bluuebunny/crossref_metadata_2025_split huggingface dataset:
COMBINED SWEEP SIGNALS FOR CORRELATION NOISE SUPPRESSION*
Authors: EDELMANN H. A. K. , WERNER H. 
Published: April 2006

AbstractComparison of both synthetic and field data shows that considerable suppression of correlation noise can be achieved with the Combisweep technique and with the Encoded Sweep Technique. In the first technique, the spectrum is shaped by superposition of linear sweeps with different frequency range; in the second technique, short sweeps of different polarity are combined to form the “alphabet” of a code.
OFFSET CONTINUATION FOR SEISMIC STACKING*
Authors: SALVADOR L. , SAVELLI S. 
Published: April 2006

AbstractWave equation migration techniques have shown the limits of traditional stacking methods with data from tectonically complicated areas. An improved stack can be obtained utilizing the dip‐moveout correction technique based on offset continuation. The properties and the limits of the algorithms used are summarized briefly.Several synthetic and real data examples are shown and compared with the results obtained using conventional processing in order to show the focusing effects and the strong improvement in signal‐to‐noise ratios, both at the stacked and migrated section level. The possibility of exploiting this technique to transform multiple coverage into increased spatial resolution is illustrated with examples.
USE OF THE SEISMIC DYNAMIC DECONVOLUTION ALGORITHM IN DIRECT RESISTIVITY INTERPRETATION*
Authors: SZARANIEC E. 
Published: April 2006

AbstractSeismic dynamic deconvolution is the mathematical basis on which a degree of unification in different prospecting methods is possible, relative to the parameter identification in horizontally stratified media. There is a basic structure which has some immediate applications to the inversion of resistivity data and possibly to other problems. For resistivity soundings there exists a key equation which is parallel to the energy conservation law in the theory of synthetic seismograms.
MAGNETOTELLURIC RESPONSE ON VERTICALLY INHOMOGENEOUS EARTH HAVING CONDUCTIVITY VARYING LINEARLY WITH DEPTH*
Authors: KAO D. 
Published: April 2006

AbstractMagnetotelluric response is studied for an inhomogeneous medium having conductivity varying linearly with depth as σ(z) =σ1+αz. For a medium having conductivity increasing linearly with depth, the phase of the impedance approaches 60° at long periods and the apparent resistivity becomes log (ρa) = 2 log (1.36/α1/3) — 1/3 log (T'). The asymptote of log (ρa, T'→∞) when plotted against log (T') has a constant gradient —1/3 and has an intercept on the log (T') axis, which equals 6 log (1.36/α1/3). When a homogeneous layer with a moderate thickness overlies an inhomogeneous half‐space, this layer does not affect the asymptote, but it affects the cut‐off period and pushes this toward the long period direction. For a medium having conductivity decreasing linearly with depth, the impedance is equivalent to that of a Cagniard two‐layer model; the intercept period related to the thickness is T'0=σ1(h2/2)2. Homogeneous multilayer approximations to an inhomogeneous layer are also investigated, and it is shown that the fit to the model variation depends on the number of layers and the layer parameters chosen.
A NUMERICAL METHOD TO COMPUTE THE RESISTIVITY TRANSFORM FROM WENNER SOUNDING DATA*
Authors: KUMAR RAKESH , CHOWDARY M. V. RAMANAIAH 
Published: April 2006

AbstractA numerical technique to compute the resistivity transform directly from the observed Wenner sounding data has been developed. In principle, the procedure is based on a decomposition method and consists of two steps: the first step determines a function that approximates the apparent resistivity data and the second step transforms this function into the corresponding kernel by an analytical operation. The proposed method is tested on some theoretical master curves. A high degree of precision is achieved with very little computer time. The applicability is shown on two field examples.
EFFICIENT 3‐D MIGRATION IN TWO STEPS*
Authors: GIBSON B. , LARNER K. , LEVIN S. 
Published: April 2006

AbstractThe addition of a third dimension to seismic data acquisition offers significant improvement of our reflection images of geologic structures, but potentially at a substantial increase in data processing cost. Here we study an approach to migration of three‐dimensional (3‐D) stacked data that can considerably reduce the burden of computation and data manipulation. Cascading two‐dimensional (2‐D) migrations in orthogonal lateral directions, we obtain an efficient 3‐D scheme that is exact for homogeneous media. As applied to examples of synthetic data from inhomogeneous media, the scheme introduces errors well below those attributable, in practice, to uncertainties in migration velocity. Application of this two‐step approach, using both Kirchhoff‐summation and finite‐difference algorithms, to a 3‐D seismic land survey over an area of known hydrocarbon traps additionally proves the importance of 3‐D migration in stratigraphic imaging.
A SIMPLE EXACT METHOD OF 3‐D MIGRATION—THEORY*
Authors: JAKUBOWICZ H. , LEVIN S. 
Published: April 2006

AbstractRecently, a heuristic and empirical analysis of a method of three‐dimensional (3‐D) migration has been presented by Gibson, Larner, and Levin. It consists of sets of two‐dimensional (2‐D) migrations in orthogonal directions. In this paper, we examine this two‐pass method using scalar wave theory and show its action to be identical to that of so‐called full 3‐D migration for constant‐velocity media. The demonstration of equivalence is carried out in both the frequency‐space and frequency‐wavenumber domains, the latter providing a particularly transparent view of the mechanism of the two‐pass approach. We then extend our analysis to consider how various approximate algorithms used in practical implementations affect the validity of our conclusion.
SIGNATURES FROM SINGLE AIRGUNS*
Authors: VAAGE S. , HAUGLAND K. , UTHEIM T. 
Published: April 2006

AbstractThe far‐field signatures from a comprehensive and systematic airgun pulse test have been analyzed. Empirical relations between the characteristic signature parameters and depth (5–12 m), pressure (100–137 bar = 10–13.7 MPa) and total chamber volume (0.65–9.5 l) have been derived. Also, the influence of using waveshape kits in different positions within the chamber has been tested.The results indicate that:

The amplitude is proportional to chamber pressure to the power 3/4.
The bubble period is nearly independent of the position of the waveshape plate.
The increase in primary/bubble amplitude ratio is inversely proportional to the chamber volume above the waveshape plate.
The amplitude is independent of airgun depth.

Suggestions and comments regarding this work from Dr B. Ursin and Dr A. Ziolkowski are appreciated. The field work was supported by the Norwegian Petroleum Directorate through the Continental Shelf Project at the Seismological Observatory, University of Bergen.An airgun allowing for continuous variation of the chamber volumes was supplied by GECO (Geophysical Company of Norway). The purchase of two airguns was financed by Norske Getty Exploration A/S.
ON THE APPLICATION OF INVERSE THEORY TO GRAVITY INTERPRETATION*
Authors: CHAVEZ R. E. , GARLAND G. D. 
Published: April 2006

AbstractNumerical investigations have been carried out on suitable interpretation schemes for gravity, based upon Backus‐Gilbert inverse theory. A two‐dimensional linear model consisting of horizontal prisms (grid shape) has been adopted for interpretation of gravity profiles. An inversion technique, which uses a linear approach suitable for rapid computation, and studies with synthetic bodies have led to improvements in the method, mainly by applying weighting factors, which improve the solution in the final stages of computational work. Gravity data from the well‐known Cyprus island positive anomaly were inverted to demonstrate the feasibility of the method.
REMOTE‐REFERENCE MAGNETOTELLURICS: EQUIPMENT AND PROCEDURES*
Authors: CLARKE J. , GAMBLE T. D. , GOUBAU W. M. , KOCH R. H. , MIRACKY R. F. 
Published: April 2006

AbstractDuring the past 3 years, major advances in the magnetotelluric technique have improved the quality of magnetotelluric data to the point where random errors in the impedance tensor and tipper are generally smaller than the uncertainty in their interpretation. The major factor in this improvement has been the introduction of the remote‐reference technique, although the use of ultrasensitive magnetometers and minicomputers for in‐field data processing has also been important. After a review of the remote‐reference technique, this paper describes the equipment and procedures used for remote‐reference magnetotellurics by the authors. Magnetometers using d.c. Superconducting Quantum Interference Devices typically have a sensitivity of 10−14 T Hz−1/2, a dynamic range of 107 in a 1 Hz bandwidth, and a slewing rate of 3 × 10−5.T s−1 at 10 kHz. The electric field measurements use conventional Cu‐CuSO4 electrodes. The remote magnetic reference signals are transmitted to the base station using FM analog telemetry. The data are collected and processed by a minicomputer based on an LSI‐11 microprocessor; the essential results—for example, the apparent resistivities and the tipper components, with their probable errors—are available in the field. Practical details are given of the handling of superconducting devices, low temperature cryostats and liquid helium in the field. Various spurious noise sources are mentioned, and techniques for minimizing their effects are described.
TEST RESULTS OF A NEW TYPE OF EFFICIENT SMALL AIRGUN ARRAY*
Authors: SAFAR M. H. 
Published: April 2006

AbstractRecently the author developed and demonstrated (Safar 1980) an efficient method for operating the airgun. The method involves the generation of a short seismic pulse from the pressure bubble pulses radiated by an airgun when fired several times at the same optimum depth but with different chamber pressures.The purpose of this paper is to present and discuss the test results obtained when implementing the same method using a two‐dimensional airgun array. The array consists of seven 0.65 liter airguns fired simultaneously at the same depth but with different chamber pressures.It is shown that the far‐field pressure pulse radiated by the seven 0.65 liter airgun array is similar to that radiated by the Flexichoc seismic source. It is concluded that the proposed airgun array can be used as a subarray to form an extremely powerful super‐long array suitable for deep seismic exploration.The author would like to thank the Chairman and Board of Directors of the British Petroleum Co. Ltd for permission to publish this paper. Thanks are also due to Mike Symes and Lovell Cox for carrying out the field tests and Seismograph Service (England) Ltd for providing the airguns.
RADIATION AND RECEIVER PATTERN OF SHEAR AND COMPRESSIONAL WAVES AS A FUNCTION OF POISSON'S RATIO*
Authors: KÄHLER S. , MEISSNER R. 
Published: April 2006

AbstractShear (= S) wave studies in addition to compressional (= P) wave surveys have revealed that S‐wave velocities are much more variable than P‐wave velocities. This strongly affects Poisson's ratio σ, especially in young sediments. It is shown that σ has a great influence on the directivity pattern, i.e., on the radiation and receiving characteristic of horizontal sources and receivers. For their calculation, well‐known surface boundary conditions and White's statement of the reciprocity relation (White 1965) are used. They also form the basis for the combination of source and receiver patterns essential for practical field work. The various combinations of horizontally and vertically arranged sources and receivers are investigated with regard to their directivity pattern, their amplitude ratio and, in general, to their usefulness in seismic prospecting.
THEORETICAL AND EXPERIMENTAL APPROACHES TO THE GEOPHONE SPURIOUS FREQUENCY*
Authors: CHEN SHI JUNG , DALTON CH. 
Published: April 2006

AbstractThe geophone spurious frequency is modeled as the resonance of the planar motion of a spider spring carrying a moving mass. An analytic solution is found using the Castigliano method by assuming that the spring arm is a single‐mode vibrating cantilever beam. The spring shape is found from this analysis. When the typical spring has a circumferentially varying cross‐section, the Castigliano method is no longer applicable. A dimensional analysis is used as an approximate method for general design. Based on the theoretical result, a rotational fixture and a translational fixture were designed for experimental purposes. A low‐noise fixture and a phase‐averaging technique provide the amplitude and spurious resonance in the frequency domain. Finally, a test is run by using a rotational fixture to compare with the approximate method of spurious frequency prediction. Very good agreement between prediction and experiment is found.
AUTOMATIC THREE‐LAYER, THREE‐DIMENSIONAL DECONVOLUTION OF THE PAYS DE BRAY ANTICLINE*
Authors: ALESSANDRELLO E. , BICHARA M. , LAKSHMANAN J. 
Published: April 2006

AbstractGravity data have been transformed into a three‐layer, three‐dimensional model by using an automatic procedure based on linear filtering. The Bouguer anomaly is first transformed by linear filtering into density variations located between two planes 1100 and 2500 m deep. These densities are then transformed into thicknesses with a constant density contrast of 0.4 g/cm3 with two geological constraints for the second and third interface:—minimum at 2500 m depth;—maximum below a variable limit given by geology.This gives the contact between the second and third layer. Differences between measured and computed gravity are then applied by a similar procedure to a layer located between depths of 0 and 500 m, giving the contact between the first and second layer. Interesting secondary anticlines and transverse faults are shown by various structural maps.
A BOREHOLE MAGNETOMETRIC RESISTIVITY EXPERIMENT*
Authors: ACOSTA J. E. , WORTHINGTON M. H. 
Published: April 2006

AbstractA borehole magnetometric resistivity (MMR) experiment is described in which an attempt is made to determine the extent and orientation of zones of fissuring within an otherwise massive sequence of carboniferous limestone. The region under investigation lies beneath a landfill site and the main parameter of interest is the direction of flow of ground water, which will be influenced by the orientation of the faults or fissures within the rock. The MMR method possesses some extremely advantageous features for application to hydro‐geological problems of this kind. The method is sensitive to electrical current channelling within an otherwise relatively resistive medium, and the detection of the magnetic field within the borehole does not depend upon electrical contact of the receiver with the ground. Consequently, the method can be used in dry or plastic‐cased boreholes. A direction of maximum electrical current flow is deduced from the MMR data which coincides with the predominant direction of jointing within the region.We are most grateful to the Water Research Centre, Medmenham, Berkshire, for supporting this research and particularly to Dr M. Fleet and Mr K. Beesley for their help and advice.
TUTORIAL: RESEARCH, APPLICATIONS, AND PUBLICATIONS IN ELECTRICAL AND ELECTROMAGNETIC METHODS*
Authors: PALACKY G. J. 
Published: April 2006

AbstractHow do papers on electrical and electromagnetic methods published in Geophysics and Geophysical Prospecting reflect the actual field application of the various techniques? Our study based on 10 volumes of the journals (1973–1982) suggests a rather good agreement. EM techniques are the most important both in the field and in print. However, an imbalance was detected in the subject matter. Too many published papers deal with theory, and too few with new instrumentation and techniques, case histories, and physical properties of rocks. Papers presented at annual meetings of the EAEG and SEG give a more even‐handed and accurate view of geophysical activities. Unfortunately, many of our colleagues around the world are not able to attend the annual meetings regularly and depend on journals for their information. Practicing and research geophysicists should submit more papers on subjects that at present are not adequately covered.
GEOELECTRICAL MODEL CALCULATIONS FOR TWO‐DIMENSIONAL RESISTIVITY DISTRIBUTIONS*
Authors: MUNDRY E. 
Published: April 2006

AbstractFor the calculation of geoelectrical model curves for a two‐dimensional resistivity distribution, the potential equation is transformed by means of a Fourier cosine transform into a two‐dimensional Helmholtz equation containing the separation parameter λ.The numerical solution of this equation for different values of λ for an irregular grid is obtained using the method of finite differences combined with the method of overrelaxation. The method by which derivatives are replaced by finite differences turned out to be very important, especially for high resistivity contrasts. After testing several methods designed to deal with any type of resistivity distribution, a method of discretization similar to that used by Brewitt—Taylor and Weaver (1976) for magnetotelluric modeling for H polarization was found the best.Examples are given of model curves for Schlumberger soundings over a vertical fault covered by overburden. The incorrect use of horizontal‐layer models leads to erroneous interpretations that are more complex than the real subsurface situations.
ON THE POSSIBILITY OF SEISMIC EXPLORATION USING SURFACE TORQUE SOURCE AND RELATED TOPICS*
Authors: KUHN M. , DRACH V. 
Published: April 2006

AbstractIn this study we derive expressions for particle displacement or particle velocity anywhere inside a stratified earth and at its surface due to horizontal torque source located in the top layer. Equivalently, invoking Green's function reciprocity theorem, the solution applies also to the case of a surface or subsurface source when the resulting displacement or velocity is measured within the top layer.In order to evaluate the closed‐form analytical solution economically and accurately it is advisable to introduce inelastic attenuation. Causal inelastic attenuation also lends the necessary realism to the computed seismic trace. To provide proof that the analytical solution is indeed correct and applicable to the multilayer case, a thick uniform overburden was assumed to consist of many thin layers. The correctness of the computed particle velocity response can be very simply verified by inspection. The computed response can also serve as a check on other less accurate methods of producing synthetic seismograms, such as the techniques of finite differences, finite elements, and various sophisticated ray‐tracing techniques.It is not difficult to construct horizontal surface torque source. It appears that such source is well suited for seismic exploration in areas with a high‐velocity surface layer. A realistic source function is analyzed in detail and normalized displacement response evaluated at different incidence angles in the near and the far fields.In an effort to distinguish the features of an SH torque seismogram from a pressure seismogram two models with identical layerings and layer parameters have been set up. As expected the torque seismogram is very different from the compressional seismogram. One desirable feature of a torque seismogram is the fast decay of multiples.Exact synthetic seismograms have many uses; some of them, such as the study of complex interference phenomena, phase change at wide angle reflection, channeling effects, dispersion (geometrical and material), absolute gain, and inelastic attenuation, can be carried out accurately and effortlessly. They can also be used to improve basic processing techniques such as deconvolution and velocity analysis.The numerical evaluation of the analytical solution of the wave equation as described in this paper has a long history. Most of the work leading to this paper was carried out by one of us (M. J. K.) in the years 1957 to 1968 at the Geophysical Research Corporation. However, the full testing of the various computer codes was carried out only very recently at the Phillips Petroleum Company.
L'EGALISATION SPECTRALE: UN MOYEN D'AMELIORER LA QUALITE DES DONNEES SISMIQUES*
Authors: COPPENS F. , MARI J. L. 
Published: April 2006

AbstractIn land seismic surveys spectrum equalization can increase the quality of seismic data in a selected frequency band. The power of lower frequencies in the spectrum of input traces is generally greater than that of higher frequencies, particularly in land seismic surveys because of ground roll. In order to improve the quality of seismic data it is necessary to raise the energy of higher frequencies to the same level as that of lower frequencies, without alteration of the phases.The first step of the method is to compute the amplitude spectrum of each input trace to determine a weighting function which is then applied to the amplitude spectrum in order to balance it. The function is the inverse of the short wavelength variation of the amplitude spectrum. The short wavelength variation can be obtained by interpolation between average values of the modulus of the amplitude spectrum computed in narrow bands within a selected band of frequencies. Another way of obtaining the short wavelength variation is to apply a low‐pass filter to the amplitude spectrum. The calculations are readily performed in the frequency domain by the Fourier transform.Spectrum equalization is automatically adjusted to each trace and does not modify the average amplitude in the time domain. However, as the frequency band and energy of the ground roll both vary according to the distance from the shot, spectrum equalization tends to make the spectrum of output traces independent of the offset distance.The use of spectrum equalization before any two‐dimensional filtering improves ground roll elimination. Continuity and resolution of horizons are also increased by spectrum equalization before CDP stack.Several examples of applications of spectrum equalization to seismic land and marine surveys are shown.
ELECTRICAL POTENTIAL DUE TO A POINT CURRENT SOURCE OVER AN INHOMOGENEOUS ANISOTROPIC EARTH*
Authors: PAL BARUN PROSAD , DASGUPTA S. P. 
Published: April 2006

AbstractA mathematical expression for potential of a direct current point source in an inhomoge‐neous anisotropic earth is derived. The coefficient of anisotropy is given by f= (σr/σz), where σr and σz are the conductivities parallel and perpendicular to the bedding plane. It is assumed that σz varies with depth, whereas σr varies transversely. This potential may be useful in interpretation of geoelectrical data in specified geological situations. Master curves for Wenner and Schlumberger configurations are presented