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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
41 - 60 of 315 results
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Prestack frequency-wavenumber (f-k) migration in a transversely isotropic medium
Authors A. Gonzalez and W. LynnPrestack frequency-wavenumber (f-k) migration (Stolt, 1978) is an efficient way to do prestack time migration. This technique is also useful for migration velocity estimation, where it is expected to yield dipindependent velocities (Li et al, 1991). In practice, however, we often find that imaging velocities do exhibit dip-dependency. This is especially noticeable with fault-plane reflections which usually image best at velocities higher than those of surrounding flat events. In a companion paper (Lynn et al, 1991), it is shown that the dip dependency of migration velocities can be explained by assuming that waves have propagated in a transversely isotropic earth. Because anisotropy affects both processing and interpretation of seismic data, it is important to identify its presence and measure its strength. In this paper we develop the necessary modifications to prestack f-k migration that compensate for such media. Moreover, we find that the transverse-isotropy adjustment parameter can be used to estimate, in an average sense, the amount of transverse isotropy.
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A unified formulation of wave field decomposition: theory and examples
Authors P. Herrmann, C. P. A. Wapenaar and A. J. BerkhoutUsing a recording by multicomponent sources and receivers, it is possible to decompose the vector wave field (velocities and stresses) into its scalar components (P and S waves) at the source and receiver side. The fully decomposed data set can constitute the starting point of a new procedure to process multicomponent data, as has been described by Berkhout and Wapenaar (1990).
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Downward extrapolation of decomposed P and S wave fields
Authors G. Haime and C. P. A. WapenaarThe use of amplitude information in pre-stack seismic data to delineate and characterize reservoirs has shown a considerable growth in recent years. Therefore, in redatuming or prestack migration techniques it is required that the used extrapolation operator contains all the necessary characteristics to treat both the phase and amplitude information in a correct manner, even in cases where the medium is strongly inhomogeneous and/or anisotropic.
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Finite-difference prestack reverse time migration using P-SV wave equation
Authors M. A. B. Botelho and P. L. StoffaPrestack Reverse Time Migration (RTM) is performed using a first-order system of hyperbolic elastic equations of motion and constitutive laws expressed in terms of particle velocities and stresses. It is well known that the RTM procedure must contain three important steps, i.e., it must apply (a) the imaging condition, (b) the reverse time extrapolation and (c) the extraction of amplitudes from the seismic data in order to build the final images. Here the above mentioned steps are implemented using the same code, which is a two-dimensional explicit finite-difference scheme. It is of second-order accuracy in time and fourth-order accuracy in space.
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Extraction of accurate P- and S-wave velocities from fixed offset well seismic profiles, UK Central North Sea
Authors J. M. Reilly and M. IdreesThe acquisition and processing of both compressional and shear wave energy from Well Seismic Profiles (WSP) is an important source of data for the investigations of seismic Amplitude Versus Offset (AVO) and anisotropy. In the marine environment we are normally dependant upon the mode conversion of conventional compressional sources (air- or watergun) for the production of shear wave energy.
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Focusing P-SV converted waves
Authors P. Y. Granger, R. Garotta and F. BaixasWith the present state of the art in data processing, converted P-SV wave sections do not show as comprehensive and detailed an image as that obtained from compressional waves. This observation alone may explain why there are objections to the use of converted waves. Results are clearly disappointing because of inaccuracies in CRP gathering and the lack of dip moveout.
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The derivation of shear velocities from log and VSP data
Authors W. L. Nutt and K. DoddsA knowledge of formation Shear Velocities is essential for many state of the art computations in Petrophysics and Geophysics. In particular mechanical properties of formations can only be accurately predicted if the corresponding formation shear velocity is equally accurately known. The interpretation and prediction of Amplitude versus Offset dependence of seismic reflection strength, AVO, is also heavily dependent on the knowledge of formation shear velocities.
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3D Multicomponent seismic reservoir characterization of South Casper Creek field, Natrona Co.
Authors R. D. Benson and T. L. DavisThe South Casper Creek field provides a unique opportunity to study a reservoir in an established oil field. A large amount of geologie and geophysical data has been collected which include weIl logs, core analyses, multicomponent vertical seismic profiles, and 3-D multicomponent surface seismic data. Interpretation of these data have contributed to an improved understanding of the reservoir. The continuing objective of this work is to develop 3-D multicomponent seismic methods to better characterize reservoir heterogeneities.
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Using mode converted shear data to map conglomerates - A case history
Authors J. D. Boyd and M. P. HarrisonThe Carrot Creek field, of Alberta, Canada produces oil from sandstones and conglomerates of the Cardium formation, which is of Upper Cretaceous (Turonian) age. A strong impedance contrast between the conglomerates and surrounding shales has made it a successful target for seismic exploration, as a purely stratigraphic play.
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Advances in airborne gamma-ray system design for geophysical and environmental applications
Authors J. R. Cox, J. Masek and J. BartosekThe purpose of this paper is to acquaint current users of Gamma- Ray equipment with the latest technology in this specialised field. Wherever possible the inforrnation is presented with a bias to Airborne applications, but of course the techniques discussed are directly applicable to Ground or Borehole applications. The presentation tries to balance an accurate explanation of these methods, with time allotted, so out of necessity, a somewhat simplified description has been used where possible.
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Radon emanometry over some kimberlites and lamproites in Southern and Western Botswana
Authors G. McDowall and H. KoketsoRadon decay product measurements have been widely used in uranium exploration, but until now this method has not been used in the exploration for kimberlites and lamproites.
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The application of transient electromagnetics to an integrated mineral exploration program
Authors N. J. Cooper and G. MaliotisFor the past 30 years, considerable geophysical interest has been focussed at mineral exploration of the Troodos Ophiolite, Cyprus. The local mining environment broadly comprises medium to small scale sulphide lenses, hosted at intermediate levels of the extrusive section. Mineral concentrations result from post-extension secondary phase alteration along ridge-parallel and transcurrent faults. Complex vertical and lateral inhomogeneities in the ophiolite section have complicated geophysical ground studies. Conventional ground exploration techniques give both inconclusive and inconsistent results when used to delineate structural features and target anomalies. Some previous success has been achieved with the I.P. method for detailed target evaluation, but this labour- and time-intensive approach is not suitable for broader scale exploration of the Ophiolite.
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On the application of geophysics in the exploration for copper and chrome ores in Albania
Authors A. Frasheri, L. Lubonia and P. AlikajThe exploration of copper and chrome ores in Albania is carried out through a wide complex of geophysical, geochemical and geological methods, which are applied in coordination to each other, according to the geological problems to be solved and the exploration phases.
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Geophysical investigations for the troll sub-sea pipe tunnel
Authors O. Kr. Fjeld, H. Deisz and B. AagaardThe Troll Pase I Project is a part of NS Norske Shell's development plans for the Troll Gas Field located 80 km west - northwest of Bergen, the Troll field is the Europes largest offshore gas field and in full production will be capable of providing 10 % of Europes total gas requirement. The wellstream will be produced on a minimally manned production platform and lead ashore for processing at a plant 40 km north of Bergen.
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Resistivity measurements on the sea bottom
Authors O. B. Lile, K. R. Backe, H. Elvebakk and J. E. BuanIn connection with the construction of a sub-sea tunnel under the 1050m wide and 35m deep Maursund strait, the Norwegian Public Roads Administration had the foilowing preinvestigations carried out: 1. Refraction seismies on the sea bottom. 2. A cored borehole from the shore 290m along the tunnel trace. 3. Acoustic tomography between the sea bottom and the borehole. 4. Resistivity measurements on the sea bottom. 5. Resistivity tomography between the sea bottom and the borehole. 6. Resistivity logging in sounding drillholes in front of the tunnel face. This paper reports the results from the resistivity measurements on the sea bottom and between the sea bottom and the diamond drillhole.
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Very high resolution 3D reflection seismic imaging of small scale structural deformations
Authors J. -P. Henriet, M. Verschuren and W. VersteegA small clay diapir under the river Scheldt in Antwerp has been used as a target for testing the possibilities of very high resolution 3D reflection seismic imaging in geotechnical practice. Results of this test may be relevant to a number of site survey cases in water-covered areas, e.g. where it is essential to elicit and analyse the spatial distribution of structural defonnations.
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Stratigraphic processing techniques for digital narrow-beam echosounding data from deep sea sediments
Authors M. Breitzke and V. SpieszContinuous high-resolution echosounding of deep sea sediments up to a maximum penetration of 200 m images sedimentation patterns and processes related to climatic and oceanographic changes. A new technical concept was applied to transmission and reception of high-frequency signals and requires arevision and adaption of classical stratigraphic processing techniques and interpretative approaches with respect to signal length and bandwidth. Selected methods are discussed in comparison to the frequency range and signal shape used in conventional (marine) seismics.
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3D Monte Carlo residual statics
By R. E. HaleStacking of common mid-point gathers assumes that reflectors corrected for normal move-out (NMO) and dip move-out (DMO) will align across the gather. If reflections do not align due to incorrect statics the stacked traces will be degraded. Static corrections for land seismic data calculated from measurements made in the field have limited accuracy so it is standard practice to pass all land data through a residual statics process before stack. The residual statics process estimates the difference between the field statics and the "true" statics required to align the reflectors. Wiggins, Larner and Wisecup (1976) described how the residual statics could be resolved into surface consistent components and this method is now called conventional residual statics. Ronen and Claerbout (1985) showed how surface consistent components could be calculated directly from stacked correlations and allowed larger residual statics to be estimated. Rothman (1985) extended this work by describing a method of estimating residual statics using simulated annealing (Monte Carlo residual statics). All three techniques were originally designed for 2-D seismic data. The conventional residual staties method generalised to 3-D seismic data with only minor modifications. The upgrade of the Monte Carlo method to accept 3-D data was more difficult and is the subject of this presentation. Real 2-D and 3-D data examples are shown which demonstrate the effectiveness of this technique.
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Attenuation of land multiple energy case studies from the Saudi Arabian Peninsula
Authors P. G. Kelamis and E. F. ChiburisThe use of Radon transforms for multiple suppression has recently gained popularity in the processing of seismic reflection data. The increased separation of events in the model space, obtained through the forward transform, is the cornerstone for multiple elimination. The application of the forward Radon transform effectively deconvolves the smearing and sharpens the image of the events. Thus, by inverse mapping the multiple energy and subtracting it from the original CMP gather. a primartes-only gather is obtained. In this paper, we first describe a simple, cost-effective methodology for multiple elimination via Radon transforms. Then, we show two distinct, case histories from the Saudi Arabian peninsula. Our processing scheme combines partlal stacking and the application of the forward/inverse Radon transform. Partial stacking just prior to the forward transform improves S/N and regularizes the geometry of the data. Thus, the Radon operators are computed only once, i.e., for the first gather. Finally, we examine the proposed Radon multiple elimination scheme in conjunction with AVO studies. We show that amplitudes and phases may be altered after Radon multiple elimination. Thus caution is advised in the application of Radon transforms for AVO studies.
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Migration of refraction events - A combination of travel time and wavefield techniques
By E. P. BrücklArbitrary lateral changes of thickness and velocity are allowed for. First order discontinuities are assumed as dominant features. Vertical velocity gradients if present should be positive and comparatively small. Topographic irregularity and layer thickness may be of comparable magnitude. The "plane layer and constant velocity"-model is considered inadequate.
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