bluuebunny/crossref_metadata_2025_split huggingface dataset:
CONVERSION POINTS AND TRAVELTIMES OF CONVERTED WAVES IN PARALLEL DIPPING LAYERS1
Authors: TESSMER GISA , BEHLE ALFRED 
Published: April 2006

AbstractFor converted waves, stacking as well as AVO analysis requires a true common reflection point gather which, in this case, is also a common conversion point (CCP) gather. The coordinates of the conversion points for PS or SP waves, in a single homogeneous layer can be calculated exactly as a function of the offset, the reflector depth and the ratio vp/vs. An approximation of the conversion point on a dipping interface as well as for a stack of parallel dipping layers is given. Numerical tests show that the approximation can be used for offsets smaller than the depth of the reflector under consideration.The traveltime of converted waves in horizontal layers can be expanded into a power series. For small offsets a two‐term truncation of the series yields a good approximation. This approximation can also be used in the case of dipping reflectors if a correction is applied to the traveltimes. This correction can be calculated from the approximated conversion point coordinates.
LONG‐OFFSET TRANSIENT EM SOUNDING NORTH OF THE RHINE‐RUHR COAL DISTRICT, GERMANY1
Authors: STEPHAN A. , SCHNIGGENFITTIG H. , STRACK K.‐M. 
Published: April 2006

AbstractDuring the past two decades electromagnetic methods have been increasingly applied to problems in hydrocarbon exploration. It has been shown that EM methods achieve satisfactory results which can be supplementary to results of the seismic method. We extend the application of the Long‐Offset Transient ElectroMagnetic Sounding (lotem) to exploration problems in the coal mining industry.A survey was conducted north of the Rhine‐Ruhr coal district using a grounded dipole source and receivers laid out along several profile lines giving areal coverage. The strong noise from cultural as well as natural electromagnetic sources was reduced using prestack data processing techniques resulting in reliable data with a good signal‐to‐noise ratio. The interpretation is self‐consistent and matches the well logs and the geological information of the area quite well. The high station density permitted the mapping of the top of the Carboniferous formation to a depth of between 800 and 1000 m below surface.
STACKING OF P‐SV SEISMIC REFLECTION DATA USING DIP MOVEOUT1
Authors: DEN ROOIJEN HUUB P. G. M. 
Published: April 2006

AbstractThe moveout of P‐SV mode‐converted seismic reflection events in a common‐midpoint gather is non‐hyperbolic. This is true even if the medium has constant P‐ and SV‐wave velocities. Furthermore, reflection‐point smear occurs even along horizontal reflectors. These effects reduce the resolution of the zero‐offset stack. In such a medium, the generalization of the dip moveout transformation to P‐SV data can be calculated analytically. The resulting P‐SV dip moveout operators solve the problem of reflection‐point smear, and image any reflector regardless of dip or depth. The viability of this technique is demonstrated on synthetic and field data.
INVERSION OF POST‐STACK SEISMIC DATA USING SIMULATED ANNEALING1
Authors: VESTERGAARD PETER D. , MOSEGAARD KLAUS 
Published: April 2006

AbstractModel‐based inversion of seismic reflection data is a global optimization problem when prior information is sparse. We investigate the use of an efficient, global, stochastic optimization method, that of simulated annealing, for determining the two‐way traveltimes and the reflection coefficients.We exploit the advantage of an ensemble approach to the inversion of full‐scale target zones on 2D seismic sections.In our ensemble approach, several copies of the model‐algorithm system are run in parallel. In this way, estimation of true ensemble statistics for the process is made possible, and improved annealing schedules can be produced.It is shown that the method can produce reliable results efficiently in the 2D case, even when prior information is sparse.
WAVELET ESTIMATION USING THE MULTITAPER METHOD1
Authors: WALDEN A. T. 
Published: April 2006

AbstractAn accurate estimate of the seismic wavelet on a seismic section is extremely important for interpretation of fine details on the section and for estimation of acoustic impedance. In the absence of well‐control, the recognized best approach to wavelet estimation is to use the technique of multiple coherence analysis to estimate the coherent signal and its amplitude spectrum, and thence construct the seismic wavelet under the minimum‐phase assumption.The construction of the minimum‐phase wavelet is critically dependent on the decay of the spectrum at the low‐frequency end. Traditional methods of cross‐spectral estimation, such as frequency smoothing using a Papoulis window, suffer from substantial side‐lobe leakage in the areas of the spectrum where there is a large change of power over a relatively small frequency range. The low‐frequency end of the seismic spectrum (less than 4 Hz) decays rapidly to zero. Side‐lobe leakage causes poor estimates of the low‐frequency decay, resulting in degraded wavelet estimates.Thomson's multitaper method of cross‐spectral estimation which suffers little from side‐lobe leakage is applied here, and compared with the result of using frequency smoothing with the Papoulis window. The multitaper method seems much less prone to estimating spuriously high coherences at very low frequencies. The wavelet estimated by the multitaper approach from the data used here is equivalent to imposing a low‐frequency roll‐off of some 48 dB/oct (below 3.91 Hz) on the amplitude spectrum. Using Papoulis smoothing the equivalent roll‐off is only about 36 dB/oct. Thus the multitaper method gives a low‐frequency decay rate of the amplitude spectrum which is some 4 times greater than for Papoulis smoothing. It also gives more consistent results across the section. Furthermore, the wavelet obtained using the multi‐taper method and seismic data only (with no reference to well data) has more attractive physical characteristics when compared with a wavelet extracted using well data, than does an estimate using traditional smoothing.
THE USE OF FORWARD‐ AND BACK‐SCATTERED P‐, S‐ AND CONVERTED WAVES IN CROSS‐BOREHOLE IMAGING1
Authors: BALCH A. H. , CHANG H. , HOFLAND G. S. , RANZINGER K. A. , ERDEMIR C. 
Published: April 2006

AbstractThe principles of imaging, for example that of prestack migration, can be applied to cross‐borehole seismic geometry just as they can to surface seismic configurations. However, when using actual cross‐borehole data, a number of difficulties arise that are rarely or never encountered in imaging surface seismic data: discontinuities may reflect or diffract incident seismic waves in any direction. If a discontinuity lies between the lines of sources and receivers, forward‐scattered, or interwell, events may be recorded. If a discontinuity lies outside the interwell region, back‐scattered, or extra‐well, events may be recorded. Many angles of incidence are possible, and all possible reflected modes (P–P, P–S, S–P and S–S) are present, frequently in nearly equal proportions. The planes of the reflectors dip from 0 to ±90°.In order to deal with these complexities we first separate propagation modes at the receiver borehole using both polarization and velocity. Next we compensate for phase distortion due to dispersion. Finally, and most importantly, we migrate or image the data in cross‐borehole common‐source gathers. To do this, a finite‐difference solution to the 2D scalar wave equation, using reverse time, for an arbitrary distribution of velocities, is used to project the separated, reflected‐diffracted wavefield back into the medium.There are four reflection modes (P–P, P–S, S–P and S–S), so we can apply four different imaging conditions. The zones outside the boreholes as well as inside the boreholes can be imaged with these conditions.These operations are repeated for each common‐source gather: each common‐source gather generates four partial images in each image space. This multiplicity of partial images can be stacked in various combinations to yield a final image of the subsurface.Our experiments using solid (not fluid) physical models indicate that when these procedures are correctly applied, high quality cross‐borehole images can be obtained. These images appear with great clarity even though some of the weak diffractions causing diffraction images may be almost totally obscured by other high‐amplitude events on the raw data.
PSEUDOREFLECTION PROFILING METHOD: AN EFFICIENT COMPLEMENT TO CDP METHOD1
Authors: TSUTSUI TOMOKI 
Published: April 2006

AbstractA method for pseudoreflection profiling (PRP) is presented. By using transmission seismograms, this method produces time sections similar to those obtained using the common depth point (CDP) method.Pseudoreflection profiles are obtained using the following procedure. Equivalent transmission responses at sites on soft deposits are derived by Wiener filter estimation using seismograms acquired on the sites and on the basement rock. These responses are substituted into the Claerbout‐Kunetz equation to obtain the pseudoreflection profiles.A 3D subsurface model is produced when the PRP method is applied to explosion seismograms. The time sections thus obtained are consistent with those obtained using the CDP method in a neighbouring area. The maximum station interval for obtaining coherent profiles with this method is estimated to be one quarter of the wavelength of the transmitted waves.This technique should be used in combination with another method such as the CDP method because it has two weak points: the velocity structure cannot be obtained directly and undesirable waves cannot be eliminated through processing.
SEPARATION OF REGIONAL AND RESIDUAL ANOMALIES BY LEAST‐SQUARES ORTHOGONAL POLYNOMIAL AND RELAXATION TECHNIQUES: A PERFORMANCE EVALUATION1
Authors: AGARWAL B. N. P. , SIVAJI CH. 
Published: April 2006

AbstractThe performances of least‐squares orthogonal polynomial and relaxation techniques in the separation of regional and residual anomalies have been evaluated with a view to minimizing personal biasing. The advantage of orthogonal over nonorthogonal polynomials is their ability to estimate an optimum order of polynomial to represent the predominant regional trend in the data using an approximate 2D difference table, the Z‐matrix. The correlation coefficients between residuals of two consecutive orders also give the same result. In the relaxation technique, a linear trend is assumed within each cell of the mesh of a square grid. A set of such linear segments can approximate any complicated regional trend. The performances of these two techniques have been evaluated using simulated gravity anomalies produced by 2D and 3D complex regional structures superimposed on residual fields due to cylinders and prismatic bodies, as well as three field examples taken from the published literature. The analyses have revealed that the relaxation technique produces excellent results when an optimum polynomial order rather than an arbitrary fixed one is used for computing the boundary conditions along the periphery of the map. Analyses have revealed that such boundary conditions provide minimum distortion near the two ends of the profile.
THE COLLOCATION APPROACH TO THE INVERSION OF GRAVITY DATA1
Authors: BARZAGHI A. , GANDINO A. , SANSO F. , ZENUCCHINI C. 
Published: April 2006

AbstractGravity data inversion can provide valuable information on the structure of the underlying distribution of mass.The solution of the inversion of gravity data is an ill‐posed problem, and many methods have been proposed for solving it using various systematic techniques.The method proposed here is a new approach based on the collocation principle, derived from the Wiener filtering and prediction theory.The natural multiplicity of the solution of the inverse gravimetric problem can be overcome only by assuming a substantially simplified model, in this case a two‐layer model, i.e. with one separation surface and one density contrast only. The presence of gravity disturbance and/or outliers in the upper layer is also taken into account.The basic idea of the method is to propagate the covariance structure of the depth function of the separation surface to the covariance structure of the gravity field measured on a reference plane.This can be done since the gravity field produced by the layers is a functional (linearized) of the depth.Furthermore, in this approach, it is possible to obtain the variance of the estimation error which indicates the precision of the computed solution.The method has proved to be effective on simulated data, fulfilling the a priori hypotheses.In real cases which display the required statistical homogeneity, good preliminary solutions, useful for a further quantitative interpretation, have also been derived.A case study is discussed.
THE TRANSIENT ELECTROMAGNETIC RESPONSE OF A POLARIZABLE SPHERE IN A CONDUCTING HALF SPACE1
Authors: LEE TERRY , THOMAS LINDSAY 
Published: April 2006

AbstractA polarizable sphere embedded in a conducting half‐space can give rise to negative voltage transients in a coincident‐loop time‐domain electromagnetic system. Such transients have been observed in field situations. Our results are obtained from a model in which the contributions of the host rock, the currents in the sphere, and the interaction between the sphere and the host rock are separated and superposed. This model uses approximations to the integral equation solutions rather than finite‐element or finite‐difference approximations, and so allows very rapid computation.The theoretical demonstration suggests that interpretation of the negative voltage transients as a polarization response is valid, but more detailed interpretation of polarization properties may not be possible, because the superposition of the polarization response on the normal response depends strongly on the position of the target.
INTEGRATED INTERPRETATION OF MARINE ENGINEERING GEOLOGICAL AND GEOPHYSICAL DATA ON THE PRINCIPLES OF EXPERT SYSTEM TECHNOLOGY1
Authors: KOVALEVSKY E. V. , KHARCHENKO V. I. 
Published: April 2006

AbstractWe describe an approach to the construction of an engineering geological expert system for identification of sub‐bottom soils in accordance with some predefined nomenclature. The following principles of integrated interpretation of engineering geophysical and geotechnical data are presented: Firstly, the transformation of physical data (compressional‐ and shear‐wave velocities, compressional‐wave attenuation coefficients, electrical conductivity, etc.) for each of the medium points into subjective probabilities for the soil belonging to each type listed in the nomenclature, and secondly, the extrapolation of local geotechnical data (primarily drilling data) to the surrounding space by means of diffusion of the initial membership function distribution, resulting in the same set of probabilities for soil types at each point in the medium under consideration. Aggregation of the fuzzy information obtained, sufficient for reaching a conclusion for most points in the medium, is carried out by means of Bayesian summation. An example is given of integrated interpretation of real data obtained from four different sources (compressional‐ and shear‐wave velocity sections Vp(x, z) and Vs(x, z), and two boreholes) related to the same profile.
A SEMBLANCE‐GUIDED MEDIAN FILTER1
Authors: REITER E. C. , TOKSOZ M. N. , PURDY G. M. 
Published: April 2006

AbstractReiter, E.C., Toksoz, M.N. and Purdy, G.M. 1992. A semblance‐guided median filter. Geophysical Prospecting41, 15–41.A slowness selective median filter based on information from a local set of traces is described and implemented. The filter is constructed in two steps, the first being an estimation of a preferred slowness and the second, the selection of a median or trimmed mean value to replace the original data point. A symmetric window of traces defining the filter aperture is selected about each trace to be filtered and the filter applied repeatedly to each time point. The preferred slowness is determined by scanning a range of linear moveouts within the user‐specified slowness passband. Semblance is computed for each trial slowness and the preferred slowness selected from the peak semblance value. Data points collected along this preferred slowness are then sorted from lowest to highest and in the case of a pure median filter, the middle point(s) selected to replace the original data point. The output of the filter is therefore quite insensitive to large amplitude noise bursts, retaining the well‐known beneficial properties of a traditional 1D median filter. Energy which is either incoherent over the filter aperture or lies outside the slowness passband, may be additionally suppressed by weighting the filter output by the measured peak semblance.This approach may be used as a velocity filter to estimate coherent signal within a specified slowness passband and reject coherent energy outside this range. For applications of this type, other velocity estimators may be used in place of our semblance measure to provide improved velocity estimation and better filter performance. The filter aperture may also be extended to provide increased velocity estimation, but will result in additional lateral smearing of signal. We show that, in addition to a velocity filter, our approach may be used to improve signal‐to‐noise ratios in noisy data. The median filter tends to suppress the amplitude of random background noise and semblance weighting may be used to reduce the amplitude of background noise further while enhancing coherent signal.We apply our method to vertical seismic profile data to separate upgoing and downgoing wavefields, and also to large‐offset ocean bottom hydrophone data to enhance weak refracted and post‐critically reflected energy.
SEISMIC RAY TRACING USING LINEAR TRAVELTIME INTERPOLATION1
Authors: ASAKAWA EIICHI , KAWANAKA TAKU 
Published: April 2006

AbstractA new ray‐tracing method called linear traveltime interpolation (LTI) is proposed. This method computes traveltimes and raypaths in a 2D velocity structure more rapidly and accurately than other conventional methods.The LTI method is formulated for a 2D cell model, and calculations of traveltimes and raypaths are carried out only on cell boundaries. Therefore a raypath is considered to be always straight in a cell with uniform velocity. This approach is suitable to tomography analysis.The algorithm of LTI consists of two separate steps: step 1 calculates traveltimes on all cell boundaries; step 2 traces raypaths for all pairs of receivers and the shot.A traveltime at an arbitrary point on a cell boundary is assumed to be linearly interpolated between traveltimes at the adjacent discrete points at which we calculate traveltimes. Fermat's principle is used as the criterion for choosing the correct traveltimes and raypaths from several candidates routinely.The LTI method has been compared numerically with the shooting method and the finite‐difference method (FDM) of the eikonal equation. The results show that the LTI method has great advantages of high speed and high accuracy in the calculation of both traveltimes and raypaths. The LTI method can be regarded as an advanced version of the conventional FDM of the eikonal equation because the formulae of FDM are independently derived from LTI. In the process of derivation, it is shown theoretically that LTI is more accurate than FDM. Moreover in the LTI method, we can avoid the numerical instability that occurs in Vidale's method where the velocity changes abruptly.
A PHYSICAL MODEL STUDY OF SCATTERING OF WAVES BY ALIGNED CRACKS: COMPARISON BETWEEN EXPERIMENT AND THEORY1
Authors: ASS'AD J. M. , TATHAM R. H. , MCDONALD J. A. , KUSKY T. M. , JECH J. 
Published: April 2006

AbstractAn approximation to plane‐wave propagation through a composite material is examined using a physical model with oriented but randomly distributed penny‐shaped rubber inclusions within an isotropic epoxy resin matrix. A pulse transmission method is used to determine velocities of shear and compressional waves as a function of angle of incidence and crack density. The experimental and theoretical results of Hudson were compared and limitations within the crack parameters used in this study have been determined. Results from both polarized shear waves (S1, S2) compare favourably with the theory for a composite with up to 7% crack density, but theory and experiment diverge at higher crack densities. On the other hand, compressional‐wave velocities at low crack densities (1% and 3%) compare favourably with the theory. It is also shown that the velocity ratio Vp/Vs for two extreme cases, i.e. propagation normal and parallel to the cracks, as a function of crack density and porosity, has a strong directional dependence.
ANISOTROPIC TRAVELTIME TOMOGRAPHY
Authors: MICHELENA R. J. , MUIR F. , HARRIS J. M. 
Published: April 2006

AbstractVelocity estimation technique using seismic data is often based on time/distance equations which are independent of direction, and even though we now know that many rocks are quite anisotropic, useful results have been obtained over the years from these isotropic estimates. Nevertheless, if velocities are significantly direction‐dependent, then the isotropic assumption may lead to serious structural interpretation errors. Additionally, information on angle‐dependence may lead to a better understanding of the lithology of the rocks under measurement. VSP and cross‐well data may each lack the necessary aperture to estimate more than two velocity parameters for each wave type, and if the data straddle a symmetry axis, then these may be usefully chosen to be the direct velocities (from time‐and‐distance measurements along the axis) and NMO velocities (from differential time‐offset measurements). These sets of two parameters define ellipses, and provide intermediate models for the variation of velocity with angle which can later be assembled and translated into estimates of the elastic moduli of the rocks under scrutiny.If the aperture of the measurements is large enough e.g. we have access to both VSP and cross‐well data, we divide the procedure into two independent steps, first fitting best ellipses around one symmetry axis and then fitting another set around the orthogonal axis. These sets of four elliptical parameters are then combined into a new, double elliptical approximation. This approximation keeps the useful properties of elliptical anisotropy, in particular the simple relation between group and phase velocities which simplifies the route from the traveltimes measurements to the elastic constants of the medium.The inversion proposed in this paper is a simple extension of well‐known isotropic schemes and it is conceptually identical for all wave types. Examples are shown to illustrate the application of the technique to cross‐well synthetic and field P‐wave data. The examples demonstrate three important points: (a) When velocity anisotropy is estimated by iterative techniques such as conjugate gradients, early termination of the iterations may produce artificial anisotropy. (b) Different components of the velocity are subject to different type of artifacts because of differences in ray coverage, (c) Even though most rocks do not have elliptical dispersion relations, our elliptical schemes represent a useful intermediate step in the full characterization of the elastic properties.