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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
251 - 300 of 315 results
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Use of walkaway VSP in imaging a pre-zechstein reservoir in the UK Southern Gas Basin
Authors J. M. Smidt, P. Conn and M. LappinThe results of many VSPs are disappointing because they have not been planned in detail, and processing has not been optimised to achieve the prime objectives due to lack of communication between the specialists involved in planning, acquiring, processing and interpreting the data. In the case study reported here the close co-operation between interpreter, technical advisor and VSP processing geophysicist resulted in a carefully designed survey and an optimised processing sequence which fulfilled the planned objective and yielded further information.
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Some characteristics of coupled wave propagation
More LessCoupled wave propagation describes propagating wavefields of different physical quantities; hereby it is possible that all those fields interact with each other. In exploration geophysics coupling effects are generally believed to be small enough to be neglegable. Comparing different material relevant to exploration, however, shows that coupling can contribute to noticable propagation effects.
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Effects of processing on three-component shear-wave data
Authors P. K. Schruth and I. BushRecent developments in seismology have led to an increase recording of three-component (3-C) data, in particular for the study of seismic anisotropy where shear-waves have vector polarizations in three dimensions. By carrying information about the symmetry structure along the raypath the polarization of shear-waves can be used to evaluate the parameters of the anisotropy and hence provide information about the internal structure of the rock (S. Crampin, 1985). Crampin has shown in numerous papers that shear-waves are particularly sensitive to changes in crack- and pore-geometry caused by changes in pore-pressure, pore-fluid properties and the orientation of the stress field that aligned the cracks.
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Seismic velocities of marine sediments in box cores with concentration on the relationship between shear strenght and shear wavevelocity
Authors I. A. Pecher, Fr. Theilen and S. NebenThe determination of geotechnical parameters at the sea floor is of growing importance for engineering purposes. The measurement of compressional (P-) and especially shear (S-) wave velocities seems to he an appropriate method for the estimation of other physical properties in marine sediments.
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P-S waveseparation using parametric inversion: An offset VSP case study
Authors D. Belaud and W. S. LeaneyThe separation of interfering compressional and shear wavefields is an important problem in the processing of three-component offset VSPs,where, because of the raypath obliquity, the amount of converted shear energy may be significant compared to the P energy.
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S-Wave velocities and shallow layering in a Campi Flegrei site (Naples) through R-wave spectral analysis
Authors F. M. Guadagno and C. NunziataThe Spectral Analysis of Surface Waves (SASW) method is a nondistructive seismic testing aimed to determine shear wave velocities and layering from surface measurements of artificially generated Rayleigh waves (R-Waves). The SASW method has been mostly employed in the evaluation of the elastic properties of pavements (Heysey et al. 1982; Nazarian and Stokoe, 1983) and was also experimented for determining shear wave velocity profiles in a liquefaction susceptible site.
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Images of rock properties estimated from a crosswell seismic velocity tomogram
More LessMethods for obtaining an accurate image of the upper crust in terms of seismic velocities have evolved rapidly during the last decade, especially with well-to-well tomography techniques: well-to- well tomography allows us to constrain velocities more than with classical reftection seismology methods. At the same time, laboratory experiments have increased our knowledge of the influence of rock properties, such as porosity, saturation or clay content, on acoustic measurements. Our goal is to use results obtained in the laboratory for the interpretation in terms of porosity of an actual cross-well survey. Three different approaches are used: applying (1) geostatistical techniques, (2) rock physics relations obtained in the laboratory, and (3) a combination of geostatistics and rock physics.
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Direct estimation of lithology or compaction from seismic and well log data
Authors O. Sylta and J. HelgesenTraditionally, compaction has been determined from seismic stacking velocities or sonic logs. The use of stacking velocities requires a considerable human effort, since consistent velocity picks must be obtained. We propose to fit a depth dependant velocity function to seismic travel time information using the semblance criterion, and thereby obtain estimates of important parameters describing lithology and compaction. The need for human interaction is thus reduced, and more reliable estimates are obtained. Furthermore, the method uses all available travel time information in the depth interval to be analyzed, including zones of weak reflections.
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A comparison of petrophysical equations for extrapolation of lithilogy beyond well locations using seismic data
Authors S. H. Archer and R. D. MartinezSeveral alternative petrophysical models exist for relating the seismic properties of rocks to the elastic parameters of the rock constituents. This paper shows the effect of some alternative petrophysical models on the extrapolation of lithology using seismic data.
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Rock samples microcracks produced by temperature, hydrostatic pressure and fluid pressure stresses: electrical responses and acoustic emissions
Authors G. Losito, M. Muschietti, A. Trova and E. BiagiDuring the last years, many laboratory experiments have been carried out to study electrical properties of rock samples subjected to temperature, hydrostatic pressure and internal pore fluid pressure cycles. For the experiments an "ad hoc" apparatus has been designed at Applied Geophysical Laboratory of University of Florence, that can condition temperature (up to 300 ·C), hydrostatic pressure (up to 40 MPa) and internal pore fluid pressure (up to 40 MPa).
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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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The F-X dip barrier removed with a pie slice
By M. GalbraithThe method of noise reduction known as FX-Deconvolution or FX-Prediction was fust introduced by Canales (1984). Gulunay (1986) pointed out that FX is incapable of correctly predicting coherent energy when more than one dip is present. Examples show that the effect of this in the real world is not as drastic as the mathematics might indicate and FX is now widely used as an effective after stack process. (or pre-stack on fully corrected gathers).
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An introduction to FKK techniques
Authors L. G. Peardon and C. W. M. BaconIn the early days of seismic data analysis, much emphasis was placed on single trace, or 1D, processing. Eventually, we became accustomed to the idea of treating the data as a two-dimensional image and 2D processing became commonplace. Today, of course, with the advent of more and more 3D surveys, there is a requirement for 3D processing techniques. In this paper we discuss a particular 3D analysis tool, namely the FKK transform, and its applications.
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A new environment for land 3D processing
By P. DixonThe traditional medium of magnetic tapes for seismic data storage and processing is being superceded by an increasing use of discs. As these plummet in cost, they are, and will increasingly be, a much more attractive medium for the seismic industry. As well as rendering significant improvements in data handling, the benefits of disc to disc processing also extend to substantial improvements in the quality of processing due to the rapid access to data in various domains.
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Complexity analysis in the double transformed radon domain
Authors M. Vissinga and J. T. FokkemaLet Dr,s(xy;xs;w) represent a data set of a splitspread seismic experiment in the freqency domain, organized for different shot and receiver coordinates xs and xy, respectively.
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On estimation of minimum-phase wavelet
By V. BardanAn important goal of seismic processing is to shape reflection events so that the final seismic section can be interpreted easily and unambiguously. The manipulation of seismic data to achieve this goal is generally called wavelet processing. Typically, some form of deconvolution is used to perform the desired shaping of the wavelets in a seismic dataset. Many different deconvolution algorithms have been developed for processing of seismic data. They all have one thing in common: their derivation are based on mathematical assumptions about the seismic trace. The most important assumptions are: the autocorrelation of the trace is a scaled version of the seismic wavelet and the seismic wavelet is minimum-phase, In these conditions we want to estimate the minimum-phase wavelet.
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Frontier exploration techniques applied to multivintage 2D seismic
Authors E. Rieser and J. D. KerrThe Laa Area lies in the North East of Austria close to the Czechoslovakian border. It is situated in the Undisturbed Molasse of Lower Austria at the edge of the Washbergzone Overthrust. The exploration targets are within the Mesozoic Malm and are changing from structural to stratigraphic traps within the Oncophora Sandstone. Multifold seismic exploration began in 1977-79 with vibroseis data being acquired.
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A Regional water-gun profile across the quaternary basin of the Southern North Sea
Authors J. Bulat, D. Cameron and C. MesdagIn 1989, a European-Community funded project was set up to study the Quatemary deposits of the southem North Sea, involving collaboration between the geological surveys and universities in Denmark, Germany, Netherlands, Belgium and the United Kingdom. The funding has enabled the drilling of nine stratigraphic boreholes and the acquistion of 1000km of digitally recorded 10cu. in air gun data.
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A case history of high resolution seismic methods for exploration of shallow oil deposits in Suriname
Authors P. Goerdajal, J. A. C. Meekes and M. H. MulderIn the coastal region of the Republic of Suriname oil is found in Paleocene sand deposits at depths of about 300 m. Although drilling costs are relatively low for these shallow depths, the structure of the sand bodies is so complex that it was decided to carry out high resolution seismic measurements with the objective to increase the succes ration of producing wells.
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Shallow seismic and gravimetric exploration of brown coal - A review
Authors H. Gaertner, R. Seitz, H. Petzold and H. SchubertFor many years coal was the main resource of primary energy in East Germany. The production of more than 300 mio. tons a year implemented a great volume of geophysical exploration activities including shallow seismic surveys which started 15 years ago. Depending on the actual tasks and seismogeological conditions several shallow seismic techniques were developed which will be presented in this paper together with gravimetric results.
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Optimum acquisition and processing for seismic reflection techniques for extractive industries
Authors I. Hill and L. VannesteSeismic reflection is a highly desirable tool for exploration for the extractive industries due to the combination of good vertical resolution with the ability to image complex structures. The disadvantages are equally apparent, high cost, complexity and delay in receiving processed sections. In addition, the technique is not weIl understood in the extractive industries and has areputation for uncertain results. Our work at Leicester has been directed towards investigating the applicability of the reflection method to a variety of geological environments, and of clearly defining the optimum acquisition and processing techniques for this type of data. With this knowledge we can develop a total system which optimises field acquisition and provides in-field processing and section display.
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Estimation of reflection co-efficients from zero-offset field data
Authors B. Arnsten, B. Ursin and M. LandrøThe classical one-dimensional inverse problem consists of predicting reflection coefficients from surface seismic data using the one-dimensional wave equation. In one spatial dimension only plane waves propagating along one axis can be described. Spherical waves originating from a point source can not be adequately described. Many authors have considered this problem, and given examples of use on field data. An early example is Bamberger et.al. (1982). This work considers the inverse problem of predicting reflection coefficients from vertically travelling spherical waves in a one-dimensional horizontally layered medium. It is a more realistic problem than the classical one in the sense that point sources are commonly used in seismic exploration.
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Calibration of seismic data for inversion
Authors A. Ziolkowski and K. -J. KosterWe present a method, based on the critical reflection theorem, for determining the system sensitivity of a seismic recording system, using only the recorded data and the known source signature.
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A massively parallel implementation of prestack Kirchhoff depth migration
By J. van TierIntegral or Kirchhoff migration methods compute the integral solution of the wave equation, and have been used extensively in seismic imaging (Schneider, 1978; Berryhill, 1979; Berkhout, 1980). For each image point in the subsurface, these methods sum amplitudes of the recorded wavefield along a diffraction curve. The shape of the diffraction curve is described by the impulse response of the image point, the Green function, which depends on the velocity field and the shot-geophone geometry.
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Sub-surface structural images from pre-stack time migrated data at an optimum velocity using a cost effective approach
Authors G. Hodgkiss, G. E. Lane and G. HorganIt has long been recognised that tbe application of dip-moveout (DMO) does not necessarilyresolve all stacking velocity conflicts in areas where: - there is a velocity gradient - where there are steep dips Such circumstances may require pre-stack time migration. A problem with full pre-stack time migration is knowledge of the velocities with which to drive the migration. In principle finding out could be a very expensive exercise. The following case-history demonstrates how to obtain high quality from an efficient approach to gaining a good image from pre-stack time migrated data at an optimum velocity.
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The modified elastic wave equation: Applied to modelling and migration
By O. SelviIn this study a modified elastic wave equation is described which simultaneously permits both upgoing and downgoing propagation. This equation can be expressed as a nonreflecting elastic wave equation Which gives reduced reflection coefficients for transrnission across material boundaries. For horrogeneous region, it becomes the full elastic wave equation.
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An interactive approach to macro model estimation
Authors B. C. Scheffers, J. W. de Bruijn and A. J. BerkhoutAdvanced seismic processing techniques are elastic and depth-oriented. With elastic, depth-oriented processing a more accurate image of the subsurface can be obtained than with acoustic time-oriented processing. Properties of the subsurface can he divided into properties that mainly influence the propagation of seismic waves (macro properties) and properties accounting for reflection of these waves (detail). If the macro properties of the sub surface are known, the propagation effects can be removed from the data, yielding a depth image of the detail. Since the elimination of propagation effects is a major processing goal, a clear understanding of the macro properties of the subsurface is necessary.
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Prestack depth migration in the double transformed radon domain
Authors R. Tatalovic and J. T. FokkemaThe proposed migration procedure is performed in the double Radon transfonned frequency domain. The main advantages of the procedure are that it does not require a detailed input velocity model, does not need decomposition to up- and downgoing waves, and does not assume laterally invariant geology.
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High-dip time migration
Authors J. L. Black, M. A. Brzostowski and H. CrookWe introduce a steep-dip time migration algorithm which exploits the strenghts of current time migration algorithms and minimizes their weaknesses. This new time migration scheme uses Stolt and phase-shift migration as a base and finite-difference migration as a residual. The data is first redatumed using a phase-shift operation and then Stolt migrated using a constant velocity over some time interval.
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Parallel implicit finite-differences prestack migration
Authors B. Moorhead and B. BiondiPrestack finite-difference depth migration is a widely used method for imaging complex geological structures (Schultz and Sherwood, 1980). This migration method not only can be used for imaging seismic data in presence of lateral variations in velocity, but its results can be also used for improving the velocity model by applying focusing analysis to the migrated wavefield (Faye and Jeannot, 1986).
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Smoothing constraints in cross-hole seismic tomography
Authors M. Pilkington and J. P. TodoeschuckThe cross-hole seismic tomography problem is ill-posed in the sense that small changes in travel times may cause excessive variations in the calculated slowness image. These artifacts can have amplitudes large enough to mask the true velocity structure and a spatial extent small enough than the resolving power of the data. Consequently, the image should be smoothed to pro duce results that are geologically meaningful.
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