bluuebunny/crossref_metadata_2025_split huggingface dataset:
ORIGIN OF THE ANOMALIES OF GRAVITY AND ITS VERTICAL GRADIENT OVER CAVITIES IN BRITTLE ROCK*
Authors: FAJKLEWICZ Z. 
Published: April 2006

ABSTRACTInvestigations show that the depth range of the gravity method for detecting cavities is considerably greater than expected on the basis of theoretical calculations that consider only the depth and shape of the cavity.The cavity generates its own gravity field that is the sum of the activity of the cavity itself and the density changes caused in the surrounding rocks by the process of their destruction as a result of the cavity's expansion. The latter factor is decisive for the cavity's detection by the gravity method in several cases.The extent of the zones of change density in the surrounding rocks and their actual shape depend on many parameters. Thus, the determination of the depth of the cavity cannot be made by comparing measured and theoretical curves of gravity anomalies. The location of the center of gravity of the area disturbed by the cavity or upper boundary of its development towards the ground surface can be determined by denoting the singular point of gravity as a result of the downward analytical continuation of Ag.The Strakhov‐Grigorieva‐Lapina (1977) method fulfils the necessary conditions in the field.
TUTORIAL INTERPRETATION OF MAGNETOTELLURIC MEASUREMENTS OVER AN ELECTRICALLY DISPERSIVE ONE‐DIMENSIONAL EARTH*
Authors: PATELLA D. 
Published: April 2006

ABSTRACTFrequency dispersion of electromagnetic parameters of earth materials has been widely documented in recent years. It is claimed that magnetotellurics (MT) may be significantly affected by dispersion. The paper studies the MT plane‐wave interpretative problem for a one‐dimensional earth characterized by the presence of dispersive layers. The theoretical properties of the MT field under the dispersion hypothesis, andthe main features of the dispersion phenomenon are synthetically reviewed. The examination of previously published MT curve responses over some models of dispersive earth section shows that ambiguity can arise when interpreting MT data with no other source of information. Thus it may be almost impossible to distinguish between the response of a dispersive section and an equally probable dispersion‐free section. The dispersion magnetotelluric (DMT) method is proposed as a means to resolve the ambiguity. The DMT method is based on the execution, at the same site, of an MT sounding and of an always dispersion‐free dc geoelectric deep sounding. The latter technique can be used to compute a synthetic dispersion‐free MT wave impedance, to be compared with the measured MT wave impedance. The apparent dispersion function is introduced as a measure of divergence between the two wave impedances.
COMPUTATION OF SIGNATURES OF LINEAR AIRGUN ARRAYS*
Authors: VAAGE S. , URSIN B. 
Published: April 2006

AbstractFar‐field signatures from an airgun array are usually obtained by carrying out extensive field measurements. In order to decrease the need for such measurements, we have developed a method for computing signatures from linear airgun arrays where the distances between the airguns are such that the non‐linear interaction among the airguns is negligible.The signature from a single airgun of a given type is computed from the following airgun parameters: airgun chamber volume, chamber pressure, airgun depth and position of the waveshape plate within the chamber. For calibration purposes, a recorded signature for one set of airgun parameters has to be provided for each type of airgun. The signatures are computed by using empirical relations between signature properties and the airgun parameters, and by treating the primary and bubble pulses separately.The far‐field signature from a linear airgun array can now be computed by summation of the delayed signatures from the airguns in the array. Practical results are shown for an array with different PAR (Bolt) 1500 C airguns.
AN ACCURATE SCHEME FOR SEISMIC FORWARD MODELLING*
Authors: TAL‐EZER H. , KOSLOFF D. , KOREN Z. 
Published: April 2006

ABSTRACTA new time integration technique for use in forward modelling programmes is introduced. The technique presents an alternative to second‐order temporal differencing. It is based on a Chebyshev expansion of the formal evolution operator to the spatially discretized wave equation. The computational effort in forward modelling based on the new technique is about the same as in methods based on temporal differencing. However, machine accuracy can be obtained. The implementation of the technique to solve the acoustic wave equation in two spatial dimensions is described. Finally, an example of using the technique to solve a problem of wave propagation in a single layer is presented.
FREQUENCY WAVENUMBER APPROACH OF THE τ‐p TRANSFORM: SOME APPLICATIONS IN SEISMIC DATA PROCESSING*
Authors: BENOLIEL S.D. , SCHNEIDER W.A. , SHURTLEFF R.N. 
Published: April 2006

AbstractThe τ‐p transform is an invertible transformation of seismic shot records expressed as a function of time and offset into the τ (intercept time) and p (ray parameter) domain. The τ‐p transform is derived from the solution of the wave equation for a point source in a three‐dimensional, vertically non‐homogeneous medium and therefore is a true amplitude process for the assumed model. The main advantage of this transformation is to present a point source shot record as a series of plane wave experiments.The asymptotic expansion of this transformation is found to be useful in reflection seismic data processing. The τ‐p and frequency‐wavenumber (or f‐k) processes are closely related. Indeed, the τ‐p process embodies the frequency‐wavenumber transformation, so the use of this technique suffers the same limitations as the f‐k technique. In particular, the wavefield must be sampled with sufficient spatial density to avoid wavenumber aliasing.The computation of this transform and its inverse transform consists of a two‐dimensional Fast Fourier Transform followed by an interpolation, then by an inverse‐time Fast Fourier Transform. This technique is extended from a vertically inhomogeneous three‐dimensional medium to a vertically and laterally inhomogeneous three‐dimensional medium.The τ‐p transform may create artifacts (truncation and aliasing effects) which can be reduced by a finer spatial density of geophone groups by a balancing of the seismic data and by a tapering of the extremities of the seismic data.The τ‐p domain is used as a temporary domain where the attack of coherent noise is well addressed; this technique can be viewed as ‘time‐variant f‐k filtering’. In addition, the process of deconvolution and multiple suppression in the τ‐p domain is at least as well addressed as in the time‐offset domain.
PROCESSING SHEAR‐WAVE VSP DATA*
Authors: KNECHT M. , EDELMANN H.A.K. 
Published: April 2006

ABSTRACTShear‐wave arrivals in vertical seismic profiling recordings made with Vibroseis can be more accurately determined by wavelet processing and spectrum whitening. This method aids discrimination of shear‐wave arrivals against P‐wave disturbances. The data, thus improved, can be used to determine the main axis of shear‐wave polarization for each sounding depth in order to maximize shear‐wave amplitude by properly adding the two horizontal component signals vectorially.
LEVINSON INVERSION OF EARTHQUAKE GEOMETRY SH‐TRANSMISSION SEISMOGRAMS IN THE PRESENCE OF NOISE*
Authors: SCHERBAUM F. 
Published: April 2006

AbstractThe Kunetz‐Claerbout equation for the acoustic transmission problem in a layered medium in its original form establishes the relation between the transmission and the reflec tion response for P‐waves in an horizontally layered medium and with vertical incidence. It states that the reflection seismogram due to an impulsive source at the surface, is one side of the autocorrelation of the seismogram due to an impulsive source at depth and a surface receiver.By adapting Claerbout's formulation to the transmission of SH‐waves, the Kunetz‐Claerbout equation also holds for reflection and transmission coefficients dependent on the incident angle. Thus, earthquake geometry SH‐transmission seismograms can be used to caculate corresponding pseudoreflection seismograms which, in turn, can be inverted for the impedance structure using the Levinson algorithm. If the average incidence angle is known, a geometrical correction on the resulting impedance model can improve the resolution of layer thicknesses.In contrast to the inversion of reflection seismograms, the Levinson algorithm is shown to yield stable results for the inversion of transmission seismograms even in the presence of additive noise. This noise stabilization is inherent to the Kunetz‐Claerbout equation.Results of inverted SH‐wave microearthquake seismograms from the Swabian Jura, SW Germany, seismic zone obtained at recording site Hausen im Tal have been compared with sonic‐log data from nearby exploration drilling at Trochtelfingen. The agreement of the main structural elements is fair to a depth of several hundred metres.
VERTICAL SEISMIC PROFILING—SEPARATION OF P‐ AND S‐WAVES*
Authors: DANKBAAR J.W.M. 
Published: April 2006

AbstractIn vertical seismic profile's (VSP's) shot with a large source offset, rays from shot to receiver can have large angles of incidence. Shear waves generated by the source and by conversions at interfaces are likely to be recorded by both the vertical and the horizontal geophones. Varying angles of incidence may give strong variations in the recorded amplitudes. Separation of P‐ and SV‐waves and recovery of their full amplitudes are important for proper processing and interpretation of the data. A P‐S separation filter for three‐component offset VSP data is presented which performs this operation. The separation filter is applied in the k‐f domain and needs an estimate of the P‐ and S‐velocities along the borehole as input. Implementation and stability aspects of the filter are considered. The filter was tested on an 1800 m offset VSP and appeared to be robust. Large velocity variations along the borehole could be handled and results were superior to those obtained by velocity filtering.
BOREHOLE COUPLING OF SEISMIC WAVES IN A PERMEABLE SOLID1
Authors: WHITE J. E. , WELSH E. 
Published: April 2006

ABSTRACTExpressions are derived for the acoustic pressure in a fluid‐filled borehole due to the passage of plane compressional or shear waves propagating in a porous and permeable medium. The derivation is based on a quasistatic description, yielding results applicable at low frequencies. Computations for a plausible sandstone (with porosity of 0.21 and permeability of 300 millidarcies) show accoustic pressure to be a surprisingly distortion‐free version of the stress waveform in a plane compressional wave. For a plane shear wave, the pressure waveform is visibly distorted.
NUMERICAL RAY GENERATION IN THE COMPUTATION OF SYNTHETIC VERTICAL SEISMIC PROFILES1
Authors: ARNTSEN B. 
Published: April 2006

ABSTRACTRay theories are a class of methods often chosen to compute synthetic seismograms due to their efficiency and ability to deal with complex, three‐dimensional inhomogeneous media. To deal with the large number of rays needed to compute synthetic seismograms, a ray generation algorithm is given which is capable of generating a numerical code describing each ray. The code describes a subset of all possible rays by considering only pre‐critical reflections. In a horizontally plane‐layered medium the generation of rays and computation of amplitudes and traveltimes can be efficiently accomplished by grouping the rays into reflection order and dynamic analogue groups. Expressions summing all unconverted rays and rays with a single mode conversion are given for source and receiver located at arbitrary positions within the medium. Examples of zero‐offset synthetic VSPs obtained by this method are given.
IMPROVEMENT OF SIGNAL‐TO‐NOISE RATIO IN RESISTIVITY PROFILES1
Authors: BERNABINI M. , BRIZZOLARI E. , PIRO S. 
Published: April 2006

ABSTRACTThe cross‐correlation technique for increasing the anomaly‐to‐noise ratio is applied to the interpretation of resistivity profiles.To verify the method theoretically, resistivity profiles above a body having the shape of a parallelepiped were simulated with correlated and uncorrelated noise. Seven different electrode geometries are discussed. As a practical test, we considered profiles of a geoelectrical survey to locate tombs at a site of archaeological interest.When the shape and dimensions of the anomaly can be foreseen, the cross‐correlation method can be applied and it gives a substantial improvement in the signal‐to‐noise ratio.
THE EFFECT OF THE ASPECT RATIO ON CRACK‐INDUCED ANISOTROPY1
Authors: DOUMA J. 
Published: April 2006

ABSTRACTMedia containing aligned cracks show anisotropy with respect to elastic wave propagation. There are several models describing the wave propagation in cracked media, most of them only valid for cracks with small aspect ratios. One of these models (Crampin's model) is compared with a model valid for all aspect ratios (Nishizawa's model). The elastic constants and the group velocities are compared for both dry and liquid‐filled inclusions with aspect ratios ranging from 0.0001 (flat cracks) up to 1 (spheres). The difference between both models is small for small aspect ratios but becomes larger for increasing aspect ratios. At a crack density of 0.05 both models give‐within an error of 5%–the same results for aspect ratios up to 0.3. Therefore Crampin's model can be applied to a large range of cracked media even if the aspect ratio of the inclusions is not small. The variation of the anisotropy as a function of the aspect ratio can be studied using Thomsen's dimensionless parameters δ, E and y. They show how inclusions with large aspect ratios result in elliptical anisotropy.
ULTRASONIC SPECTROSCOPY IN ROCKS: AN EXPERIMENTAL STUDY OF HIGHLY POROUS SYNTHETIC SANDSTONES1
Authors: ALMOSSAWI H. I. H. 
Published: April 2006

ABSTRACTBroad‐band ultrasonic impulses and the attenuation spectral ratio technique have been used to investigate the characteristics of synthetic rocks for different saturants, by obtaining the attenuation coefficients in the frequency range of 0.1‐1.0 MHz.The general trend of the data indicates that the experimental attenuation coefficient increases with an increase in the grain/pore size. Results show that large grains/pores can increase the attenuation coefficient in all mechanisms, and water‐saturated synthetic sandstone rocks result in a higher attenuation coefficient than similar oil‐saturated rocks.
NUMERICAL SOLUTION OF THE ACOUSTIC AND ELASTIC WAVE EQUATIONS BY A NEW RAPID EXPANSION METHOD1
Authors: KOSLOFF DAN , FILHO ANIBAL QUEIROZ , TESSMER EKKEHART , BEHLE ALFRED 
Published: April 2006

AbstractWe present a new rapid expansion method (REM) for the time integration of the acoustic wave equation and the equations of dynamic elasticity in two spatial dimensions. The method is applicable to spatial grid methods such as finite differences, finite elements or the Fourier method. It is based on a Chebyshev expansion of the formal solution to the appropriate wave equation written in operator form. The method yields machine accuracy yet it is faster than methods based on temporal differencing. Its disadvantages are that it does not apply to all types of material rheology, and it can also require much storage when many snapshots and time sections are desired. Comparisons between numerical and analytical solutions for simple acoustic and elastic problems demonstrate the high accuracy of the REM.
SENSITIVITY ANALYSIS OF MAGNETOTELLURIC MEASUREMENTS IN RELATION TO STATIC EFFECTS1
Authors: SASAKI YUTAKA 
Published: April 2006

AbstractThe relation between the horizontal profiles of the subsurface resistivity and surface magnetotelluric data can be described by the input and output of a moving‐average filter. The impulse response of this spatial filter, which characterizes the averaging process of the magnetotelluric measurements, is given by the sensitivity profile. Thus, the sensitivity analysis can provide insight into the characteristics of the measurements and hence the mechanism of the static effects. The sensitivity analysis presented here consists of constructing the vertical section of the sensitivity distribution using the finite‐element method and then Fourier transforming the selected horizontal profiles. When the dipole is assumed for measuring the electric field, the static effects can be explained by the high‐pass filter characteristics for the near‐surface. When the electrode separation is taken into account, the sensitivity can be obtained by averaging the sensitivities for the dipoles over the horizontal distance equal to the electrode separation. Therefore, the higher‐frequency components at each depth decrease with increasing electrode separation. Thus, although the static effects can be reduced simply by increasing the electrode separation, information on the resistivity variation at depth is also lost. However, such an adverse effect can be reduced by making the EMAP‐type measurements followed by the spatial filtering of the profile data using the tapered weighting function.
APPLICATION OF ELASTIC MODELLING IN PROCESSING AND INTERPRETATION OF VSP DATA: A CASE HISTORY1
Authors: AMINZADEH F. 
Published: April 2006

ABSTRACTData from offshore Norway is used to study applications of elastic VSP modelling in detecting shear waves and observing the effects of successive mode conversion in field‐recorded VSP data. The shear‐wave velocities and densities from log data are used in conjunction with compressional wave velocities determined from surface seismic and log data in the VSP modelling.The time domain non‐normal incidence elastic VSP modelling technique of Aminzadeh and Mendel is used as the modelling algorithm. Two surface seismograms are computed first. One is the vertical component and the other is the horizontal component for plane waves that have specified incident angles. A downward continuation method is then applied to generate seismograms at different depth points. The collection of these seismograms constitutes non‐normal incidence VSPs. Both vertical and horizontal components of VSP data can be obtained by this procedure.In this paper non‐normal incidence VSPs are generated for a 12.5° incident plane wave. The modelling results of layered earth systems of thin layers and thick layers are both compared with field data, and the effect of mode conversions in thin layers is observed. Several events in the field data can be explained by this elastic VSP modelling.Comparison of the model data and field data enabled a probable tube wave or out‐of‐plane event to be identified, the removal of which significantly improved the final VSP section. This study also shows how the VSP data helped the interpretation of the surface 3D data.
ELASTIC EXTRAPOLATION OF PRIMARY SEISMIC P‐ AND S‐WAVES1
Authors: WAPENAAR C. P. A. , HAIMÉ G. C. 
Published: April 2006

AbstractThe elastic Kirchhoff‐Helmholtz integral expresses the components of the monochromatic displacement vector at any point A in terms of the displacement field and the stress field at any closed surface surrounding A. By introducing Green's functions for P‐ and S‐waves, the elastic Kirchhoff‐Helmholtz integral is modified such that it expresses either the P‐wave or the S‐wave at A in terms of the elastic wavefield at the closed surface. This modified elastic Kirchhoff‐Helmholtz integral is transformed into one‐way elastic Rayleigh‐type integrals for forward extrapolation of downgoing and upgoing P‐ and S‐waves. We also derive one‐way elastic Rayleigh‐type integrals for inverse extrapolation of downgoing and upgoing P‐ and S‐waves. The one‐way elastic extrapolation operators derived in this paper are the basis for a new prestack migration scheme for elastic data.
EFFICIENT COMPUTATION OF TRANSIENT SOUNDING CURVES FOR WIRE SEGMENTS OF FINITE LENGTH USING AN EQUIVALENT DIPOLE APPROXIMATION1
Authors: STOYER C. H. 
Published: April 2006

AbstractCalculations of emf induced in a vertical axis coil by a current step in a straight wire segment of finite length were carried out using an equivalent dipole placed along the source wire. An approximation is valid for homogeneous and layered earth models. The location of the equivalent dipole is calculated by forcing the early‐ and late‐time asymptotes of the transient sounding curve of the equivalent dipole to match those of the finite wire. This approximation works because the early‐time asymptote of the emf depends on the component of the receiver position which is parallel to the wire while the late‐time asymptote does not. Analytical integration of the early‐time asymptote yields an expression for the equivalent dipole location in closed form.The coil can be placed anywhere in the vicinity of the finite wire. Square or rectangular loop sources can be simulated by one or more finite wire segments depending on the source‐receiver geometry. The equivalent radius calculated for central loop soundings agrees well with the value derived using a circular loop with the same area as the square loop.Results show that acceptable sounding curves can be generated by the equivalent dipole for coils placed as close as 0.5 source lengths from the finite wire segment. Higher accuracy can be obtained by splitting the finite wire into two or more subsegments. Results for layered models are slightly better than homogeneous earth models when the resistivity increases with depth and slightly worse for models with resistivity decreasing with depth. Approximate calculations are about 10 times less expensive than exact calculations depending on the method used for the numerical integration.
OPTIMIZED FAST HANKEL TRANSFORM FILTERS1
Authors: CHRISTENSEN NIELS BØIE 
Published: April 2006

AbstractIn the linear digital filter theory for calculation of Hankel transforms it is possible to find explicit series expansions for the filter coefficients. A method is presented for optimizing the Hankel filters calculated in this way. For a certain desired accuracy of computation, the sampling density and filter length are minimized by choosing the parameters determining the filter characteristics according to the analytical properties of the input function. A new approach to the calculation of the filter coefficients has been developed for these optimized filters. The length of the filters may be further reduced by introducing a shift in the sampling scheme.
A COMPARISON BETWEEN KIRCHHOFF AND GRT MIGRATION ON VSP DATA1
Authors: DILLON P. B. 
Published: April 2006

AbstractA modern approach to migration is to perform wavefield extrapolation, subject to an imaging condition. Correct wavefield extrapolation requires that the boundary conditions at the array of geophones satisfy the wave equation. A sufficient condition is to perform the survey with a single stationary source. Contrary to this condition, many VSPs are conducted in deviated wells, where the source is maintained vertically above the down‐hole geophone at each well station. Such a survey fails to provide the boundary conditions theoretically necessary for wave‐equation migration.A recently published inversion scheme, referred to as acoustic generalized Radon transform migration (GRT migration), was developed to handle any configuration of sources and geophones, including moving‐source deviated‐well VSP surveys. GRT migration may be viewed as a weighted version of the generalized Kirchhoff migration, derived in this paper from the exploding‐reflector model.When a VSP‐survey geometry has been specified, GRT migration can be expressed in terms of array parameters, and compared with the equivalent expression for Kirchhoff (wave‐equation) migration. The differences between the two integrals are significant and their effect is demonstrated on VSP data.