bluuebunny/crossref_metadata_2025_split huggingface dataset:
PHYSICAL PROPERTIES OF SYNTHETIC SANDSTONE ROCKS1
Authors: ALMOSSAWI H. I. H. 
Published: April 2006

ABSTRACTA novel process is used to coat glass spheres with a polymerizing thermo‐setting resin, only microns thick. Synthetic rocks of known grain size distribution and pore space characteristics are then made by heating the resin‐coated glass spheres under compression in a special mould. The dynamic Young's moduli of these rocks are found to be affected by the percentage of resin content (cement) and the synthetic diagenesis rather than the grain size and permeability.
THE TIME‐DOMAIN ELECTROMAGNETIC RESPONSE OF POLARIZABLE BODIES: AN APPROXIMATE CONVOLUTION ALGORITHM1
Authors: SMITH R. S. , WALKER P. W. , POLZER B. D. , WEST G. F. 
Published: April 2006

ABSTRACTIt is now believed that the negative transients observed in coincident‐loop transient electromagnetic (TEM) measurements are caused by polarizable bodies (bodies whose conductivity increases as a function of frequency). Ordinarily the TEM response of polarizable bodies is obtained by calculating the frequency‐domain response at many frequencies and transforming it to the time domain via Fourier, Laplace or Hankel transforms. This is normally a computationally laborious task. However, for some simple non‐polarizable bodies the time‐domain response is analytical and can be computed easily. When these simple bodies are weakly polarizable an approximate response can be obtained by convolving the easily‐calculated, non‐polarizable response with the impulse response of the polarization. The approximate response is found to be very similar to the exact response for the polarizabilities normally seen in geological materials.
APPLICATION OF PETROPHYSICAL MEASUREMENTS TO THE PREDICTION OF SEISMIC RESPONSES OF DIFFERING LITHOLOGYOR PORE FLUIDS1
Authors: RAFISON B. J. 
Published: April 2006

ABSTRACTDetermination of petrography and pore fluid content is an ultimate goal of an integrated seismic‐petrophysical study. For lack of a general inversion technique, forward modelling is useful in studying the relations between lithology, stratigraphy, pore fluid content and the seismic response. This report describes a study of two clastic sequences in Utah, from which 32 rock samples were analysed. A detailed petrographic study was done. Laboratory measurements were made of ultrasonic compressional‐ and shear‐wave velocity as a function of pressure. We computed the velocities at seismic frequencies for the samples when dry, over‐pressured, brine saturated, and oil saturated. The velocities were sensitive to the porosity, carbonate cementation and the depositional facies. We generated velocity profiles for hypothetical reservoirs for a range of saturation states. The velocity profiles were used to generate synthetic seismic shot gathers to study the seismic response of these clastic reservoirs. The fluid‐saturation strongly affects the seismic respone, as does the presence of a coal seam. An amplitude change with offset is often observed. However, stratigraphy appears to have a stronger effect on the seismic response.
TWO‐PASS 3D MIGRATION AND LINEARIZED INVERSION IN THE (x, t)‐DOMAIN1
Authors: JAKUBOWICZ H. , MILLER D. 
Published: April 2006

Abstract3D Kirchhoff migration and acoustic Born inversion of zero‐offset seismic data in a constant‐velocity medium can be uniformly factored as a cascade of two 2D diffraction integrals. The formal argument is based on a straightforward implementation of the original time‐domain approach of Gibson, Larner and Levin. The factorization differs from the factorization described by Jakubowicz and Levin in omitting all time‐dependent filters from the 2D operators in favour of ID filtrations performed as a preprocess and a postprocess.
SEISMIC MIGRATION IN ELLIPTICALLY ANISOTROPIC MEDIA1
Authors: VERWEST BRUCE J. 
Published: April 2006

AbstractThe study of wave propagation in media with elliptical velocity anisotropy shows that seismic energy is focused according to the horizontal component of the velocity field while the vertical component controls the time‐to‐depth relation. This implies that the vertical component cannot be determined from surface seismic velocity analysis but must be obtained using borehole or regional geological information. Both components of the velocity field are required to produce a correctly focused depth image. A paraxial wave equation is developed for elliptical anisotropic wave propagation which can be used for modelling or migration. This equation is then transformed by a change of variable to a second paraxial equation which only depends on one effective velocity field. A complete anisotropic depth migration using this transformed equation involves an imaging step followed by a depth stretching operation. This allows an approximate separation or splitting of the focusing and depth conversion steps of depth migration allowing a different velocity model to be used for each step. This split anisotropic depth migration produces a more accurate result than that obtained by a time migration using the horizontal velocity field followed by an image‐ray depth conversion using the vertical velocity field. The results are also more accurate than isotropic depth migration and yield accurate imaging in depth as long as the lateral variations in the anisotropy are slow.
Q‐LOG DETERMINATION ON DOWNGOING WAVELETS AND TUBE WAVE ANALYSIS IN VERTICAL SEISMIC PROFILES1
Authors: MARI J. L. 
Published: April 2006

ABSTRACTSeismic attenuation introduces modifications in the wavelet shape in vertical seismic profiles. These modifications can be quantified by measuring particular signal attributes such as rise‐time, period and shape index. Use of signal attributes leads to estimations of a seismic‐attenuation log (Q‐log).To obtain accurate signal attributes it is important to minimize noise influence and eliminate local interference between upgoing and downgoing waves at each probe location. When tube waves are present it is necessary to eliminate them before performing separation of upgoing and downgoing events. We used a trace‐by‐trace Wiener filter to minimize the influence of tube waves. The separation of upgoing and downgoing waves was then performed in the frequency domain using a trace‐pair filter.We used three possible methods based on signal attribute measurements to obtain g‐log from the extracted downgoing wavefield. The first one uses a minimum phasing filter and the arrival time of the first extremum. The two other methods determine the Q‐factor from simple relations between the amplitudes of the first extrema and the pseudo‐periods of the down‐going wavelet.The relations determined between a signal attribute and traveltime over quality factor were then calibrated using field source signature and constant‐Q models computed by Ganley's method. Q‐logs thus obtained from real data are discussed and compared with geological information, specifically at reservoir level.Analysis of the tube wave arrivals at the level of the reservoir showed a tube wave attenuation that could not be explained by simple transmission effects. There was also a loss of signal coherence. This could be interpreted as tube wave diffusion in the porous reservoir, followed by dispersion. If this interpretation can be verified, tube wave analysis could lead to further characterization of porous permeable zones.
DETECTION OF LATERAL VARIATIONS IN GEOLOGICAL STRUCTURES USING ELECTRICAL RESISTIVITY GRADIENT PROFILING1
Authors: SHETTIGARA V. K. , ADAMS W. M. 
Published: April 2006

ABSTRACTHydrogeological problems which involve the determination of lateral variations in structures or physical properties may be solved with electrical resistivity gradient profiling if there are significant variations in electrical resistivity, spatially or temporally. The method is explained, evaluated for sensitivity, compared with other methods, and applied to the location of volcanic dike zones that are impounding an anomalous water body near Schofield, Oahu, Hawaii. From the electrical soundings and other independent data, the lateral positions of the boundaries have been refined and their nature estimated.
3D ELECTROMAGNETIC INVERSION USING INTEGRAL EQUATIONS1
Authors: EATON PERRY A. 
Published: April 2006

ABSTRACTUtilizing electromagnetic data in geophysical exploration work is difficult when measured responses are complicated by the effects of 3D structures. 1D and 2D models may not be capable of accurately simulating the physical processes that contribute to a measured response. 3D conductive‐host modelling is difficult, costly and time‐consuming. Using a 3D inverse procedure it is possible to automate the interpretation of controlled‐source electromagnetic data. This procedure uses an inverse formulation based on frequency‐domain, volume integral equations and a pulse‐basis representation for the internal electrical field and anomalous conductivity. Beginning with an initial model composed of a 3D inhomogeneous region residing in a laterally homogeneous (layered‐earth) geoelectrical section, iterative least‐squares algorithms are used to refine the geometry and the conductivity of the inhomogeneity. This novel approach for 3D electromagnetic interpretation yields a reliable and stable inverse solution provided constraints on how much the variable can change at each iteration are incorporated. Integral‐equation‐based inverse formulations that do not correctly address the non‐linearity of this inverse problem may have poor convergence properties, particularly when dealing with the high conductivity contrasts that are typical of many exploration problems.While problems associated with contamination of the data by random noise and non‐uniqueness of solutions do not usually influence the inverse solution in an adverse manner, problems associated with model inadequacy and errors in an assumed background conductivity structure can produce undesirable effects.
SPONTANEOUS POTENTIALS AND RESISTIVITY SURVEYS APPLIED TO HYDROGEOLOGY IN A VOLCANIC AREA: CASE HISTORY OF THE CHAÎNE DES PUYS (PUY‐DE‐DÔME, FRANCE)1
Authors: FOURNIER CLAUDE 
Published: April 2006

ABSTRACTTwo geophysical methods [resistivity soundings and spontaneous potentials (SP)] are used to investigate aquifers in the Quaternary volcanic formations of the Chaîne des Puys (France). The interpretation of the resistivity soundings required a specific work of determination of the resistivity of the formations concerned. The basement topography, characterized by axial horst and graben structures and perpendicular palaeovalleys, was revealed. SP was developed experimentally in the field and theoretically. It is shown that the SP anomalies can be considered as double‐layer potentials, with their source on the aquifers' interfaces. The most significant of them is the water table, which creates an SP anomaly of similar but inverted form. SP anomalies due to flows in unconfined aquifers, in palaeovalleys, and in volcanic ranges are computed and compared with the observed ones. These two methods have determined the extent and the boundary conditions of the different hydrogeological basins and have determined the major drainage axes and the groundwater flow pattern.
INVERSION OF MAGNETOTELLURIC DATA: A NON‐LINEAR LEAST‐SQUARES APPROACH1
Authors: PEDERSEN L. B. , RASMUSSEN T. M. 
Published: April 2006

AbstractThe inverse problem of magnetotellurics over a horizontally stratified earth is described, with emphasis on practical application. The inversion is divided into basically two steps. The construction of some best solution, and the analysis of that solution with regard to uncertainty and complexity. For the construction of best solutions a robust non‐linear solver was developed, and for the estimation of parameter errors a modified eigenvalue‐eigenvector analysis is performed to better describe non‐linear effects. The choice of the number of layers is shown to be intimately connected with the structure of data errors and the misfit between model and data. An example from the Siljan impact structure in Sweden illustrates the power of the technique.
MODEL‐BASED TRANSFORMATIONS OF COMMON MIDPOINT GATHERS1
Authors: NÆSS O. E. 
Published: April 2006

ABSTRACTA method for transforming Normal Moveout corrected CMP‐gathers is proposed. The method is based upon the availability of a model of the CMP‐gather. However, the transformation can be performed with any degree of accuracy in the model. Ideally the employed model should be a synthesis of all available a priori information about the particular data set.Mathematically the transformation is performed as follows. The CMP‐gather is considered to be a matrix. This matrix is first decomposed into a set of submatrices of the same dimensions. Each submatrix consists of non‐zero elements or samples with the same relative amount of noise. By reducing each of these submatrices to a vector (a trace) we get a new set of traces. This set then represents the transformed CMP‐gather.The purpose of the transformation is to organize the CMP data in a form which makes it easier both to analyse the noise distribution and to take the necessary steps to improve the signal‐to‐noise ratio at the stacking stage. In principle the method incorporates the exploitation of multichannel recordings with the use of models. Several examples of transformed gathers and their applications to the improvement of real seismic data are shown.
A SIMPLE EFFICIENT METHOD OF DIP‐MOVEOUT CORRECTION1
Authors: JAKUBOWICZ H. 
Published: April 2006

AbstractMuch of the success of modern seismic data processing derives from the use of the stacking process. Unfortunately, as is well known, conventional normal moveout correction (NMO) introduces mispositioning of data, and hence mis‐stacking, when dip is present. Dip moveout correction (DMO) is a technique that converts non‐zero‐offset seismic data after NMO to true zero‐offset locations and reflection times, irrespective of dip. The combination of NMO and DMO followed by post‐stack time migration is equivalent to, but can be implemented much more efficiently than, full time migration before stack.In this paper we consider the frequency‐wavenumber DMO algorithm developed by Hale. Our analysis centres on the result that, for a given dip, the combination of NMO at migration velocity and DMO is equivalent to NMO at the appropriate, dip‐dependent, stacking velocity. This perspective on DMO leads to computationally efficient methods for applying Hale DMO and also provides interesting insights on the nature of both DMO and conventional stacking.
INVERSE THEORY APPLIED TO MULTI‐SOURCE CROSS‐HOLE TOMOGRAPHY.
Authors: PRATT R. GERHARD , WORTHINGTON M. H. 
Published: April 2006

AbstractFrequency‐domain methods are well suited to the imaging of wide‐aperture cross‐hole data. However, although the combination of the frequency domain with the wavenumber domain has facilitated the development of rapid algorithms, such as diffraction tomography, this has also required linearization with respect to homogeneous reference media. This restriction, and association restrictions on source‐receiver geometries, are overcome by applying inverse techniques that operate in the frequency‐space domain.In order to incorporate the rigorous modelling technique of finite differences into the inverse procedure a nonlinear approach is used. To reduce computational costs the method of finite differences is applied directly to the frequency‐domain wave equation. The use of high speed, high capacity vector computers allow the resultant finite‐difference equations to be factored in‐place. In this way wavefields can be computed for additional source positions at minimal extra cost, allowing inversions to be generated using data from a very large number of source positions.Synthetic studies show that where weak scatter approximations are valid, diffraction tomography performs slightly better than a single iteration of non‐linear inversion. However, if the background velocities increase systematically with depth, diffraction tomography is ineffective whereas non‐linear inversion yields useful images from one frequency component of the data after a single iteration. Further synthetic studies indicate the efficacy of the method in the time‐lapse monitoring of injection fluids in tertiary hydrocarbon recovery projects.
THE OPTIMUM GATE LENGTH FOR THE TIME‐VARYING DECONVOLUTION OPERATOR1
Authors: DIMRI VIJAY P. , SRIVASTAVA KIRTI 
Published: April 2006

AbstractThe time‐varying deconvolution operator designed by dividing time‐varying sequence has been extended to include an optimal division of the input data. A numerical example illustrates that the error energy is less in the case of optimally divided input in comparison with arbitrary division.
3D AND 2½ D MODELLING OF GRAVITY ANOMALIES WITH VARIABLE DENSITY CONTRAST1
Authors: RAO D. BHASKARA , PRAKASH M. J. , BABU N. RAMESH 
Published: April 2006

AbstractThe decrease of density contrast in sedimentary basins may be approximated by a quadratic function. A sedimentary basin may be viewed as a number of prisms placed in juxtaposition. Equations in closed form for the gravity anomalies of 3D and 2½ D prismatic models are derived. Approximate equations for these models are also derived for rapid calculations. Efficient methods are developed for anomaly calculation by an appropriate use of the exact and approximate equations, and hence, for 3D and 2½ D modelling. The depths to the basement are adjusted iteratively by comparing the calculated anomalies with the observed anomalies. These methods are applied for analysis of the residual anomaly map of the Los Angeles Basin, California.
METHODS FOR CALCULATING FRÉCHET DERIVATIVES AND SENSITIVITIES FOR THE NON‐LINEAR INVERSE PROBLEM: A COMPARATIVE STUDY1
Authors: McGILLIVRAY P. R. , OLDENBURG D. W. 
Published: April 2006

AbstractA fundamental step in the solution of most non‐linear inverse problems is to establish a relationship between changes in a proposed model and resulting changes in the forward modelled data. Once this relationship has been established, it becomes possible to refine an initial model to obtain an improved fit to the observed data. In a linearized analysis, the Fréchet derivative is the connecting link between changes in the model and changes in the data. In some simple cases an analytic expression for the Fréchet derivative may be derived. In this paper we present three techniques to accomplish this and illustrate them by computing the Fréchet derivative for the ID resistivity problem. For more complicated problems, where it is not possible to obtain an expression for the Fréchet derivative, it is necessary to parameterize the model and solve numerically for the sensitivities ‐ partial derivatives of the data with respect to model parameters. The standard perturbation method for computing first‐order sensitivities is discussed and compared to the more efficient sensitivity‐equation and adjoint‐equation methods. Extensions to allow for the calculation of higher order, directional and objective function sensitivities are also presented. Finally, the application of these various techniques is illustrated for both the 1D and 2D resistivity problems.
A FAST ALGORITHM FOR THE COMPUTATION OF RADON TRANSFORMS1
Authors: HANEVELD C. J. , HERMAN G. C. 
Published: April 2006

AbstractA fast algorithm is presented for numerical evaluation of forward and inverse Radon transforms. The algorithm does not perform exact one‐to‐one mapping as the discrete Fourier transform but, due to the use of band‐limited basis functions, it is robust and sufficiently accurate for seismic applications. By rewriting the transform as a convolution, a computational speed is obtained similar to the speed of the 2D fast Fourier transform.
ELASTIC WAVE PROPAGATION IN TRANSVERSELY ISOTROPIC MEDIA USING FINITE DIFFERENCES1
Authors: TSINGAS C. , VAFIDIS A. , KANASEWICH E. R. 
Published: April 2006

AbstractWhen treating the forward full waveform case, a fast and accurate algorithm for modelling seismic wave propagation in anisotropic inhomogeneous media is of considerable value in current exploration seismology. Synthetic seismograms were computed for P‐SV wave propagation in transversely isotropic media. Among the various techniques available for seismic modelling, the finite‐difference method possesses both the power and flexibility to model wave propagation accurately in anisotropic inhomogeneous media bounded by irregular interfaces. We have developed a fast high‐order vectorized finite‐difference algorithm adapted for the vector supercomputer. The algorithm is based on the fourth‐order accurate MacCormack‐type splitting scheme. Solving the equivalent first‐order hyperbolic system of equations, instead of the second‐order wave equation, avoids computation of the spatial derivatives of the medium's anisotropic elastic parameters. Examples indicate that anisotropy plays an important role in modelling the kinematic and the dynamic properties of the wave propagation and should be taken into account when necessary.
SEISMIC CHARACTER MAPPING OVER RESERVOIR INTERVALS1
Authors: LENDZIONOWSKI V. , WALDEN A. T. , WHITE R. E. 
Published: April 2006

AbstractThis paper presents some results from an investigation into the utility of pattern recognition methods in seismic interpretation. The seismic instantaneous attributes of amplitude, phase and frequency provide a way of quantifying the character of a simple reflection. Measures of character can be developed from cross‐plots and cluster analysis of these attributes. It is demonstrated that such seismic character can produce better‐defined maps than a single attribute. These procedures can be extended to attributes derived from seismic trace segments, such as trace energy and centre frequency, and to multitrace attributes, but more effort is then needed to analyse the attributes and search out useful ones.An introduction is given to projection pursuit which has proved a useful exploratory tool for the anlysis of attribute relationships.It is important to stress that pattern recognition techniques simply help bring relationships and patterns in the data to the attention of the interpreter and the most persistent problem in applying these techniques is the evaluation of potentially interesting patterns. The decision on what use can be made of them is highly interpretive and their calibration is difficult. Well control is vital but it normally allows only very limited supervision of a seismic classifier. An example is presented to illustrate these problems.
SEQUENTIAL WIENER DECONVOLUTION TO IMPROVE SEISMIC RESOLUTION1
Authors: FERBER R.‐G. , KOITKA H. 
Published: April 2006

AbstractTwo different techniques for performing time‐variable Wiener deconvolution are compared using stacked seismic data. The conventional technique involves the empirical division of the data into a number of gates and the determination of time‐invariant deconvolution filters for each gate. In the second technique, the deconvolution filter is recomputed after each time increment from a fixed‐length data gate sliding along the trace. This scheme has the advantage that no a priori segmentation of the data is needed.