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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
161 - 180 of 315 results
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Optimal estimation of crack strike
Authors C. MacBeth and G. S. YardleyIt has been suggested (Crampin et al. 1989) that the study of shear-wave splitting may yield valuable infonnation about the stress- and crack-geometries within reservoirs for hydrocarbon production. Shear-waves upon entering an anisotropic medium split into (usually) two components, with the faster component polarized parallel to the strike of vertical parallel cracks in the rockmass. Detennining the polarization of this leading split shear-wave for sub-vertical raypaths, will give infonnation about the strike of aligned vertical cracks, and hence the preferential directions of fluid flow within a reservoir. The time-delay between the arrival of the first and second split shear-waves gives the percentage anisotropy of the medium along the particular raypath, and from this crack density and porosity may be inferred.
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Orthorhombic media: Elastic waves in a vertically fractured earth
Authors M. Schoenberg and K. HelbigAnisotropy in the Earth's crust seems to be well modeled by orthorhombic symmetry (three mutually orthogonal planes of symmetry. In many places rocks can be regarded as a transversely isotropie (t.i.) background with a vertical symmetry axis (due mainly to fine horizontal layering ) in which is embedded a system of vertical parallel fractures (equivalent to aligned microcracks ), which themselves are assumed to be orthorhombic with respect to the natural coordinate system, with x3 positive downward and x1 perpendicular to the fractures (Schoenberg and Douma, Geophysical Prospecting 36, 571-.590, 1988).
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Observations of shear-wave splitting from mining induced stress-failures
Authors G. Graham and S. CrampinLocal networks operated in the gold mines of South Africa record mining-induced events with local magnitude as low as -4 (Legge and Spottiswoode 1987) and occasionally exceeding 5 on the Richter scale. Nearly all events of magnitude 2 and above are also recorded and located by the National Seismological Network of the South African Geological Survey (Femandez and Du PIessis 1988).
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Shear-wave point singularities in sedimentary basins
By S. CrampinShear waves may not behave uniformly for all directions of propagation in sedimentary basins. Splitting of seismic shear-waves (bi-refringence) is observed in many sedimentary basins, and appears to be caused by propagation through a combination of two phenomena: 1) the azimuthal isotropy of fine horizontal layering or Iithology, leading to transverse isotropy with a vertical axis of symmetry, and velocity anisotropy of anything up to 30 or 40%; and 2) the azimuthal anisotropy of stress-aligned fluid-fiIled cracks, microcracks, and preferentially oriented pore-space, leading to transverse isotropy with a horizontal axis of symmetry, and much weaker velocity anisotropy (often less than 5 %). The combination of the two transverse isotropies with orthogonal axes of summetry leads to an orthorhombic symmetry system.
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Anisotropic and velocity dispersion of finely layered media
Authors J. M. Carcione and B. RommelFine layering is one of the geological systerns that more frequently contribute to the formation of sedimentary basins. Moreover, the constituents of a fine layered medium are among the cornponents of reservoir rocks: sandstones and Iirnestones, for instance, which are the recipient rocks, and shales which form the seal rocks. By fine layering we mean that the dominant wavelength of the seismic putse is much longer than the thicknesses of the individual Iayers. When this occurs, effective anisotropy takes place. Moreover, in the presence of hydrocarbons, the media may show substantial attenuation properties and velocity dispersion. This is the case with poreus or cracked rocks like sandstones and limestones, respectively, and even shale formations with considerable fluid content.
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Coupled wave propagation
More LessWave propagation in exploration geophysics is usually modeled elastically or viscoelastically, Phenomena, which include non-seismic quantities are conventionally neglected, when determining elastic constarits of rocks. Coupled wave propagation has been observed in laboratory measurements for a long time; however in field experiments observations have been less convincing.
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An alternative approach to the Cagniard de Hoop method
Authors N. Bleistein and J. K. CohenThe Cagniard-de Hoop method is ideally suited to the arialysis of wave propagation problems in stratified media.
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Selected 3D LOTEM interpretation case histories in exploration situations
Authors A. Hördt, G. Karlik, K-M. Strack and P. WolfgramIn the past few years the activities of TEM measurements in oil exploration have significantly increased. One reason why these methods could not yet reach a breakthrough is that the interpretation is usually based on layered earth models. However, many cases which are interesting for hydrocarbon exploration include 3D structures, such as overthrusts or anticlines.
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An introduction to transient EM imaging via interpretation of current distributions
Authors B. A. James and A. ZerilliA method of subsurface resistivity imaging with transient electromagnetics (TEM) has been created for generaI mapping of geoelectric structure. The method first maps the distributed current flow in the earth as a function of time. The geoelectric structure is then imaged from the interpreted current networks.
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Experimental electromagnetic prospecting for hydrocarbon exploration in the Venezuelan mountain fronts
Authors H. Passalacqua, A. S. Orange, C. Marquez, J. Copley and R. B. FurgersonINTEVEP, S.A. has been involved in the evaluation of alternative techniques to the seismic reflection since 1983. The first activity of this project was related with design and construction of a digital multifonetional data acquisition system, TEM and MT, and related software, which was used for an initial evaluation of these techniques in the Eastem Basin of Venezuela
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Airborne bathymetry in ice covered waters
Authors P. Walker and S. HolladayThe Through-Ice Bathymetry System, or TIBS, is a helicopter electromagnetic system currently under development by the Canadian Hydrographic Service. The aim of the system is to provide reliable bathymetric coverage in areas where bathymetric data would otherwise be difficult to acquire. Development is currently focused on assessing the performance of the system in ice covered Arctic waters with depths up to 50 meters, but TIBS could also be used to map areas where the presence of shoals or other hazards renders conventional ship-borne bathymetric sounding dangerous or impractical.
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Magnetotelluric analysis in the wave number domain
Authors C. Torres-Verdin and H. F. MorrisonOur study is an attempt to a scertain practical vertical and lateral resolution bounds with which geoelectric features in the subsurface can be resolved from lateral measurements of magnetotelluric (MT) fields. For this purpose, we have linearized the relationship between a perturbation in subsurface resistivity and the ensuing perturbation in the surface MT fields (Torres-Verdin and Bostick, 1990). Even though the linearized MT field solutions apply only to low-rontrast models, they hold for any geometrical complexity in the subsurface. In fact, the linear-system analysis yields a set of response equations than can be easily particularized to the cases of one-, two-, and three-dimensional (1-D, 2-D, and 3-D, respectively) subsurface resistivity distributions.
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Two level magnetovariational measurements for the determination of the underground resistivity distribution
Authors D. Patella and A. SiniscalchiThe natural electromagnetic field suggests some different types of measurement to determinate the electrical conductivity of the Earth's crust and upper mantle. In particular, if a measurement at two levels is possible, we can obtain the impedance from purely electric or purely magnetic data (Berdichevskii et al., 1969). In this note we study this second case which can have interesting developments in some applicative fields, as e.g. offshore or in presence of tunnels.
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Development and improving of georadar system as a geophysical method for mining exploration
By M. PiccoloThe scope of this report is to summarize all the experiences arose during the project "DEVELOPMENT AND IMPROVING OF GEORADAR SYSTEM AS A GEOPHYSICAL METHOD FOR MINING EXPLORATION", supported by EEC funds.
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Downward continuation of some stationary geophysical fields
More LessThis paper offers a new method for downward continuation of some stationary geophysical (geothermal or geo-electrical) fields in 3D gradient media.
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On the application of statics in marine 3D data processing
Authors G. P. Lourantos, F. Kirchheimer and H. KoitkaIn the process of of land seismic data static corrections play a firmly established role and a wide spectrum of rnethods for the estimation of statics has been evolved. This is quite in contrast with the situation for offshore surveys, Where statics have been largely neglected. Little is published about the specific problems of estimating and correcting the effects of near-seabottom velocity anomalies.
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Recent advances in seismic processing provide the key to evaluating the commerciality of the Wealden oil play in the Celtic Sea Basin
Authors J. Pender, L. Godfrey and S. CoperudeEsso Exploration discovered oil in the Cretaceous Wealden of the Celtic Sea Basin in the early 1970'5. Since then, other operators have found oil in the Wealden but have been unable to justify the development of these finds. The main obstacle to commerciality has been the inability to predict the extent and distribution of oil- or gas-bearing reservoir sand using seismic data.
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A Comparative study of pre-salt imaging techniques in the pre-Caspian Basin
Authors K. Schleicher, M. A. Brzostowski, P. V. Zahharov and G. F. MillsWe compare different techniques for imaging pre-salt reflectors In the Pre-Caspian Basin of the Sovlet Union. This basin is the site of the newly discovered Tenglz Field. Until recently only shallow fields could be imaged In this basin. Modern seismic processing techniques have allowed the imaging of Devonlan events below Permian salt.
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Seismic data processing and interpretation problems encountered beneath rapidly changing water depths: A case history from the carbonate banks of the Seychelles
Authors A. H. Roberts and S. A. ThornThe Seychelles Archipelago consists of a series of steep-sided carbonate banks that were formed by deposition of a thick, high velocity , shallow marine carbonate sequence. Deposition of these carbonates has been concentrated on local highs and has kept pace with the Tertiary thermal subsidence which followed the rifting between India and the Seychelles.
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Model based depth conversion and processing - A case study offshore mid-Norway
Authors K. Willersrud, A. I. Evensen, K. Mathisen and I. MagnusAccurate depth maps and correctly depth converted cross sections are primary products of geologic and geophysical investigations. Because the time structures on our seismic sections do not necessarily mirror the true depth structures, depth conversion may become a demanding task. Despite the obvious importance of reliable depth models, approximations in the depth conversion process often reduce the quality of the final products.
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