bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimum suppression of coherent signals with linear moveout in seismic data
Authors: Simaan M. , Love P. L.
Published: October 2002

This paper presents a new technique for suppression of coherent signals with linear moveout in seismic data. This technique is implemented in two schemes which take an array of traces as input and produce either one single trace or another array of traces as output. Two seismic applications are discussed in detail. The first is in the area of separation of upward and downward travelling signals in vertical array seismology, and the second is concerned with the attenuation of ground roll in conventional land data and cable noise in marine data, respectively. In each case, an example of real seismic data is presented to illustrate the effectiveness of this technique.
Anisotropy and dispersion in periodically layered media
Authors: Helbig K.
Published: October 2002

Elastic waves propagating in a periodically layered medium exhibit transverse isotropy, provided the wavelength is long compared to the spatial period of the layer sequence. The elastic constants of the long‐wave equivalent transversely isotropic medium can be calculated in several ways, all of which are based on the low‐frequency limit: one either determines from the outset the macroscopic (in average homogeneous) strain response of a representative block of the periodically layered medium to (in average homogeneous) stresses, or one determines the dispersion equation for such media and evaluates this equation for low frequencies. Both approaches yield the same replacement medium. The replacement of a layered medium by a homogeneous transversely isotropic medium is justified if all wavelengths are sufficiently long. High‐resolution techniques, the increasing use of shear waves, and attention to stratigraphic detail require a quantitative evaluation of what is sufficiently long, as well as a study of what happens for wavelengths between that limit and the limit of resolution. Such information can be obtained through a numerical evaluation of the general dispersion equation: one obtains the frequency as a function of the spatial wave vector k. The phase velocity is the v=ωk/(k⋅k) and the group velocity [Formula: see text]. In general, both dispersion and anisotropy are to be expected. This method has been applied to SH-waves in periodically layered media in general, and the dispersion equation has been evaluated numerically for two representative media. The most significant result is that the long‐wavelengths approach, i.e., a nondispersive transversely isotropic replacement medium, is strictly valid for wavelengths larger than three times the spatial period of layering. For small angles against the vertical, dispersion for shorter wavelength is significant. However, for directions making an angle of more than about 30 degrees with the vertical, dispersion sets in at much shorter wavelengths and is in general much more gentle.
Noise correlation lengths in remote reference magnetotellurics
Authors: Goubau W. M. , Maxton P. M. , Koch R. H. , Clarke J.
Published: October 2002

In remote reference magnetotellurics one uses a reference magnetometer, located typically several kilometers from the magnetotelluric site, to obtain estimates of the impedance tensor that are unbiased by random noise. Measurements have been made of the correlation length of this noise by using a third SQUID magnetometer as a local reference. The impedance tensor was determined as a function of the separation between this magnetometer and the magnetotelluric site, and compared with the tensor obtained simultaneously using the remote reference. This study yielded the surprising result that a separation of as little as 200 m was sufficient to obtain an unbiased estimate of the impedance tensor. The magnitude of the magnetic noise was determined with two different techniques: one involving the impedance tensor, and the other involving a magnetic transfer tensor. The second method produced a substantially lower estimate of the magnetic noise at periods longer than about 1 sec. It is concluded that a component of the noise arises from the non‐plane wave nature of the magnetic and electric fields.
Simultaneous correction for divergence and deconvolution without changing industrial practice
Authors: Mendel Jerry M.
Published: October 2002

In 1978 Mendel and Kormylo (see, also, Kormylo and Mendel, 1980; and Mendel, 1983) proposed a technique for simultaneously correcting for spherical divergence and performing deconvolution. We showed that when the traditional starting point for deconvolution, namely, the convolutional sum model of a seismogram, is cast into state‐variable format, so that deconvolution can be performed by Kalman filtering and optimal smoothing techniques, e.g., minimum‐variance deconvolution (MVD) or maximum‐likelihood deconvolution (MLD), then one should not make the commonly made divergence correction on the data. Instead, that correction, which is a time‐varying one, should be put into the state‐variable model for deconvolution. This is possible because MVD and MLD are applicable to time‐varying and/or nonstationary systems.
Interval gravity‐gradient determination concepts
Authors: Butler Dwain K.
Published: October 2002

Considerable attention has been directed recently to applications of gravity gradients, e.g., Hammer and Anzoleaga (1975), Stanley and Green (1976), Fajklewicz (1976), Butler (1979), Hammer (1979), Ager and Liard (1982), and Butler et al. (1982). Gravity‐gradient interpretive procedures are developed from properties of true or differential gradients, while gradients are determined in an interval or finite‐difference sense from field gravity data. The relations of the interval gravity gradients to the true or differential gravity gradients are examined in this paper. Figure 1 illustrates the concepts of finite‐difference procedures for gravity‐gradient determinations. In Figure 1a, a tower structure is illustrated schematically for determining vertical gradients. Gravity measurements are made at two or more elevations on the tower, and various finite‐difference or interval values of vertical gradient can be determined. For measurements at three elevations on the tower, for example, three interval gradient determinations are possible: [Formula: see text]; [Formula: see text]; [Formula: see text]; where [Formula: see text] and [Formula: see text] etc. For a positive downward z-;axis, these definitions for [Formula: see text] and [Formula: see text] will result in positive values for the vertical gradient. Relations of the interval gradients to each other and to the true or differential gradient are examined in this paper.
Synthetic full waveform acoustic logs in cased boreholes
Authors: Tubman Kenneth M. , Cheng C. H. , Toksöz M. Nafi
Published: October 2002

A general expression is derived for the dispersion relations and the impulse response of a radially layered borehole. The model geometry consists of a central fluid cylinder surrounded by an arbitrary number of solid annuli. A Thomson‐Haskell type propagator matrix is used to relate stresses and displacements across the layers. Although the model is completely general, the geometries considered here are restricted to those of a cased hole. Layers of steel, cement, and formation surround the innermost fluid layer. Synthetic microseismograms containing all body and interface waves are calculated for a variety of model parameters. Formation body wave arrivals are relatively unaffected by the presence of a casing. They may, however, be hard to identify cement velocities are close to or larger than those of the formation. The Stoneley and pseudo‐Rayleigh wave arrivals are strongly influenced by the casing parameters. They respond to the combined effects of the steel, the cement, and the formation.
Diffusion of electromagnetic fields into a two‐dimensional earth: A finite‐difference approach
Authors: Oristaglio M. L. , Hohmann G. W.
Published: October 2002

We describe a numerical method for time‐stepping Maxwell’s equations in the two‐dimensional (2-D) TE‐mode, which in a conductive earth reduces to the diffusion equation. The method is based on the classical DuFort‐Frankel finite‐difference scheme, which is both explicit and stable for any size of the time step. With this method, small time steps can be used at early times to track the rapid variations of the field, and large steps can be used at late times, when the field becomes smooth and its rates of diffusion and decay slow down. The boundary condition at the earth‐air interface is handled explicitly by calculating the field in the air from its values at the earth’s surface with an upward continuation based on Laplace’s equation. Boundary conditions in the earth are imposed by using a large, graded grid and setting the values at the sides and bottom to those for a haft‐space. We use the 2-D model to simulate transient electromagnetic (TE) surveys over a thin vertical conductor embedded in a half‐space and in a half‐space with overburden. At early times (microseconds), the patterns of diffusion in the earth are controlled mainly by geometric features of the models and show a great deal of complexity. But at late times, the current concentrates at the center of the thin conductor and, with a large contrast (1000:1) between conductor and half‐space, produces the characteristic crossover and peaked anomalies in the surface profiles of the vertical and horizontal emfs. With a smaller contrast (100:1), however, the crossover in the vertical emf is obscured by the halfspace response, although the horizontal emf still shows a small peak directly above the target.
Enhancement of transient electromagnetic soundings: A metallic model study
Authors: Doll W. E. , Skibicky T. V. , Clay C. S.
Published: October 2002

Transient electromagnetic (TEM) soundings can be enhanced by additions of matched filtered signals from different coil separations. Results of laboratory metallic model studies are used to demonstrate this technique. A short pulse of current is transmitted from one coil. For each of the signals received in the other coil, the half‐space response (surface wave) is subtracted from the total signal to determine the reflected signal. These reflected signals are passed through matched filters corresponding to the proper separation and a possible layer depth. All of the matched filtered signals corresponding to a particular possible depth are added, and the summed output is stored. The filtering and summing process is repeated for a set of possible interface depths. The sum of matched filtered signals for the correct depth has a peak value of 1 and is symmetric about the peak. We believe that short‐range early‐time values of the transient response can be used to estimate the conductivity of the first layer. This estimate is used for numerical computations of half‐space responses. We have used several magnetic dipoles to represent the finite‐sized coils to show good agreement between theoretical and experimental half‐spaces. Theoretical reflection signals for matched filters can be computed numerically. The stacking of matched filtered signals is an electromagnetic analog of the common‐depth‐point (CDP) seismic technique. It should reduce the effects of nearsurface inhomogeneities and improve the resolution. As an incidental experiment, we show comparisons of the reflected signal and the “image transmission.” The reflected and transmitted signals are essentially the same and support the concept of hybrid‐ray theory for electromagnetic sounding.
Simple inversion of time‐domain electromagnetic data
Authors: Barnett C. T.
Published: October 2002

The eddy currents induced in a thin confined conductor by a fixed‐loop time‐domain EM system can be represented by a single equivalent current filament. The equivalent current filament stays in the plane of the conductor at all times during the decay of the secondary field, but tends to migrate from a position of maximum primary field coupling at early time toward the center of the conductor at late time. This filament approximation is used in the design of a least‐squares inversion procedure which fits circular or rectangular current filaments to an observed eddy current distribution. The inversion procedure provides a rapid but precise means of estimating the position, size, and attitude of a conductor which has been detected by a time‐domain EM survey.
Offset panel
Authors: Fulton Thomas K. , Darr K. Michele
Published: October 2002

The offset panel is a display of basic seismic data which combines single‐channel profiles from successive offsets (source‐to‐receiver distances) into one format. The profiles are displayed one below another and arranged vertically by offset and horizontally by common‐depth‐point. This arrangement allows for comparison of variations observed at one offset to those at another offset. Alteration of the data due to near‐surface geologic variations generates geometric patterns on the display which are different from patterns due to changes in seismic source or receiver. This display has utility in data processing to verify field recording geometry, monitor the seismic source (primarily a marine application), and determine processing parameters. It aids data interpretation by allowing for the detection of an anomalous velocity zone in the near‐surface which may affect deeper structural interpretation. Utilization of the offset panel in identification of shallow events of high amplitude also allows identification of shallow drilling hazards in the marine environment with conventional seismic data.
On: “Mineral discrimination and removal of inductive coupling with multifrequency IP” by W. H. Pelton, S. H. Ward, P. G. Hallof, W. R. Sill, and P. H. Nelson (GEOPHYSICS, 43, 588–609).
Authors: Soininen Heikki
Published: October 2002

The authors discussed the behavior of the resistivity spectra by means of the Cole‐Cole dispersion model. They also discussed the corrections with which the petrophysical resistivity spectrum can be reduced into an apparent resistivity spectrum caused by a polarizable body embedded in an unpolarizable environment. The application of the Cole‐Cole dispersion model is a marked step forward in spectral IP analysis. However, closer attention must be paid to the assumptions and approaches on which the authors base the relations between the petrophysical and apparent spectra. The authors based their relations between the true and apparent spectra on the use of the dilution factor [Formula: see text]. In accordance with the definition by Seigel (1959), they assumed that [Formula: see text] is a real constant (independent of frequency) over the whole frequency range under consideration. First consider the justification for the assumption of the existence of a constant factor [Formula: see text] in the light of an example calculated for phase spectra. Similar considerations could also be made with the aid of amplitude spectra.
DIRECT METHOD OF INTERPRETATION IN RESISTIVITY PROSPECTING
Authors: Pekeris C. L.
Published: October 2002

A method is given of determining the variation of conductivity with depth for the case of a horizontally layered earth.
THREE‐LAYER RESISTIVITY CURVES FOR THE ELTRAN ELECTRODE CONFIGURATION
Authors: West S. S.
Published: October 2002

A method is described by which standard curves of apparent resistivity vs. electrode separation can be calculated for certain electrode configurations from the published data for the Wenner configuration. Data are given for plotting curves representing 15 sets of values of the three layer parameters in the case of the Eltran configuration.
NOTE ON THE CHANGE IN FREQUENCY OF THE REFLECTION FROM BASEMENT AS THIS REFLECTING HORIZON INCREASES IN DEPTH
Authors: Harris Sidon , Haskell N. A.
Published: October 2002

Summary—A table is presented showing the variation in frequency of the reflection obtained from “basement” as this horizon increases in depth. The data were derived from seismograms obtained in the San Joaquin Valley, north of Bakersfield, California. The so‐called “basement” reflecting horizon is followed from a depth of about 1300 feet on the east side to a depth of about 4400 feet farther out toward the middle of the Valley. The results indicate that the frequency of the reflection decreases by about 27 per cent while the depth of the reflecting horizon increases approximately 340 per cent. Gutenberg’s equation, [Formula: see text], where T is the period of the wave, D is the total distance travelled, and a is a constant, gives an approximate representation of these observations if [Formula: see text] and a=0.05 when T is expressed in thousandths of a second and D in feet.
WELL LOGGING BY RADIOACTIVITY
Authors: Green W. G. , Fearon R. E.
Published: October 2002

A discussion of the relation of radioactivity curves to physical properties of strata studied, including the influence of the history of the rocks involved. Comparisons are made between the probable areal extent of correlatable horizons in various types of formations, and the ability of various logging methods to distinguish these layers is considered as an index of the geological usefulness of the various logging methods. The possibility of distinguishing altered zones which do not show marked physical differences fom the neighboring rocks (such as faults and unconformities), is discussed, and its bearing on the usefulness of radioactivity logging is considered. Illustrations of the correlating power of radioactivity logs are given. Comparisons with electrical logs and with geological sample logs are shown to illustrate the points discussed. A brief discussion of the development of the radioactivity logging method and of the technique of carrying out field operations is also given.
MICHIGAN WEATHERING
Authors: Zirbel N. N.
Published: October 2002

In this paper the weathering conditions found in the Michigan Basin are discussed. It is shown that the depth of the drift cannot generally be determined from weathering profiles and a method for correcting reflection depths under these conditions is suggested.
On: “Higher order autospectra by maximum entropy method” by R. O. Plaisted and H. G. Peña (GEOPHYSICS, 48, 1409–1410, October, 1983)
Authors: Nikias C. L. , Raghuveer M. R.
Published: October 2002

As stated in the short note by R. O. Plaisted and H. G. Peña, they derived what they believed to be “a maximum entropy (MEM) representation for bispectra” and they claimed that their result “can be generalized to an Nth‐order MEM auto‐spectrum.” The purpose of this discussion is to point out the faults in the derivation and show that the claim of MEM is not correct. Furthermore, the parametric representation of the bispectrum that the authors actually attempted to derive is not new but can be found elsewhere (Huber et al., 1971). Plaisted and Peña stated that the random variables [Formula: see text] in their equation (1) can be assumed generally to be normally distributed with zero mean and variance [Formula: see text]. This is a serious mistake. With this assumption the third‐order moment sequence of the [Formula: see text] process given by their equation (3) vanishes, i.e., [Formula: see text] for all (k, l) and therefore the bispectrum of [Formula: see text] is identical to zero. The random variables [Formula: see text] in their equation (1) and hence [Formula: see text] have to be non‐Gaussian for the bispectrum of [Formula: see text] to exist. As a matter of fact, bispectral analysis has been used to obtain information regarding deviations of a process from Gaussianity (Godfrey, 1965).
A standard quantitative calibration procedure for marine seismic sources
Authors: Fricke J. R. , Davis J. M. , Reed D. H.
Published: October 2002

Marine source evaluation has traditionally been a subjective and qualitative study. This is particularly true in the case of source spectral characteristics. As a result, source evaluation, development, and array design based on seismic energy over a specified frequency band have been impossible. Further, the transient nature of source wavelets lends itseft to energy analysis rather than the traditional relative power analysis. The methodology required to achieve a calibrated quantitative estimate of marine seismic source energy is detailed here. The procedure involves two steps. First, a calibrated high‐fidelity measurement of the source signature is required; and second, an analysis of the signatures by a suite of computer algorithms designed to extract quantified measures of amplitude and energy is necessary. Results of the analysis include a time‐domain signature calibrated in American National Standards Institute (ANSI) units of pressure and a frequency domain spectrum calibrated in ANSI units of energy flux. These results provide figures of merit for evaluation of individual source characteristics (e.g., energy flux over seismic bandwidth, and total source energy). Additionally, the results provide data for a quantitative comparison of any two or more sources. Illustrative examples of source studies are included.
History of geophysical technology through advertisements in GEOPHYSICS
Authors: Sheriff Robert E.
Published: October 2002

Exploration geophysics has been largely a free‐enterprise venture and new developments have been “sold” through advertisements in the journal Geophysics. Thus, a review of advertisements provides an eclectic history of geophysics. The following is the view obtained from advertisements alone. The dates cited are usually when ads for innovations first appeared. New features often had been applied earlier, before they were advertised.
The effectiveness of offshore three‐dimensional seismic surveys—Case histories
Authors: Horvath P. S.
Published: October 2002

Gulf began investigating three‐dimensional seismic surveys in the mid‐1960s through Gulf Research and Development Company. During the late 1960s, models were constructed to simulate acquisition and processing. Three‐dimensional (3-D) migration was achieved in the early 1970s, and Gulf began field testing 3-D seismic data acquisition in 1974. By 1978, 3-D seismic surveys were available as a commercial service through contractors. Some advantages that 3-D seismic surveys have over 2-D seismic surveys are: they can help refine both structure and stratigraphic interpretations; they assist in defining the paleogeology and reveal details otherwise not apparent; they help determine the reservoir limits through improved interpretation of the structure and hydrocarbon indicators; they enable the acquisition of subsurface control under surface obstructions, such as platforms, rigs, etc.; they provide the opportunity to construct profiles in any direction desired; and they lend themselves to interactive interpretation. When using 3-D seismic surveys, improved seismic resolution is expected. This in turn improves drilling success and finding new reserves, makes the development drilling program more efficient, and provides the best possible location for a wildcat survey. The results achieved in 16 3-D seismic surveys that cover 26 blocks in the offshore Gulf of Mexico reveal that offshore 3-D seismic surveys can be a cost‐effective way of finding and developing hydrocarbons.
Well logging—A 25‐year perspective
Authors: Snyder Donald D. , Fleming David B.
Published: October 2002

Developments in the field of well logging over the last 25 years are reviewed. Surface and borehole instrumentation have evolved significantly, taking advantage of modern digital and analog integrated circuits. Most open‐hole petroleum well logs are now recorded digitally. Digital logs are also frequently acquired in cased‐hole petroleum, mineral, and geotechnical applications. Nuclear well‐log measurements have become accepted and reliable. New measurements include borehole compensated density and neutron‐porosity, sidewall epithermal neutron‐porosity, and most recently litho‐density. The neutron decay log, developed early in the 25‐year period, has undergone a number of major improvements since its introduction. Probes which make spectral measurements of natural gamma‐ray emission, and gamma‐ray emission from neutron interactions with matter have also been developed. Resistivity measurements are now made with probes which combine three or more sensors each with different depths of investigation so that information about the borehole invasion profile can be acquired. Acoustic logging methods have expressed major developments and improvements. The compensated sonic measurement was introduced early in the period along with the cement bond logging method. Interest in measurement of shear‐wave velocity has produced new direct shear‐wave measurements as well as improved acoustic probes for full‐waveform acoustic logging. Other interesting or promising methods which have been developed or improved during the period include the borehole televiewer, the borehole gravimeter, and the nuclear magnetic resonance log. The digital computer provides powerful capabilities for well‐log analysis both at the well site and in the office. Analysis of complex sand‐shale and carbonate formations using two or more logs in a simultaneous solution of a litho‐porosity model is now routine. Powerful signal processing techniques are being applied to “deconvolve” well logs, to enhance or synthesize images of the wellbore, and to estimate or extract information from full‐waveform acoustic logs. While new or improved measurements have been introduced and log analysts now have access to powerful computers and graphic work stations, understanding of the petrophysical significance of the measurements lags behind the basic hardware measurement and interpretation technology.
A simple parametric model for the electromagnetic response of an anomalous body in a host medium
Authors: West G. F. , Edwards R. N.
Published: October 2002

A simple approximate representation of the spectral response of an arbitrary kind of electromagnetic (EM) prospecting system to a small conductive target in a conductive environment has been derived. The representation contains the direct response from the layered host medium and the first‐order effects of eddy current induction, current channeling, magnetic induction, and the coupling between eddy current and magnetic inductions in the anomalous body, as modified by the host medium. The only significant computational task in the representation is evaluation of a few Green’s functions for the host medium. As a guide to establishing proper approximations, a fundamental study of integral equations is presented. Very simple solutions for the secondary or scattering sources which represent the EM effect of the body are obtained for a few basic cases. Equations for more general cases are complicated by additional terms in the Green’s functions which represent ac interaction between scattering sources and the host medium, the effects of layering in the host, or interactions between the different types of scattering sources. Through a supplementary study of the response of a conductive disk embedded in a conductive host to an axisymmetric field, the ac interaction between the host medium and scattering sources in the disk is shown to be relatively unimportant. Hence, interaction with layering in the host is also minimal. Green’s functions in the integral equation can consequently be simplified. The representation as a whole has been tested by comparing its predictions of the spectral form of a response with two published data sets, responses of a plate model in a conductive host. The fits for targets of moderate and large size, relative to the scale of the particular experiment, range from excellent to good qualitative resemblance. The representation should find application in making expensive numerical or analog model studies more useful and understandable to the field geophysicist, in converting real and model data between the time‐ and frequency‐domain formats, in constructing practical inverse algorithms, and in predicting the form of EM effects in induced polarization and magnetometric resistivity surveys.
Remote sensing—A geophysical perspective
Authors: Watson Ken
Published: October 2002

In this review of developments in the field of remote sensing from a geophysical perspective, the subject is limited to the electromagnetic spectrum from 0.4 μm to 25 cm. Three broad energy categories are covered: solar reflected, thermal infrared, and microwave. The reflected solar region has been the most intensely studied. Photointerpretation of images from individual spectral bands or from color composites remains the most widely used method of analysis. New instrumentation and digital processing, based on analysis of laboratory and field spectra, provide significant advances that are beginning to be applied to resource exploration. Color compositing techniques have been effectively used to detect the characteristic spectral reflectance features of iron oxides and hydroxyl‐bearing materials in satellite multispectral data for mapping areas of hydrothermal alteration. Airborne spectrometers can now detect individual spectral features of many minerals which are diagnostic of different stages of hydrothermal alteration. Evolution was from discrimination, based on empirical experience, to mineralogic identification and leading to quantification. Current developments also indicate new, promising extensions to vegetated terrains. Advances in thermal infrared studies are due to development of thermal models that permit mapping of physical property variations and to detection of spectral differences that provide important compositional information. Analysis techniques are still in their infancy, and thermal satellite data remain appropriate only for regional investigations. Microwave data have been acquired mainly with radar systems, which can provide very high resolution from space, but use of textural and slope information has had limited application. Long‐wavelength radiation has been shown to penetrate dry materials, and this may be applicable in extremely arid regions.
Two‐dimensional transient electromagnetic responses
Authors: Adhidjaja Jopie I. , Hohmann Gerald W. , Oristaglio Michael L.
Published: October 2002

The time‐domain electromagnetic (TEM) modeling method of Oristaglio and Hohmann is reformulated here in terms of the secondary field. This finite‐difference method gives a direct, explicit time‐domain solution for a two‐dimensional body in a conductive earth by advancing the field in time with DuFort‐Frankel time‐differencing. As a result, solving for the secondary field, defined as the difference between the total field and field of a half‐space, is not only more efficient but is also simpler and eliminates several problems inherent in the solution for the total field. For example, because the secondary field varies slowly both in space and time, it can be modeled on a coarse grid with large time steps. In addition, for a simple body the field is local; therefore, because the field can be assumed to satisfy a simple boundary condition in the earth computation is greatly simplified. Our tests show that for the same accuracy, the secondary‐field solution is roughly five times faster than the total‐field solution. We compute and analyze the magnetic field impulse response for a suite of models, most of which consist of a thin body embedded in a conductive half‐space—with or without overburden. The results indicate the conductive half‐space will both delay and attenuate the response of the body and even obscure it if the conductivity contrast is small. The results also suggest that the conductive host can alter the decay rate of the response of the body from its free‐space counterpart. Our results for multiple bodies illustrate the importance of early‐time measurements to obtain resolution, particularly for measurements of the horizontal magnetic field. The vertical magnetic field, however, can be used to infer the dip direction of a dipping body by studying the migration of the crossover. The results for models which include overburden show that the effect of a conductive overburden, in addition to the half‐space effect, is to delay the response of the body, because the primary current initially tends to concentrate and slowly diffuse through the overburden, and does not reach the body until later time. This effect also complicates the early‐times profiles, becoming more severe as the conductivity of the overburden is increased.
Fundamental characteristics of an approximate correction method for electromagnetic coupling in frequency‐domain induced polarization
Authors: Wang J. , Zhan K. , Shien L. , Yan L.
Published: October 2002

Our studies have determined that both gradient and dipole‐dipole arrays may produce a rather strong electromagnetic (EM) coupling effect under certain conditions. Variations in these apparent effects are closely correlated with changes of surface current density and also influenced by topographic relief. The dipole‐dipole array can provide EM coupling anomalies with the same sign and shape as bona fide induced‐polarization (IP) anomalies. It has been shown by theoretical calculations and model studies that response curves of frequency effects of EM coupling in logarithmic coordinates have distinctive features of sectional linearization. This can provide a basic method to correct approximately EM coupling. Compared with methods being used inside and outside China, the suggested correction method has the advantage of wide application. This has been already proven by a few field examples.
Stretched automatic amplitude adjustment of seismic data
Authors: Coruh C.
Published: October 2002

The effectiveness of automatic amplitude adjustment before Vibroseis® correlation (Coruh and Costain, 1983) to whiten seismic data leads to this paper which presents a general method based on automatic amplitude adjustment after stretching ordinary seismic data. The central feature of this method is the ability to separate spectral components of data in time and then to balance separated spectral components for whitening before data are compressed back. The method whitens data in a time‐variant fashion and is called stretched automatic amplitude adjustment or stretched automatic gain control (SAGC).
Study of the leaking channel modes of in‐seam exploration seismology by means of synthetic seismograms
Authors: Franssens G. R. , Lagasse P. E. , Mason I. M.
Published: October 2002

If the dispersion characteristics of coal‐seam channel guides do not extend smoothly at frequencies below cut‐off, the number of parameters required to process inseam seismic data increases. Unwanted modes become more difficult to suppress, the arriving pulses from targets become more difficult both to identify and to pulse compress. The connection between leaking and normal channel mode dispersion is established here by first synthesizing and then analyzing theoretical in‐seam seismograms. Calculation of synthetic seismograms is based on numerical evaluation of the spatial Fourier integral for elastic displacements in the complex wavenumber plane. Theoretical seismograms are presented for three‐layered models. Phase velocity characteristics are recovered from these signals and compared with those obtained from the zero loci of the period equation in the complex wavenumber plane. Under cut‐off the former method yields smooth extensions of the normal mode dispersion characteristics, in contrast to the velocity curves obtained from the period equation only. It is found that the dispersion characteristics obtained from analyzing the seismograms can be used to recompress the dispersed arrivals.
The electromagnetic response of an inhomogeneous layered earth—A general one‐dimensional approach
Authors: Bezvoda V. , Segeth K.
Published: October 2002

The electromagnetic response is studied for a model three‐layer earth formed by constant conductivity in the first and the third layers and conductivity varying with depth in the second layer (i.e., the inhomogeneous transition layer). A generalization to the case of many constant or variable conductivity layers is presented, too. The model problem is addressed by numerically solving an initial value problem for an ordinary differential equation for the inhomogeneous transition layer. The applicability of the procedure proposed, which is limited by the numerical method used, is discussed. As an illustration, the computed apparent resistivities and phases are compared with the results of Kao and Rankin (1980). The technique presented is applied to the computation of the response in the very‐low frequency (VLF) method. Application to other methods employing the plane wave is similar.
A nonlinear signal detector for enhancement of noisy seismic record sections
Authors: Kong S. M. , Phinney Robert A. , Roy‐Chowdhury Kabir
Published: October 2002

In an area of complicated structure a stacked record section is likely to be characterized by a low signal‐to‐noise ratio, even after substantial velocity analysis and other processing. The interpreter identifies signals showing phase coherence across many traces at physically allowable velocities and compiles them into a line drawing. We have developed a nonlinear filter designed to mimic this process, which passes only signals showing spatial coherence and having slowness within an allowed range. In this algorithm, called the “SSD filter,” overlapping M-trace windows are converted into a p-τ representation, obtained by multiplying the stack along the relevant slant line by the smoothed semblance. The results from all windows are composited in the p-τ domain, then retransformed into x-t. The principal tunable parameter is the width M of the correlation window, adjusted to provide an output which agrees well with the event picks made by an experienced interpreter on a test panel of data. The method was developed to enable production of cleaned‐up sections from the often noisy stacks produced in deep crustal seismic studies. An example from the COCORP Southern Appalachian profile illustrates how removal of incoherent noise from the stacked section substantially enhances the quality of the migrated section.
Geophysical exploration with audiofrequency natural magnetic fields
Authors: Labson V. F. , Becker A. , Morrison H. F. , Conti U.
Published: October 2002

Experience with the AFMAG method has demonstrated that an electromagnetic exploration system using the Earth’s natural audiofrequency magnetic fields as an energy source is capable of mapping subsurface electrical structure in the upper kilometer of the Earth’s crust. We resolved the limitations of this method by adapting the tensor analysis and remote reference noise bias removal techniques from the geomagnetic induction and magnetotelluric methods to computation of the tippers. After a thorough spectral study of the natural magnetic fields, we designed lightweight magnetic field sensors capable of measuring the magnetic field throughout the year. We also built a digital acquisition and processing system with the ability to provide audiofrequency tipper results in the field. This new instrumentation was used in a study of the Mariposa, California site previously mapped with AFMAG. This study once again demonstrates the usefulness of natural magnetic field data in mapping an electrically conductive body. Reoccupation of one of the sites in several different seasons proves the high level of repeatability of these data. As well as being repeatable, the tensor data provide additional information on the geometry of the conductive body. Different electrical conductivity features can be distinguished using a broad band of frequencies.
Linear inversion of gravity data using the spectral expansion method
Authors: Chavez Rene E. , Garland George D.
Published: October 2002

Inversion of gravity anomalies in terms of an anomalous mass distribution with irregular outline but uniform density leads to a nonlinear inverse problem. An alternative approach based on the thin‐sheet approximation can, however, be formulated as a linear inverse problem provided the structures are two‐dimensional. The anomalous mass is represented by a thin sheet, which is located at a depth [Formula: see text] and is divided into M strips with N < M data points. Thus, an underdetermined system of linear equations is obtained which is solved by the spectral expansion method for the surface density distribution of each segment. This set of parameters is then transformed into a prismatic structure with variable depth but uniform density. The modeling procedure involves a noniterative method. A gravity problem is investigated, and the solution obtained compares well with previous interpretations.
Measurements of attenuation from vertical seismic profiles by iterative modeling
Authors: Dietrich Michel , Bouchon Michel
Published: October 2002

We present a numerical simulation of vertical seismic profiles (VSP) using the discrete horizontal wavenumber representation of seismic wave fields. The theoretical seismograms are computed in the acoustic case for flat layered media, and they include the effects of absorption and velocity dispersion. A study using the synthetic seismograms was conducted to investigate the accuracy and resolution of attenuation measurements from VSP data. It is shown that in finely layered media estimates of the anelastic attenuation obtained by use of the reduced spectral ratio method are usually inaccurate when the attenuation is measured over a small vertical extent. An iterative method is presented which improves the resolution of the measurements of intrinsic dissipation. This method allows determination for synthetic data of the quality factor over depth intervals of about one wavelength of the dominant seismic frequency.
On the lateral resolution achieved by Kirchhoff migration
Authors: Safar M. H.
Published: October 2002

I show that the implementation of the Kirchhoff summation migration technique is equivalent to the generation of a synthetic focused array. The focused array theory, which is well developed in underwater acoustics, is used to deal with the factors limiting lateral resolution achieved when carrying out point diffractor imaging by Kirchhoff summation. Lateral resolution of a focused array, defined as the horizontal distance between two identical point diffractors which are just resolvable, is obtained from the width of the array response at the 0.35 amplitude points. The focused array theory approach should help in obtaining more insights into the practical aspects of Kirchhoff summation imaging technique, namely, choice of aperture and spatial sampling interval and the influence of velocity errors and seismic frequency. Examples given show how the lateral resolution can be improved by either increasing the array aperture or by using higher seismic frequency. I use the concept of the depth of focus to demonstrate the deterioration in the lateral resolution caused by migration velocity errors. Finally, I illustrate how imaging noise is generated when using a large spatial sampling interval.
Two‐dimensional acoustic modeling by a hybrid method
Authors: Shtivelman V.
Published: October 2002

This paper follows previous work (Shtivelman, 1984) in which a hybrid method for wave‐field computation was developed. The method combines analytical and numerical techniques and is based upon separation of the processes of wave scattering and wave propagation. The method is further developed and improved; particularly, it is generalized for the case of an inhomogeneous medium above scattering objects (provided the inhomogeneity is weak, i.e., the effects of scattering can be neglected) and is represented by a simpler and more convenient form. Several numerical examples illustrating application of the method to the problems of two‐dimensional acoustic modeling are considered.
Shear velocity logging in slow formations using the Stoneley wave
Authors: Stevens Jeffry L. , Day Steven M.
Published: October 2002

We apply an iterative, linearized inversion method to Stoneley waves recorded on acoustic logs in a borehole. Our objective is to assess inversion of Stoneley wave phase and group velocity as a practical technique for shear velocity logging in slow formations. Indirect techniques for shear logging are of particular importance in this case because there is no shear head wave arrival. Acoustic logs from a long‐spaced sonic tool provided high‐quality, low‐noise data in the 1 to 10 kHz band for this experiment. A shear velocity profile estimated by inversion of a 60 ft (18 ⋅ 3 m) section of full‐wave acoustic data correlates well with the P‐wave log for the section. The inferred shear velocity ranges from 60 to 90 percent of the sound velocity of the fluid. Formal error estimates on the shear velocity are everywhere less than 5 percent. Moreover, application of the same inversion method to synthetic waveforms corroborates these error estimates. Finally, a synthetic acoustic waveform computed from inversion results is an excellent match to the observed waveform. On the basis of these results, we conclude that Stoneley‐wave inversion constitutes a practical, indirect, shear‐logging technique for slow formations. Success of the shear‐logging method depends upon availability of high‐quality, low‐noise waveform data in the 1 to 4 kHz band. Given good prior estimates of compressional velocity and density of the borehole fluid, only rough estimates of borehole radius and formation density and compressional velocity are required. The existing inversion procedure also yields estimates of formation Q inferred from spectral amplitudes of Stoneley waves. This extension of the method is promising, since amplitudes of Stoneley waves in a slow formation are highly sensitive to formation Q. Attenuation caused by formation Q dominates over attenuation caused by fluid viscosity if the viscosity is less than about [Formula: see text]. However, Stoneley‐wave amplitudes are also sensitive to gradients in shear velocity in the direction of propagation. In some cases, correction for the effects of shear‐velocity gradients is required to obtain the formation Q from Stoneley‐wave attenuation.