bluuebunny/crossref_metadata_2025_split huggingface dataset:
A METHOD TO DETERMINE THE VELOCITIES OF THE SEAFLOOR AND NEAR‐SURFACE SEDIMENTS*
Authors: BERGE A. M. , BESKOW B. 
Published: April 2006

ABSTRACTShotpoint gathers from conventional reflection seismic surveys contain both reflected and refracted waves. In this study shot records were processed and analyzed, and the data were modeled with reflected, refracted, and reflected‐refracted waves to fit the recorded data. The result is a detailed velocity model. The inverse problem for refracted waves was solved by using the Wiechert‐Herglotz inversion.A 500‐km‐long 26‐fold reflection seismic line from the Barents Sea, north of Norway, has been investigated. The data show high velocities, multiple reflections, and various types of noise. To test the method a total of 34 shot gathers were analyzed along this line. The aim of the interpretation was to determine the velocity in the seafloor and the near‐surface sediments. It is possible to map the vertical as well as the lateral velocity distribution in detail. Depending on the length of the streamer and the velocity gradient in the sediments, the calculated depth varies between 300 and 500 m below the seafloor. These velocities were also compared to the stacking velocities obtained from the reflection seismic data to see how the velocities determined by different methods were related.The velocity distribution in the sediments is one of the key factors in seismic interpretation. The technique discussed in this paper can contribute to velocity information both in the processing and interpretation of seismic data.
VERTICAL SEISMIC PROFILING IN COAL*
Authors: GREENHALGH S. A. , SUPRAJITNO M. 
Published: April 2006

AbstractThe intellection of seismic wave propagation in coal measures demands direct observation of the wavefield progression. Two vertical seismic profiles with high spatial and temporal sampling, were recently recorded in the Sydney Basin coalfields as part of an experimental coal seismic program.Static corrections and interval velocities were obtained by an automated system to determine first kicks and pulse rise times. Upgoing and downgoing waves were separated in the f—k‐plane using a novel technique of contour slice filtering. The isolated upgoing waves clearly display reflections from the major coal seams within the stratigraphic sequence. The downgoing wave spectra were subjected to attenuation analysis. The deduced specific quality factor Q for Permian coal measure rocks lies in the range 20–70. Similar estimates were obtained in the time domain from measurements of pulse broadening.Synthetic VSP seismograms, computed using an exact recursive formulation, are an indispensable aid to interpretation. They illustrate the filtering effects of coal seams and sequences, and the effects of the contribution of internal and free‐surface multiple reflections in the recorded wavetrains.
ESTIMATION OF MARINE SOURCE SIGNATURES FROM DIRECT ARRIVALS TO HYDROPHONE GROUPS*
Authors: KRAVIS S. P. 
Published: April 2006

AbstractDeterministic deconvolution of seismic data recorded with a non‐minimum phase source, such as a sparker, requires an estimate of the source signature. Present methods of deterministic signature estimation and deconvolution require additional field equipment (near‐ or far‐field hydrophones), or else make a deterministic estimate of the effective source signature (the free‐field source signature convolved with source and receiver ghosts). By analyzing the direct arrival signal to normal hydrophone groups it is shown that extraction of the free‐field source signature from this signal is possible for a spherically symmetric source, and it is demonstrated that the use of this signature for a preliminary deterministic deconvolution gives better results on sparker data than minimum‐phase whitening deconvolution applied on its own. The applicability of the method to non‐spherically symmetric sources, such as arrays, is also discussed.This paper is published with the permission of the Director, Bureau of Mineral Resources, Geology and Geophysics, Canberra, Australia.
INTERPRETATION AND DEPTH OF INVESTIGATION OF GRADIENT MEASUREMENTS IN DIRECT CURRENT GEOELECTRICS*
Authors: SCHULZ R. 
Published: April 2006

AbstractGradient measurements in a homogeneous electrical primary field can easily be interpreted for simple models. The simplified solution (conducting or resistant body in a homogeneous space in a homogeneous electrical field) is often sufficiently accurate, as comparisons with the exact solution (body of finite resistivity in a homogeneous half‐space in a quasihomogeneous electrical field) show. The exact geometry of the body cannot be determined by gradient measurements; the same anomaly of apparent resistivity can be caused by different bodies. In particular, the similarity between a sphere and a cube of the same volume is very high.There is a distinct influence of the resistivity of the overburden: the higher this resistivity is, the stronger is the effect caused by a buried body.If a deviation of 10% of the apparent resistivity is assumed as the lower boundary at which a buried body can be detected by gradient measurements, the depth of investigation for a three‐dimensional body is approximately equal to its width; in the two‐dimensional case the thickness of the overburden can be twice the width. If the overburden has a resistivity which is higher than the resistivity of the substratum, these depths are greater. The greatest possible depth is approximately three times the width of the body.
SIGNAL‐TO‐NOISE RATIO ENHANCEMENT IN SEISMIC MULTIFOLD DATA USING BAYESIAN STATISTICS*
Authors: WALTHAM D.A. , BOYCE J.F. 
Published: April 2006

ABSTRACTBayesian statistics are applied to the problem of signal‐to‐noise ratio enhancement from a common‐midpoint gather. By maximizing the a posteriori probability distribution of the gather with respect to the minimum‐offset trace and suppressing multiples via a semblance criterion, a statistically biased stack is formed with signal‐to‐noise ratio comparable to that of the usual stack while preserving the resolution and registration of the original noisy trace.Application of the algorithm to real data reveals geologically significant features which are indistinct in the standard stacked section.
EXPERIMENTAL EVIDENCE OF S*‐WAVE*
Authors: KIM J.Y. , BEHRENS J. 
Published: April 2006

ABSTRACTDuring development of theoretical methods to compute synthetic seismograms, a new type of wave called S*‐wave was discovered by Hron and Mikhailenko. This wave propagates with the shear‐wave velocity and can be interpreted as a non‐geometrical wave arrival with large amplitudes strongly depending on the depth of a pure P‐point source.In this first experimental verification of the existence of S*‐waves by means of two‐dimensional model‐seismics it is demonstrated that:1. the S*‐wave exists and depends on the source distance from the free surface; 2. the S*‐wave is generated as an ordinary shear wave on the free surface at the point located directly above the P‐source, as illustrated in the synthetic seismograms. The measured seismograms agree remarkably well with the computed ones.
RESISTIVITY AND IP MODELING OF THE THREE‐ARRAY DOWN‐HOLE PROSPECTING TECHNIQUE*
Authors: BUSBY J.P. , DABEK Z.K. 
Published: April 2006

ABSTRACTThe resistivity and induced polarization responses due to a number of IP targets for a down‐hole three‐array have been calculated numerically. The IP targets are approximated by triangle‐faceted polyhedra; number and size of the facets are limited by the numerical procedure. Despite this limitation a number of profiles have been obtained for a sphere, lens, prolate spheroid and thin dipping slab. The profiles obtained do not differentiate between the shapes of the IP targets, but depend upon the vertical extent of the target within the search‐radius of the borehole. The type curves produced could be of use for preliminary interpretation and in the field for planning the next stages of a geophysical survey.The lateral and vertical search radii of the borehole are increased by the use of such an array. For the bodies used it is estimated that the search radii are extended by 2.0 dipole units.
IN‐LOOP PULSE EM RESPONSE OF A STRATIFIED EARTH*
Authors: RAI S. S. , SARMA G. S. 
Published: April 2006

ABSTRACTThe in‐loop pulse electromagnetic response of a stratified earth has been expressed in terms of an apparent resistivity‐ time plot using the PEM response over a homogeneous half‐space which is typically unipolar with monotonic decay. This half‐space response characteristic provides a unique relationship between Crone PEM channel amplitude and the apparent half‐space resistivity. The possibility to resolve a thin intermediate conductive and resistive layer with the in‐loop PEM system has been investigated. The system is well in shallow geoelectric mapping.
RESISTIVITY SOUNDING ON A HORIZONTALLY STRATIFIED MULTI‐LAYERED EARTH*
Authors: RAGHUWANSHI S.S. , SINGH BIJENDRA 
Published: April 2006

ABSTRACTThe electrical potential due to a point source of current placed on the ground surface is studied for a multi‐layered earth consisting of homogeneous overburden of constant conductivity over a stack of transition layers where conductivity varies with depth according to power and exponential laws in even and odd layers, respectively. The general recursion relations are derived and are used to obtain expressions for the apparent resistivities for Schlumberger and Wenner electrode arrays. Their asymptotic behavior has been studied. The solutions for some particular cases are given: (i) odd layers with conductivities exponentially varying with depth while all even layers (and the first) have constant conductivities; (ii) even layer conductivities varying as a power law while odd number layers are of constant conductivity; and (iii) any two successive layers as transition layers and all others having constant conductivities. Further it is shown that Patella's theory is a particular case of the present study. It is concluded, therefore, that the present treatment is more general as all earlier models consisting of trasition layers can be derived from this study.
GEOELECTRIC AXIAL DIPOLE SOUNDING CURVES FOR A CLASS OF TWO‐DIMENSIONAL EARTH STRUCTURES*
Authors: PATELLA D. , TRAMACERE A. 
Published: April 2006

ABSTRACTWith the aim of studying the behaviour of geoelectric axial dipole vertical soundings over complex geology, a systematic theoretical approach is presented for a class of earth structures characterized by horizontal and vertical parallel boundary planes. The two‐dimensional cylindrical bodies of infinite length and rectangular cross‐section are constrained to have resistivities satisfying Alfano's condition at every intersection line of the graticule, in order to adopt the image‐point theory. A detailed analysis is performed for models with any number of horizontal boundaries and two vertical discontinuities. The apparent resistivity formulas are obtained and selected apparent resistivity curves are drawn for different parameter combinations and various directions of the sounding expansion axis. The class under consideration contains as a particular case the HVC model elaborated in Alpin's monograph, where only a small collection of master curves is available for the axial array. The reconstruction of those curves by the present formulation shows the existence of large discrepancies. A test based on the transformation to equivalent half‐Schlumberger sounding curves supports the conclusion that an unidentified error must exist in some part of the theoretical approach of the Russian researchers. Finally, some field sounding curves based on geothermal and volcanological surveys are presented and interpreted by complete curve matching, essentially to show the applicability of the theoretical solutions.
THE ABEL‐FOURIER METHOD OF HANKEL TRANSFORMATION: APPLICATIONS TO SEISMIC DATA*
Authors: WILSON C.R. 
Published: April 2006

AbstractThe formal equivalency between a Hankel transform and an Abel transform followed by a Fourier transform is used as a basis for the computation of synthetic seismograms and for performing plane‐wave decomposition of both synthetic and field seismograms. The Abel‐Fourier method performs better for near‐offset or small ray‐parameter, although at greater computational effort than calculations based upon the asymptotic Hankel transform.
THEORETICAL TRANSIENT EM RESPONSE CURVES OVER A THIN DIPPING DYKE IN FREE SPACE–SEPARATED INLINE LOOP CONFIGURATION*
Authors: OGILVY R.D. 
Published: April 2006

ABSTRACTTheoretically exact type curves are presented to show the effect on response shape of changing dip, strike, transmitter‐receiver separation, depth of burial and target conductance. Relatively small dip variations produce a marked asymmetry in response shape. For steeply dipping conductors, strike can be determined from the cross‐over width of the anomaly. The curves illustrate the dynamic character of the transient process, and suggest how the spatial disposition and quality of large sheet conductors might be inferred from the extent of peak‐amplitude displacement with time, and rate of eddy‐current diffusion. A generalized interpretation scheme is proposed based on dimensionless response characteristics which will facilitate the rapid determination of conductor dip, conductance and depth of burial for any time regime.
ON REVERSE‐TIME MIGRATION*
Authors: GAZDAG J. , CARRIZO E. 
Published: April 2006

ABSTRACTMigration is a process whereby events in ‘image space’ are mapped into their correct positions in ‘object space’. The wave equations associated with this mapping may be defined and solved numerically either in image space or in object space. In the former the CMP section, which represents the initial conditions, is extrapolated toward increasing depths, and the migrated data are recovered at zero time. In the latter, the wave‐field extrapolation takes place in the coordinate frame of the depth section, and the CMP data serve as boundary conditions at the surface. Computations begin at the last sample of the record section and continue ‘reverse time’ until time zero.This paper describes a reverse‐time migration (RTM) method and compares its performance with that of an image‐space method based on the idea of phase shift plus interpolation (PSPI). Synthetic zero‐offset sections serve as examples for migration experiments with the RTM and PSPI methods. It is shown that the RTM approach to migration is rather expensive, but its robustness and accuracy are difficult to surpass.
ON THE POSSIBILITY OF THE TELLURIC METHOD: SOME RESULTS ON FAULTED STRUCTURES*
Authors: ILICETO V. , SANTARATO G. 
Published: April 2006

ABSTRACTThe telluric method of geophysical investigation is re‐evaluated and several improvements are suggested. Besides the traditional Jacobian, several new parameters that bear information on the structure under investigation are proposed as sounding tools. These parameters are calculated for several two‐dimensional models and plotted as pseudo‐sections.The model results, calculated over a wide frequency band (10‐4‐10‐2 Hz) suggest that these new parameters can give very detailed information about the subsurface structures.By way of example, we present experimental results obtained along two profiles across familiar structures. The interpretation of the results leads to two‐dimensional models of the subsurface that are in good agreement with known geophysical and geological features of these two sections.
CONSIDERATIONS IN ESTIMATING THE MINIMUM‐PHASE PROPERTIES OF SAMPLED DATA*
Authors: ELLENDER S.A. 
Published: April 2006

ABSTRACTAnalysis of the phase spectra from the signatures, impulse responses and other wavelets observed in seismic data leads to the construction of equivalent minimum‐phase functions. The accuracy of such computations using digitally sampled data is questioned with special reference to Texas Instruments DFS IV and DFS V recording filters. Results vary with the lengths and sample rates of the time functions, and further errors may be introduced when implementing the Hilbert transform. Such problems are related to poor resolution in the low amplitude areas of the spectrum. Techniques for correction are described. With appropriate shaping a reasonably accurate phase spectrum may be computed for the minimum‐phase function. The generation of minimum‐phase wavelets within the processing sequence is briefly discussed.
COMPLETE INVERSION OF ZERO‐OFFSET SEISMIC DATA*
Authors: URSIN B. 
Published: April 2006

ABSTRACTThe one‐dimensional seismic inverse problem consists of recovering the acoustic impedance (or reflectivity function) as a function of traveltime from the reflection response of a horizontally layered medium excited by a plane‐wave impulsive source. Most seismic sources behave like point sources, and the data must be corrected for geometrical spreading before the inversion procedure is applied. This correction is usually not exact because the geometrical spreading is different for primary and multiple reflections.An improved algorithm is proposed which takes the geometrical spreading from a point source into account. The zero‐offset reflection response from a stack of homogeneous layers of variable thickness is used to compute the thickness, velocity and density of each layer. This is possible because the geometrical spreading contains additional information about the velocities.
RESISTIVITY AND INDUCED POLARIZATION RESPONSES OF ARBITRARILY SHAPED 3‐D BODIES IN A TWO‐LAYERED EARTH*
Authors: DAS U.C. , PARASNIS D.S. 
Published: April 2006

ABSTRACTNumerical computations using the integral equation method are presented for resistivity and IP responses due to arbitrarily shaped 3‐dimensional bodies in a layered earth. The unknown surface charge density distribution is expressed as the solution of Fredholm's integral equation of the second kind. Use of moment method (with pulse basis function and point‐collocation) yields the matrix equations for the unknowns. The contributions to Green's function are solved (a) analytically for the primary and (b) by convolution for the secondary contributions resulting in a fast algorithm. The further step of computing potential, apparent resistivity, chargeability etc., for any electrode system, is straightforward. Our results show a good agreement with those from finite difference methods and physical tank experiments. The CPU time is only 138 s on a super‐minicomputer for an apparent resistivity pseudo‐section, even with 96 elementary cells as used for discretization. A large number of models for different geological situations were studied; some are presented here.
FREQUENCY ELECTROMAGNETIC SOUNDING USING A VERTICAL MAGNETIC DIPOLE*
Authors: MUNDRY E. , BLOHM E.‐K. 
Published: April 2006

ABSTRACTA horizontal transmitter loop (vertical magnetic dipole) is used for frequency electromagnetic (FEM) soundings. The frequency ranges from approximately 6 Hz to about 4000 Hz. The vertical and radial magnetic field components are measured for 20 frequencies per decade several hundred meters from the transmitter loop. A very small bandwidth is selected for amplification using a reference signal. An Apple computer is used for data acquisition.A computer program for Marquardt inversion optimizes the parameters for the n‐layer case: the resistivities and thicknesses of individual beds and a correction factor for the primary magnetic field. Interpretation of each component individually yields practically the same parameters. Examples from the field are given with interpretation; comparison with dc resistivity measurements is provided.The ratio of vertical/radial magnetic field components vs. frequency can be transformed simply into apparent resistivity vs. apparent depth. This can be done in the field to obtain an estimation of the depth of the layer boundaries. FEM results are compared with Schlumberger d.c. sounding obtained at the same site.
THE GENERALIZED SPECTRAL FUNCTION OF A PLANE LAYERED MEDIUM*
Authors: FERBER R.‐G. 
Published: April 2006

ABSTRACTThe spectral function of a plane layered medium, which represents the net downgoing energy in the first layer due to a normally incident impulsive plane wave, plays an important role in the solution of the one‐dimensional inverse problem in reflection seismology. Hitherto the extension to non‐normal incidence was known only for a medium with free surface. By giving the extension for arbitrary surface reflection coefficients, this paper fills a gap.
EVANESCENT EFFECTS IN ACOUSTICAL WAVE PROPAGATION*
Authors: KUHN M. J. 
Published: April 2006

ABSTRACTThe propagation of transient acoustic pressure waves in a layer enclosed between two, not necessarily identical, half‐spaces is considered. The source and the receivers are always located in the same half‐space and at the same depth. The source excitation function is a narrow causal spike. Several thicknesses of the layer are examined including the case in which the embedded layer vanishes.The phenomena of ‘constricted’ head waves and wide‐angle reflections in the layer are examined in detail using a ‘numerical experimentation’ approach. First, a closed‐form solution is numerically evaluated. Then this solution is developed in series and each term is evaluated separately using the same numerical techniques. When the contribution of an individual high‐order term becomes unimportant, all higher order terms are discarded, and the response is constructed by superposition of the previously computed low‐order terms only.Propagation by wide‐angle reflections from inside the layer is of interest. When the thickness of the layer is reduced to a fraction of the wavelength, these events consist typically of a low amplitude, high frequency, geometrical acoustics arrival, followed by higher amplitude, low frequency, non‐geometrical coda. When all important low‐order terms are added, the non‐geometrical events tend to interfere destructively, leaving a waveform nearly identical to that obtained by integration of the closed‐form solution.When the thickness of the embedded layer is measured in fractions of the dominant wavelength, none of the individual terms of the series development can be duplicated by asymptotic ray tracing. However, because the codas of the various terms interfere destructively, the total response may be well‐represented by the addition of a few low‐order rays, using asymptotic approximation. This discovery extends the usefulness of Huygens‐Kirchhoff ray tracing to modeling of wave propagation in thin layers.
ANALYSIS AND IMPROVEMENT OF THE PRIMARY FIELD WAVEFORM OF THE GROUND TEM METHOD*
Authors: QIAN BIN 
Published: April 2006

ABSTRACTMany ground TEM systems use an on‐off type of primary field waveform and measure the secondary field only in the transmitter off‐time, so as to realize the pure secondary field measurement. The width of the trailing edge of the primary pulses and the duration of the transmitter on‐time form a time‐constant (Tc) window. Most effective excitation is achieved if the target conductor has a Tc falling into this window.To be able to measure in the transmitter on‐time, two primary field waveforms are proposed. The first is a series of unipolar pulses with identical steep ramps at both leading and trailing edges. Measurements made in the on‐time after the leading edges simply reduce the stacking time needed and power consumption by half. Also, long‐ Tc conductors are better excited. The second is a series of bipolar pulses with long ramps as the leading edges and short ramps as the trailing edges. This waveform presents two different Tc‐windows at the same time and helps to improve the detection of long‐Tc conductors in the presence of short‐ Tc ones.