalachian data
Authors: Kaufman S.
Published: October 2002

The data packages for 348 km of seismic lines in the Georgia‐Tennessee area of the southern Appalachian region are now available for the cost of reproduction and shipping. These data were obtained by the Consortium for Continental Reflection Profiling (COCORP) are applying sophisticated continuous seismic reflection techniques to the solution of geologic problems of the earth’s crust and upper mantle. The operation is part of the U.S. Geodynamics Project sponsored by the National Academy of Sciences and funded by the National Science Foundation. The executive committee of the consortium consists of members from Cornell University, University of Houston, Princeton University, University of Texas at Austin, University of Wisconsin, and Shell Oil Co. Cornell University has the operational responsibility.
Relationship of magnetic anomalies due to subsurface features and the interpretation of sloping contacts
Authors: Rao D. Atchuta , Babu H. V. Ram , Narayan P. V. Sanker
Published: October 2002

A study of the magnetic anomalies produced by sloping geologic contacts, thin dikes, and horizontal cylinders has revealed that a single relationship exists among the magnetic anomalies created by them. The magnetic anomaly due to a horizontal cylinder, the first horizontal derivative of the magnetic anomaly due to a thin dike, and the second horizontal derivative of the magnetic anomaly due to a sloping contact are found to be identical in shape. Gay (1963, 1965) presented standard curves to interpret the magnetic anomalies over long tabular bodies (1963) and long horizontal cylinders (1965). It is shown here that the same curves can also be used to interpret the total, vertical and horizontal magnetic anomalies due to sloping geologic contacts.
Some recent magnetic induced‐polarization developments—Part II: Survey results
Authors: Howland‐Rose A. W. , Linford G. , Pitcher D. H. , Seigel H. O.
Published: October 2002

Because magnetic induced‐polarization (MIP) field measurements reflect polarization current both in the interior of polarizable bodies and in the external medium (return current), an interesting variety of response types, depending upon the distribution of ground conductivities and polarization, has been observed in field surveys. Most of the survey results obtained to date have been amenable to interpretation on the basis of the theoretical and model studies, although some unexplained aspects exist. One interesting type of response is that associated with massive, highly conducting sulfide bodies. These often show reversed polarity anomalies because they provide a channel for the return polarization current due to disseminated sulfides which are adjacent to them. The massive sulfide bodies themselves do not produce appreciable induced‐polarization (IP) effects due to their high‐conductivity contrast with surrounding rocks. The results of ten MIP surveys over known metallic mineral deposits are presented from Australia, and one is presented from Canada. Both time‐domain (transient) and frequency‐domain (complex conductivity) measurements have been made on different deposits.
Errors in stacking velocity—True velocity conversion over complex geologic situations
Authors: Blackburn G.
Published: October 2002

The problems inherent in the conversion of stacking velocities to true vertical velocities have been studied by nonzero offset raypath tracing of primary P‐waves through a suite of varied geologic sections. It is shown that conversion errors can be caused by timing errors as a result of migration and also by raypath distortions due to complex overburdens. The raypath modeling shows that (1) the customary assumption that stacking velocities approximate root‐mean‐square velocities is erroneous in areas of geologic complexity. (2) Essential velocity information from horizons overlain by geologic irregularities is often difficult to recognize and is apt to be ignored or misidentified. (3) Standard fitting of hyperbolic curves to produce normal moveout traces is erroneous in areas of geologic complexity. Consequently there is a need for correct migration, which takes into account all overburden complexities, of all traces in a common‐depth‐point gather prior to velocity determination so that realistic velocities can be obtained.
Radioactivity method
Authors: Duval J. S.
Published: October 2002

Radioactivity measurements have played an important role in geophysics since about 1935, and they have increased in importance to the present. The most important areas of application have been in petroleum and uranium exploration. Radioactivity measurements have proved useful in geologic mapping, as well as in specialized applications such as reactor‐site monitoring. The technological development of the method has reached a plateau, and the future of the method for some applications will depend upon development of more sophisticated data processing and interpretation.
Our Society and our industry
Authors: Silverman D. , Domenico S. N.
Published: October 2002

The Constitution of the Society of Exploration Geophysicists states in Article II, Section 1: The objectives of this Society shall be to promote the science of geophysics, especially as it relates to exploration and research, to foster the common scientific interests of geophysicists and to maintain a high professional standing among its members.
On: “Role of borehole geophysics in defining the physical characteristics of the Raft River geothermal reservoir, Idaho” by W. S. Keys and J. K. Sullivan (GEOPHYSICS, June 1979, p. 1116–1141).
Authors: Evans Hilton B.
Published: October 2002

There is an error in equation (6) on p. 1140 of Keys and Sullivan; the equation should read: [Formula: see text] However, Table 4, p. 1140 of the same paper appears to have been computed using the correct form of the equation, as given above.
Effects of terrain on borehole gravity data
Authors: Hearst J. R , Schmoker J. W. , Carlson R. C.
Published: October 2002

The effect of terrain on gravity measurements in a borehole and on formation density derived from borehole gravity data is studied as a function of depth in the well, terrain elevation, terrain inclination, and radial distance to the terrain feature. The vertical attraction of gravity [Formula: see text] in a borehole resulting from a terrain element is small at the surface and reaches an absolute maximum at a depth of about one and one‐half times the radial distance to the terrain element, then decreases at greater depths. The effect of terrain on calculated formation density is proportional to the vertical derivative of [Formula: see text] and is maximum at the surface, passes through zero where |[Formula: see text]| is greatest, and reaches a second extremum of opposite sign to the first and of much lower magnitude. Accuracy criteria for borehole‐gravity terrain corrections show that elevation accuracy requirements are most stringent for a combination of nearby terrain features and near‐surface gravity stations. Sensitivity to terrain inclination is also greatest for this combination. The measurement of the free‐air gradient of gravity, commonly made’slightly above the ground surface, is extremely sensitive to topographic irregularities within about 300m of the measurement point. The effect of terrain features 21.9 to 166.7 km from the well [Hammer’s (1939) zone M through Hayford‐Bowie’s (1912) zone O] on calculated formation density is nearly constant with depth. At these distances, the terrain correction will be equivalent to a dc shift of about [Formula: see text] of average elevation above or below the correction datum. The effect of topography beyond 166.7 km is not likely to exceed [Formula: see text].
Local functions to represent regional and local geomagnetic fields
Authors: Alldredge Leroy R.
Published: October 2002

Local functions can be used to represent adequately regional and local magnetic fields. A procedure is described in which separate local functions are used in each of many overlapping areas. Weighting functions remove any discontinuities at area boundaries. This procedure was applied to the 1‐degree quadrangle means of the horizontal magnetic component for the United States using ten overlapping areas. Use of separate local functions could have distinct advantages over use of one function for the entire area, because more detail can be retained and the total area can be extended easily to include surrounding areas. Local anomaly maps must retain wavelengths as short as a few kilometers. A local function representation of part of the anomaly map of Nevada, developed by employing a double Fourier series, reduced the computer storage required to approximately one‐fifth that required to store the original anomaly values, and displayed a partial interpretation of the map in terms of directional trends and depth to sources.
On: “Resolution studies in airborne resistivity surveying at VLF,” by S. A. Arcone (GEOPHYSICS, May 1979, p. 937–946).
Authors: Thiel David V.
Published: October 2002

Arcone (1979) uses the vector integral equation [equation (17)], taken from Stratton (1941, p. 467), to solve for the electric field at any point contained inside a closed surface comprising an infinitely large horizontal surface with a particular resistivity containing a small circular region having a different resistivity.
Depth migration after stack
Authors: Judson D. R. , Lin J. , Schultz P. S. , Sherwood J. W. C.
Published: October 2002

The conventional methods for migrating a seismic section, e.g., the finite‐difference method and the Kirchhoff summation method, are inadequate in the presence of significant lateral variations in velocity. For this type of velocity distribution, the basic migration output should be in true depth, although for practical purposes it may be preferable to display it with a nonlinear depth scale. A finite‐difference method has been implemented for obtaining migrated depth sections. The concept underlying this involves all the usual assumptions of a dip line and primary reflections only, with the seismic section considered as the surface measurement of an upcoming wave field which we process with downward continuation in small increments of depth, rather than the customary increments of traveltime. The specified velocity variation laterally along a thin layer results in transmission time changes which must be corrected by a small static time shift applied to each seismic trace. This additional operation within the migration algorithm can be difficult and expensive to implement and is the main reason for its prior omission. Results are given of depth migration applications to both synthetic and real seismic data.
Algorithms for least‐squares linear prediction and maximum entropy spectral analysis—Part I: Theory
Authors: Barrodale I. , Erickson R. E.
Published: October 2002

Experience with the maximum entropy spectral analysis (MESA) method suggests that (1) it can produce inaccurate frequency estimates of short sample sinusoidal data, and (2) it sometimes produces calculated values for the filter coefficients that are unduly contaminated by rounding errors. Consequently, in this report we develop an algorithm for solving the underlying least‐squares linear prediction (LSLP) problem directly, without forcing a Toeplitz structure on the model. This approach leads to more accurate frequency determination for short sample harmonic processes, and our algorithm is computationally efficient and numerically stable. The algorithm can also be applied to two other versions of the linear prediction problem. A Fortran program is given in Part II.
Wavefront curvatures in three‐dimensional laterally inhomogeneous media with curved interfaces
Authors: Hubral Peter
Published: October 2002

The curvatures of a wavefront along a ray in laterally inhomogenous three‐dimensional velocity media separated by curved first‐order interfaces can be computed by a system of equations that includes a Riccati‐type differential matrix equation. This has a simple analytic solution along circular raypaths in media of constant velocity gradients.
Frequency‐wavenumber analysis of geothermal microseisms at Norris Geyser basin, Yellowstone National Park, Wyoming
Authors: Oppenheimer David H. , Iyer H. M.
Published: October 2002

Microseisms were recorded by two separate arrays within 5 km of Norris Geyser basin, Yellowstone National Park, Wyoming. The data were analyzed using frequency‐wavenumber (f-k) spectral techniques to investigate whether the microseisms are originating at the geyser basin and, if so, whether body waves emanating from a deep source could be distinguished from surface waves on the basis of phase velocity. Array aperture and seismometer spacing were systematically varied to examine a continuous wavenumber range of 0 to 100 cycles/km. Results from high‐resolution f-k analysis show that the microseisms indeed originate at the geyser basin in the frequency range 1.5 to 6.3 Hz with phase velocities of 1.1 to 2.5 and 2.0 to 4.0 km/sec on arrays southwest and east of the geyser basin, respectively. Although we could not distinguish between surface waves and body waves originating near the surface solely on the basis of phase‐velocity information, observed velocities clearly preclude the possibility that a deep hydrothermal system is responsible for body‐wave microseisms in this area.
Frequency‐independent background internal friction in heterogeneous solids
Authors: Armstrong Baxter H.
Published: October 2002

A source of elastic wave internal friction is proposed for complex, randomly heterogeneous solids. It arises from the dissipation of long‐range thermal currents due to temperature waves excited by dilatational strain fluctuations between extended elastically dissimilar regions in a solid containing multiple heterogeneities. Dissipation of thermal currents driven by temperature waves across grain boundaries is a part of conventional theory in the high‐frequency limit where only individual anisotropies immediately adjacent to each other are considered, and the range of the waves is small compared to a grain size. It is argued that such dissipation will also become important toward low frequency if a random distribution of elastic heterogeneity exists on a scale exceeding both the thermal skin depth and the grain size. In this event, nearly adiabatic temperature differences arise, in response to an incident elastic wave, between contiguous regions the size of the skin depth due to the difference in average elastic properties. Fast local relaxation on the scale of the grain size as described by prior theory equalizes temperature over such regions, but the relaxation between them is controlled by the skin depth rather than the structure of the inhomogeneities. Dissipation due to this long‐range relaxation produces a figure of merit Q of magnitude 100–1000 for most rocks which is independent of (or nearly independent of) frequency, depending upon the properties and distribution of the inhomogeneities. Qualitative comparison with experiment is given for five types of rock, three glasses, and several other complex solids in the kilocycle frequency region which supports the hypotheses. For sufficiently small grain or effective crystallite size, the effect persists well into the megacycle range. Existing attenuation data for solids with granular or disordered molecular structure, or pure crystalline materials with high concentrations of dislocations, appear consistent with the predictions of the theory.
Fourier transform of the general expression for the magnetic anomaly due to a long horizontal cylinder
Authors: Murthy K. S. R. , Mishra D. C.
Published: October 2002

The general expression for the magnetic anomaly due to a long horizontal cylinder (Gay, 1965) applicable to all three components, i.e., ΔT, ΔZ, and ΔH, is transformed to obtain its characteristics in the frequency domain. The transformed expression clearly demonstrates that its decay is primarily controlled by the depth of the model, while its value at ο = 0 depends upon the nature of magnetization and the cross‐section of the body. It is possible, therefore, to derive the parameters of the causative model directly from the computed transform of the measured anomaly.