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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
121 - 140 of 315 results
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Minor fault geometries and their effect on seismic data in the West African Salt Basin
Authors P. M. Trayner, G. G. Grant and D. StaughtonSeismic data recently acquired by GECO in the offshore West Africa Salt Basin image numerous minor faults in the post-Miocene section. The faults have normal throw and the majority dip shoreward, ie opposite to the present bathymetric slope. This direction is contrary to that which would be expected in a model of gradual downslope creep and their consistency on dip lines has led to the images being dismissed as spurious features introduced during data processing.
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Improved reservoir description by use of seismic amplitude maps at Veslefrikk Field, North Sea
Authors L. Pedersen, S. Befrin, E. Berg and E. HegreThe Veslefrikk field was declared commercial based on observations from only two exploratlon wells and a 3-D seismic survey. The field has been in production for one year now.
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Reservoir characterization: A case history of the Sarir Field, Sirte Basin, Lybia
Authors R. O. Lindseth and N. A. BerruienThe elevation of the oil/water contact within the massive Sarir field, located in the Sirte basin of Libya, is relatively uniform, suggesting continuous communication of the reservoir over the entire field. Yet, preliminary injectivity tests and drilling of peripheral injectors suggested otherwise. In some cases injection of water was found to he poor and distribution of fluid did not appear uniform. Also, the location of some peripheral injectors was well within the productive limit because the water/oil contact was poorly defined.
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An integrated approach to reservoir petrophysical parameters evaluation
Authors E. Poggiagliolmi and D. LowdenPetrophysical parameters necessary for reservoir characterisation are normally derived from borehole data. Such information can be very accurate in the depth direction but has a small lateral penetration. Surface seismic, on the other hand, has low vertical resolution but is laterally continuous. Calibration of the seismic data to borehole information through integrated processing permits reservoir petrophysical parameters to be accurately mapped from seismic data. The relationship between seismic and petrophysical properties is addressed with reference to a reservoir evaluation study. The relationships are used to calibrate seismic data and to map heterogeneities in reservoir description parameters.
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G.I. Gun sea trials
More LessThe search continues for the ideal marine seismic source. Each of the currently available sources has its good and bad points. Air guns suffer from long-period bubble oscillation, and so have to be used in large arrays. Water guns have good short signatures, but these are non minimum phase. A 'bubble-free' airgun would be a very attractive source, and the G.I Gun is an attempt to attain this (Pascouet 1989). This paper presents results from trials of the G.I. gun, and a comparison between G.I. guns and water guns.
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Modelling of G.I. Gun signatures
By M. LandrøTwo years ago A. Pascouet (1989) presented a new airgun called the GI gun. The main characteristic of this gun is its ability to control (i.e. damp) its own bubble oscillation. The removal of the bubble osciallion is achieved through a tuning of air injection within the bubble. In order to achieve optimal bubble suppression the amount of injected air must be of the order of two times the original amount of air used to create the primary pressure pulse (see Pascouet, 1989). It has been shown however (SSI-brochure), that even if the volume ratio between injected air and original air is reduced to 1.0 (i.e. the gun is operated in the harmonic mode), farfield signatures with a very good primary to bubble ratio can be achieved.
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The low frequency response of high resolution airgun arrays
Authors D. J. Painter, R. H. Seymour and P. J. SmithDuring the past ten years the development and deployment of sleeve airguns and water guns have significantly improved the high frequency content on the final seismic section with a consequent gain in resolution. A critical issue is to what extent the industry's quest for improved high frequency has resulted in loss of low frequency energy and a reduction in penetration. This paper uses a simple theoretical model to examine the compromises, and illustrates the conclusion using results from seismic surveys.
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Modelling air gun array performance specifications
Authors W. R. Cotton and D. J. MonkAir gun arrays have been the preferred source for hydrocarbon exploration for nearly 25 years. One of the challenges over that period has been the computer simulation of the array signatures. The acoustic signal from an array of many air guns is sufficiently complex to thwart many early attempts at accurate modelling. As a result, it was not possible to predict the performance of an array under deteriorating conditions of operation such as the failure of specific guns. operating specifications were deduced from sparse information obtained from far field signature recordings and they carried the risk of being conservatively described without being comprehensive in coverage.
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Comparison of airgun clusters
Authors S. Strandenes, S. Vaage, G. Zaalberg-Metselaar and A. SodalLast year, Vaage, Strandenes & Metselaar [1] reported results from measurements of several symmetrical two gun clusters with varying separation distance, depth, firing pressure and source type. The source-types involved were Bolt 600 C airguns with volumes 10, 20 and 40 cu. inch, Sleeve airguns with volumes 10, 40, 70 and 150 cu. inch and Bolt Combo airguns with volumes 10, 40 and 90 cu. inch. It was found that the optimal gun separation in asymmetrical two gun cluster, based on this database, was 2.4 times the equilibrium radius, that the improvement in primary/bubble ratio on the average was a factor of 2.4 times the ratio for two corresponding non interacting guns and that the loss in primary peak amplitude at the optimal distance was 15 %.
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Signature measurements concerning the exploitation of interaction in the design of compact airgun arrays
Authors Mo. H. Safar, V. B. Hewett and P. HaskeyIn a paper presented at the 37th EAEG Annual Meeting held in Bergen, June 1975, Safar proposed a method (UK Patent No. 1542261) for efficient design of compact airgun array. The proposed method (Safar, 1976) involves the damping of the bubble pressure pulses by exploiting the interaction between a number of properly spaeed identical airguns.
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A New marine vibrator based on Terfenol-D
Authors R. Fritsvold, B. Ursin-Holm and R. TenghamnThe Vibroseis technique was introduced to the geophysical industry in the early 1960'5 and has been used successfully in land seismic acquisition world-wide. In the early 1980'5, steps were taken to convert the technology from land to the marine environment. The Hydraulic based Marine Vibrators developed 50 far, are mainly constructed as large and complex mechanical systems that are intricate to operate from a vessel.
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Synthetic seismograms from extended ray tracing
Authors A. Hanyga and H. B. HelleIn [1,2,3] a method of numerical computation of wave fields is developed. It is an extension of Asymptotic Ray Theory (ART) applicable in regions containing caustics. The essential features of the new method. called Extended Ray Tracing (ERT), consist in application of expressions which are well-defined at caustics in combination with complex ray tracing in caustic shadows.
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Extrapolation operators by beam tracing
Authors S. R. G. Kremer, J. T. Fokkema and C. P. A. WapenaarThe key process in seismic imaging is given by migration. The accuracy and efficiency of the migration process is directly determined by the involved downward extrapolation operators. Taking into account that a downward extrapolation operator equals the response at the surface due to a point source in a subsurface grid point of a macro model, the computation of point source responses in macro models is of vital importance for the seismic method.
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2D Modelling of thin layers
Authors G. Bernasconi and G. DrufacaForward modelling of seismic response of thin layers is not perforrned satisfactorily by conventional techniques. Finite-Difference methods stumble upon small grids, Ray-Tracing becomes cumbersome with conversions, multiples and post critical effects, and Reflectivity works only with horizontal layers. Hybrid schemes that join ray methods with reflectivity might offer a useful low-cost alternative [1,2]. Our scheme uses rays to transport the reflectivity of thin-layered small-scale discontinuities across large scale homogeneities. Conversely transmissivity - that takes into account interbed reverberation and conversions - correctly energizes transmitted rays, even in post critical situations.
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Reflectivity revisited
Authors A. J. Berkhout, C. P. A. Wapenaar and C. G. Mde BruinThe seismic method is based on the fundamental property that downward travelIing seismic source wave fields are returned to the surface by reflection (and/or refraction). Hence the reflectivity property of the subsurface plays a key role in seismic exploration. Structural information is gene rally derived from 'average' reflectivity. Lithologic information can be derived as weIl, provided angle-dependent reflectivity is taken into account.
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Generalized born scattering and Quasi-Shear ray coupling in anisotropic media
Authors C. H. Chapman and R. T. CoatesThe propagation of high-frequency seismic waves through inhomogeneous media can often be described well by asymptotic ray theory. Nevertheless, situations in which ray theory breaks down are widespread and of interest: rays may be focused at caustics causing amplitude singularities; wavefronts may contain amplitude discontinuities causing diffracted signals, e.g. at critical points or shadows; signals may be scattered by inhomogeneities causing low-frequency reflections; and rays may couple due to rapid variations in the ray solutions. In this paper, we concentrate on extensions of ray theory that solve the last two problems - scattering and coupling between ray solutions due to inhomogeneities in the model and ray solution.
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Computation of ray-born synthetic seismograms for anisotropic, inhomogeneous media
Authors R. L. Gibson and M. N. ToksözScattering of seismic waves by inhomogeneity in the earth leads to significant effects on observations of seismic energy. We apply an algorithm similar to that proposed by Beydoun and Mendes (1989) to model these scattered wavefields by combining the Born approximation and ray theory. Scattered displacements generated by a perturbation to a known background earth model are described by the Born approximation, whereby the inhomogeneity is treated as a secondary source radlating energy as it interacts with the incident waves from the true source. Variations in density act as single force sources, while perturbations to any of the 21 independent elastic constants lead to moment tensor secondary sources.
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Seismic traveltime equations for Azimuthally Anisotropic and Isotropic media
By A. G. SenaThe interpretation of traveltime curves over layered anisotropic media would give more physical insight if analytical (although approximate) formulas for such traveltime curves in terms of the subsurface anisotropy were available. In this paper, we derive the analytical expressions for traveltime-offset curves over multilayered weakly azimuthally anisotropic and isotropic media in terms of the elastic properties of each layer. This method is based upon an approximate skewed hyperbolic moveout formula involving three measurement bulk velocities for each reflector: the average vertical velocity and horizontal and skew moveout velocities. This technique allows for fast traveltime computation and extremely rapid estimation of the interval elastic parameters.
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Electrical conductivity of the earth crust in Europe
By L. VanyanFar from relatively narrow conducting anomalies (graphite shist belts, fracture zones, etc.) the background electrical resistivity of the upper part of crystaline Earth crost reaches 105 -106 Ohm.m. It coincides with laboratory measurements if taken into account porosity value and water in pores. One could expect an increase of the resistivity with depth due to closing of pores. But unexpectedly resistivity is decreasing at dep th of 10-20 km practically for all investigated areas of Europe. Magnetotelluric soundings of the last decade showed that the conductivity value of the lower crost is similar to that of the sediment cover. This fact is in a sharp disagreement with high resistivity of the dry lower crost rocks derived from laboratory measurements.
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Integrated gravity, seismic and geological modelling in the Rhine Graben
More LessA combined geophysical and geological interpretation of the Rhine Graben and vicinity in SW Germany is presented in the form of three 2-D, 250km profiles, starting from the S edge of the Rhenish shield and adjacent Lorraine and Saar-Nahe basins, crossing the Rhine Graben in three separate sections, and extending across the Mesozoic platform to the fore-alpine Molasse Basin.
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