bluuebunny/crossref_metadata_2025_split huggingface dataset:
SEISMIC INTERPRETATION OF UPPER ELK POINT (GIVETIAN) CARBONATE RESERVOIRS OF WESTERN CANADA1
Authors: BROWN R. J. , ANDERSON N. L. , HILLS L. V. 
Published: April 2006

AbstractThe seismic signatures of three reefs of the Upper Elk Point Subgroup (Givetian Stage) of the Western Canada Sedimentary Basin are documented and analysed on the basis of variations in seismic image of particular lithologic units, lateral amplitude and/or phase changes, structural relief and velocity‐generated relief, as rendered by the reflection data. The effects on seismic signatures of spatial geological variations resulting from such phenomena as differential compaction, reef‐focused salt dissolution, palaeotopography, lateral and vertical facies variations, regional dip, and reservoir morphology are discussed. The usefulness of seismic data in clarifying relationships between reefs and their adjacent sedimentary sections, particularly in cases where well control is sparse, is also considered. Such documentation of seismic signatures from known reefs using geophysical and geological analysis can establish criteria to enable recognition of similar buildups elsewhere.Three example reefs are presented, each typical of a particular area and environment of W. Canada. The first is from the Winnipegosis Formation of SE Saskatchewan, the second and third from the Rainbow Member and Upper Keg River Reef Member, respectively, of the Keg River Formation of NW Alberta. All three of these carbonate buildups developed in the evaporitic Elk Point Basin. However, the degree of salt encasement and subsequent dissolution varied greatly, as do the resulting seismic effects. For these three reef types, the typical elements of their seismic signatures have been compiled and are here summarized.
AN ADVANCED CMP RAY‐TRACING1
Authors: KOREN ZVI 
Published: April 2006

AbstractThe ray‐tracing algorithm presented in this paper is based on formulae derived for the common reflecting element (CRE) stacking method. A 2D, smooth, laterally‐varying media is assumed where offset rays and traveltimes are evaluated from normal‐incidence (central) rays. The method uses a second‐order asymmetrical approximation for rays and an additional oblique spherical approximation of the central wavefronts for calculating offset traveltimes. In order to solve the two‐point ray‐tracing problem for the common midpoint (CMP) configuration of source‐receiver pairs located symmetrically around the CMP stations, the central rays are perturbed to satisfy the above‐mentioned asymmetrical distribution. Although the accuracy of the calculations is limited for far offsets, it is still good for distances of the order of the reflecting depths. Since only a few normal‐incidence rays are traced through the medium, the method is very fast and is found to be most attractive for iterative inversions in macromodel estimation.
DIRECTIONAL DECONVOLUTION OF MARINE SEISMIC REFLECTION DATA: NORTH SEA EXAMPLE1
Authors: ROBERTS G. A. , GOULTY N. R. 
Published: April 2006

AbstractDirectional deconvolution of the signature from a marine seismic source array may be achieved in combination with prestack migration or dip moveout (DMO) processing. The benefit is demonstrated using an example profile from the southern North Sea. In particular, shallow, dipping reflectors have improved continuity and frequency content. The method could be extended to 3D data to remove both in‐line and cross‐line directivity effects.
ANISOTROPY DETECTED IN SHALLOW CLAYS USING SHEAR‐WAVE SPLITTING IN A VSP SURVEY1
Authors: DOUMA JAN , DEN ROOIJEN HENK , SCHOKKING FLORIS 
Published: April 2006

AbstractShear waves can provide valuable information about seismic anisotropy. On entering an anisotropic medium, a shear wave generally splits (shear‐wave splitting) into a fast and a slow quasi‐shear wave with polarizations fixed by the elastic properties of the medium and direction of travel. If the medium contains planar discontinuities with common normals, the fast shear wave will be suitably propagated if its polarization lies in the plane of the discontinuities. Measuring this polarization, using a VSP geometry with oriented three‐component geophones in the borehole, offers the possibility of monitoring the orientation and density of the discontinuities as a function of depth.Such a shear‐wave VSP was carried out in an uncased 0.3 m diameter borehole drilled to a depth of 120 m in the north of The Netherlands. The upper 80 m of the sequence, consisting of a glacial till and sands and clays of Pleistocene age, was studied. The clays in this sequence have been subjected to glacial deformation and as a result are overconsolidated and locally fissured.In our shallow VSP experiment, shear‐wave splitting and therefore anisotropy was identified at various geophone depths for one source offset. Hodograms showed a consistent polarization of the fast shear‐wave component over a large depth interval. Under the assumption that the anisotropy was caused by planar discontinuities with common normals, this polarization direction gives the strike of the fissures in this interval. The polarization direction of the fast S‐wave did not correspond exactly with the strike which was obtained from geological information on the fissures. The geological information was from undisturbed oriented 70 mm core samples taken at 3 m intervals in the borehole. The discrepancy, however, could be explained in terms of dipping fissures, and such a dip was confirmed by the geological and geotechnical information.The orientation of fissures is an important factor in the directional deformation and strength characteristics of clays as far as geotechnical behaviour is concerned. This study thus illustrates a practical application of shear‐wave splitting observed in shallow shear‐wave VSP for geology and geotechnical engineering.
3D GRAVITY INVERSION OF THE CHESHIRE BASIN1
Authors: ABDOH A. , COWAN D. , PILKINGTON M. 
Published: April 2006

AbstractThree‐dimensional inverse gravity modelling is used to determine the structure of the Carboniferous sediments underlying the Jurassic and Permo‐Triassic formations in the Cheshire Basin. The gravity effect of the Jurassic and Permo‐Triassic rocks was first removed from the observations and the resulting residual gravity field was then inverted to map variations in depth of the Carboniferous formations. The inversion technique is performed in the frequency domain, enabling rapid computation of solutions for large gridded data sets. Evaluation of results shows that there is good agreement between calculated and observed values, with interpreted depths to the base of the Carboniferous in excess of 6 km. Gravity inversion results also delineate areas of faulting on the south‐western and eastern edges of the basin.
PULSE DISTORTION IN CAUSTIC REGIONS1
Authors: GELIUS L.‐J. , STAMNES J. J. , HEIER H. 
Published: April 2006

AbstractIt is well known that there is a phase shift of π associated with the propagation of a seismic pulse through a focus. But this simple behaviour is true only if the two points at which the pulse is observed lie sufficiently far from the focus, one on either side. Near the focus matters are more complicated and the shape of the pulse may be severely distorted because of diffraction. To examine this problem we consider a pulse generated by a point source and focused by a spherical reflector. Using a general combined method of ray tracing and diffraction, we compute seismograms for receivers lying both inside and outside the focal region, and determine how the shape of the pulse changes on propagation through the focus. To assist the physical interpretation of the numerical results, we also present relevant exact and asymptotic results for comparison.
RESOLUTION ANALYSIS OF BAND‐LIMITED AND OFFSET‐LIMITED SEISMIC DATA FOR PLANE‐LAYERED SUBSURFACE MODELS1
Authors: VAN RIJSSEN E. P. F. , HERMAN G. C. 
Published: April 2006

AbstractA singular‐value decomposition technique is presented for quantifying the information content of band‐limited and offset‐limited seismic reflection data for the case of plane‐layered subsurface models. With the aid of this method, vertical resolution, tuning effects and ambiguities between different types of lithological parameters can be analysed simultaneously. The method is applied in a model study for quantifying the effects of offset‐limitation and the presence of different wavetypes on the ability to discriminate different types of parameters.
INTEGRATED EXPLORATION FOR LOW‐TEMPERATURE GEOTHERMAL RESOURCES IN THE HONEY LAKE BASIN, CALIFORNIA1
Authors: SCHIMSCHAL ULRICH 
Published: April 2006

AbstractAn integrated exploration study is presented to locate low‐temperature geothermal reservoirs in the Honey Lake area of northern California. Regional studies to locate the geothermal resources included gravity, infra‐red, water‐temperature, and water‐quality analyses. Five anomalies were mapped from resistivity surveys. Additional study of three anomalies by temperature‐gradient and seismic methods was undertaken to define structure and potential of the geothermal resource. The gravity data show a graben structure in the area. Seismic reflection data indicate faults associated with surface‐resistivity and temperature‐gradient data. The data support the interpretation that the shallow reservoirs are replenished along the fault zones by deeply circulating heated meteoric waters.
ANALYSIS OF THE COMMON MIDPOINT GATHER BY DECOMPOSITION INTO ELEMENTARY WAVEFRONTS1
Authors: THORE PIERRE , DE BAZELAIRE ERIC 
Published: April 2006

AbstractThe imaginary wavefront of a common midpoint gather of seismic data is analysed in terms of elementary wavefronts. The wavefronts emitted by the sources are split into a sequence of elementary circular wavefronts. Each such wavefront is independently transmitted through the subsurface and received by a group of receivers associated with the sources. The theory is first demonstrated in the case of a one‐layer medium and is then extended to the case of a multilayer medium.We show that pairs of imaginary stationary points F and F' occur in the first medium through which all the rays, corresponding to the group of sources and receivers associated with the elementary wavefront pass. The location of the points is determined by 2n equations with 2n unknowns; (2n ‐ 1) of these equations correspond to n layers of the medium, and an additional equation is given by the geometry of the CMP gather. Applying Fermat's principle between the points F and F', we obtain an algorithm for CMP ray tracing. The wavefront sampling problem is considered but not theoretically solved. Results obtained in the general case (a multilayer medium with lateral velocity variation) are presented and compared with those obtained by other modelling techniques.
PARAXIAL RAY TRACING IN 3D INHOMOGENEOUS, ANISOTROPIC MEDIA1
Authors: GIBSON RICHARD L. , SENA ARCANGELO G. , TOKSÖZ M. NAFI 
Published: April 2006

AbstractMany investigations of the propagation elastic waves within the earth require a technique for producing synthetic seismograms which is capable of modelling 3D propagation effects. Ray methods are an excellent option for these problems, because they can be made fully 3D and allow a relatively quick and flexible computation of synthetic seismograms. However, the two point problem of finding the ray which connects exactly a specific source and receiver, may still be difficult and time consuming. Therefore, application of the paraxial method, which allows extrapolation of the information on a given ray to nearby receiver locations, is very valuable. With this approach, great savings in computation time and significant simplification of computer codes are possible.We investigate the application of the paraxial ray method to two problems in which the effects of 3D seismic wave propagation are important. The first is a model of a reef structure. In this case, we consider synthetic seismograms for a VSP experimental configuration. When the SV source and well are located along the axis of the reef, only 2D propagation effects are observed. If the source‐receiver plane is located to the side of the reef, however, the 3D shape of the reef causes significant amplitudes to be predicted for shear‐wave arrivals on the transverse component of the synthetic seismograms.The second example is a ID, layered earth model, but it includes two layers which are azimuthally anisotropic due to the presence of aligned, vertical fractures. This anisotropy leads to 3D raypaths. Synthetic seismographs are presented for a cross‐hole geometry both for an equivalent isotropic model and for the direction parallel to the fractures and at an angle of 45° to the fractures in the anisotropic case. These synthetics show that the differences between the isotropic case and the case for source and receivers aligned with the plane of the fractures are small and subtle. On the other hand, the predictions for the direction at 45° to the cracks show shear‐wave splitting and significant transverse component signal.These results have important implications for both modelling and for applications such as tomography. It is clear that for some of the cases considered, a 2D algorithm will lead to errors in interpretation of data. In addition, ray‐based tomographic techniques will have great difficulty in obtaining a well‐defined 2D planar image when the signals are propagating in regions outside the image plane.
EXAMINATION OF A SPECTRAL METHOD FOR MEASURING THE EFFECTS OF ANISOTROPY1
Authors: MACBETH COLIN , CRAMPIN STUART 
Published: April 2006

AbstractThe most diagnostic effect of anisotropy on shear waves is shear‐wave splitting. This phenomenon creates a distinctive signature in the 3D particle motion. Methods to extract the effects of anisotropy from shear‐wave data attempt to measure details of this motion. Many techniques have been published recently which process the shear waves in the time or frequency domain. Here we examine the way in which information on the interference effects between the split shear waves is contained within the frequency domain, and suggest some criteria which may be used in future processing algorithms. The time‐delay between the split shear waves, and the polarization direction of the leading shear wave can be converted into easily measured features from analysis of the Fourier spectrum of the shear‐wave signal on each component of motion. These features arise in the spectral interference patterns which are formed by the interaction between the two closely‐spaced and similar waveforms. The interference patterns are interpreted for synthetic and observed seismogram data.
NEAR‐SOURCE PULSE PROPAGATION: APPLICATION TO Q‐DETERMINATION1
Authors: JONGMANS D. 
Published: April 2006

AbstractAmong the approaches generally used to measure attenuation from field data, the study of the first pulse broadening appears to be one of the more promising methods to estimate the quality factor Q for different geological formations including soils. Using a numerical scheme, we studied the evolution of the pulse shape in the neighbourhood of the source in order to establish the limits of the method. It was found that the pulse width variations depend strongly upon source depth. At short distances from the source, the pulse shape is controlled mainly by the near‐field terms and/or the onset of surface waves. The investigations proved that the pulse‐broadening method is reliable for distances greater than about 1.2 wavelengths. From numerical experiments, the maximum error in Q‐determination is found to be 10% in the half‐space case.
GEOELECTRICAL SURVEYS OF DIPPING STRUCTURES1
Authors: BERNABINI M. , CARDARELLI E. 
Published: April 2006

AbstractAmong resistivity methods, models containing two dipping discontinuity surfaces with a conductive medium between them have been considered in this study. The theoretical apparent resistivity curves obtained for such models were calculated using Alfano's integral equation for various dip angles of planes at different array distances from the contacts. The results obtained showed that it is possible to achieve the dip values of the discontinuities under particular conditions, but ambiguities cannot be ruled out.
APPLICATION OF THE MULTIFREQUENCY HORIZONTAL‐LOOP EM METHOD IN OVERBURDEN INVESTIGATIONS1
Authors: PALACKY G. J. 
Published: April 2006

AbstractIn recent years, geophysical methods (shallow seismic, electromagnetic, resistivity, ground penetrating radar) have been increasingly applied to overburden investigations. Their effectiveness has been found to depend significantly on local geological conditions. Compared with advanced seismic techniques, EM methods are faster and hence more cost‐effective, but they have not been considered sufficiently accurate.Analysis is carried out of data obtained with the multifrequency horizontal‐loop method (HLEM) in northeastern Ontario, where the overburden consists of Quaternary glacial and glaciolacustrine sediments. Surveying along 1‐6 km long profiles permitted recognition of bedrock inhomogeneities and selection of sites suitable for HLEM data interpretation using the layered model. Phasor diagrams and computer inversion based on the ridge regression technique were used to interpret HLEM soundings obtained at eight frequencies. Interpreted layer resistivities and thicknesses were correlated with the results of Rotasonic drilling at 70 sites. Relatively accurate estimates of overburden thickness (within 10%) could be obtained in about 80% of the cases. Nine examples of HLEM soundings are given and discussed: three each of one‐, two‐ and three‐layer situations. An appropriate interpretation model cannot be selected simply by minimizing the rms error or by analysing the parameter resolution matrix. Frequently, the most effective way of evaluating a solution is to consider whether resistivity values determined by inversion fit any of the ranges determined by statistical analyses of sediment resistivities. A previously published study of electrical properties of Quaternary sediments indicated that resistivities of clay, till and sand are stable within a fairly large area, such as the one under investigation. While the application of HLEM methods to mapping of Quaternary sediments can be considered a success, interpretation of EM data in regions covered by glacial sediments is more difficult than in weathered terrains, where near‐surface layering is more predictable. The problem of equivalence causes non‐uniqueness in interpretation. Thickness equivalence, which results in poor resistivity estimates, was found to affect areas convered by sand and till. Conductance equivalence caused poor resolution of thickness and resistivity for thin clay layers (less than 10 m).
ACOUSTIC MODELLING ON A GRID OF VERTICALLY VARYING SPACING1
Authors: JASTRAM CORD , BEHLE ALFRED 
Published: April 2006

The Fourier spectral method and high‐order differencing have both been shown to be very accurate in computing spatial derivatives of the acoustic wave equation, requiring only two and three gridpoints per shortest wavelength respectively. In some cases, however, there is a lack of flexibility as both methods use a uniform grid. If these methods are applied to structures with high vertical velocity contrasts, very often most of the model is oversampled. If a complicated interface has to be covered by a fine grid for exact representation, both methods become less attractive as the homogeneous regions are sampled more finely than necessary.In order avoid this limitation we present a differencing scheme in which the grid spacings can be extended or reduced by any integer factor at a given depth. This scheme adds more flexibility and efficiency to the acoustic modelling as the grid spacings can be changed according to the material properties and the model geometry. The time integration is carried out by the rapid expansion method. The spatial derivatives are computed using either the Fourier method or a high‐order finite‐difference operator in the x‐direction and a modified high‐order finite‐difference operator in the z‐direction. This combination leads to a very accurate and efficient modelling scheme. The only additional computation required is the interpolation of the pressure in a strip of the computational mesh where the grid spacing changes.
SEISMIC PHASE UNWRAPPING: METHODS, RESULTS, PROBLEMS1
Authors: SHATILO A. P. 
Published: April 2006

AbstractSix known methods of seismic phase unwrapping (or phase restoration) are compared. All the methods tested unwrap the phase satisfactorily if the initial function is a simple theoretical wavelet. None of the methods restore the phase of a synthetic trace exactly.An initial validity test of the phase‐unwrapping method is that the sum of the restored wavelet phase spectrum and the restored pulse‐trace phase spectrum (assuming the convolutional model for the seismic trace) must be equal to the restored phase spectrum of the synthetic trace. Results show that none of the tested methods satisfy this test. Quantitative estimation of the phase‐unwrapping accuracy by correlation analysis of the phase deconvolution results separated these methods, according to their efficiency, into three groups. The first group consists of methods using a priori wavelet information. These methods make the wavelet phase estimation more effective than the minimum‐phase approach, if the wavelet is non‐minimum‐phase. The second group consists of methods using the phase increment Δø(Δω) between two adjacent frequencies. These methods help to decrease the time shift of the initial synthetic trace relative to the model of the medium. At the same time they degrade the trace correlation with the medium model. The third group consists of methods using an integration of the phase derivative. These methods do not lead to any improvement of the initial seismic trace.The main problem in the phase unwrapping of a seismic trace is the random character of the pulse trace. For this reason methods based on an analysis of the value of Δø(Δω) only, or using an adaptive approach (i.e. as Δω decreases) are not effective. In addition, methods based on integration of the phase derivative are unreliable, due to errors in numerical integration and differentiation.
SOME FADS AND FALLACIES IN SEISMIC DATA ANALYSIS1
Authors: WALDEN A. T. , WHITE R. E. 
Published: April 2006

AbstractIn many branches of science, techniques designed for use in one context are used in other contexts, often with the belief that results which hold in the former will also hold or be relevant in the latter. Practical limitations are frequently overlooked or ignored. Three techniques used in seismic data analysis are often misused or their limitations poorly understood: (1) maximum entropy spectral analysis; (2) the role of goodness‐of‐fit and the real meaning of a wavelet estimate; (3) the use of multiple confidence intervals.It is demonstrated that in practice maximum entropy spectral estimates depend on a data‐dependent smoothing window with unpleasant properties, which can result in poor spectral estimates for seismic data.Secondly, it is pointed out that the level of smoothing needed to give least errors in a wavelet estimate will not give rise to the best goodness‐of‐fit between the seismic trace and the wavelet estimate convolved with the broadband synthetic. Even if the smoothing used corresponds to near‐minimum errors in the wavelet, the actual noise realization on the seismic data can cause important perturbations in residual wavelets following wavelet deconvolution.Finally the computation of multiple confidence intervals (e.g. at several spatial positions) is considered. Suppose a nominal, say 90%, confidence interval is calculated at each location. The confidence attaching to the simultaneous use of the confidence intervals is not then 90%. Methods do exist for working out suitable confidence levels. This is illustrated using porosity maps computed using conditional simulation.