bluuebunny/crossref_metadata_2025_split huggingface dataset:
MODELLING CHANNEL WAVES WITH SYNTHETIC SEISMOGRAMS IN AN ANISOTROPIC IN‐SEAM SEISMIC SURVEY1
Authors: LIU ENRU , CRAMPIN STUART , ROTH BRUCE 
Published: April 2006

AbstractIn‐seam seismic surveys with channel waves have been widely used in the United Kingdom and elsewhere to map coal‐seams and to detect anomalous features such as dirt bands, seam thinning and thickening, and particularly in‐seam faulting. Although the presence of cleat‐induced anisotropy has been recognized in the past, almost all previous analyses have assumed homogeneous isotropic or transversely isotropic coal‐seams. Channel waves, however, exhibit properties which cannot be fully explained without introducing anisotropy into the coal‐seam. In particular, Love‐type channel waves are observed for recording geometries where, in a homogeneous isotropic or transversely isotropic structure, the source would not be expected to excite transverse motion. Similarly, modes of channel‐wave propagation display the coupled three‐component motion of generalized modes in anisotropic substrates, which would not be expected for Rayleigh and Love wave motion in isotropy or in transversely isotropic media with azimuthal isotropy.We model the observed in‐seam seismic channel waves with synthetic seismograms to gain an understanding of the effects of cleat‐induced anisotropy on the behaviour of channel waves. The results show a reasonable good match with the observations in traveltime, relative amplitudes, dispersion characteristics and particle motions. We demonstrate that anisotropy in the surrounding country rocks contributes significantly to the coupling of channel wave particle motion, although its effect is not as strong as the anisotropy in the coal‐seam. We conclude that the effects of cleat‐ and stress‐induced anisotropy are observed and can be modelled with synthetic seismograms, and that anisotropy must be taken into account for the detailed interpretation of channel waves.
FREQUENCY‐SELECTIVE DESIGN OF THE KIRCHHOFF MIGRATION OPERATOR1
Authors: GRAY SAMUEL H. 
Published: April 2006

AbstractIntegral migration techniques perform a sum over an aperture of input traces to obtain output at a single point. The length of the aperture is limited by a spatial Nyquist criterion, which typically prohibits imaging very steep dips at very high frequencies without generating severe migration artifacts (migration operator aliasing). For time‐domain Kirchhoff migration, this can be a fatal shortcoming. The standard way to address this problem is to interpolate traces spatially before migration. This reduces the trace spacing, thereby increasing the frequency content which can be migrated without aliasing at steep dips.An alternative remedy to the operator aliasing problem is to modify the phase response of the Kirchhoff migration operator. This operator is frequency‐selective across the migration aperture: it passes all temporal frequencies of the input traces in the innermost portion of the aperture (referring to the shallow dips), and gradually cuts out the higher frequencies as it approaches the outer portion of the aperture. Thus, while all frequencies of the input data contribute to the shallow‐dip portion of the migrated image, only the permissible low frequencies of the input data contribute to imaging the steepest dips.Using a simple realization of a frequency‐selective Kirchhoff migration operator, this technique is illustrated on a synthetic data set involving greater than vertical dips.
REFLECTION AND POLARIZATION OF TUBE WAVES AS SEEN IN VSP DATA1
Authors: MJELDE R. 
Published: April 2006

AbstractP‐wave and S‐wave data acquired with vertical seismic profiling (VSP) often include tube waves propagating in the borehole, although considerable efforts are generally made to ensure that these waves are not recorded. However, several theoretical studies have indicated that tube waves could provide important information about the rock formation and thus should not be considered as pure noise.In order to study some of these aspects experimentally, tube waves were acquired by VSP in a well in the Paris Basin both before and after casing. A sparker was used as source inside the borehole, which ensured that the data recorded contained high‐amplitude tube waves.It is shown that the casing is an obstacle which prevents the study of formation parameters, and thus further tube‐wave acquisitions should be carried out in open holes only.The before‐casing tube‐wave reflection log is compared to a synthetic log computed from the sonic log. The high resolution of the tube waves is of particular interest, revealing layers that are too thin to be detected in body‐wave surveys.It has recently been suggested that the projection of the tube‐wave polarization in the horizontal plane can be used to determine directions of stress‐induced anisotropy in the rock formation. Strong polarization anomalies are observed in the data sets but are attributed to tool problems rather than any rock‐formation feature.
MODELLING OF GI GUN SIGNATURES1
Authors: LANDRØ MARTIN 
Published: April 2006

AbstractIn 1989 a new type of marine seismic source was introduced. This new air‐gun, which consists of two air chambers instead of one, is called the GI gun. The main feature of this gun is that the bubble created by the gun is stabilized by an injection of extra air from the second chamber at a later time. This injection mechanism reduces the amplitude of the bubble oscillations, which also means that the acoustic signal from a GI gun shot is characterized by a very clean primary pulse followed by very small bubble oscillations. A method for calculating the acoustic signal generated by a GI gun is presented. Based on the solution of a damped Kirkwood–Bethe equation, the far‐field pressure of single GI guns and of arrays of GI guns is calculated.It is shown that the optimal values for injection start time and injection period vary with injector volume and gun depth. It is also shown that the precision in the firing time for the injector should be of the order of 4 ms, while the precision of the injection period should be of the order of 8 ms. Modelled and measured far‐field signatures have been compared, and the relative error energy is found to be less than 3.5% for all examples.
ANISOTROPIC Q AND VELOCITY DISPERSION OF FINELY LAYERED MEDIA1
Authors: CARCIONE JOSÉ M. 
Published: April 2006

AbstractWhen a seismic signal propagates through a finely layered medium, there is anisotropy if the wavelengths are long enough compared to the layer thicknesses. It is well known that in this situation, the medium is equivalent to a transversely isotropic material. In addition to anisotropy, the layers may show intrinsic anelastic behaviour. Under these circumstances, the layered medium exhibits Q anisotropy and anisotropic velocity dispersion.The present work investigates the anelastic effect in the long‐wavelength approximation. Backus's theory and the standard linear solid rheology are used as models to obtain the directional properties of anelasticity corresponding to the quasi‐compressional mode qP, the quasi‐shear mode qSV, and the pure shear mode SH, respectively. The medium is described by a complex and frequency‐dependent stiffness matrix. The complex and phase velocities for homogeneous viscoelastic waves are calculated from the Christoffel equation, while the wave‐fronts (energy velocities) and quality factor surfaces are obtained from energy considerations by invoking Poynting's theorem.We consider two‐constituent stationary layered media, and study the wave characteristics for different material compositions and proportions. Analyses on sequences of sandstone‐limestone and shale‐limestone with different degrees of anisotropy indicate that the quality factors of the shear modes are more anisotropic than the corresponding phase velocities, cusps of the qSV mode are more pronounced for low frequencies and midrange proportions, and in general, attenuation is higher in the direction perpendicular to layering or close to it, provided that the material with lower velocity is the more dissipative. A numerical simulation experiment verifies the attenuation properties of finely layered media through comparison of elastic and anelastic snapshots.
COMMENT ON ‘ASPECTS OF CHARGE‐ACCUMULATION IN D.C. RESISTIVITY EXPERIMENTS’ BY Y. LI AND D. W. OLDENBURG1
Authors: SZARKA L. 
Published: April 2006

AbstractThe paper by Li and Oldenburg (1991) gives an important insight into d.c. charge accumulation problems. Nevertheless, their derivation concerning the role of the permittivity of the medium is not as straightforward as it could be. Another question, worth discussing, is the problem of double layers, which is missing from the authors’ paper.
REFLECTION AMPLITUDES AND MIGRATION AMPLITUDES (ZERO‐OFFSET SITUATION)1
Authors: BORTFELD R. , KIEHN M. 
Published: April 2006

AbstractThe effect of wave‐equation migration on amplitudes is determined. This effect is derived for zero‐offset traces and for second‐order approximations of the traveltimes. Three steps are followed: firstly, the amplitudes of zero‐offset traces are established; secondly minus half the traveltimes are used as input for downward continuation in migration (forward in space and time); thirdly, the amplitudes of the migrated events are determined by downward continuation (at zero‐traveltimes).Layered models (piles of homogeneous layers) with smooth interfaces are used. The determinants of the 2 × 2 matrices B0 obtained for these models are responsible for the main effect on migration. The migration result primarily depends on the overburden as the inverse of det (B0). Drastic effects can occur over small distances. For weakly reflecting media, it is confirmed that wave‐equation migration gives “correct” results (but the input data must be multiplied by V0T0), i.e. amplitudes proportional to the reflection coefficient. For any velocity changes, the inverse of det (B0) will, in general, give inaccurate migration amplitudes and inaccurate lithological interpretations. In a simple step, true amplitude migration, or exact migration, is derived from our results.It is assumed that no focus phenomena are present. The effect of buried foci is discussed briefly.
FIXED LOOP SOURCE EM MODELLING RESULTS USING 2D FINITE ELEMENTS1
Authors: VALLA PIERRE 
Published: April 2006

AbstractAmong electromagnetic sounding techniques, the Mélos method possesses the specific feature of including an apparent resistivity computation. This acts as a normalizing scheme so that 2D modelling results can be obtained without accounting for a true 3D source. However, in order to get reliable numerical modelling results for a 2D magnetic dipole source, improved algorithms are required in order to apply the standard finite‐element technique: quadratic basis functions must be used in place of linear basis functions, and a more sophisticated method than conventional ones is necessary for properly solving the resulting system of linear equations.Such modelling results have been used to study theoretical responses for the Mélos method in the search for conductive bodies in mineral exploration. Two sets of models are presented and discussed. They show that the typical Mélos response to a conductive target is a bipolar anomaly on the apparent resistivity pseudo‐section, with a conductive pole at low frequency which is centred above the target.
COMMENT ON ‘THE RESOLUTION OF NARROW LOW‐VELOCITY ZONES WITH THE GENERALIZED RECIPROCAL METHOD’ BY DERECKE PALMER1
Authors: WHITELEY R. J. 
Published: April 2006

AbstractThe following discussion concentrates on one of the synthetic examples used by Palmer (1991) to justify his method of seismic refraction interpretation (Fig. 2, p. 1035) and on his field data and interpretation of the first field example (a collapsed doline) which appeared on pp. 1050–1053.
ASPECTS OF 1D SEISMIC MODELLING USING THE GOUPILLAUD PRINCIPLE1
Authors: SLOB EVERT , ZIOLKOWSK1 ANTON 
Published: April 2006

AbstractA reflection response function for a 1D discretized earth model can be obtained using ray‐theory and Z‐transforms with the Goupillaud model. This is usually done by taking the source function as a plane wave impinging normally on the layered earth. Two important problems have been tackled with this basic idea. The first, extraction of the source wavelet, and the second, a description of the free‐surface related problems.In the Goupillaud model, the one‐way traveltime in each layer is taken to be the same time interval At, which is also the time unit for the Z‐transform. The two‐way traveltime in any layer is 2Δt, corresponding to a multiplication by Z2. The reflection impulse response therefore contains only even powers of Z. The convolution of the reflection response with the wavelet yields a seismogram whose Z‐transform contains both odd and even powers of Z. However, even though the seismogram contains more coefficients than unknowns, the wavelet cannot be extracted, because the coefficients are not independent: later coefficients are functions of earlier ones, which does not make sense physically. To overcome this physical problem for the reflection seismogram, the two‐way traveltime through the layer should be Δt. It is then impossible to extract the wavelet, as there are fewer coefficients in the seismogram than unknowns.Szaraniec has proposed a modification to the Goupillaud model, known as the odd‐depth model, that includes the free surface and a top layer whose two‐way traveltime Δt is half the two‐way traveltime 2Δt of all the other layers. Using what Szaraniec calls the fundamental identity of the odd‐depth model, it is possible to extract the source wavelet from the seismogram. We show that this fundamental identity holds only if reflection coefficients of deeper interfaces are functions of the reflection coefficients of shallower interfaces; that is, for extremely improbable geologies.Neither of these approaches offers a solution to the deconvolution problem. It is better to obtain the source signature from measurements in the field. Only Szaraniec's model offers the possibility of tackling the problem of the free surface but because of an inherent flaw in the model, it fails to address the problem.
DETERMINATION OF ELECTRICAL PROPERTIES OF THE GROUND AT SHALLOW DEPTH WITH AN ELECTROSTATIC QUADRUPOLE: FIELD TRIALS ON ARCHAEOLOGICAL SITES1
Authors: TABBAGH ALAIN , HESSE ALBERT , GRARD RÉJEAN 
Published: April 2006

AbstractKnowledge of both the electrical resistivity ρ and the dielectric permittivity ɛ of the ground is important for the determination of characteristics such as granularity, porosity, moisture and salt content. Whereas the measurement of ρ is very common and can be achieved using either the d.c. resistivity method or a wide variety of EM devices, ɛ remains practically unknown in the low‐frequency domain between the IP domain and the high‐frequency domain. Following the principle of the quadrupole technique used in d.c. prospecting and of the quadrupolar probe for measuring the complex permittivity of the ionosphere, we propose a new approach which does not require any galvanic contact between the poles and the ground. The transfer impedance can be evaluated using the quasi‐static approximation for low frequency or the full EM theory for higher frequencies. The conditions under which both calculations apply are discussed. At low frequencies and for low resistivity ground, the electrostatic quadrupole measures ρ exactly as with the d.c. technique; for higher resistivities or frequencies the simultaneous measurement of both properties becomes possible. Examples in archaeological prospecting are presented and checked against independent d.c. resistivity measurements or excavation analyses.
DIELECTRIC PROPERTIES OF WET SOILS IN THE FREQUENCY RANGE 1–3000 MHz1
Authors: WENSINK W.A. 
Published: April 2006

AbstractThe effective relative dielectric constant ɛe, r and the effective conductivity σe have each been determined as a function of frequency in the range 1–3000 MHz at volumetric water contents of up to approximately 0.74 for clays, 0.83 for a peat and 0.56 for a silt.At frequencies above about 25 MHz (depending on soil type), ɛe, rincreases with water content for all samples. However, at lower frequencies, ɛe, ronly increases with water content as long as the wet density also increases, which is the case for water contents up to a critical value lying between 0.35 and 0.48. At higher water contents, ɛe, rand the wet density decrease with increasing water content. Consequently, curves of ɛe, rversus frequency for two wet samples with different water contents, at least one of them higher than the critical value, are seen to cross at about 25 MHz. Below the critical value the curve of the sample with the lower water content is below the other curve at all freqencies applied. At a given frequency, σe has a maximum as a function of water content. This is tentatively explained by assuming that σe is the sum of pore water conductivity (increasing with water content until all salts in the soil are dissolved into the water and then decreasing) and surface water conductivity (increasing with wet density and therefore increasing with water content up to the critical value and then decreasing).At frequencies higher than 1000 MHz, ɛe, rdepends only weakly on salinity (which is represented by the measured conductivity). It shows an increasing dependence if the frequency is decreased towards 1 MHz.The highest values of ɛe, rand σe, measured in this work, occur for a sample of wet, nearly saturated silt originating from a location below sea‐level near to the Dead Sea, Israel: ɛe, rdecreases continuously from a value of about 104 at 3 MHz to about 102 at 200 MHz, while σe rises from about 4 S/m to 5 S/m at these respective frequencies. The dependence of the wavelength on the loss‐tangent is strong and the wavelength is considerably smaller than it would be in a dielectric. This is the only sample for which σe increases with water content, even if the latter is above its critical value. Therefore it is assumed that the pore water conductivity is greater than the surface water conductivity if the volumetric water content is lower than 0.564, the maximum value applied. The measurements give evidence for the presence of a relaxation at about 3 MHz for all samples examined.
A RECONCILIATION OF MATHEMATICAL MODELS FOR SPONTANEOUS MINERALIZATION POTENTIALS1
Authors: FURNESS PETER 
Published: April 2006

AbstractTwo methods for computing spontaneous mineralization potentials in the region external to the source body are reviewed. The first of these is a long‐established technique in which the causation is assumed to be a distribution of simple current source on the boundary of the mineralization. The second is a more recent technique which assumes a surface distribution of current dipole moment (double layer) along the boundary of the source body.The former technique is a special case of a more general spontaneous potential (SP) model in which the source is a density of current dipole moment (current polarization) distributed throughout the mineralization. As far as the potentials in the region external to the source body are concerned this current polarization can be simply related to an equivalent double layer source function, i.e. the potential discontinuity produced over the boundary of the mineralization by an equivalent double layer model.This simple relationship suggests an integral equation technique for the exact numerical solution of boundary value problems appropriate to the polarization model for spontaneous mineralization potentials. The technique is applied to exploring the justification of interpreting mineralization self‐potentials by the traditional approach.
DIFFRACTION BY A WEDGE IN AN ACOUSTIC CONSTANT DENSITY MEDIUM1
Authors: BERG PER , IF FLEMMING , NIELSEN PER , SKOVGAARD OVE 
Published: April 2006

AbstractA new method is presented for solving the 2D problem of diffraction of a plane wave by a wedge of arbitrary angle in a purely acoustic, constant‐density medium with different constant compressional wave speeds inside and outside the wedge. The diffraction problem is formulated as integral equations, and a wavenumber–frequency representation of the scattered field is obtained. With the aid of the Cagniard–de Hoop method, exact analytical expressions in the space–time domain are obtained for the different wave constituents, i.e. geometric optical scattered waves and edge diffracted waves including head waves. These expressions can be computed to any degree of accuracy within reasonable computation times on a computer, and the semi‐analytical method of solution presented thus constitutes a means of constructing reference solutions for wedge configurations. Such highly accurate reference solutions are of importance for verification of results that include diffraction phenomena modelled by general numerical approximate methods, e.g. finite differences, finite elements and spectral methods. Examples of such applications of the method of solution are given.
Relation of in situ resistivity to water content in salt rocks1
Authors: Yaramanci U. 
Published: April 2006

AbstractThe investigation of water in salt‐rock formations is of particular relevance to underground nuclear waste repositories. In the Asse salt‐mine (Germany) a study into the relationship of in situ resistivity to water content has been made. Measurements were carried out in older rock‐salt using an electrode array in boreholes, an electrode profile in a drift and small resistivity sensors in and around a drift seal. Further measurements were made on moist zones in a contact area of younger rock‐salt and carnallitite and also in older rock‐salt with anhydrite bands using electrode profiles in the drifts. The resistivities range from 102Ωm to 106Ωm. Corresponding probes have water contents from 0.01% to 1.3%. A definite relationship between resistivity and water content is revealed which can be described by Archie's law using a cementation factor m of 1.9. Porosities are between 0.08% and 1.4% and the saturations vary considerably. An explicit influence of saturation on resistivity cannot be discovered using the present data. The results enable us to estimate the in situ water content and the order of the in situ porosity using resistivity surveys at different scales. This increases significantly the safety of a nuclear repository site.
Shallow fault location in coal measures using offset Wenner resistivity profiling1
Authors: Hobbs B.A. , Reading A.M. 
Published: April 2006

AbstractThe five‐electrode resistivity configuration of Barker, who introduced the concept of offset resistivity measurements, is used in a profiling mode in the search for lateral variations of resistivity down to depths of a few tens of metres. Theoretical computations show that plotting simple pseudosections of offset measurements over faults is sufficient to reveal the fault position. The method is subsequently applied in the field for locating a buried fault in coal measures. Although offset pseudosections are often all that is required, apparent resistivity pseudosections are examined and compared to collations of 1D inversions of the sounding profile data and to computations over 2D models. It is shown that apparent restivity pseudosections may present a rather misleading picture.