bluuebunny/crossref_metadata_2025_split huggingface dataset:
STRUCTURE OF THE SOUTHWESTERN FRASER RIVER DELTA AS DETERMINED FROM GEOELECTRIC SOUNDING1
Authors: NOBES D.C. , HAMILTON T. S. , CARTWRIGHT P. 
Published: April 2006

AbstractGeoelectrical sounding profiles were collected on the southern part of the Fraser River delta, to provide a geophysical estimate of the subsurface structure and geotechnical properties. The differences between emergent and intertidal areas were assessed, and the geoelectric technique was found to be a viable one in an intermittently exposed tidal‐flat environment. The subsurface geoelectric structure provides a link between reflection seismic data sets for Georgia Strait and the lower mainland. The survey was intentionally designed to complement the conventional exploration information for this basin and the shallow high‐resolution seismic and drilling which focused on the unconsolidated Quaternary section. The electrical models consist of three layers: (I) electrically‐conductive, porous, saturated and under‐saturated marine silts, sands and gravels, overlying (II) less conductive and more consolidated marine clays, and variably reworked glaciomarine deposits together with weathered clastic sedimentary bedrock, which in turn overlies (III) less porous, more resistive, relatively unweathered bedrock. Estimates of thickness and geotechnical properties are obtained for shallow layers which are not available from either the short boreholes or shallow high‐resolution seismic lines. This information is particularly useful in appraising the liquefaction potential of the unconsolidated layers due to earthquake risk.
3D ACOUSTIC PRESTACK REVERSE‐TIME MIGRATION1
Authors: CHANG WEN‐FONG , McMECHAN GEORGE A. 
Published: April 2006

AbstractA prestack reverse‐time migration algorithm which operates on common‐source gathers, recorded at the Earth's surface, from 3D structures, is conceived, implemented and tested. Reverse‐time extrapolation of the recorded wavefield (a boundary‐value problem), and computation of the excitation‐time imaging condition for each point in a 3D volume (an initial‐value problem), are both performed using a second‐order finite‐difference solution of the full 3D scalar wave equation. The algorithm is illustrated by processing synthetic data for a point diffractor, an oblique wedge, and the French double dome and fault model.
TRACE INTERPOLATION BY SLANT‐STACK MIGRATION1
Authors: NOVOTNÝ M. 
Published: April 2006

AbstractThe slant‐stack migration formula based on the Radon transform is studied with respect to the depth step Δz of wavefield extrapolation. It can be viewed as a generalized trace‐interpolation procedure including wave extrapolation with an arbitrary step Δz. For Δz= 0 the formula yields the familiar plane‐wave decomposition, while for Δz > 0 it provides a robust tool for migration transformation of spatially undersampled wavefields. Using the stationary phase method, it is shown that the slant‐stack migration formula degenerates into the Rayleigh‐Sommerfeld integral in the far‐field approximation. Consequently, even a narrow slant‐stack gather applied before the diffraction stack can significantly improve the representation of noisy data in the wavefield extrapolation process. The theory is applied to synthetic and field data to perform trace interpolation and dip reject filtration. The data examples presented prove that the Radon interpolator works well in the dip range, including waves with mutual stepouts smaller than half the dominant period.
TRANSFORMATION OF LINE‐SOURCE RESISTIVITY DATA TO POINT‐SOURCE DATA AND VICE VERSA USING THE MATRIX METHOD1
Authors: NIWAS SRI 
Published: April 2006

AbstractInvestigations have shown the existence of a linear relationship between point‐source resistivity data and line‐source resistivity data through a matrix operator, which paves the way for the efficient transformation of line‐source data to the corresponding point‐source data and vice versa. The power of these equations has been established by computational examples. The relationship will be useful in the modelling and inversion of resistivity data from 2D structures.
GEOMETRIC CORRECTIONS IN ATTENUATION MEASUREMENTS1
Authors: KLIMENTOS THEODOROS 
Published: April 2006

AbstractSeismic wave attenuation in porous rocks consists of intrinsic or anelastic attenuation (the lost energy is converted into heat due to interaction between the waves and the rocks) and the extrinsic or geometric attenuation (the energy is lost due to beam spreading, transmission loss and scattering). The first is of great importance because it can give additional information on the petrophysical properties of rocks (permeability, degree of saturation, type of saturant, etc.). The most difficult problem in attenuation measurements is estimating or eliminating extrinsic attenuation, so that the intrinsic attenuation can be obtained. To date, in laboratory attenuation measurements using wave propagation, several methods have been used. The difficulties vary with the method. The coupling effect and the geometric divergence or beam spreading are the major problems.Papadakis’ diffraction corrections have been used extensively by Winkler and Plona in their modified pulse‐echo high‐pressure attenuation measurements. These corrections are computed for homogeneous liquid media and their failure to fit data for solid material implies that these corrections must be used with caution, especially for high Q values.Three new methods for laboratory ultrasonic attenuation measurements are presented. The first is the ‘ultrasonic lens’ method for attenuation measurements at atmospheric pressure, in which an ultrasonic lens placed between transmitter and sample transforms the initially oblique incident beam into normal incidence so that the geometric divergence is eliminated. The second method is the ‘panoramic receiver’, in which the beam spreading can be eliminated by integrating the ultrasonic energy over a large area. The third method is called 'self‐spectral ratio’ and is applicable for all pressure conditions. Attenuation is estimated by comparing two signals recorded on the same rock but with two slightly different thicknesses under the same pressure conditions. Hence the extrinsic attenuation for both thicknesses is approximately the same. A comparison between the self‐spectral ratio method and that of Winkler and Plona demonstrates a very good agreement for a broad band of frequencies. Hence the Winkler‐Plona technique and Papadakis’ diffraction corrections can be accepted as reliable in any future work.
LAYERED PERMEABLE SYSTEMS1
Authors: SCHOENBERG MICHAEL 
Published: April 2006

AbstractPermeability is a second rank tensor relating flow rate to pressure gradient in a porous medium. If the permeability is a constant times the identity tensor the permeable medium is isotropic; otherwise it is anisotropic. A formalism is presented for the simple calculation of the permeability tensor of a heterogeneous layered system composed of interleaved thin layers of several permeable constituent porous media in the static limit. Corresponding to any cumulative thickness H of a constituent is an element consisting of scalar H and a matrix which is H times a hybrid matrix function of permeability. The calculation of the properties of a medium equivalent to the combination of permeable constituents may then be accomplished by simple addition of the corresponding scalar/matrix elements. Subtraction of an element removes a permeable constituent, providing the means to decompose a permeable medium into many possible sets of permeable constituents, all of which have the same flow properties. A set of layers of a constituent medium in the heterogeneous layered system with permeability of the order of 1/h as h→ 0, where h is that constituent's concentration, acts as a set of infinitely thin channels and is a model for a set of parallel cracks or fractures. Conversely, a set of layers of a given constituent with permeability of the order of h as h→ 0 acts as a set of parallel flow barriers and models a set of parallel, relatively impermeable, interfaces, such as shale stringers or some faults. Both sets of channels and sets of barriers are defined explicitly by scalar/matrix elements for which the scalar and three of the four sub‐matrices vanish. Further, non‐parallel sets of channels or barriers can be ‘added’ and 'subtracted’ from a background homogeneous anisotropic medium commutatively and associatively, but not non‐parallel sets of channels and barriers reflecting the physical reality that fractures that penetrate barriers will give a different flow behaviour from barriers that block channels. This analysis of layered media, and the representations of the phenomena that can occur as the thickness of a constituent is allowed to approach zero, are applicable directly to layered heat conductors, layered electrostatic conductors and layered dielectrics.
ADAPTIVE PICKING OF REFRACTED FIRST ARRIVALS1
Authors: SPAGNOLINI UMBERTO 
Published: April 2006

AbstractStatic correction computations require knowledge of the refracted traveltimes. Zero‐phase wavelet sources cannot be picked reliably when incoherent picking techniques are used.Assuming a complex convolutional model for Vibroseis, a coherent picking technique based on the matched filter is described. In order to match the filter to the first arrival wavelet an adaptive algorithm is used. This allows the filter to change both with shot and offset so that all the properties of matched filtering such as improvement of S/N and resolution can be exploited. Incoherent picking is used before coherent picking to improve the convergence of adaptive picking.
A FIRST‐ORDER CORRECTION TO THE THEORY FOR THE ELECTROMAGNETIC RESPONSE OF A THIN CONDUCTING SHEET1
Authors: HURLEY D. G. , SIEW P. F. 
Published: April 2006

AbstractThe eddy currents induced in a thin conducting sheet by a sinusoidally varying primary magnetic field are investigated in the low‐frequency limit when the depth of penetration of the primary field is much greater than the thickness of the sheet. A perturbation scheme in terms of the small parameter, δ, which is the ratio of the thickness of the sheet to the length scale of the primary field is initiated. The zero‐order terms in δ give the familiar results that were first obtained by Maxwell. The first‐order terms in δ depend on the tangential as well as the normal component of the primary field and give eddy‐current distributions that vary quadratically across the sheet. The boundary conditions that determine the accompanying secondary magnetic field are derived. Detailed results are given when the primary field is that due to a dipole.
A SIMULATED ANNEALING APPROACH TO SEISMIC MODEL OPTIMIZATION WITH SPARSE PRIOR INFORMATION1
Authors: MOSEGAARD KLAUS , VESTERGAARD PETER D. 
Published: April 2006

AbstractIt is well known that seismic inversion based on local model optimization methods, such as iterative use of linear optimization, may fail when prior information is sparse. Where the seismic events corresponding to reflectors of interest remain to be identified, a global optimization technique is required.We investigate the use of a global, stochastic optimization method, that of simulated annealing, to solve the seismic trace inversion problem, in which the two‐way traveltimes and reflection coefficients are to be determined. The simulated annealing method is based on an analogy between the model‐algorithm system and a statistical mechanical system. We exploit this analogy to produce improved annealing schedules. It is shown that even in cases of virtually no prior information about two‐way traveltimes and reflection coefficients, the method is capable of producing reliable results.
DETECTION OF AN AIR‐FILLED DRAINAGE GALLERY BY THE VLF RESISTIVITY METHOD1
Authors: OGILVY R. D. , CUADRA A. , JACKSON P. D. , MONTE J. L. 
Published: April 2006

AbstractVLF wave impedance measurements have been successfully used to detect air‐filled drainage galleries near the town of Alcala, Spain. The galleries are detectable by H‐polarization electric field measurements due to the electric field anomalies associated with the galleries and overlying gravel deposits. The forced deviation of the primary current flowlines around the 2D void results in a higher‐than‐normal apparent resistivity and a relative phase low above the gallery.The findings support earlier theoretical predictions that at very low frequencies (VLFs), galvanic current effects may dominate over vortex currents in moderately conductive terrains. Theoretical modelling confirmed that for a resistive target no detectable E‐polarization response can be expected from either magnetic or electric field measurements since current line deviations and vortex effects are negligible under such circumstances.The results demonstrate the importance of using at least two orthogonal VLF transmitters in order that anomalies arising from both galvanic and inductive effects may be identified, irrespective of orientation.
THE INSENSITIVITY OF REFLECTED SH WAVES TO ANISOTROPY IN AN UNDERLYING LAYERED MEDIUM1
Authors: SCHOENBERG MICHAEL , COSTA JESSE 
Published: April 2006

AbstractPropagation in the plane of mirror symmetry of a monoclinic medium, with displacement normal to the plane, is the most general circumstance in anisotropic media for which pure shear‐wave propagation can occur at all angles. Because the pure shear mode is uncoupled from the other two modes, its slowness surface in the plane is an ellipse. When the mirror symmetry plane is vertical the pure shear waves in this plane are SH waves and the elliptical SH sheet of the slowness surface is, in general, tilted with respect to the vertical axis. Consider a half‐space of such a monoclinic medium, called medium M, overlain by a half‐space of isotropic medium I with plane SH waves incident on medium M propagating in the vertical symmetry plane of M. Contrary to the appearance of a lack of symmetry about the vertical axis due to the tilt of the SH‐wave slowness ellipse, the reflection and transmission coefficients are symmetrical functions of the angle of incidence, and further, there exists an isotropic medium E with uniquely determined density and shear speed which gives exactly the same reflection and transmission coefficients underlying medium J as does monoclinic medium M. This means that the underlying monoclinic medium M can be replaced by isotropic medium E without changing the reflection and transmission coefficients for all values of the angle of incidence. Thus no set of SH seismic experiments performed in the isotropic medium in the symmetry plane of the underlying half‐space can reveal anything about the monoclinic anisotropy of that underlying half‐space. Moreover, even when the underlying monoclinic half‐space is stratified, there exists a stratified isotropic half‐space that gives the identical reflection coefficient as the stratified monoclinic half‐space for all angles of incidence and all frequencies.
MULTICOMPONENT COMMON‐RECEIVER GATHER MIGRATION OF SINGLE‐LEVEL WALK‐AWAY SEISMIC PROFILES1
Authors: JACKSON G. M. , MASON I. M. , LEE DELMAN 
Published: April 2006

AbstractSeismic data are usually separated into P‐waves and S‐waves before being put through a scalar (acoustic) migration. The relationship between polarization and moveout is exploited to design filters that extract the desired wavetype. While these filters can always be applied to shot records, they can only be applied to a triaxial common‐receiver gather in special cases since the moveout of scattered energy on the receiver gather relates to path differences between the surface shots and the scatterer while the polarization is determined by the path from scatterer to downhole geophone. Without the ability to separate wavefields before migration, a ‘vector scalar’ or an elastic migration becomes a necessity.Here the propagation of the elastic wavefield for a given mode (e.g. P‐S) is approximated by two scalar (acoustic) propagation steps in a ‘vector scalar’ migration. ‘Vector’ in that multicomponent data is migrated and 'scalar’ in that each propagation step is based on a scalar wave equation for the appropriate mode. It is assumed that interaction between the wavefields occurs only once in the far‐field of both the source and receiver. Extraction of the P, SV and SH wavefields can be achieved within the depth migration (if one assumes isotropy in the neighbourhood of the downhole receiver) by a projection onto the polarization for the desired mode. Since the polarization of scattered energy is only a function of scatterer position and receiver position (and not source position), the projection may be taken outside the migration integral in the special case of the depth migration of a common‐receiver gather. The extraction of the desired mode is then performed for each depth migration bin after the separate scalar migration of each receiver gather component.This multicomponent migration of triaxial receiver gathers is conveniently implemented with a hybrid split‐step Fourier‐excitation‐time imaging condition depth migration. The raytracing to get the excitation‐time imaging condition also provides the expected polarization for the post‐migration projection. The same downward extrapolated wavefield can be used for both the P‐P and P‐S migrations, providing a flexible and efficient route to the migration of multicomponent data.The technique is illustrated on a synthetic example and a single‐level Walk‐away Seismic Profile (WSP) from the southern North Sea. The field data produced images showing a P‐P reflector below the geophone and localized P‐P and P‐S scatterers at the level of the geo‐phone. These scatterers, which lie outside the zone of specular illumination, are interpreted as faults in the base Zechstein/top Rotliegendes interface.
PARAMETRIZATION OF GRT INVERSION FOR ACOUSTIC AND P–P SCATTERING1
Authors: IKELLE L. T. , KITCHENSIDE P. W. , SCHULTZ P. S. 
Published: April 2006

AbstractThe generalized Radon transform (GRT) inversion contains an explicit relationship between seismic amplitude variations, the reflection angle and the physical parameters which can be used to describe the earth efficiently for inversion purposes. Using this relationship, we have derived parametrizations for acoustic and P–P scattering so that the variations in seismic amplitude with reflection angle for each parameter are sufficiently independent. These parametrizations show that small offset and large offset amplitudes are related to different physical parameters. In the case of acoustic scattering, the small‐offset amplitudes are related to impedance variations while large‐offset amplitudes are related to velocity variations. A similar result has been established for P–P scattering.The Born approximation (which is used to derive the GRT inversion) does not correctly predict the amplitude due to velocity variations at large offsets, and thus the inversion of velocity is not as satisfactory as the inversion of impedance.
A THEORETICAL TREATMENT OF THE EFFECT OF MICROSCOPIC FLUID DISTRIBUTION ON THE DIELECTRIC PROPERTIES OF PARTIALLY SATURATED ROCKS1
Authors: ENDRES ANTHONY L. , KNIGHT ROSEMARY 
Published: April 2006

AbstractMicroscopic fluid distribution can have a significant effect on the dielectric properties of partially saturated rocks. Evidence of this effect is found in the laboratory data presented by Knight and Nur in which different methods for controlling saturation produced very different results for the dependence of the dielectric response on water saturation. In this study, previously derived models for the dielectric response of a heterogeneous medium are generalized and the case of a pore space occupied by multiple pore fluids is considered. By using various geometrical distributions of water and gas, it is observed that both the pore geometry in which saturation conditions are changing and the gas–water geometry within a given pore space are critical factors in determining the effective dielectric response of a partially saturated rock.As an example, data for a tight gas sandstone undergoing a cycle of imbibition and drying are analysed. Previous research has demonstrated that significantly different microscopic fluid distributions result from the application of these two techniques to control the level of water saturation. By approximating these microscopic fluid distributions using simple geometrical models, good agreement is found between experimental data and calculated dielectric properties.
BURGER'S EQUATION AS A MODEL FOR THE IP PHENOMENON1
Authors: WYLLER J. , WELLANDER N. , LARSSON F. , PARASNIS D. S. 
Published: April 2006

AbstractThe dynamic response characterizing the induced‐polarization (IP) phenomenon is modelled by a non‐linear diffusion equation (Burger's equation) supplemented by relevant initial and boundary values. The analysis of the model yields a voltage step response and an impedance curve in the frequency domain which agree qualitatively with experimental measurements. Curve fits based on the model have been made in the case of electrochemical cell measurements. The diffusion coefficients estimated by means of these curves are of the same order of magnitude as those calculated using experimental measurements. The normalized transient with these diffusion coefficients agrees with observations, but probably has a shorter discharge time. We have also carried out a comparison with predictions obtained from a linear, finite diffusion layer model, thus showing that for most practical situations the nonlinear term modelling the migration effect can be neglected.
WHEN LEAST‐SQUARES SQUARES LEAST1
Authors: AL‐CHALABI M. 
Published: April 2006

AbstractThere is a general lack of awareness among ‘lay’ professionals (geophysicists included) regarding the limitations in the use of least‐squares. Using a simple numerical model under simulated conditions of observational errors, the performance of least‐squares and other goodness‐of‐fit criteria under various error conditions are investigated. The results are presented in a simplified manner that can be readily understood by the lay earth scientist. It is shown that the use of least‐squares is, strictly, only valid either when the errors pertain to a normal probability distribution or under certain fortuitous conditions. The correct power to use (e.g. square, cube, square root, etc.) depends on the form of error distribution. In many fairly typical practical situations, least‐squares is one of the worst criteria to use. In such cases, data treatment, ‘robust statistics’ or similar processes provide an alternative approach.
ELASTIC REDATUMING OF MULTICOMPONENT SEISMIC DATA1
Authors: WAPENAAR C. P. A. , COX H. L. H. , BERKHOUT A. J. 
Published: April 2006

AbstractElastic redatuming can be carried out before or after decomposition of the multicomponent data into independent PP, PS, SP, and SS responses. We argue that from a practical point of view, elastic redatuming is preferably applied after decomposition. We review forward and inverse extrapolation of decomposed P‐ and S‐wavefields. We use the forward extrapolation operators to derive a model of discrete multicomponent seismic data. This forward model is fully described in terms of matrix manipulations.By applying these matrix manipulations in reverse order we arrive at an elastic processing scheme for multicomponent data in which elastic redatuming plays an essential role. Finally, we illustrate elastic redatuming with a controlled 2D example, consisting of simulated multicomponent seismic data.