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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
81 - 100 of 315 results
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Hole-to-surface seismic reflection surveys for opencast coal exploration
Authors E. Kragh and N. R. GoultyOpencast coal mines in such a densely populated country as the UK are mostly of small size by world standards, but they are highly profitable. Annual production of opencast coal is currently about 17 million tonnes, constituting some 20% of the UK coal industry's total output. Transport costs are low as the coal is produced for domestic consumption, and consequently opencast coal is a very important national resource.
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Self-potential survey for a preliminary geothermal research in a Southern Italy coastal area
Authors R. Quarto and D. SchiavoneSystematic self-potential exploration of large areas proved to be effective in geothermal research. In this field, SP measurements directly depend on the fluid flow through thermoelectric and electrokinetic phenomena. Thus, due to its low prospecting costs, a preliminary geophysical survey with the SP method can be particularly useful in new areas of geothermal interest.
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Crustal imaging of the Larderello geothermal field, Italy, from teleseismic P waveforms
Authors J. E. Foley, M. N. Toksöz and F. BatiniThe seismic velocity structure of the Larderello Geothermal Field of central Italy is investigated by inversion of seismic travel travel-time and waveform observations made on the 26-station seismic monitoring network operating in that area. We have divided the program of imaging the velocity structure of the Larderello area into two separate stages. The first stage utilizes abundant low frequency (0.5 to 3.0 Hz) teleseismic P-waveform data. Both teleseismic travel-time residual and waveform analysis techniques are used to produce a broad picture of the crust to depths of about 40 km. The second stage utilizes higher frequency (4.0 to 12 Hz) P and S wave travel-time observations from local earthquakes, and images the shallow and mid-crust to depths of about 15 km with higher resolution. The results of the first stage techniques are presented in this paper and the results of the second stage inversion in the following report (Block et al.).
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Crustal velocity structure of the Larderello geothermal field determined from local earthquake arrival time data
Authors L. Block, M. N. Toksöz and F. BatiniA joint hypocenter-velocity inversion of local earthquake arrival time data has been performed in the Larderello Geothermal Field in Tuscany, Italy. More than two thousand local earthquakes have been recorded in the Larderello area since 1977. Most of the earthquakes occur shallower than 8 km depth, although a few occur as deep as 30 km. The corner frequency of the data is typically 25 Hz. P wave and S wave arrival times from 269 of these earthquakes were used to simultaneously determine the hypocenter parameters and the three-dimensional P wave velocity and Vp/Vs structures. Due to a variety of rock types present in the uppermost 2 km, as well as a complex tectonic history, the shallow velocity structure is very complicated. This shallow structure was determined from seismic reflection and vertical seismic profiling (VSP) data and kept fixed during the joint hypocenter-velocity inversion.
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3D Surface seismic and well seismic applied to the delineation of geothermal reservoirs in metamorphic formations in the Larderello area
More LessThe delineation of deep (>3000m) reservoirs is a primary objective in the exploration of the LardereIlo and Mt Amiata (Italy) geothermal fields. Drilling wells at such depths in relatively hard rocks is expensive, therefore risk must be minimized through optimum application of surface exploration techniques. Since 1974 ENEL has performed extensive reflection seismic surveys. They made it possible to identify well defined groups of reflectors within a metamorphic basement composed of phyllites, micoschists and gneisses (Batini et at, 1978). An exploratory well, the San Pompeo 2 well, revealed the presence of a permeable zone filled with geothermal fluids at very high temperature (>400°C) and pressure (>240 bars) and corresponding to the main reflector or K marker. Subsequent deep drilling targeted at reflections with lateral continuity over 1-2 km above the K marker demonstrated the existence of productive zones associated with these reflections. Because these reflections are less continuous, their delineation is a major objective. A 3D seismic reflection survey combined with multi-offset well seismic profiles, well logs and measurements on core samples (ERL/MIT, 1989) was therefore planned as a pilot study. It was carried out at the Monteverdi test site where an extensive drilling programme was to take place.
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Inversion of combined data of the CRE nad CEE imaging
Authors A. Berkovitch, B. Chelchinsky, S. Keydar and V. ShtivelmanThe present paper is devoted to solution of an inverse problem - construction of 2D "true model" of a studied medium, using data obtained by the Common Reflecting (CRE) and Common Evolute Element (CEE) methods. The CRE and CEE methods are components of a novel system of processing and interpretation of seismic data, which is based on fundamental topological ideas, new different types of stack and homeomorphic images of studied media. The mentioned and other components of the homeomorphic imaging system were presented on Adelaide ASEG, Dallas and San Francisco SEG, Berlin EAEG, Istanbul IASPEl and Jerusalem Mathematical Geophysics Meeting and several journal papers.
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Depth focusing analysis: The effects of elevation corrections and structural DIP
More LessPrestack depth migration, combined with depth-focusing analysis (DFA), is a useful method for imaging and velocity estimation in areas of complex structure. However, such regions often possess significant topographic relief that leads to large elevation-static corrections prior to migration. The result is a distorted wavefield that yields erroneous depth-focusing information. Of further consequence are the limits imposed by the zero-dip approximation to the wave equation used for velocity estimation.
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Migration velocity computation using plane wave decomposed data
By M. T. TanerIn general, seismic data is acquired and processed as point source illuminated reflections. CDP or CMP processing is the central point of the processing sequence. Imaging is referred to as pre- or post- stack migrated data. Conventional CDP processing requires several velocity analyses, each for a different stage of the processing sequence to produce the final subsurface imaging. These velocities do not strictly relate to the subsurface physical parameters. However, the pre-stack shot domain imaging is the only procedure where the data satisfies the wave equation with no simplifying approximations and the velocities used for image construction are directly related to the subsurface physical characteristics.
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Migration velocity analysis by migration of velocity spectra
Authors C. J. Beasley and R. KlotzMigration of seismic data is required to collapse diffractions and position reflectors in the correct spatiallocation. Today's accurate migration algorithms applied to high-resolution seismic data can, in theory, produce accurate images of nearly vertical structures. However, in practice, migration accuracy depends not only on the accuracy of the migration algorithm, but also on the accuracy of the migration-velocity field. Indeed, as dip increases, accuracy in migration becomes critically dependent on the migration velocity.
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Velocity model development for prestack depth migration at different geological environments
Authors P. Farmer, S. Eidsvig and A. HeggernesIn the presence of lateral velocity variations depth migration rather than time migration should be used to obtain a proper image of the subsurface. If the velocity model is known and if the stack is a good estimate of zero-offset section, post-stack depth migration often gives good results. Normal common-midpoint stacking relays on the assumption of hyperbolic moveout curvature. Prestack depth migration, however, is not dependent on the validity of the common-midpoint assumption, and therefore it should be used in complex areas with strong lateral velocity variations to develope the velocity model and to image the structure properly.
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The hyperbolic radon transform
Authors E. de Bazelaire, S. Desaunay and J. R. ViallixReducing to a point the moveout curve of the reflections in common midpoint is the oldest problem facing seismic processing. This is currently achieved through a dynamic correction and multicoverage stacking. The process is costly, both in computing and preparation time, and of limited efficiency for multiple attenuation. So much so that, in recent years, new methods have been sought.
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Multiple suppression in the Barents Sea using the generalised radon transform as a notch velocity filter
Authors R. G. Williams, A. Haugen and N. CooperThe generalised Radon transform has been proposed as a means of multiple attenuation on CDP records. Initial results, especially on synthetic data, suggested an improvement in both velocity resolution and amplitude preservation compared to conventional FK techniques. The generalised Radon transform discriminates between primaries and multiples by measuring their curvature with respect to offset unlike the FK transform which discriminates on the basis of gradient. The use of curvature offers the possibility of separating primaries from multiples even at very short offsets. It is also possible to separate multiples with smaller velocity differences than is possible with FK methods. However, despite the initial optimism, generalised Radon transform techniques have failed to replace FK methods as the industry standard. This is partly because of the increased cost and partly because of technica1 prob1ems.
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A t-p inversion technique including amplitude
More LessTo improve the reliability of a velocity-depth function v(z), it is desirabie to use the amplitude information throughout the entire inversion process instead of interpretating it only in the x-t-domain ( x-distance, t-tirne ), which is of ten arnbigous because of crossing traveltime curves and shadow zones.
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Angular coverage compensation in transmission tomography
Authors G. Boehm, P. Carrion, F. Pettenati and A. VesnaverIn seismic tomography, the quality of reconstruction depends on different parameters such as angular coverage, signal-to-noise-ratio, local ray geometry, imposed constraints, etc. For the basic definitions and references on geotomography we send the reader to Nolet (1987) and Carrion (1987). In this study, we pursue the following goals: to give a detailed account on recoverability versus angular coverage, to study image blurring due stochastic characteristics of noise and to show the role of constraints in those cases when angular coverage is incomplete.
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Acoustic tomography for enhanced oil recovery and reservoir description
Authors J. H. Justice, A. A. Vassiliou, M. E. Mathisen, W. H. Troyer and P. S. CunninghamAs large commercial oil fields become increasingly difficult to find, attention has turned to the extensive reserves which still remain unproduced in our existing oil fields. Many of these fields are in decline and continued economic production depends on secondary (waterflood) or tertiary enhanced oil recovery (EOR) procedures. EOR programs are generally expensive and are often carried out in complex reservoir environments where even small improvements in efficiency may carry major economic benefits.
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Imaging reflectors in a crosswell seismic experiment
Authors M. Becquey, J. O. Bernet-Rollande, J. Raulent and G. NoualCrosshole seismic surveys can give improved resolution because the source and receivers can be lowered as close to the target as possible. CrossweIl tomography is now extensively used. But, to image reservoir layers, this technique needs wells drilled rather far below the reservoir, an uncornmon configuration for production wells. The crosshole reflection method does not entail such a limitation.
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High resolution cross-hole seismic tomography at the Backhorn test facility in Illinois, USA
Authors H. Saito, T. Takahashi, T. Imayoshi and S. SakashitaHigh resolution cross-hole seismic experiments were conducted at the Backhorn Test Facility in Illinois, U.S.A., where we drilled five 270 m boreholes over the oil producing field for the purpose of various borehole geophysical experiments. According to the exploration and production histories of the field, the oil reservoir is the porous zone of Silurian Kankakee "White Lime" carbonate which is horizontally distributed at the depth of about 200 m, and has no more than 10 m thickness. In fact, two boreholes out of five we drilled produce oil from the Kankakee formation. The results of several kinds of logging measurements conducted just after drilling five boreholes indicate a good correlation between formations from borehole to borehole. However, they also suggest significant lateral changes in porosity within the Kankakee formation.
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New developments in wave equation tomography
Authors Y. Luo and G. T. SchusterA new methodology is described which allows for wave equation inversion of skeletalized parts of data records. Skeletalized data is a reduced subset of the original raw data that is mainly influenced by only one type of model parameter; e.g. first arrival traveltimes skeletalized from seismograms are mainly influenced by the velocity (not density) parameters.
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Iterative geophysical diffraction tomography
Authors L. -J. Gelius and J. J. StamnesA new iterative diffraction-tomography method, tailored for geophysical applications, is described. The main problem in geophysics is the complicated background medium. Therefore, we have derived a reconstruction algorithm which take into account that the point-spread function is space variant. Using this algorithm, we compensate for the limited-view problem in an iterative manner.
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Traveltime tomography in anisotropic media: some examples
Authors R. G. Pratt, C. H. Chapman and W. J. McGaugheyTravel-time tomography is often applied to cross-hole seismic data and images of the velocity distribution are used to provide stratigraphic and/or structural interpretations. The technique is often applied in regions where one might expect significant anisotropy (for example in complex, layered stratigraphy or in fractured rocks). Without accounting for anisotropy, the tomographic images suffer from distortions that can render the results un-interpretable.
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