bluuebunny/crossref_metadata_2025 huggingface dataset:
Microseisms in geothermal exploration—studies in Grass Valley, Nevada
Authors: Liaw Alfred L. , McEvilly T. V.
Published: October 2002

Frequency(f)‐wavenumber(k) spectra of seismic noise in the bands 1 ⩽ f ⩽ 10 Hz in frequency and |k| ⩽ 35.7 cycles/km in wavenumber, measured at several places in Grass Valley, Nevada, exhibit numerous features which can be correlated with variations in surface geology and sources associated with hot spring activity. Exploration techniques for geothermal reservoirs, based upon the spatial distribution of the amplitude and frequency characteristics of short‐period seismic noise, are applied and evaluated in a field program at this potential geothermal area. A detailed investigation of the spatial and temporal characteristics of the noise field was made to guide subsequent data acquisition and processing. Contour maps of normalized noise level derived from judiciously sampled data are dominated by the hot spring noise source and the generally high noise levels outlining the regions of thick alluvium. Major faults are evident when they produce a shallow lateral contrast in rock properties. Conventional seismic noise mapping techniques cannot differentiate noise anomalies due to buried seismic sources from those due to shallow geologic effects. The noise radiating from a deep reservoir ought to be evident as body waves of high‐phase velocity with time‐invariant source azimuth. A small two‐dimensional (2-D) array was placed at 16 locations in the region to map propagation parameters. The f‐k spectra reveal shallow local sources, but no evidence for a significant body wave component in the noise field was found. With proper data sampling, array processing provides a powerful method for mapping the horizontal component of the vector wavenumber of the noise field. This information, along with the accurate velocity structure, will allow ray tracing to locate a source region of radiating microseisms. In Grass Valley, and probably in most areas of sedimentary cover, the 2–10 Hz microseismic field is predominantly fundamental‐mode Rayleigh waves controlled by the very shallow structure.
Role of borehole geophysics in defining the physical characteristics of the Raft River geothermal reservoir, Idaho
Authors: Keys W. Scott , Sullivan James K.
Published: October 2002

Numerous geophysical logs have been made in three deep wells and in several intermediate depth core holes in the Raft River geothermal reservoir, Idaho. Laboratory analyses of cores from the intermediate depth holes were used to provide a qualitative and quantitative basis for a detailed interpretation of logs from the shallow part of the reservoir. A less detailed interpretation of logs from the deeper part of the reservoir is based on much less corroborative evidence. Extensive use was made of computer plotting techniques to arrive at some interpretations. Both the stratigraphic correlation utilizing a full suite of logs and the attitude of bedding calculated from acoustic televiewer logs indicate gentle dips throughout most of the reservoir with some local flexures. Televiewer logs permitted the location and orientation of numerous fractures and several features that may be faults. Temperature and flowmeter logs provide evidence that these fractures and faults are conduits that conduct hot water to the wells. One of the intermediate depth core holes penetrated a hydrothermally altered zone that includes several fractures producing hot water. This altered production zone could be distinguished by several logs. Borehole gamma spectrometry can be used to identify anomalous concentration of uranium, thorium, and potassium which are probably due to transportation by hydrothermal solutions. Computer crossplotting was used as an aid to the identification of such rock types as quartzite, quartz monzonite, and biotite schist in the deeper wells. Alteration of biotite schist to chlorite schist was also recognizable on these logs using computer‐based analysis. Borehole geophysics not only provided much information on the Raft River geothermal reservoir but also permitted the lateral and vertical extrapolation of core and test data and bridged the gap between surface geophysical data and core analyses.
Wave‐equation datuming
Authors: Berryhill John R.
Published: October 2002

Wave‐equation datuming is the name given to upward or downward continuation of seismic time data when the purpose is to redefine the reference surface on which the sources and receivers appear to be located. This technique differs from conventional datuming methods in the repositioning of seismic reflections laterally as well as vertically in response to observed time dips. The most interesting applications of the technique are those in which the redefined reference surface is an actual geologic interface having an irregular topography and a large velocity contrast. Wave‐equation datuming can remove the deleterious effect such an interface has on seismic reflections originating below it. Wave‐equation datuming also is applicable in seismic modeling. The computations required in wave‐equation datuming are related to those of migration. The Kirchhoff integral formulation of the wave equation can provide a basis for computation to deal with the irregular surfaces and variable velocities that are central to the problem. The numerical implementation of the Kirchhoff approach can be reduced to an efficient procedure involving summations and convolutions of seismic traces with short shaping and weighting operators.
The multicoil II airborne electromagnetic system
Authors: Fraser Douglas C.
Published: October 2002

A helicopter‐towed electromagnetic (EM) system has been developed with two orthogonal transmitter coils. Units currently in service have either two or three orthogonal receiver coils. The associated software yields in‐phase and quadrature channels which are generally free of the conductive response of overburden and of the magnetic induction response of magnetite. These geologic noise sources can mask the response of bedrock conductors for all previously developed airborne EM (AEM) systems. The new technique involves energizing conductors with two orthogonal transmitter coils, both operating at approximately the same frequency (e.g., 900 Hz). The subtracting of the secondary field components of one maximum‐coupled coil pair from the other yields in‐phase and quadrature difference channels. These channels may contain as much as an order‐of‐magnitude increase in the signal/noise (S/N) ratio for bedrock conductors in a geologically noisy environment. The new system can also indicate whether a steeply dipping conductor is thin (e.g., width less than 3 m) or thick (greater than 10 m). The thickness parameter provides a new criterion for conductor sorting, complementing the usual parameters of conductance and strike‐length. The geophysical data are digitally recorded, and the profile records and maps are plotted by computer. The traces include the basic recorded data, the EM difference channels, a channel of resistivity, and a channel of conductance. The conductance channel essentially is an automatic anomaly selector calibrated in mhos. The usual application of the system is for metallic mineral prospecting. However, the system has also been used for sand and gravel detection using two well separated frequencies. For such applications, apparent resistivities are computed for each frequency and are displayed as channels on the profile record and as contour maps.
Terrain corrections for borehole gravity measurements
Authors: Beyer Larry A.
Published: October 2002

This note presents examples of terrain corrections calculated for borehole gravity surveys made in a variety of topographic settings. The effect of terrain corrections on vertical density profiles calculated from borehole gravity measurements also is shown.
Density determinations of basic volcanic rocks of the Yellowknife supergroup by gravity measurements in mine shafts—Yellowknife, Northwest Territories
Authors: Gibb R. A. , Thomas M. D.
Published: October 2002

Gravity measurements were made in two gold mine shafts sunk in the Archean Yellowknife greenstone belt to determine the in‐situ densities of basic volcanic rocks of the Kam formation, Yellowknife supergroup. Thirteen stations were occupied between the surface and a depth of 608 m at an average interval of about 50 m in the C shaft of Giant Yellowknife Mines Limited, and 14 stations were occupied between the surface and a depth of 1598 m at an average interval of about 120 m in the Robertson shaft of Con mine, Cominco Limited. Densities were computed using the terminology of borehole gravimetry with appropriate corrections for surface terrain and underground voids such as shafts, drifts, and stopes. Weighted mean in‐situ densities of [Formula: see text] (36 to 608 m depth) and [Formula: see text] (surface to 1598 m depth) were obtained from the gravity measurements for the Giant and Robertson sections, respectively; these values compare with mean densities of 2.82 and [Formula: see text] obtained from rock samples collected at the underground gravity stations. Sheared specimens and massive specimens collected at both underground and surface gravity stations have mean densities of 2.80 and [Formula: see text], respectively. Unaltered surface samples collected at stratigraphic intervals of about 150 m throughout the entire volcanic sequence have a mean density of [Formula: see text]. Core samples obtained from holes drilled from the bottom of C shaft extend the vertical density profile for the Giant section from a depth of 608 to 1416 m; the mean density of these samples is [Formula: see text]. The lower bulk densities obtained from the mine shaft experiments reflect in part the high proportion of sheared rocks and in part the presence of lower‐density members of the Kam formation (andesite, dacite, tuff, breccia, and agglomerate) in the vicinity of the shafts, as opposed to purely massive basaltic rocks. A density of [Formula: see text] based on the proportion of low‐ and high‐density rocks in the volcanic belt is considered to be more representative of the Kam formation as a whole.
Seismic velocities in transversely isotropic media, II
Authors: Levin Franklyn K.
Published: October 2002

P‐wave, SV‐wave, and SH‐wave velocities are computed for transversely isotropic solids formed from two isotropic solids. The combinations are shale‐sandstone and shale‐limestone solids of an earlier paper (Levin, 1979), but one velocity of the nonshale component is allowed to vary over the range of Poisson’s ratios σ = 0 to σ = 0.45, i.e., from a rigid solid to a near‐liquid. When the S‐wave velocity of either the sandstone or limestone is varied, the ratio of horizontal P‐wave velocity to vertical P‐wave velocity goes through a maximum as σ increases and subsequently falls to values less than unity as σ approaches 0.5. The P‐wave velocity that would be found with a short surface spread also goes through a maximum and, at σ = 0.5, is less than the P‐wave velocity of either isotropic component. SV‐wave velocities found for data from a short spread are unreasonably large; SH‐wave velocities decrease monotonically as σ increases, but the ratio of horizontal SH‐wave velocity to vertical SH‐wave velocity goes through a minimum of unity.
Seismic method
Authors: Allen S. J.
Published: October 2002

The fifty years from 1930 to 1980 almost completely cover the entire history of the reflection seismograph exploration industry. During its growth from infancy to maturity there have been many major technological breakthroughs and innumerable evolutionary advances. At the conclusion of any time period, there is a tendency to believe that the peak has been reached, only to see significant breakthroughs which start a whole new round of amazing developments. So it is today with a veritable explosion of channel capacity in the recording systems and three‐dimensional surveys finally just becoming a practical reality. The prospects for major improvements in the resolving power of the seismic exploration method have probably never been better.
Magnetic method
Authors: Reford M. S.
Published: October 2002

Developments which have led to an enormous increase in the use of magnetic surveys during the past 50 years are reviewed here. The prime factor was the development of the airborne magnetometer. Its application quickly showed the value of magnetic maps as aids in geologic mapping and in determining basement structure. Developments have included new types of magnetometers with greater sensitivity, and magnetic gradiometers, combined with the use of digital recording and processing routines. The worldwide coverage available is stimulating the preparation of magnetic maps on a continental scale for tectonic studies.
Interpretation of velocity spectra
Authors: Cunningham Allen B. , Heffring Harland H.
Published: October 2002

One major effort in processing seismic data is that of correcting for the geometric effects or normal moveout introduced by multichannel data gathering procedures. Usually, this involves detecting coherent reflected signals, determining a time (t) versus moveout (Δt) relationship for these signals, time correcting the individual receiver traces, and combining selected traces into a stacked or composite section. Most of these operations are automated. Several velocity analysis techniques are available for detecting reflections automatically, and individual trace correcting and combining procedures are trivial. However, determination of a moveout function which will discriminate among valid reflections, multiple reflections, and noise is still largely a human operation. We describe a machine method devised to determine automatically these moveout, or t‐Δt, functions, as they are commonly called. The method makes extensive use of the observed phenomenon that the t‐Δt function is well approximated by a segmented straight‐line curve when plotted on a logarithmic moveout (Δt) versus linear time (t) graph. Criteria for determining these straight‐line functions, which include a constraint on the line segment slopes, are discussed; several examples resulting from their use are shown. In the examples, seismic sections corrected by our machine‐derived moveout functions are compared with those corrected by more conventionally determined functions.
Precision continuous temperature logging and comparison with other types of logs
Authors: Reiter Marshall , Mansure Arthur J. , Peterson Brian K.
Published: October 2002

For a variety of well environments, continuous temperature logs at different speeds, taken with appropriate equipment and fast time‐response probes, can yield temperature data often reproducible to several hundredths of a degree centigrade. Larger differences in reproducibility (several tenths of a degree centigrade) probably result from changes in the well bore. Below the liquid level, data need not be filtered through time‐response characteristics of the probe to give accurate geothermal gradients (typically ± a few percent) that correlate with lithology. Resulting temperature‐gradient logs are qualitatively correlated to other logs, such as induction‐conductivity, gamma‐ray, seismic, bulk‐density, and lithologic logs. Qualitative correlation with the induction‐conductivity log appears best. Temperature gradients generally increase when other logs indicate the formation is becoming shaly or clayey (less sandy, silty, or limy), and/or less competent. The practical worth of the temperature‐gradient log may be its ability to sense formation properties inside casing and tubing. This characteristic of the temperature‐gradient log may allow lithologic identification in shut‐in wells and permit useful logging after casing has been set.
On: “Complex seismic trace analysis” by M. T. Taner, F. Koehler, and R. E. Sheriff (GEOPHYSICS, June 1979, p. 1041–1063).
Authors: Saha J. G. , Rao N. D. J. , Agrawal M. C.
Published: October 2002

In Appendix B, the authors derive an analytical expression for the conjugate component of a Ricker wavelet and state that the different attributes of a wavelet with a peak frequency of 25 Hz are listed in Table 1 of their paper. The quadrature component of a Ricker wavelet should be read as:[Formula: see text]where [Formula: see text] for [Formula: see text] and 1.3…, [Formula: see text] for [Formula: see text]
Velocity analysis in transversely isotropic media
Authors: Lucas A. L. , O’Brien P. N. S. , Thomas J. H.
Published: October 2002

In transversely isotropic media, the moveout velocity obtained from common‐depth‐point (CDP) analysis may be significantly different from the horizontal velocity of the pseudo‐P wave. In Levin’s (1978) paper, he discusses, among other things, the problem of velocity determination in a medium in which the pseudo‐P wave surface produced by a point source is an ellipsoid of revolution. He points out that one would expect many sedimentary rocks to be transversely isotropic with a vertical axis of symmetry. In his Appendix he proves that an ellipse (using two dimensions for convenience) is one possible shape for the wave surface in such a medium. He also shows, as have others, that in this case CDP velocity analysis measures the velocity of horizontal propagation.
The influence of the planted geophone on seismic land data
Authors: Hoover G. M. , O’Brien J. T.
Published: October 2002

Characteristics of the seismic data acquisition system that previously have been ignored become important as more sophisticated interpretive methods based on broader frequency bandwidths are developed to extract stratigraphic information from land data in hydrocarbon and mineral exploration. Theoretical and experimental results indicate that the geophone plant can be approximated by a damped oscillatory coupling, properties dependent upon the geophone mass, dimension of earth contact, and local soil consolidation. A massive geophone with minimal earth contact exhibits a low‐frequency plant resonance with weak damping and acts as a low‐pass filter to eliminate the high‐frequency components of a recorded signal. A lightweight geophone with large earth contact exhibits a high‐frequency plant resonance with strong damping and, consequently, filtering effects are minimal if the plant resonance is well above the signal bandwidth. Although signal filtering influences are weak for strong damping, phase delays can introduce errors of several milliseconds which resemble static errors. Additional complications arise since these time shifts are frequency dependent and, consequently, not identical for all reflection events in a seismic trace. The resonant frequency of the geophone plant increases with increased soil consolidation; however, damping demonstrates only a weak dependence upon consolidation. All of these factors can limit the effectiveness of common‐depth‐point (CDP) stacking methods if the proper technique is not practiced in the acquisition of broad‐bandwidth seismic data.
Design parameters for aerial gamma‐ray surveys
Authors: Pitkin James A. , Duval Joseph S.
Published: October 2002

The initial parameter to consider when planning an aerial gamma‐ray survey is whether the survey will obtain total‐count or spectrometric data. This decision is often controlled by the money available, because spectrometric surveys are at least three times more expensive than total‐count surveys. Other parameters important in the planning of an aerial survey are flight‐line spacing, survey altitude, and detector volume. These parameters are shown in graphs that provide a better understanding of their interaction and can be used in survey planning. The graphs are based upon the concepts of the infinite source yield and the circle of investigation. These concepts relate an aerial gamma‐ray detector to the ground area viewed by the detector. Concentric rings that show the ground area viewed by a stationary detector are constructed using the radius of the circle of investigation for fixed percentages of the infinite source yield as a function of detector altitude above ground level. The data permit calculation of the width of a strip of ground along the flight line viewed by a moving detector for several flight altitudes. Velocity was found not to affect the detection of finite sources seriously in calculated gamma‐ray data, and the measurement time interval was found to have effects similar to those of velocity. Graphs of percent ground coverage and estimated ground resolution provide means of determining flight‐line spacing and survey altitude. The effect on the measured gamma‐ray signal by an anomalous source within the circle of investigation is calculated using concentric rings that represent one sample period along the flight line. Required detector volume can be estimated from graphs that provide approximate count rates for given detector volumes for total count and spectrometric data. A table shows estimated errors for several detector volumes and portrays the statistical uncertainty that results from the smaller volumes. The various curves presented can be used to define survey design parameters. Hypothetical examples illustrate their use and the fact that frequent compromises occur during the planning process.
Measurements of attenuation from vertical seismic profiles
Authors: Hauge Paul S.
Published: October 2002

Values of attenuation have been extracted from five detailed velocity surveys. Significant amounts of attenuation are observed in all five wells. Measured values of attenuation vary by a factor of 10, ranging from less than 0.1 to 0.9 dB/wavelength. Correlation of attenuation with lithology is good, considering the resolution limits of the surveys. One of the surveys was performed in West Texas, and the other four were carried out in the Gulf Coast area. The total number of recording depths in each survey varied from 31 to 208, with downhole recordings taken every 10 or 20 ft over key stratigraphic zones. In all five surveys, a monitor geophone was placed near the well to check for source consistency. Our procedure for measuring attenuation involved constructing the spectral ratio between each downhole pulse and one particular reference pulse. A value of cumulative attenuation (CA) was then extracted from each spectral ratio and plotted as a function of geophone depth. This method allowed us to use many measurements in determining the attenuation over each depth interval.
A Born‐WKBJ inversion method for acoustic reflection data
Authors: Clayton Robert W. , Stolt Robert H.
Published: October 2002

Density and bulk modulus variations in an acoustic earth are separately recoverable from standard reflection surveys by utililizing the amplitude‐versus‐offset information present in the observed wave fields. Both earth structure and a variable background velocity can be accounted for by combining the Born and WKBJ approximations, in a “before stack” migration with two output sections, one for density variations and the other for bulk modulus variations. For the inversion, the medium is considered to be composed of a known low‐spatial frequency variation (the background) plus an unknown high‐spatial frequency variation in bulk modulus and density (the reflectivity). The division between the background and the reflectivity depends upon the frequency content of the source. For constant background parameters, computations are done in the Fourier domain, where the first part of the algorithm includes a frequency shift identical to that in an F-K migration. The modulus and density variations are then determined by observing in a least‐squares sense amplitude versus offset wavenumber. For a spatially variable background, WKBJ Green’s operators that model the direct wave in a medium with a smoothly varying background are used. A downward continuation with these operators removes the effects of variable velocity from the problem, and, consequently, the remainder of the inversion essentially proceeds as if the background were constant. If the background is strictly depth dependent, the inversion can be expressed in closed form. The method neglects multiples and surface waves and it is restricted to precritical reflections. Density is distinguishable from bulk modulus only if a sufficient range of precritical incident angles is present in the data.
Interpretation of magnetic anomalies due to dikes: The complex gradient method
Authors: Rao D. Atchuta , Babu H. V. Ram , Narayan P. V. Sanker
Published: October 2002

A method to interpret the magnetic anomaly due to a dipping dike using the resultant of the horizontal and vertical gradients of the anomaly is suggested. The resultant of both the gradients is a vector quantity and is defined as the “complex gradient.” A few characteristic points defined on the amplitude and phase plots of the complex gradient are used to solve for the parameters of the dike. For a dike uniformly magnetized in the earth’s magnetic field, the amplitude plot is independent of [Formula: see text], the index parameter, which depends upon the strike and dip of the dike and the magnetic inclination of the area. The phase plot of the complex gradient is an antisymmetric curve with an offset value equal to [Formula: see text]. For a dike whose half‐width is greater than its depth of burial, two maxima at equal distances on either side of a minimum value appear on the amplitude plot. For a dike whose half‐width is equal to or less than its depth of burial, the amplitude plot is a bell‐shaped symmetric curve with its maximum appearing directly over the origin. In the case of a thin dike, the amplitude function falls off to half its maximum value at the same point on the abscissa where the phase function reaches, i.e., [Formula: see text]. A combined analysis of the amplitude and phase plots of the complex gradient yields all the parameters of the dike. The method is applicable for the magnetic anomaly in either the total, vertical, or horizontal field. A field example is included to show the applicability of the method.
COCORP: Laramie Range Area, Wyoming
Authors: Kaufman S.
Published: October 2002

The Consortium for Continental Reflection Profiling (COCORP) announces the availability of the data package and tapes for the Laramie Range area, Wyoming, for the cost of reproduction. The COCORP 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, the University of Texas at Austin, the University of Wisconsin, and the U.S. Geological Survey. Cornell University is the operating institution.
Rapid convergence of some seismic processing algorithms
Authors: Rosenbaum J. H. , Boudreaux G. F.
Published: October 2002

The rate of convergence of many numerical algorithms can be greatly improved by repeated application of the method of summation by parts. Cases of interest in seismology arise when we need to resample the spectrum of a function at unevenly spaced frequency values. Related examples include the time‐domain evaluation of the Hilbert transform and the extrapolation to the real axis of spectra evaluated in the complex domain. A formula of this type was first presented by Lanczos (1956). The validity of the algorithms depends upon the fact that summation by parts of the cardinal‐series representation is justified as long as a function is somewhat oversampled.
A three‐dimensional study of seismic diffraction patterns from deep basement sources
Authors: Schilt F. Steve , Kaufman Sidney , Long George H.
Published: October 2002

In 1975 the Consortium for Continental Reflection Profiling (COCORP) acquired 37 km of 12‐fold common‐midpoint (CMP) stacked seismic reflection profiles in Hardeman County, Texas (Oliver et al, 1976). The entire crustal section was the object of study, so a record length of 15 sec (two‐way traveltime) was recorded. Three separate profiles were arranged in an H‐shaped pattern to provide two‐dimensional (2-D) surface coverage. In addition to numerous reflections, the seismic sections exhibit pronounced apparent diffractions. These deep diffractions are more numerous and pronounced here than on any other COCORP survey to date. In many cases diffractions on different profiles intersect at the profile junctions. If these intersecting diffractions represent the same structural feature observed along different profiles, the diffraction time‐surface provides an estimate of the location of the diffractor and an effective velocity (analogous to a stacking velocity) to the depth of the diffractor. Arrival times of these diffraction events were read, and a least‐squares technique was developed to model intersecting diffractions simultaneously and derive location and velocity assuming point or line diffractors. Nine apparent diffraction events were identified, occurring mostly in the 5–9 sec range (∼15–30 km). Point diffractor models generally give a closer fit than line models, with most of the resulting velocities between 6.0 and 7.5 km/sec. However, the introduction of a radial dimension (such as a dome instead of a point) significantly lowers these velocities without affecting the quality of fit. The locations of the point models, when projected to the surface. are distributed roughly along a northwest‐southeast trend. The trend is subparallel to the Wichita Mountains and may indicate deep intrusive and/or tectonic structures related to their evolution. These results are important evidence that strong heterogeneities on the scale of a few kilometers are present at midcrustal depths.
Linear inversion of body wave data—Part II: Attenuation versus depth using spectral ratios
Authors: Jacobson R. S. , Shor George G. , Dorman LeRoy M.
Published: October 2002

We present a method for determining the specific quality factor [Formula: see text] by the use of spectral ratios of seismic refraction data. The only modifications to standard refraction profiles are that the source and receivers be sufficiently far away from reflective boundaries to eliminate the angular dependence of the source spectrum itself and of the observation of the source signal by the receiver. The last requirement may not be too significant. Application of this method for determining attenuation as a function of depth was applied to data obtained in a thick sedimentary section in the Bay of Bengal. Once an adequate velocity‐depth function is determined, attenuation as a function of depth is readily calculated by the use of linear inverse theory. Several preferred solutions are discussed, including a model of constant [Formula: see text] of 0.0115. A comparison of our data with a compilation of data from the published literature of attenuation with depth reveals a reasonable concordance, but with some slight differences. A peak of attenuation at 600-m depth probably represents the depth of lithification of the sediments into mudstone.
Integral equation modeling of three‐dimensional magnetotelluric response
Authors: Ting Sam C. , Hohmann Gerald W.
Published: October 2002

We have adapted a three‐dimensional (3-D) volume integral equation algorithm to magnetotelluric (MT) modeling. Incorporating an integro‐difference scheme increases accuracy somewhat. Utilizing the two symmetry planes of a buried prismatic body and a normally incident plane wave source greatly reduces required computation time and storage. Convergence checks and comparisons with one‐dimensional (1-D) and two‐dimensional (2-D) models indicate that our results are valid. We show theoretical surface anomalies due to a 3-D prismatic conductive body buried in a half‐space earth. Instead of studying the electric and magnetic fields, we have obtained impedance tensor and magnetic transfer functions by imposing two different source polarizations. Manipulation of the impedance tensor and magnetic transfer functions yields the following MT quantities: apparent resistivity and phase, impedance polar diagrams, tipper direction and magnitude, principal directions, skew, and ellipticity. With our preliminary analyses of these MT quantities, we have found that three‐dimensionality is usually revealed by all of them. Furthermore, we have recognized two pairs of complementary parameters: apparent resistivity and phase, and skew and ellipticity. Because of surface charges at conductivity boundaries, low‐frequency 3-D responses are much different from 1-D and 2-D responses. Thus, in many cases 3-D models are required for interpreting MT data. Although an overall 3-D MT interpretation is still not practical due to high computer costs and the complicated structure of the true earth, combined 2-D and 3-D modeling can be applied to yield a gross 3-D structure, which is composed of a cross‐section and its strike extent. In doing so, we suggest that the cross‐section be obtained from higher frequency 2-D E‐perpendicular [Formula: see text] mode modeling, and that the strike extent be derived by matching with lower frequency E‐parallel [Formula: see text] mode results due to corresponding 3-D models. In addition, we have indicated that some simple 3-D features, e.g., location above conductive zone, corners, and symmetry lines, can easily be recognized from the surface MT response.
An oceanographic data recorder
Authors: Davis Raymond E. , Ambuter Bruce P.
Published: October 2002

The authors have designed and built a low‐power, low‐cost tape recorder capable of extended operation in either analog or digital recording modes. Its small size permits use inside 6‐inch ID pressure vessels that would normally preclude a commercial recorder of similar specifications. Data are recorded on 1800 ft of 1/4‐inch audio tape using standard 5‐inch diameter tape reels. Total continuous analog recording time at 1/40 inches/sec is 10 days. In the digital mode, total storage capacity at 2500 bits per inch is [Formula: see text] bits. This permits 14 hours continuous recording at 125-Hz sample rate or 7 hours at a 250-Hz sample rate. A single motor‐gearhead combination drives the capstan flywheel and tape take‐up fluid clutch through urethane belts. Guides are of polished stainless steel and are fixed, in keeping with simplicity. Cost to manufacture, including fabrication and parts, is $1,000.00, plus $150.00 for a four‐track analog head or $500.00 for a seven‐track head. During the last 2 years, the recorders have performed in the analog mode in eight Woods Hole Oceanographic Institution ocean bottom hydrophone systems and in the digital mode in six U.S.G.S. ocean bottom instrument packages.
An inverse ray method for computing geologic structures from seismic reflections—Zero‐offset case
Authors: May B. T. , Covey J. D.
Published: October 2002

Seismic interpretation of structures usually involves identifying and mapping marker reflections in the time domain; however, forward modeling has shown that it can be difficult to map the complex reflection images arising from geologic structures. Inverse modeling by ray techniques offers the potential of computing a structure in the depth domain where it is comparatively easy to evaluate a structural target. An interactive algorithm is presented which has its basis in the eikonal equation and results in a practical procedure to compute models with complex geometries and inhomogeneous layers. Input consists of interpreted reflection times from a CDP‐stacked section and spatial velocity functions determined externally to the algorithm. Output is a two‐dimensional (2-D) model having curvilinear reflectors that can terminate within the model, at faults, and at unconformities. Benefits of structural inverse modeling are realized in the rapid construction of models that have velocity fields defined in the depth domain, explicitly accounting for ray curvature and ray kinking. To illustrate the inverse technique, examples of a complex synthetic thrust fault model and a field‐recorded growth fault model are included. The capability to inverse model steeply dipping structures is of particular interest because it completes a full modeling cycle of (1) theoretical prediction that steep‐dip reflections should be observable, (2) processing of field‐recorded CDP trace data to produce interpretable steep‐dip reflections, and finally (3) computation of steep‐dip reflector positions in the depth domain. An interesting benefit is the application of this algorithm to computing image rays on complex structures and the subsequent implications about time migration of CDP‐stacked sections.
Migration of seismic data from inhomogeneous media
Authors: Hatton Les , Larner Ken , Gibson Bruce S.
Published: October 2002

Because conventional time‐migration algorithms are founded on the implicit assumption of locally lateral homogeneity, they leave events mispositioned when overburden velocity varies laterally. The ray‐theoretical depth migration procedure of Hubral often can provide adequate first‐order corrections for such position errors. Complex geologic structure, however, can so severely distort wavefronts that resulting time‐migrated sections may be barely interpretable and thus not readily correctable. A more accurate, wave‐theoretical approach to depth migration then becomes essential to image the subsurface properly. This approach, which transforms an unmigrated time section directly into migrated depth, more completely honors the wave equation for a medium in which variations in interval velocity and details of structural shape govern wave propagation. Where geologic structure is complicated, however, we usually lack an accurate velocity model. It is important, therefore, to understand the sensitivity of depth migration to velocity errors and, in particular, to assess whether it is justified to go to the added effort of doing depth migration. We show a synthetic data example in which the wave‐theoretical approach to depth migration properly images deep reflections that are poorly resolved and left distorted by either time migration or ray‐theoretical depth migration. These imaging results are, moreover, surprisingly insensitive to errors introduced into the velocity model. Application to one field data example demonstrates the superior treatment of amplitude and waveform by wave‐theoretical depth migration. In a second data example, deep reflections are so influenced by anomalous overburden structure that the only valid alternative to performing wave‐theoretical depth migration is simply to convert the unmigrated data to depth. When the overburden is laterally variable, conventional time migration of unstacked data can be as destructive to steeply dipping reflections as is CDP stacking prior to migration. A schematic example illustrates that when migration of unstacked data is judged necessary, it should normally be performed as a depth migration.
Seismic Loss Mechanisms
Authors: Dutta N. C.
Published: October 2002

In the petroleum industry there is considerable current interest in understanding the causes of seismic energy loss. During the past few years, considerable progress has been made in the field of seismic wave attenuation as a result of both controlled laboratory studies of the static and the dynamic properties of rock specimens and theoretical modeling. During the 49th Annual International Meeting of the Society of Exploration Geophysicists in New Orleans, a research workshop on Seismic Loss Mechanisms was organized. The purpose of this workshop was to assess current understanding of the physical processes that cause attenuation of seismic energy. The workshop, organized by Kenneth Larner of Western Geophysical and the author, featured a multispeaker format involving invited papers from both industry and academia. The presentations were followed by discussions lasting approximately 90 minutes which included the speakers, the audience, and a panel of experts in the field of seismology.
Direct reconstruction of velocity and density profiles from scattered field data
Authors: Raz S.
Published: October 2002

A direct Born inversion algorithm is presented which separates the effects of stratified medium velocity and density gradients, thus facilitating the simultaneous reconstruction of the velocity and density profiles from scattered field data. A significant but not unexpected conclusion is that velocity and density variations cannot be separated and resolved from data acquired by a single source‐receiver configuration. Measurements at a minimum of two offset angles are required.
Linear prediction error filter design
Authors: Hildebrand S. T.
Published: October 2002

The design of a linear prediction error filter can be formulated as a four‐step procedure. These steps, in part, consist of a variational calculus approach that leads directly to the normal equations and an entropy conservation principle. From the normal equations, we obtain parameters called partial correlation coefficients that minimize the prediction error at the kth filter design step. Partial correlation coefficients determine the best prediction error filter for different seismic trace models (autoregressive, autoregressive‐moving average, and phase‐intercept‐shifted autoregressive traces). A result shows that simple phase distortion can increase the length of the prediction error filter and affect the estimates of the reflection sequence.
The Cole‐Cole model in time domain induced polarization
Authors: Lee T.
Published: October 2002

Recently Pelton et al. (1978) used a Cole‐Cole relaxation model to simulate the transient voltages that are observed during an induced‐polarization survey. These authors took the impedance of the equivalent circuit Z(ω) to be [Formula: see text]They then gave the expression for the transient voltage [Formula: see text] as [Formula: see text]In equation (2), [Formula: see text] was misprinted as [Formula: see text]. In these equations, [Formula: see text] and [Formula: see text], [Formula: see text] and τ are constants to be determined for the given model. [Formula: see text] is the height of the step current that will flow in the transmitter. A disadvantage of equation (2) is that it is only slowly convergent for large t/τ. Pelton et al. (1978) used a τ which ranged from [Formula: see text] to [Formula: see text]. The purpose of this note is to provide an alternative expression for [Formula: see text] that is valid only at the later stages but which does not have this disadvantage. The trivial case of c = 1.0 is ignored.
On: “A grounded vertical long‐wire source system for plane wave magnetotelluric analog modeling” by R. N. Edwards (GEOPHYSICS, October 1980, p. 1523–1529).
Authors: Wait James R.
Published: October 2002

In an interesting analysis, Edwards shows that a vertical long wire source will produce electromagnetic (EM) fields that satisfy simple impedance relationships for a homogeneous half‐space model of the earth. The important restriction is that the radial distance to the observer be large compared with an electrical skin depth. Certainly the vertical wire structures provide a very convenient modeling scheme for the “average prospector” to interpret magnetotelluric (MT) data collected over confined inhomogeneities within the conductive host region.
An indirect seismic method for determining the thickness of a low‐velocity layer underlying a high‐velocity layer
Authors: Kaila K. L. , Tewari H. C. , Krishna V. G.
Published: October 2002

We present an indirect method for determining the thickness of a low‐velocity layer (LVL) underlying a high‐velocity layer (HVL) in seismic prospecting. Comparison of the average velocity‐depth function determined from the first arrival refraction data with that obtained from reflection data in the same region, especially below the LVL, makes it possible to recognize the presence of the LVL and to estimate its probable thickness. The applicability of the method has been demonstrated in a field case where the presence of an LVL is indicated by geologic evidence. It has been shown that thickness estimates of an LVL and an HVL can be made reliably in situations where the velocity in the LVL can be accurately estimated from nearby exposures or in a drilled well. For the field case analyzed, a thickness of 0.75 km was estimated for an LVL (probably Mesozoic sediments) underlying a 0.25 km thick HVL (probably basalt). The velocity of propagation in the LVL was taken from seismic data on nearby exposed Mesozoics as 4.0 km/sec, and the velocity of the HVL is 5.4 km/sec, based on the refraction data. In areas where the velocity in the LVL cannot be inferred accurately, an upper limit of this velocity can be obtained which permits estimation of the maximum possible thickness of the LVL. In the field example presented, we show that the velocity in the LVL cannot exceed 4.17 km/sec.
An investigation of finite‐element modeling for electrical and electromagnetic data in three dimensions
Authors: Pridmore D. F. , Hohmann G. W. , Ward S. H. , Sill W. R.
Published: October 2002

The finite‐element method can be used to solve the differential equations which describe electrical and electromagnetic (EM) field behavior. The equations are, respectively, Poisson's equation and the vector, damped wave equation. The finite‐element equations are derived, in both cases, using the minimum theorem. While both tetrahedral and hexahedral elements may be used for the modeling of the resistivity problem, only hexahedral elements give satisfactory results for the EM problem. A disadvantage of the relatively simple mesh design used in the approach described here is the presence of long thin elements. Such elements have very poor interpolating properties, and they adversely affect the rate of convergence of the overrelaxation technique used in solving the resulting system of linear equations. For the modeling of resistivity data over an earth with one plane of symmetry, the system of equations typically has about 9000 unknowns. About 50,000 unknowns are needed to give a satisfactory solution to an EM problem where the earth has one plane of symmetry. The advantage of solving these problems with a technique such as the finite‐element method is that earths with an almost arbitrary distribution of conductivity can be modeled. On the other hand, an integral‐equation method can be far more cost effective for small inhomogeneities. The results from the resistivity algorithm show the adverse effect of an irregular, conducting, and polarizable overburden on dipole‐dipole, induced polarization surveys. Modeling of a horizontal loop EM survey illustrates the importance of assessing the host rock conductivity before attempting to interpret inhomogeneity responses.
Magnetotelluric response on vertically inhomogeneous earth having conductivity varying exponentially with depth
Authors: Kao D.
Published: October 2002

Magnetotelluric (MT) response is studied for a vertically inhomogeneous earth, where conductivity (or resistivity) varies exponentially with depth as [Formula: see text]. Horizontal electric and magnetic fields in such an inhomogeneous medium are given in terms of modified Bessel functions. Impedance and apparent resistivity are calculated for (1) an inhomogeneous half‐space having conductivity varying exponentially with depth, (2) an inhomogeneous half‐space overlain by a homogeneous layer, and (3) a three‐layer model with the second layer as an inhomogeneous or transitional layer. Results are presented graphically and are compared with those of homogeneous multilayer models. In the case of resistivity increasing exponentially with depth, the results of the above inhomogeneous models are equivalent to those of Cagniard two‐layer models, with [Formula: see text]. In the case of resistivity decreasing exponentially with depth, the homogeneous multilayer approximation depends upon the number of layers and the layer parameters chosen; |Z/ωμ| as a function of frequency is more useful than the apparent resistivity in determining the values of p and [Formula: see text].