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2nd International Congress of the Brazilian Geophysical Society
- Conference date: 27 Oct 1991 - 01 Nov 1991
- Location: Bahia, Brazil
- Published: 28 October 1991
81 - 100 of 203 results
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Amplitude Anomalies,and AVO
By W. EckhardtIn the last decade Petrobrás drilIed a great number of wells based on the presence of amplitude anomalies mostly bright spots in seismic data. Not all of these wells proved to be producers. This study aims to investigate whether the application of amplitude versus offset (AVO) or angle analysis techniques would have increased the success rate of drilling. A second aim of this study is to point out for what type of data AVO analysis is feasible and what suite of preprocessing is needed before the data are submitted to AVO analysis.
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Influęncia do Binômio Fm. Barreiras/Carbonatos na Qualidade Sismica na Regiăo de Japaratuba (SE)
By F.A.F. GomesA área do Baixo de Japaratuba e adjacęncias (fig. 1) ostenta má qualidade sísmica e como reflexo apresenta uma baixa densidade de linhas sísmicas para o atual estágio exploratório da Bacia Sergipe-Alagoas. Na área afloram clásticos da Fm. Barreiras sobrepostos a carbonatos das Fms. Riachuelo e Cotinguiba.
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Wavelet Estimation and Deconvolution by the Vertical Double-Streamer Experiment
Authors H. Huck, M. Tygel and A. MatthiesIn this paper, we give a deterministic estimation scheme for an arbitrary (i.e. mixed delay) source wavelet and the reflectivity of a stack of horizontal layers. From a marine vertical double-streamer seismogram the wavelet estimation and the deconvolution can be performed without any a priori information on the souree pulse or the reflection coefficients of the layers. For this purpose, the recordings at two depth levels in the uppermost (water) layer (see Fig.1) are decomposed in their plane wave contributions. The decomposition lead's to the consecutive recovery of the medium reflectivity and the source wavelet in the slowness-frequency (p -w)-domain. The proposed scheme is based completely on the point source response of a layered acoustic medium (bounded by a free surface above and a homogenous half-space below) given by the wave equation (see Ziolkowski et al. (1987), Tygel and Hubral (1987), Tygel et al. (1991)).
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The Accuracy of Linearized Elastic Parameters Estimation
Authors B. Ursin and E. TjĺlandElastic parameters can be found from amplitude versus offilet (AVO) curves. A fourth-order polynomial can be fitted to the AVO-curve and estimated by a least squares technique. The polynomial coefficients are linearly related to a set of elastic constants. In this paper the statistical properties of the estimated polynomial coefficients and elastic model parameters are investigated. An eigenanalysis is also performed of the normalized covariance matrices.
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Where are the Fault Plane Reflections ?
Authors A. González, W. Lynn, W.F. Robinson IV and S. MackayTwo imaging methodologies that we expect to give comparable results are 1) DMO followed by CMP stack and poststack time migration and 2) prestack f-k time migration. For the most part, these two approaches do indeed give similar images; yet in one important area they are nearly always different. Specifically, the approach of DMO-stackmigration routinely images reflections from fault planes; whereas the approach of prestack f-k migration routinely does not. This observation is not limited to anyone geologic basin, but is seen worldwide. This paper investigates several possible reasons that could explain the differences in fault-plane imaging: 3-D effects, vertical velocity variation, acquisition geometry issues, and transverse isotropy. We find that only transverse isotropy can single-handed explain the different responses of the imaging approaches. We further show that the sensitivity of prestack f-k migration to transverse isotropy can be used to estimate the amount, in some average sense, of transverse isotropy. This has importance not only for selection of imaging parameters, but also for interpretation and time-to-depth conversion. In this paper we also show that using an aneliptic dispersion relation in prestack f-k migration compensates for the effects of transverse isotropy, thus reducing the dip dependency of migration velocities.
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Interactive Imaging and Interval Velocity Analysis in Complex Geological Areas
More LessWe present an interactive procedure forthe estimation of interval velocity models and the corresponding depth images. This procedure utilizes internal velocity analysis in conjunction with both prestack and poststack migrations and seems to yield reliable results. A major advantage of the procedure is that geological interpretation rnay be used throughout the process.
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Seismic Imaging of Salt Structures in the Gulf of Mexico
Authors D.W. Ratcliff, S.H. Gray and N.D. WhitmoreMajor hydrocarbon accumulations are often associated with salt structures in the Gulf of Mexico. Correctly imaging the salt structures has been and will continue to be an important exploration tool. Defining the salt boundaries and the surrounding sediments enables the geophysicist to explore more effectively for those hydrocarbon accumulations. For example, imaging steeply dipping salt face reflections can assist in optirrizing salt flank well locations. Properly migrating salt overhang reflections that have travelled through salt can aid in determining updip limits of possible reservoir sands. Imaging below salt can help identify structural features that trap significant hydrocarbon resources. Salt imaging technology can also be used to reduce some of the risks associated with salt flank, salt overhang and subsalt drilling.
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The Evolution of Stacking Reflection Data in Areas of Complex Geology
By W.S. FrenchSignificant changes occur every few years in the way we process seismic reflection data from areas of complex geology.
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A Case Study of Pre-Salt Imaging
Authors M.A. Brzostowski and W. RitchieConventional imaging techniques operating in the time domain are recognized to poorly resolve complex geologic environments. Complex subsurface geology with rapid spatial changes in interval velocity requires pre-stack imaging techniques operating in depth. However, these techniques rely on good definition of the velocity field. A common approach to defining this field involves multiple iterations of pre-stack depth migration through successively deeper velocity layers - an expensive and time-consuming process. In this paper, we describe a technique to cost and time effectively develop the required velocity model using travel-time inversion, of followed by replacement of the complex velocity field with a simplified field thereby removing nonhyperbolic moveout effects and allowing implementation of the most cost and time effective imaging solution. The technique is applied to data from the Pre-Caspian Basin of the Soviet Union where the ability to image below significant accumulations of Permian salt and overcome the false time pull-ups due to the fast saIt velocity is essential to accurate mapping of this basin. Following development of the velocity model using travel-time inversion, its use in conjunction with both pre and post-stack time and depth migration techniques is demonstrated in a set of seismic data comparisons. The conclusion is that the concept of Replacement Dynamics provides an attractive, cost-effective alternative to other computationally intensive techniques in unraveling many areas of complex subsurface geology.
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An Asperity-Deformation Model for Effectivity Pressure
Authors A.F. Gangi and R.L. CarlsonThe variations of the mechanical and transport properties of cracked and/or porous rocks under isotropic stress depend upon both the confining pressure (Pc) and the pore-fluid pressure (Pp). To a first approximation these rock properties vary with differential pressure, Pd = Pc - Pp; at least for low differential pressures. However, at higher differential pressures. the properties vary in a more complicated, way upon the two pressures. The concept of effective pressure, Pe, is used to denote this variation; that is, the rock properties are constant for constant effective pressure even though the confining and pore pressures change.
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Macro Model Optimization and Depth-Migration
By R. MarschallAny procedure to go from time to depth requires (among other features) a macro-model, i.e. a velocity model. This macro-model therefore has to be specified if one wants to apply any type of depth-migration, pre-or poststack. However, the same macro-model specification is needed if one wants to depth-convert an interpretation of e.g. a 2-D time section. The ever-lasting problem here was given by the question: how good is the final resulting depth map away from the fixpoints, i.e. wells, especially when the depthing is done by applying e.g. APPARENT VELOCITIES. In this work we describe the CONCEPT OF VELOCITY-CHECKiNG BY WAVEFRONTS which is applicable to both, i.e. conventional depthing problems as well as to depth-migration related problems, simply by using the given redundancy of normal seismic data.
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Wave Theoretic Multiple Suppression and Source Signature Estimation
By A.B. WegleinMultiple suppression has a long history in exploration seismology. Although our current methods (NMO-stack, p-r and f-k filtering and modeling} are often effective there are also many cases where muttiples remain a problem. The goal of a recent wave theoretic mutliple suppression method (Weglein and Stotl, 1991, and Carvalho, Waglein and Stolt, 1991) is the suppression of multiples that are not susceptible to our current mutliple removal methods. Two categories of multiples are targeted: (1) those that satisfy the assumptions of moveout separation methods (homogeneous overburden, horizontal reflectors, and knowledge of the velocity of a primary) but are too close to a primary in moveout, and {2} those that do not satisfy one of (or a combination of) the above listed assumptions. The method we are proposing Is a nonlinear migrationinversion procedure based on the T operator of scattering theory. Each successive term in the series solution requires the same migration operation.
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Seismic Imaging of Rift-Related Subsalt Structures Using Prestack Depth Migration
Authors S. MacKay, D. Tufekcic and A. GonzálezThe North Sea and the Gulf of Suez contain classic examples of rift-type graben structures. Typical of such geologic regimes, substantial thicknesses of evaporites were accumulated during the inijiation of rifling. Evaporites, especially salt, provided ideal seals for the faulted pregraben and graben-fill sediments. Subsequent burial then yielded the mechanism for halokinesis and the resultant traps associated wijh salt diapirislm. Anhough contributing to many of the conditions necessary for hydrocarbon trapping, salt presents a severe impediment to seismic imaging. Irregular masses of salt act as distorting lenses to refiected seismic energy. To form accurate images of the subsurface, prestack depth migration is needed.
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Finite-Difference Reverse Time Migration of Multi-ConfigurationMarine Seismic Data
Authors M.A.B. Botelho and P.L. StoffaPre-stack reverse time migration requires that each subsurfacee grid point have a unique imaging time. This time is used to stop the reverse time propagation. Here we compute the required imaging times by using a finite difference algorithm to solve the forward problem. These times, which correspond to the arrival time of the direct wave, are usually found by ray tracing. The procedure we present consists of forward finite difference modelling and detection of the energy of the direct arrival at each subsurface grid point.
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Climate Change in the Northern Hemisphere: The Record from Borehole Temperatures
By H.N. PollackRock temperatures at shallow depths within the Earth are an archive of temperature changes that have occurred at the surface of the Earth in the recent past. Thus subsurface temperatures comprise a valuable complement to surface meteorological data in understanding the Earth's surface temperature history, particularly for times prior to the establishment of a worldwide network of meteorological stations. The subsurface observations are relevant to an assessment of the role of atmospheric greenhouse gases in the global warming of the 20th century.
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Reconstruçăo de Mudanças CIimáticas a Partir de Perfis Geotérmicos: Avanços Teóricos
By F.B. RibeiroA identificaçăo e remoçăo de perturbaçőes climáticas no cálculo de densidade de fluxo térmico e a reconstituiçăo das variçőes climáticas recentes săo problemas clássicos da geotermia. O reconhecimento de que tais perturbaçőes ocorrem de forma mais generalizada do que se imaginava a pouco mais de uma década e também a crescente preocupaçăo com o meio ambiente tęm, recentemente, aumentado o interęsse pelo problema. A primeira parte deste trabalho apresenta uma revisăo dos métodos de simulaçăo de perturbaçőes climáticas sobre perfis. Na segunda parte, discute-se a aplicaçăo dos métodos de inversăo na reconstituiçăo das variaçőes climáticas a partir de perfis geotérmicos.
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Variaçăo Temporal de Perfis Geotérmicos Rasos Associada a Alteraçőes MicrocIimáticas na Amazônia
Authors J.R.S. Souza, R.L.C. Araújo and M. MakinoA energia solar incidente na superficie terrestre é fundamentalmente refletida, reirradiada (transporte de calor sensivel) ou convertida em calor latente de vaporizaçăo, fotossintese, bem como em outros processos biológicos. Além disso, uma parte apreciavel dessa eneria é transmitida e armazenada abaixo da superficia afetando o regime térmico do solo (Sellers, 1974; Souza et al, 1989).
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Evidęncias Geotérmicas sobre Variaçőes Climáticas Recentes no Hemisfério SuI
By V.M. HamzaAs flutuaçőes climáticas exercem influęncias significativas sobre o balanço energético da superfície da Terra, com consequęncias notáveis, tanto na geosfera como na biosfera. Os efeitos térmicos das alteraçőes energéticas, na superfície, penetram em profundidade numa forma muito semelhante ŕs penetraçőes das variaçőes diurnas e sazonais da radiaçăo solar.
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Tendęncias e Flutuaçőes Seculares da Pluviometria no Brasil
Authors T.M.B.S. Xavier and A.F.S. XavierA pluviometria é um parâmetro muito utilizado para detectar e avaliar mudanças climáticas. Neste trabalho săo estudadas tendęncias e flutuaçőes de séries longas de chuva, no Brasil, referentes a períodos úmidos do ano e anos hidrológicos. A metodologia envolve o uso de técnicas de séries temporais, para detectaçăo de tendęncias e a análise de quase-ciclos. Discutem-se os resultados preliminares obtidos, quanto a sua possível interpretaçăo em termos da influęncia de fatores físicos e antrópicos sobre o clima.
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Aspectos Climáticos da Variabilidade de Cotas Máximas e Minimas do Rio Negro: 1903 - 1985
Authors A.F.S. Xavier, O. Massambani and T.M.B.S. XavierA análise de vazőes ou de alturas de rios é importante, no contexto de estudos hidrometeorológicos, desde que esses dados representam uma integraçăo da chuva e do escoamento superficial nas respectivas bacias coletoras, bem como, de outros processos de alimentaçăo. Por outro lado, o vies inerente a observaçőes de tipo puntual fica praticamente eliminado.
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