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53rd EAEG Meeting
- Conference date: 26 May 1991 - 30 May 1991
- Location: Florence, Italy
- ISBN: 978-90-73781-03-0
- Published: 28 May 1991
261 - 280 of 315 results
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Surface waves: Nothing but noise?
Authors C. Schneider, A. Virtlmayer, R. Misiek and L. DresenIn exploration geophysics, seismic surface techniques have been developed to a high degree. Information about the underground is solely based on body waves. Dispersive surface waves are believed to be undesired noise. Thus, field layouts try to avoid surface waves, and data processing metbods are used to attack Love- and Rayleigh-waves. Methods to generate and process dispersive surface waves to search tbe underground, as well known from earthquake observations or in-seam seismic techniques, are most unusual in exploration geophysics. However, ignoring surface waves means not seeing valuable signaIs, at least from the near surface underground.
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Determination of elastic constants pertaining to a symmetry plane of an orthorhombic medium under arbitrary overburden from three-component offset VSP measurements
By K. HelbigThe propagation of elastic waves in the 1-3 plane of an orthorhombic medium (3-axis vertically downwards) is controlled by six elastic stiffnesses: c11, c13, c33, c44, c55, and c66. For propagation in the 2-3 plane c1 and c13 are replaced by c22 and c23. The remaining elastic stiffness c12 influences propagation in the horizontal (symmetry) plane. Propagation in oblique directions involves all nine elastic stiffnesses.
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A new approach to studying the anisotropy of seismic waves velocities
More LessThe traditional approach to studying seismic anisotropy is based on using elastic parameters Cij according to generalised Hook's law. The geological medium is approximated by a homogeneous anisotropic model in which seismic rays are straight lines.
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The wave packages change in the wave propagation
More LessThe investigation of the non-linear effects during the propagation of the seismic waves like behaviour of the structure of the wave field from the intensivity, non-linear interaction of the seismic waves, soliton property etc. is one way to receive additional information for nonlinearity of the media.
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Observations of seismic anisotropy on the Norwegian continental shelf
Authors T. Stordal and R. KanestrømWe have investigated the P wave anisotropy at three different locations on the Norwegian Continental Shelf. Two of the locations are in the North Sea and one location is in the Mid- Norwegian Shelf. In one of the locations the seismic data consisted of zero-offset VSP data and reflection and refraction data along two perpendicular lines crossing through the well. In the other two locations the seismic data consisted of zero-offset VSP data and walk-away VSP-data along perpendicular profiles symmetrical around the well. For all the wells sonic logs and density logs were available.
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Acoustic anisotropy of synthetics with controlled crack geometries
Authors J. S. Rathore, E. Fjaer, R. M. Holt and L. RenlieThe knowledge of crack distributions is of vital importance for the withdrawal of fluids from reservoirs in which cracks exist as penetrating fractures, serving as primary channels for fluid flow. The presence rif cracks affects the acoustic wave velocities giving rise to anisotropy. It is to be able to predict theoretically whether the observed anisotropy is caused by a large number of small, isolated microcracks, or by a few similarly oriented, larger and possibly connected fractures. Several theoretical models for the acoustic behaviour in cracked and jointed media exist (Hudson, 1981, Thomsen, 1986), yet none of them have been confirmed in a controlled experiment, one in which sizes, shapes, amounts and orientational distributions of cracks are known. In geomaterials these are generally unknown parameters. Hence we have undertaken the task of producing a synthetic material in which the above crack parameters are known.
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Anisotropic scalar imaging
Authors M. Karrenbach and F. MuirWith increased interest in anisotropic propagation effects and integration of different acquisition geometries and scales into one data set, one would like to be able to conveniently use existing well-known scalar algorithms. Many generic seismic processing algorithms are based in one way or another on an isotropic assumption. Here we show a practical way, how to extend existing scalar migration or modeling algorithms to simple anisotropic symmetry systems. We compare exact anisotropic dispersion relations to their double elliptic approximation. These schemes apply only to scalar wave fields. A scalar Eigenfield is a scalar representation (not necessarily complete) of a vector Eigen field, such as reflected p-vector wave field, s-vector wave fields, Means for obtaining a scalar Eigenfield include operations like: rotations or sophisticated removal of free surface effects.
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Comparison of theoretical formulations for deriving the elastic constants of finely-layered cracked media
By I. BushThe main causes of seismic anisotropy in sedimentary rocks are the layer-induced anisotropy of finely stratified isotropic rock (fine-layer anisotropy) and the anisotropy due to distributions of stress-aligned fluid-filled micro-cracks (extensive-dilatancy anisotropy). Shear-wave propagation in finely-layered cracked media is characterized by the presence of point singularities, which occur at different directions (incidences and azimuths) of shearwave propagation.
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Anisotropic physical modelling of multioffset profiles over an orthorhombic medium
Authors R. J. Brown, D. C. Lawton and S. P. CheadleIn reflection seismic surveys involving an anisotropic medium, the apparent velocity variation is no longer hyperbolic because the NMO (normal-moveout or stacking) velocity is, in general, no longer invariant with direction of propagation. This variation of velocity with direction in symmetry planes has been studied by many (Postma 1955; Crampin and Kirkwood 1981; among others) and recently Thomsen (1986) presented approximate equations for qP, qSV and SH velocities for transversely isotropic media (valid also in symmetry planes of any anisotropic symmetry system) correct to the fust order in quantities that are small for weak anisotropy. Thomsen's (1986) expressions are for phase velocity but he reaffirms that the difference between group and phase velocity is a second-order quantity. We examine this relationship to enable comparison of experimentally observed NMO (group) velocities with phase velocities from Thomsen's approximate relations.
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Information content of amplitude versus offset of shear-waves in anisotropic media
Authors G. S. Yardley, G. Graham and S. CrampinMany recent publications (Alford 1986, Martin and Davis 1987, Squires et al 1989) have analyzed anisotropy in shear-wave reflection surveys. Such studies yield values of the percentage anisotropy over wide depth ranges in the shear-wave section. Percentage anisotropy is usually calculated from the time delays between the arrivals of the fast and slow split shear-waves and gives information about the crack density and porosity in the section. The polarization direction of the fast split shear-wave is determined by the crack and stress geometries in the rock and is Iikely to yield information about preferential directions of fluid flow within a reservoir.
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Inversion of VSP field data for anisotropy with singularities
By C. MacBethInversion of the parameters characterizing shear-wave splitting, provide a way of obtaining a good starting model for full-wave modelling, improving the convergence to a final waveform match. This is necessary as the shear-waves behave in a highly non-linear fashion with respect to anisotropy. To arrive at such a model for the subsurface. a two stage process is involved. Firstly, estimates for the parameters which specify the morphology of the split shear-waves are obtained, and secondly, these values are interpreted in terms a range of physically plausible anisotropic models. The estimation stage in this work is still in its infancy, however MacBeth (1990) demonstrated that it is possible using current rudimentary algorithms, to achieve a well resolved inversion result using the polarization of the leading split shear-waves, provided VSP data for several azimuths of direction are recorded. The subject of this present work is to show that these results may be used to facilitate the final fit using full-wave modelling.
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Processing anisotropy with the shear-wave analysis package (SWAP)
By P. WildOver the three years that the Edinburgh Anisotropy Project has been running, a significant number of computer programs have been written with a view to analyzing the effects of seismic anisotropy from multi-component data sets. The computer programs contain a number of important new tools for understanding and evaluating the behaviour of split shear-waves.
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Implementing pre-stack rotation for four-component shear-wave reflection data
Authors X. -Y. Li and S. CrampinSource-geophone rotation has now become a conventional procedure for processing four-component (two-horizontal sources and two-horizontal receivers) shear-wave reflection data, as demonstrated by Alford (1986), and Squires et al. (1989) and among others. To apply source-geophone rotation, a post-stack rotation analysis (Alford 1986; Squires et al. 1989), or a least square fitting procedure (Murtha 1989) is required to determine the optimum rotation angle. Since it is computing intensive and time consuming, pre-stack rotation is an unwelcome additional procedure (Sriram et al. 1990).
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Interpretation of seismic anisotropy parameters
Authors F. M. Lyakhovitsky and L. Y. BrodovIn accordance with experimental data, in many cases sedimentary deposits are found to be anisotropic media for long seismic waves. With sufficient approximation the model of these rocks is transversely isotropic media (TIM) with horizontal or vertical plane of isotropy, and they are called polar or azimuthal anisotropy, respectively.
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Modelling anisotropic waves in three dimensions
Authors H. Igel, P. Mora and D. RodriguesA finite difference approximation to the elastic wave equation has been implemented on the parallel connection machine (CM). Realistic three dimensional Earth models can be created by a joint surface modeling package (GOCAD). To each homogeneous domain anisotropic stiffnesses may be attributed using Hudson's theory for cracks [1] or the group-theoretical formulations from Schoenberg and Muir [2]. The resulting anisotropic symmetry system can have any spatial orientation. Wavefronts in three dimensions and seismograms for a saltdome structure including anisotropic layers are shown.
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Wavefield decomposition for anisotropic media
By D. NicholsMany authors have proposed processing multicomponent data by treating each wavetype as a separate scalar wavefield. While this is not valid in general media it is a very convenient approximation that has proven to be useful in practice. The first stage in using this method is to decompose the displacement fields measured at the surface into fields corresponding to the amplitudes of the different wavetypes.
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Migration velocity analysis: A comparison of two approaches
Authors K. L. Schleicher, D. J. Grygier and M. A. BrzostowksiWave equation processing algorithms are available to image very complex structures. In most cases these processes are limited by our ability to estimate the correct velocity field. There are two types of migration Velocity Analysis (MVA) that use migration as a tooI to estimate velocity. The first type, velocity scanning MVA, prestack migates the data with a suite of velocities.
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Processing of reflection data with the CRE method
Authors H. Steentoft and W. RabbelDiviations from horizontal layering are often observed and lead to imperfectly stacked sections. In order to compensate shortcomings of the Common - Midpoint (CMP) - method additional processing steps such as Dip Moveout (DMO) are usually performed.
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Orthogonalized noise subtraction
Authors F. Poletto and A. CragliettoExperience gained processing data recorded in a seismic experiment with a drilling-bit source suggests that the acquisition could be improved by integrating it with a monitoring of the principal sources of the noise due to engines and yard activity.
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Removal of water multiples - A new F-K approach
Authors D. E. Lokshtanov, O. A. Sandvin and E. S. HusebyeHere we propose a frequency-discrete wavenumber (F-K) method for removing water multiples from shot-record reflection data. The method is based on the acoustic wave equation in 2-D media. modelling the seismic reflection response from a stack of sub surface interfaces. which can have irregular boundaries. However, no knowledge of the subsurface geology below the water bottom is required in order to remove water multiples.
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