bluuebunny/crossref_metadata_2025 huggingface dataset:
Algebraic reconstruction of a two‐dimensional velocity inhomogeneity in the High Hazles seam of Thoresby colliery
Authors: Mason I. M.
Published: October 2002

A high proportion of faces started in mechanized coal mines run into underground faults. The faults take many forms, from the splitting of a seam through a hidden stress pattern caused by subsidence or folding, to a washout or a vertical throw. All faults reduce face output. A throw of only 1.5 m can lead to a face being abandoned. Faults of this order cannot be mapped reliably from the surface. They may be mapped in an underground seismic survey. Roadways of a mine may give access to fast refracting horizons above and below a coal seam. Waveguiding in the plane of the seam, if it occurs, simplifies migration of wave trains reflected from discrete faults. The reduction problem involved in a fault mapping is only two‐dimensional. Given guiding, whether leaky or not, it is possible to map distributed faults of low reflectivity by shooting in transmission across a panel of coal. Algebraic reconstruction techniques are used here to reduce first break times‐of‐flight through a 425 × 950 m rectangular block of coal into the profile of a velocity inhomogeneity. Input data are derived by static correction from hand‐picked arrival times. The reduction itself is effected using an algorithm which accommodates underground site access restrictions. In back projecting first break velocities, a truncated cosine is used to weight the relative contributions of rays passing at different distances from any given mapping point. The reconstructed velocity field suggests that the coal panel is bisected by a ridge of higher velocity. The suspicion of a ridge is reinforced by results of an aberration test based on a standard Huygens‐Kirchhoff migration. The ridge is found to follow the general line of a system of pillars left in place during the mining of a lower horizon. It is concluded that channel waves may be used to map subsidence into old workings underground. Coal seams apparently share, with other sedimentary rocks, the property of a pressure‐sensitive seismic velocity.
The use of near‐infrared spectroscopy to determine the degree of serpentinization of ultramafic rocks
Authors: Hunt Graham R. , Evarts Russell C.
Published: October 2002

Visible and near‐infrared (0.35 to 2.5 μm) bidirectional reflection spectra were recorded for a suite of particulate samples from mineralogically well‐characterized serpenatinized ultramafic rocks. The reflection spectra typically exhibit well‐defined minima due to electronic and vibrational processes in the individual mineral constituents. The contrast of near‐infrared spectral features of primary magnesian silicate minerals and secondary hydrous‐serpentine group minerals can be used to indicate the degree of serpentinization of the rock, provided less than about 1 percent of finely divided magnetite is present. The effect of magnetite, apparent in rocks with more than 50 percent serpentine, is to reduce the overall reflectance and the contrast of spectral bands. Near‐infrared spectrometry is potentially a rapid and reliable technique for detecting the highly serpentinized rocks which constitute target areas for asbestos exploration.
On: “SEG delegation visit to People’s Republic of China” by SEG delegation members (October 1980 GEOPHYSICS, p. 1534–1550).
Authors: Pan Cheh
Published: October 2002

I read the report “SEG delegation visit to People’s Republic of China” with great interest. It is very nice to see that SEG is doing so well in the PRC Exchange Program. This will certainly be of benefit to both the U.S. and China.
Spectral balancing in the time domain
Authors: Tufekčić D. , Claerbout J. F. , Rašperić Z.
Published: October 2002

The recent interest in deconvolution and the variety of new deconvolution methods in use demonstrates the importance and complexity of this problem. Each of the existing methods has some advantages for optimum enhancement of definition of seismic data. We believe that the spectral balancing method considered here is appropriate for data in which the frequencies of the organized noise are rather complex, e.g., land data shot by surface sources.
Seismic Q—Stratigraphy or dissipation
Authors: Spencer T. W. , Sonnad J. R. , Butler T. M.
Published: October 2002

The basic problems encountered in extracting estimates of seismic dissipation from data recorded on vertical seismic profiles are analyzed. Because anomalous dissipation in the subsurface is likely to be associated with conditions or lithologies of limited vertical extent, a knowledge of the factors which influence the spatial resolution of an attenuation measurement is of considerable importance. By introducing a statistical perspective, it is possible to simulate multiple measurements in an inhomogeneous interval and to draw conclusions which apply to an entire class of impedance structures. Theoretical seismograms are analyzed to demonstrate that for small receiver separations neither a single measurement nor the mean value determined from multiple measurements is likely to give a good estimate of the attenuation for an inhomogeneous depth interval. For small receiver separations, the attenuation computed from the amplitude ratios method is much more strongly influenced by the local stratigraphy in the immediate vicinity of the seismometer than by the attenuation in the depth interval between seismometers. As the seismometer separation increases, there is a dramatic decrease in the variability in the attenuation values determined from multiple measurements. A critical distance can be defined which is a measure of the spatial resolution. Beyond the critical distance the mean value approaches a quantity which is the sum of two components: the effective dissipation for an inhomogeneous interval and a stratigraphic term which describes the attenuation in a nondissipative system.
The effect of dipping beds on a borehole gravimeter survey
Authors: Brown A. R. , Lautzenhiser T. V.
Published: October 2002

We study the effects of dipping beds on a borehole gravimeter (BHGM) survey. A dipping bed solution is presented which uses an anomalous bed density in a regional framework. A method of application is shown which provides an estimate of the regional framework density and dip angle of the beds. The method of application requires measuring local formation density independently.
Response of a layered earth to the Crone pulse electromagnetic system
Authors: Verma S. K. , Rai S. S.
Published: October 2002

The response of a layered earth to the Crone pulse electromagnetic (PEM) system, which measures the decay of the time derivative [Formula: see text] has been computed. The transient vertical magnetic field component [Formula: see text] due to a vertical magnetic dipole is obtained by applying a Fourier series summation approach and using digital linear filters to compute the response at individual frequencies. Oscillations in [Formula: see text] due to Gibb’s phenomena are suppressed with Lanczos weights, and the derivative [Formula: see text] is computed numerically by using a linear difference approximation over five points. Decay curves for various half‐spaces are found to cross each other at different values of time. Thus, a single channel response cannot be used to estimate the half‐space resistivity uniquely. This can be achieved, however, by making use of responses at two different channels. Conductivity‐aperture diagrams for half‐space models are plotted for both [Formula: see text] and [Formula: see text]. For [Formula: see text], all the channel amplitudes show well‐defined peaks in the range of 0.3 to 5 ω-m, whereas for [Formula: see text] these peaks lie in the range of 0.7 to 20 Ω-m. This supports the finding by earlier workers that for higher conductivities a total field measuring system responds better than a derivative measuring system. A theoretical formulation is presented to compute the Crone PEM response of an n‐layered earth model. After checking the accuracy of the program, some results are presented for models with different numbers of layers. However, detailed investigation is reported for two‐layered earth only. It is found that overburden parameters can be determined by utilizing responses at two different channels. Nomograms to determine these parameters are presented. There nomograms reveal that for the resistive basement case the best discrimination can be done for the overburden resistivities in the range of 3 to 10 Ω-m and for thicknesses in the range of 0.5 L to 1.5 L (L being the coil separation). For the conductive basement situation the corresponding values are 3 to 15 Ω-m and 0.25 L to 1 L.
Induced‐polarization response of zeolitic conglomerate and carbonaceous siltstone
Authors: Nelson P. H. , Hansen W. H. , Sweeney M. J.
Published: October 2002

Three case studies investigating induced‐polarization (IP) responses of a zeolite‐bearing conglomerate and of two carbonaceous siltstones are presented. The IP response of these noneconomic geologic materials can either mask or mimic the response from sulfide mineralization which is sought by electrical field surveys. The nonsulfide rock types which produced unusually high responses on IP field surveys were sampled by core drilling for chemical, mineralogical, and electrical laboratory study. The electrical response of core samples was measured in a four‐electrode sample holder over the 0.03–1000 Hz range. Geologic description of the core, petrographic examination of thin sections, mineral identification by x‐ray diffraction (XRD), and chemical analysis of samples supplemented the electrical measurements. A surface phase response of 20 mrad was obtained from field surveys over the Gila conglomerate at an Arizona location. Core samples of the Gila were examined in thin section, and clast surfaces were found to be coated with a thin layer of zeolites. These zeolites project into pore spaces in the conglomerate, and thus are in intimate contact with formation waters. A series of laboratory experiments suggests that zeolites cause most of the observed IP response. Phase responses as high as 100 mrad were measured with field surveys over siltstone and limestone sequences in western Nevada. Samples recovered from the Luning and Gabbs‐Sunrise formations include siltstones containing small amounts of amorphous carbon. These siltstones are very conductive electrically, and the high‐phase response is attributed to polarization of the carbon‐pore water interface. Low porosity in these carbonaceous siltstones enhances the phase response.
Critique of terrain corrections for gravity stations
Authors: Hammer Sigmund
Published: October 2002

The terrain correction problem for gravity stations continues to attract perennial interest. Since the publication of Hayford and Bowie’s method (1912) and Hammer’s detailed tables (1939), no fewer than 35 papers have reported changes and improvements (in various languages, including Russian) the latest by Olivier and Simard (1981) in a recent issue of Geophysics. However, in none of these papers is there explicit discussion of the weighting for topography within the radial distance across a topographic compartment. This is not a negligible factor, especially for inner topographic zones.
Inversion of self‐potential data from the Cerro Prieto geothermal field, Mexico
Authors: Fitterman David V. , Corwin Robert F.
Published: October 2002

Self‐potential (SP) data from the Cerro Prieto geothermal field in Baja California, Mexico have been inverted using a model consisting of a vertical contact separating regions of different electrical properties. A temperature source is assumed to coincide with the vertical contact between materials with different thermoelectric coupling coefficients. A derivative‐free Levenberg‐Marquardt algorithm is used to estimate values for the depth, vertical extent, length, and intensity of the source region. The depth to the top of the source is estimated to be about 1.3 ± 0.2 km, which agrees quite well with the depth to the top of the production zone determined from drilling. The vertical extent and length of the source region are estimated to be 11 ± 3 km and 9.9 ± 0.4 km, respectively. There appears to be geologic evidence for the presence of a fault or fault zone within the geothermal field that roughly coincides in location with the self‐potential source region. The conductivity on the east side of the production zone is estimated to be 80 percent of the value to the west, which is in general agreement with field resistivity measurements. Thermoelectric coupling coefficients measured in the laboratory on samples of reservoir rock are not large enough to explain the −340 ± 40 mV source intensity predicted by the model, possibly because the laboratory measurements were made at temperatures about 300°C lower than the reservoir value. These results do not rule out the possibility of a streaming potential source mechanism.
Technique for reflection prospecting in the Rub’ Al‐Khali
Authors: Robinson Don K. , Al‐Husseini Moujahed I.
Published: October 2002

The Rub’ Al‐Khali desert with its extensive and massive sand dunes presents difficult technical and logistic problems for reflection prospecting. The steep angle of repose of the sand dune faces and the low velocity of the air‐filled sand cause significant delays to the reflected waves. In the dunes, in order to maintain the fidelity of both the downgoing and upcoming waves, small dynamite source and receiver arrays are each contoured to a single elevation. In the flat sabkhas, larger arrays are maintained. After application of dune statics (which are derived initially from an average velocity for the sand and then refined with residual refraction statics), the small arrays are summed so as to simulate the larger arrays used on the flat sabkhas. When the terrain is dominated by dunes, increased use of small arrays reduces the maximum spread length to the detriment of the deeper reflections. Where this occurs, a second set of dynamite patterns is fired so as to increase the effective receiver arrays from 120 to 240. In this manner the simulated spread length is maintained at a minimum of 12,000 ft. The traces resulting from the array simulation on the dunes are then gathered, along with the sabkha traces, into mixed fold common‐depth‐point (CDP) gathers and processed conventionally. This technique results in the recording of high‐quality, broadband reflection profiles with improved continuity beneath the dunes.
Hole‐to‐surface resistivity measurements
Authors: Daniels Jeffrey J.
Published: October 2002

Hole‐to‐surface resistivity measurements over a layered volcanic tuff sequence illustrate procedures for gathering, reducing, and interpreting hole‐to‐surface resistivity data. The magnitude and direction of the total surface electric field resulting from a buried current source is calculated from orthogonal potential difference measurements for a grid of closely spaced stations. A contour map of these data provides a detailed map of the distribution of the electric field away from the drill hole. Resistivity anomalies can be enhanced by calculating the difference between apparent resistivities calculated from the total surface electric field and apparent resistivities for a layered earth model. Lateral discontinutities in the geoelectric section are verified by repeating the surface field measurments for current sources in several drill holes. A qualitative interpretation of the anomalous bodies within a layered earth can be made by using a three‐dimensional (3-D) resistivity model in a homogeneous half‐space. The general nature of resistive and conductive bodies causing anomalies away from the source drill holes is determined with the aid of data from several source holes, layered models, and 3-D models.
Autoregressive recovery of the acoustic impedance
Authors: Walker Colin , Ulrych Tad J.
Published: October 2002

This paper presents a method of recovering the acoustic impedance from reflection seismograms using autoregressive (AR) modeling, an approach originally applied to deconvolution by Lines and Clayton (1977). The algorithm which we describe is novel both in the manner in which the missing low‐ and high‐frequency information is predicted, and in the fact that the prediction may be constrained if acoustic impedance information is available. The prediction of the low frequencies treats the missing data as a gap which extends from the low‐frequency cut‐off in the negative frequency band to the corresponding frequency in the positive frequency band. The conjugate symmetry which governs the behavior of the spectrum in the band is taken into account in the prediction. The missing high frequencies are predicted using a modified minimum entropy norm in the frequency domain. Both synthetic and field examples are presented and illustrate the robustness of the new AR algorithm under a variety of conditions. The field example also compares the results obtained using the AR algorithm with the linear programming method of Oldenburg et al (1983). The agreement in results is particularly gratifying in view of the differences in the two inversion schemes.
Application of seismic reflection data to discriminate subsurface lithostratigraphy
Authors: Sinvhal Amita , Khattri Kailash
Published: October 2002

A correlation between lithology and quantitative parameters abstracted from seismic reflection data is established. The concept and methodology developed on synthetic data has been successfully applied to discriminate between two different kinds of lithologies. A particular hydrocarbon‐bearing formation in a sedimentary basin in Western India has been considered, part of which is dominantly sandy (lithological composition: sand = 53 percent, shale = 21 percent, coal = 26 percent) and another part which is dominantly shaly (sand = 37 percent, shale = 60 percent, coal = 3 percent). These two different lithologies are mathematically modeled using one‐step Markov chains. Their seismic responses when scrutinized in time and frequency domain and subjected to statistical discriminant analysis give a fair idea about synthetic subsurface lithostratigraphy. Seismic reflection data from the same area were considered for a similar analysis. On subjecting the data to discriminant analysis, it was again possible to discriminate between the two lithologies. Also, seismograms from different areas of the same basin could be assessed in terms of subsurface lithology. Seven seismic discriminators of subsurface lithostratigraphy have been identified, three of which are abstracted from the autocorrelation function and four from the power spectrum of the seismogram. This analysis is a potential tool for diagnosing subsurface lithology from seismic data and may ultimately help discriminate an oil‐bearing stratigraphic trap from its barren surroundings in a sedimentary basin.
The influence of out‐of‐plane surface properties on unmigrated time sections
Authors: Cohen Jack K. , Bleistein Norman
Published: October 2002

Both analytic and numerical means are used to demonstrate amplitude and phase behavior of reflections from three‐dimensionally curved interfaces. In particular, the high‐frequency geometrical optics approximation for the backward scattered wave field demonstrates that the phase of a reflection from a syncline is determined by the (in general, different) positions of focal regions for the two principal curves at the specular reflection point. Thus the amplitudes and phases of events observed on seismic sections are influenced as much by the shape of the reflector “out‐of‐the‐plane” as by the shape in the plane of the section. Results of the numerical examples suggest possible pitfalls in traditional interpretation of two‐dimensional seismic data.
Noise attenuation by Vibroseis whitening (VSW) processing
Authors: Coruh Cahit , Costain John K.
Published: October 2002

Reflection seismic data recorded on the Atlantic Coastal Plain to examine suspected faulting of the shallow (200 m) basement were contaminated by severe ground roll noise with an apparent velocity of 587 m/sec, 25 Hz dominant frequency, and 23 m apparent wavelength. A receiver interval of 5 m and a total spread length of 285 m were used to obtain 24‐fold coverage. Although the noise problem was severe, overlapping source and/or receiver arrays on the surface were not used for the recording geometry in order to avoid a decrease in resolution caused by the smoothing effect of overlapping subsurface coverage. Instead, the large‐amplitude surface waves were attenuated by a process called Vibroseis® whitening (VSW). VSW is based on the application of time‐varying amplitude scaling before Vibroseis crosscorrelation. A conventional method of automatic digital gain control was found to be effective for this purpose. This scaling results in a signal‐to‐noise ratio improvement equal to the gain expected from crosscorrelation. The scaling window length and the length of sweep are the only parameters required to define the signal‐to‐noise improvement for a given swept‐frequency band. A shorter scaling window and longer sweep length give better results because of the higher gain obtained by crosscorrelation. An increase in the quality of Atlantic Coastal Plain data clearly showed that VSW processing resulted in high resolution on stacked sections and made it possible to map shallow basement reflections at 0.2 sec, revealing faults with up to 10 m offset. Reflections from the sedimentary section above basement also became distinct.
Werner deconvolution for automated magnetic interpretation and its refinement using Marquardt’s inverse modeling
Authors: Ku Chao C. , Sharp John A.
Published: October 2002

We present a deconvolution for automated magnetic interpretation based on Werner’s (1953) simplified thin‐dike assumption which leads to the linearization of complex nonlinear magnetic problems. The usefulness of the method is expanded by the fact that the horizontal gradient of the total field caused by the edge of a thick interface body is equivalent to the total field from a thin dike. Statistical decision, numerical iteration, and a seven‐point operator are used to improve approximations of susceptibility, dip, depth, and horizontal location of the source. Marquardt’s nonlinear least‐squares method for inverse modeling is then used to refine automatically the first approximation provided by the deconvolution. Synthetic and real total‐field data are used to demonstrate the process.
Elliptical anisotropy—Its significance and meaning
Authors: Helbig K.
Published: October 2002

Nonspherical wavefronts in anisotropic media are often assumed to be approximately elliptical. However, in transversely isotropic media only the wavefront of SH waves is always an oblate ellipsoid. The wavefront of SV waves is never an ellipsoid, and the wavefront of P waves is an oblate ellipsoid if and only if the expression [Formula: see text] vanishes. This cannot happen if the anisotropy is due to lamellation (periodic layering with a spatial period small in comparison to the wave length). The occurrence of elliptical wavefronts cannot be detected on the basis of surface observations of times alone, since the complete wave field can be transformed into one with spherical wavefronts everywhere by simple stretching of layers. Neither arrival times nor apparent slownesses (and thus Snell’s law) are affected by this transformation. All concepts and algorithms applicable to spherical wavefronts are applicable to elliptical wavefronts, in particular the determination of a velocity as the zero‐offset limit of the stacking velocity. Since the resulting velocity is invariant under the stretching transformation, it can represent only a velocity that is itself an invariant, viz., the velocity at right angles to the direction of stretching. However, amplitude observations of SH waves give an independent indication of elliptical anisotropy. Although the wavefronts of P and SV waves can never be ellipsoids if the anisotropy is the result of lamellation, pieces of the wavefront can be represented with sufficient accuracy by an ellipsoid. This representation allows a simple determination of the ratio “zero‐offset limit of stacking velocity/vertical velocity.” Constraints on the parameters of the thin layers that constitute a lamellated medium can be translated into constraints of the above velocity ratio. For P waves this ratio is centered around unity for a wide range of constituent parameters.
IDB2—A Fortran program for computing extremal bounds in gravity data interpretation
Authors: Huestis Stephen P. , Ander Mark E.
Published: October 2002

It is well known that the interpretation of gravity anomaly data suffers from a fundamental nonuniqueness. No matter how complete a gravity data set may be, there are an unlimited number of subsurface density solutions compatible with it. This generally remains true even when the class of mathematically acceptable solutions is limited by imposed constraints based on physical or geologic arguments. The common practice constructing a single solution fitting, or only approximately fitting, the anomaly data is therefore of limited value.
Electrical conductance in a porous medium
Authors: Bussian A. E.
Published: October 2002

The Hanai‐Bruggeman equation is applied to a model of a conductive rock matrix immersed in water. The result is a general two‐component equation which relates the electrical properties of a formation to the electrical properties of the rock matrix and the water at any frequency. The low‐frequency limit is compared with previous theoretical and experimental work on sand formations. From a comparison of the three‐resistor model of Wyllie and Southwick, the Waxman‐Smits model, the dual water model of Clavier et al (1977) and the proposed model, it is found that the only representation which is viable, conceptually sound, and in agreement with experiment, is the last one.
Cross‐hole seismic surveys: Applications for studying subsurface fracture systems at a hot dry rock geothermal site
Authors: Fehler Michael , Pearson Chris
Published: October 2002

The use of cross hole seismic surveys for delineating the location and size of subsurface fracture systems is investigated. The radiation pattern for P and S waves emitted by a seismic source in a borehole is derived. Experimental work in relatively homogeneous granite suggests that the derived relationship adequately describes the radiation pattern for both explosive sources and acoustic transducers placed in fluid‐filled boreholes. Using the above functional expressions for the S‐ and P‐wave amplitudes, we have developed a technique to estimate Q and locate discrete fractures in crystalline rock that compose the Hot Dry Rock Geothermal Reservoir at Fenton Hill, New Mexico. To calculate Q, we measure the P‐ and S‐wave amplitudes as a function of distance from the source, compensate for the radiation pattern and geometrical spreading effects, and match the relative attenuation to a function of the form [Formula: see text] using a least‐squares regression technique. For undisturbed parts of the reservoir, we obtain values of Q on the order of 160 for P waves and from 170 to 150 for S waves. Using our method we are able to detect a decrease in the average Q due to extensive fracturing following heat extraction from the Fenton Hill reservoir. We have also been able to locate discrete large‐scale fractures in the reservoir by noting regions characterized by a sudden change in signal amplitude, wave form or frequency content that cannot be explained by radiation pattern effects or increases in source‐receiver separation. These localized attenuating regions in the Fenton Hill Hot Dry Rock reservoir are probably open fractures.
Diffraction of axisymmetric waves in a borehole by bed boundary discontinuities
Authors: Chew W. C. , Barone S. , Anderson B. , Hennessy C.
Published: October 2002

This paper presents the calculation of the diffraction of axisymmetric borehole waves by bed boundary discontinuities. The bed boundary is assumed to be horizontal and the inhomogeneities to be axially symmetric. In such a geometry, an axially symmetric source will produce only axially symmetric waves. Since the borehole is an open structure, the mode spectrum consists of a discrete part as well as a continuum. The scattering of a continuum of waves by bed boundaries is difficult to treat. The approach used in the past in treating this class of problem has been approximate in nature, or highly numerical, such as the finite‐element method. We present here a systematic way to approximate the continuum of modes by discrete modes. After discretization, the scattering problem can be treated simply. Since the approach is systematic, it allows derivation of the solution to any desired degree of accuracy in theory; but in practice, it is limited by the computational resources available. We also show that our approach is variational and satisfies both the reciprocity theorem and energy conservation.
Analysis of equivalence for the Schlumberger resistivity methods using the RHO-R and RHO-C curves in the resistivity transform domain
Authors: Murakami Yutaka
Published: October 2002

The problem of equivalence in the resistivity methods is studied in the resistivity transform domain. The RHO-R and RHO-C curves, which are, respectively, determined by the transverse resistance and longitudinal conductance, are shown to define the asymptotes of the resistivity transform curve. Models which give almost identical asymptotes also give almost identical resistivity transform curves. Thus, equivalent models can be found by obtaining models which do not change the shape of these asymptotes. For a given layered model, a plot of the RHO-R, RHO-C, and resistivity transform curves graphically shows the range of equivalence of the model. Equivalent models can be obtained by changing either resistivity, thickness, or depth of the given model through a simple graphical procedure.
On: “Geothermal prospecting with Shallo‐Temp Surveys” by L. A. LeSchack and J. E. Lewis (GEOPHYSICS, v. 48, p. 975–996, July, 1983).
Authors: Larson Timothy H. , Heigold Paul C.
Published: October 2002

LeSchack and Lewis discuss the use of the time lag method for determining the thermal diffusivity of a homogeneous and isotropic layer of earth material. Their expression for diffusivity is [Formula: see text] where [Formula: see text], x = thickness of layer (cm), P = period of the annual temperature wave (s) (assumed to be sinusoidal), and [Formula: see text].
Simulation of induction logging by the finite‐element method
Authors: Chang Shu‐Kong , Anderson Barbara
Published: October 2002

The response of an electromagnetic induction logging tool passing through many invaded thin beds is simulated by the finite‐element method. This simulation has achieved high accuracy by using a difference potential which enables the transmitter‐receiver mutual coupling to be treated analytically. Consequently the removal of the mutual coupling from the induction tool response has no numerical ill effects. The finite‐element model is truncated at a very large distance with a zero field outside the model. In order to achieve both accuracy and computational efficiency, the grid is projected to the truncation surface by gradually increasing its size according to an estimated error analysis of the finite‐element method. The numerical results were verified against analytical solutions for limiting cases and excellent agreement was obtained. In the presence of skin effect, which is beyond Doll’s analysis by geometrical factory theory, the finite‐element solution conveniently provides a way to check and improve the interpretation of induction logs. It also lends itself to future applications in tool design, signal processing, and resistivity inversion schemes.
Velocity analysis based on minimum entropy
Authors: De Vries D. , Berkhout A. J.
Published: October 2002

Seismic resolution is determined by the sparsity of reflection events together with the dispersion of the wavelets representing those events. In this paper, minimum entropy (ME) norms are introduced as a measure of spatial resolving power. It is shown that the lateral dispersion of inverted diffractor responses (inverted spatial wavelets) increases with increasing velocity error. Using this property, minimum entropy velocity analysis (MEVA) is proposed to extract velocity information from diffraction energy. MEVA can be successfully applied to zero‐offset (including poststack) data and common‐offset data with a sufficient amount of diffraction energy. In addition, MEVA can be used as an alternative to existing CMP velocity estimation techniques.
Traveltime inversion of offset vertical seismic profiles—A feasibility study
Authors: Lines L. R. , Bourgeois A. , Covey J. D.
Published: October 2002

Traveltimes from an offset vertical seismic profile (VSP) are used to estimate subsurface two‐dimensional dip by applying an iterative least‐squares inverse method. Tests on synthetic data demonstrate that inversion techniques are capable of estimating dips in the vicinity of a wellbore by using the traveltimes of the direct arrivals and the primary reflections. The inversion method involves a “layer stripping” approach in which the dips of the shallow layers are estimated before proceeding to estimate deeper dips. Examples demonstrate that the primary reflections become essential whenever the ratio of source offset to layer depth becomes small. Traveltime inversion also requires careful estimation of layer velocities and proper statics corrections. Aside from these difficulties and the ubiquitous nonuniqueness problem, the VSP traveltime inversion was able to produce a valid earth model for tests on a real data case.
Sound speed and attenuation measurements in gassy sediments in the Gulf of Mexico
Authors: Edrington Thomas S. , Calloway Terry M.
Published: October 2002

The speed and attenuation of sound in gassy marine sediments in the Mississippi delta area have been measured by means of small (1 to 500 mg) explosive charges buried 30 m below the mud line. The respective values obtained were 800 m/sec and 1.4 dB/kHz-m. Effective gas content was estimated to be approximately 0.06 percent.
New three‐point formulas for inductive coupling removal in induced polarization
Authors: Coggon J. H.
Published: October 2002

A simple method of separating inductive coupling effects from induced polarization (IP) effects in low‐frequency IP measurements was described by Hallof (1974). The basis of the method is that the polarization effect can be approximated as constant phase over a limited frequency range, while the inductive phase response varies as different powers of the frequency. Hallof used linear and quadratic frequency variations, to fit curves to phase measurements at two and three frequencies, respectively. Three‐point formulas for separation of a constant‐phase polarization effect from an inductive‐phase response varying as frequency and [Formula: see text] are now commonly applied to multifrequency IP measurements.
Causes of compressional‐wave anisotropy in carbonate‐bearing, deep‐sea sediments
Authors: Carlson R. L. , Schaftenaar C. H. , Moore R. P.
Published: October 2002

Forty indurated sediment samples from DSDP site 516 were studied with the principle objective of determining which of several proposed mechanisms is the cause of acoustic anisotropy in carbonate‐bearing deep‐sea sediments. Recovered from sub‐bottom depths between 388 and 1222 m, the samples have properties exhibiting the following ranges: wet‐bulk density, 1.90 to [Formula: see text]; fractional porosity, 0.46 to 0.14; carbonate content, 34 to 88 percent; compressional‐wave velocity (at 0.1 kbar), 1.87 to 4.87 km/sec; anisotropy, 1 to 13 percent. Velocities were measured in three mutually perpendicular directions through the same specimen in 29 of the 40 samples studied. Calcite fabric has been estimated by x‐ray pole figure goniometry. The major findings of this study are. (1) Carbonate‐bearing deep‐sea sediments may be regarded as transversely isotropic media with symmetry axes normal to bedding. (2) Calcite c‐axes are weakly concentrated in a direction perpendicular to bedding, but the preferred orientation of calcite does not contribute significantly to velocity anisotropy. (3) The properties of bedded and unbedded samples are distinctly different. Unbedded sediments exhibit low degrees of acoustic anisotropy (1 to 5 percent). By contrast, bedded samples show higher degrees of anisotropy (to 13 percent), and anisotropy increases markedly with depth of burial. Thus, bedding must be regarded as the principal cause of acoustic anisotropy in calcareous, deep‐sea sediments.
A computationally fast approach to maximum‐likelihood deconvolution
Authors: Chi Chong‐Yung , Mendel Jerry M. , Hampson Dan
Published: October 2002

In this paper we derive and implement a maximum‐likelihood deconvolution (MLD) algorithm, based on the same channel and statistical models used by Kormylo and Mendel (1983a), that leads to many fewer computations than their MLD algorithm. Both algorithms can simultaneously estimate a nonminimum phase wavelet and statistical parameters, detect locations of significant reflectors, and deconvolve the data. Our MLD algorithm is implemented by a two‐phase block component method (BCM). The phase‐1 block functions like a coarse adjustment of unknown quantities and provides a set of good initial conditions for the phase‐2 block, which functions like a fine adjustment of unknown quantities. We demonstrate good performance of our algorithm for both synthetic and real data.
Use of seismic stratigraphy for Minnelusa exploration, northeastern Wyoming
Authors: Raffalovich Reverend Francis D. , Daw Terrell B.
Published: October 2002

While Minnelusa sands have yielded significant reserves in Wyoming’s Powder River Basin, geologic complexities have made these sands an elusive target. This paper briefly describes the development of a technique which was used successfully in the exploration of Minnelusa sands. This tehnique can be applied to many stratigraphic exploration programs. Sonic logs, which are key logs in defining Minnelusa sands, in the C-H field were used to construct synthetic seismograms. These synthetics were then organized in cross‐section form to define whether a change in Minnelusa sands would yield an identifiable change on the synthetics. The “idealized” seismic response did show an obvious lateral change from upper sand to no upper sand conditions, and a pilot seismic line was shot using a Vibroseis® source. This line, which was shot through the C-H field, successfully showed the updip limits of the upper Minnelusa sands. A subsequent seismic program was acquired and other leads and prospects were identified, including prospects that were drilled and successfully completed in the Rozet area. However, a number of other wells conformed to Murphy’s law. In addition to standard processing techniques, high‐resolution processing and seismic attribute processing was done on some of the seismic data, yielding differing degrees of success. By closely coordinating geologic and geophysical principles, a useful stratigraphic‐seismic methodology was developed which has application to a wide variety of exploration problems. ™Trade and service mark of Conoco Inc.
The effect of the invaded zone on full wavetrain acoustic logging
Authors: Baker L. J.
Published: October 2002

Most theoretical studies of acoustic borehole logging have employed the simple model of a fluid borehole in an infinite solid. This work attempts to account for the invaded zone using a more sophisticated model that additionally includes finite concentric shells surrounding the borehole. By appropriately choosing the physical parameters of these cylindrical shells, one can study the effect of the invaded zone, mudcake, or a cased hole on acoustic wavetrain components. The model accounts for geometric attenuation, but it assumes the formation is perfectly elastic. A key consideration is the distance that the acoustic log “sees” into the formation. The body of this report is devoted to studying the effect of the invaded zone, mudcake, or steel casing on the components of the full wavetrain (P-wave, S-wave, reflected modes, and Stoneley mode). I conclude that a sonic logging tool has a very shallow depth of investigation. This depth, for P- and S-waves, depends upon the spacing between the source and receiver. An approximate rule of thumb is that if the source‐to‐receiver separation is n feet, the logging tool sees n inches into the formation. Thus, a conventional logging tool sees less than 6 inches into the surrounding formation. Since attenuation of the P- and S-wave arrivals places a practical constraint on realistic source‐to‐receiver separations, the depth of investigation is limited to a maximum of about 2 ft. Therefore, an acoustic logging tool is usually capable of measuring properties of the invaded zone only. For the reader interested in the mathematical details of this work, the method of computation is described in Appendix A.
On S-wave directivity patterns
Authors: Pilant Walter L.
Published: October 2002

Plane‐wave directivity patterns for both P- and S-waves approaching a free surface are well known (Knopoff et al., 1957, Figure 3–5). These have been shown to apply in a reciprocal manner to time‐harmonic S-waves emanating from vertical and horizontal sources (Miller and Pursey, 1954; Cherry, 1962) in both two‐dimensional (2-D) and three‐dimensional (3-D) cases. Knopoff and Gilbert (1959) showed that the plane‐wave directivity patterns also apply to the first motions seen in the impulsive‐source case (3-D) and Pilant (1979, sec. 9–6) showed that they held in the equivalent 2-D problem. Theoretical expressions for these patterns are given by Pilant (ibid) as [Formula: see text] and [Formula: see text] where [Formula: see text] is measured from the vertical and the positive z-axis is into the medium. The x-axis lies along the free surface and the quantity [Formula: see text]. For angles greater than critical [Formula: see text], the proper expression for the square root is given by [Formula: see text] Thus for angles of incidence (or take‐off) greater than [Formula: see text], both [Formula: see text] and [Formula: see text] become complex numbers and lead to phase‐shift induced waveform changes as the S-waves interact with the free surface. The functions [Formula: see text] and [Formula: see text] are shown in Figure 1 for the angular range 34–37 degrees which includes the angle [Formula: see text] degrees. For this example, [Formula: see text] corresponding to a Poisson’s ratio equal to one‐quarter. The null in [Formula: see text] and the maximum in [Formula: see text] are clearly seen.
Inapplicability of pulse train time‐domain measurements to spectral induced polarization
Authors: Soininen Heikki
Published: October 2002

The method of induced polarization (IP) is based on the frequency dependence of resistivity of rocks. In spectral IP the apparent resistivity is measured at a wide‐frequency band (e.g., 1/1024…4096 Hz). The apparent resistivity depends upon the distribution of the resistivity of the earth according to the laws of electromagnetism. On the basis of their spectral measurements Pelton et al. (1978) proposed that variations in mineral texture give rise to variations in the frequency spectra of resistivity. It should thus be feasible to use these spectra to discriminate between, say, graphite and sulfides. The frequency domain and the time domain are equivalent in a linear and causal system, the domains being interrelated through the Fourier transform. The time domain is attractive in that the whole transient can be recorded in a single measurement. Hence, there are devices in commercial use that record spectra in the time domain by measuring the voltage transient at a number of instances after the current pulse has been switched off. The primary current signal in these devices is generally a pulse train composed of pulses of finite duration. The pulse train has advantages over the single pulse because it permits the measurements to be repeated and thus improves the signal‐to‐noise (S/N) ratio of the measurements.
A time‐domain EM system measuring the step response of the ground
Authors: West G. F. , Macnae J. C. , Lamontagne Y.
Published: October 2002

A wide‐band time‐domain EM system, known as UTEM, which uses a large fixed transmitter and a moving receiver has been developed and used extensively in a variety of geologic environments. The essential characteristics that distinguish it from other systems are that its system function closely approximates a stepfunction response measurement and that it can measure both electric and magnetic fields. Measurement of step rather than impulse response simplifies interpretation of data amplitudes, and improves the detection of good conductors in the presence of poorer ones. Measurement of electric fields provides information about lateral conductivity contrasts somewhat similar to that obtained by the gradient array resistivity method.
Time‐domain electromagnetic soundings at the Nevada Test Site, Nevada
Authors: Frischknecht F. C. , Raab P. V.
Published: October 2002

Structural discontinuities and variations in the resistivity of near‐surface rocks often seriously distort dc resistivity and frequency‐domain electromagnetic (FDEM) depth sounding curves. Reliable interpretation of such curves using one‐dimensional (1-D) models is difficult or impossible. Short‐offset time‐domain electromagnetic (TDEM) sounding methods offer a number of advantages over other common geoelectrical sounding methods when working in laterally heterogeneous areas. In order to test the TDEM method in a geologically complex region, measurements were made on the east flank of Yucca Mountain at the Nevada Test Site (NTS). Coincident, offset coincident, single, and central loop configurations with square transmitting loops, either 305 or 152 m on a side, were used. Measured transient voltages were transformed into apparent resistivity values and then inverted in terms of 1-D models, Good fits to all of the offset coincident and single loop data were obtained using three‐layer models. In most of the area, two well‐defined interfaces were mapped, one which corresponds closely to a contact between stratigraphic units at a depth of about 400 m and another which corresponds to a transition from relatively unaltered to altered volcanic rocks at a depth of about 1000 m. In comparison with the results of a dipole‐dipole resistivity survey, the results of the TDEM survey emphasize changes in the geoelectrical section with depth. Nonetheless, discontinuities in the layering mapped with the TDEM method delineated major faults or fault zones along the survey traverse. Schlumberger resistivity soundings expanded to an AB/2 of 1220 m detect only the first interface mapped by the TDEM method. Schlumberger and large‐offset, frequency‐domain, sounding curves appear to be more distorted by lateral variations than the TDEM sounding curves.