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4th International Congress of the Brazilian Geophysical Society
- Conference date: 20 Aug 1995 - 24 Aug 1995
- Location: Rio de Janero, Brazil
- Published: 20 August 1995
1 - 20 of 330 results
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Caracterizaçăo Sismica do Tectonismo Transcorrente na Bacia do Jatoba
By A. A. PeraroA Bacia do Jatobá, situada no Estado de Pernambuco, possui aproximadamente 6.000 km2 de área e marca a inflexăo da direçăo geral do sistema de riftes intracontinentais do Tucano- Jatobá, de N-S para N 70° E. (Figura 1). Sua morfologia geral, quando observada em seçăo NW-SE, e dominada por um meio "graben" com o substrato constituído predominantemente por blocos rotacionados e prcigressivamente mais baixos em direçăo a NW. As principais falhas internas sao aproximadamente paralelas a falha de borda da bacia, a Falha de Ibimirim.
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The Timing of Maturation, Pressure Development and Secondary Migration in the Northern Gulf Basin
Authors A. H. Ross lr., T. F. Schwarzer and F. W. SchroederBasin modeling of maturation, fluid flow and abnormal pressure development has allowed significant advances in the understanding of the Tertiary hydrocarbon system of South Louisiana. Identification of source rocks through matching of oils and extracts allowed the migration pathway options to be examined in detail. Fluid flow modeling(using Bethke's BASIN2 and IFP's TEMISPACI demonstrated the necessity for faults as migration pathways since without the faults sufficient permeability does not exist to allow migration from source to reservoir. Abnormal pressure modeling in conjunction with maturation modeling allowed the development of a timing scenario to explain the relationships between maturation, abnormal pressure development in reservoirs, the formation of traps and effective migration pathways. The synthesis of the results of these techniques allows for a complete understanding of the workings of a hydrocarbon system. These techniques applied here in a normal listric fault, Tertiary clastic setting can be applied to any type of structural stratigraphic setting and obtain similar results.
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Ray Reflection Tomography: A Comparison Between Layer-Stripping and Simultaneous Updating Methods
More LessIn this work we compare two traveltime inversion methods in terms of robustness and stability. The first one, which was formulated in [Soares, 1994], is a layer-stripping procedure where the geological model reconstruction is carried out layer to layer, from above downward. The second method consists of an iterative process where all the parameters are updated simultaneously. We show that for suitable medium parametrizationa, large offsets aquisitlons, low to moderate lateral velocity variation and constraints imposed in the Tikhonov's sense [Tikhonov, 1963], both methods work very well. We show that the reconstruction of a geological medium described by sixty-four B-splines coefficients is carried out in only three iterations. Moreover, both methods show the superiority of the interfaces reconstruction compared to the velocity resolution, as it was analytically demonstrated by Bube et al. for an horizontal layer [Bube et al., 1991). Nevertheless, although the second method reduces the cumulative error resulting from the errors in the upper layers retrieva.ls, the layer stripping method is more robust and stable.
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Zero-Phase Conversion of Marine Data Using One-Parameter Phase Filters and Kurtosis Maximization
Authors G. Cambois and N.D. HargreavesConsiderable research and development effort in seismic signal processing has been applied over the years to the problem of estimating the phase of the seismic wavelet. This is information that is lacking in the second order statistics (the autocorrelation) that are the usual input to industry standard processing techniques. Wavelet phase can be estimated by the minimum-phase condition, but several authors have shown that likely errors in the input to the computation can produce large errors in the resulting phase estimates. The central assertion of this paper is that in many cases the phase spectrum of the processed wavelet has a very simple general form. For processing conforming to the minimum-phase assumption. the phase of the processed wavelet is inversely proportional to frequency, and can be simply characterised by one-parameter which is the constant or proportionality. We reach this conclusion from both an experimental investigation of zero-phase processing and from a theoretical analysis of the likely errors in minimum-phase computation. We tested the assertion by reviving the maximum kurtosis technique of Fourmann (1985). This leads to an extension of that technique in which. rather than the constant phase-rotation originally proposed by Fourmann, we use zero-phase conversion filters of the form P/f, P a constant to be determined by kurtosis maximization.
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O Refletor Plano Versus o Curvo na Estabilizaçăo de um Operador de Inversăo de Modelos Sismico-Geológicos com Variaçăo Vertical de Velocidade
More LessExistem uma infinidade de perturbaçőes que podem ser feitas no campo de vagarosidades que săo incapazes de perturbar os tempos de viagem de reflexăo colhidos em um modelo de referenda, sujeito a algumas condiçőes, mesmo em uma aquisiçăo infinita e contínua, enquanto que em presence das mesmas condiçőes, qualquer perturbaçăo no refletor produz perturbaçőes nos dados, mesmo quando estes se apresentam de forma discreta (Bube et al., 1989). O estudo aqui apresentado está assentado em um modelo bidimensional (acredito que, caso alguns cuidados năo sejam tornados, o problema da má determinaçăo dos parâmetros do campo de vagarosidades em relaçăo ŕqueles do refletor se reproduzirăo, no caso tridimensional, com uma dramaticidade ainda maior).
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The CRE Method: An Alternative to CMP stacking
Authors Joao Carlos Ribeiro Cruz, P. Hubral and M. TygelThis paper presents new theoretical aspects concerning seismic stacking methods. We study in particular the homeomorphic imaging method called common-reflecting-element (CRE). For that, we consider a two-dimensional layered medium consisting of horizontal layers with constant velocities. The seismic data are a set of seismograms related to primary reflections that are sorted in a common-mid-point (CMP) gather. The CRE method is based on the reconstruction of a wavefront that has a circle as geometrical approximation and hypothetically originates at a reflection point. The formula used for the local time-correction (moveout) depends on the radius of curvature of the wavefront and the velocity of the first layer. Because this formula does not depend on a reference time, there is no problem with stretching effects. The consistency and accuracy of the CRE method are analysed considering situations where the wavefront does not exactly match a circle.
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Decomposition of a Mixed-Phase Causal Signal
Authors M.J. Porsani and B. UrsinA new algorithm for the decomposition of a mixed-phase signal into its minimum-phase and maximum-phase componentes is described. It is only valid for signals whose transform has no zeros on the unit circle. The method is based on solving a number of extended Yule-Walker equations to estimate the number of zeros inside the unit circle. A numerical example is shown to demonstrate the effectiveness of the method. It is also shown that a mixed-phase inverse filter based on the signal decomposition performs much better than minimum-phase deconvolution.
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A Brief Study on Square and Long-Pixels Model Parameterization for Cross-Well Seismic Tomography Purposes
By J.L. MartinsA brief analysis concerning discretization of depth models for borehole-to-borehole seismic tomography implementation has been performed. It focused on the application of rectangular cells (long pixels) to the discretization of the model parameters rather than the conventional square cells. Results have successfully shown the ability of x-direction long pixels in increasing resolution of the tomograms. In principle, horizontal layering and the cross-well shot-receiver configuration supported such ability.
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Traçado de Raios Através de Algoritmos Genéticos
Authors C.A. da Cunha Filho and F.A. de Castro ThedyInúmeras técnicas de traçado de raios e de cômputo dos tempos de trânsito vęm sendo amplamente usadas na Sísmica, uma grande parte tendo como fundamento a teoria do raio. Estes métodos estăo sujeitos ŕs restriçőes impostas pela teoría do raio, entre as quais destacam-se a aproximaçăo de alta frequęncia e a premissa de que as variaçőes de velocidade năo sejam bruscas em relaçăo ao comprimento de onda da fonte.
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Modelagem de Dados Sísmicos em Meios Elásticos Porosos
Authors E. Ambrosini Thedy and M.A. BarsotteliA detecçăo de estruturas formadas por rochas porosas preenchidas por hidrocarbonetos é um dos objetivos da Sísmica de Exploraçăo. Um conhecimento dos efeitos causadas pelas propriedades petrofísicas do reservatorio nos dados sísmicos, por exemplo, orosidade, permeabilidade e razăo de Poisson, é de grande importância na interpretaçăo desses dados.
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Deconvoluçăo Adaptativa da Assinatura da Fonte Utilizando urn Algoritmo Tipo Levinson para Janelas de Tempo
Authors J.F. Bezerra Montenegro and M.J. PorsaniO método denominado deconvoluçăo do traço sísmico é frequentemente empregado para comprimir o pulso sísmico e portanto melhorar a definiçăo das reflexőes dos dados registrados. Outra aplicaçăo da deconvoluçăo é remover reverberaçőes. A econvoluçăo baseia-se na obtençăo do operador de prediçăo correspondente ao filtro inverso do pulso sísmico.
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Inversăo Elástica 1D Utilizando Algoritmos Genéticos
Authors P.R. dos Santos, M.J. Porsani and P.L. StoffaO objetivo deste trabalho é a inversăo de dados sísmicos reais e sintéticos, com e sem ruídos aleatórios visando a obtençăo da velocidade das ondas P e S, densidade, impedância é razăo de Poisson. Para tanto utilizou-se os algoritmos genéticos, que săo metodos de pesquisa global, baseados na mecânica da seleçăo natural e da genética. A modelagem direta assume para a subsuperficie um modelo 1D, constituído por camadas plano-paralelas e isotrópicas. Trabalhou-se no domínio ro-p e r-p, onde os cálculos săo mais rápidos, utilizandose o método da refletividade. A inversăo dos dados sintéticos, sem ruido, foi excelente. Com a adiçăo de ruídos aleatórios, a performance năo foi tăo alta, mas os resultados săo satisfatórios. Os dados sísmicos reais săo provenientes da parte sul do Mar de Barents, na Noruega, e apresentam excelente qualidade. As maiores dificuldades foram a estimativa da assinatura da fonte e a equalizaçăo das amplitudes em relaçăo aos dados da modelagem direta. Quando se compara os dados de entrada aos dados calculados pelos algoritmos genéticos, observa-se que os eventos principais estăo presentes nos traços sintéticos e o resultado da inversăo é, portanto, razoável. As principais diferençăs devem-se ŕ forma da onda utilizada na modelagem direta e aos problemas relacionados ŕ amplitude, que năo foram totalmente eliminados.
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Turning-Ray Tomography Velocity Analysis: A Case History from Southern Trinidad
Authors N. G. Tilander, M. E. Wuenscher and J. P. StefaniTurning-ray tomography is a method for estimating near-surface velocity structure using first-break arrival times in areas where conventional refraction statics techniques fail due to poor data or lack of a smooth refractor/velocity structure. We applied this technique to an exploration area of onshore southern Trinidad. The velocity estimates correlate accurately with both sonic logs and near-surface geologic features, and they accurately predicted near-surface velocities which were later confirmed by drilling. We save combined the shallow tomographic velocity information with deeper velocity information from well and seismic data to construct velocity models for ray-trace modeling and depth migration. As a result of this work, we believe that the refraction tomographic method is a viable technique for deriving useful velocity information from first-break arrival time data.
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Algoritmo Rapido para 0 Calculo de Tempos de Transite com Interpolacao do Campo de Tempo
Authors R.S. Nunes de Bragança and S.V. GoldinProgramas de modelamento direto e inversăo de dados sísmicos, em geral necessitam solucionar o problema cinemático direto urn grande mimero de vezes. Portanto, é imprescindível que o algoritmo destinado ŕ esse fim, seja o mais eficiente possivel.
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Regularizacao em Tomografia nao Linear de Transmissao: Criterios para Selecao de Ordem e Fator de Regularizacae
Authors T.J. Mafra de Sá and A. BassreiO termo regularizaçăo ou suavizaçăo, refere-se ŕ gama de métodos que procuram resolver problemas inversos mal-condicionados através da minimizaçăo da derivada primeira ou superiores, dos parâmetros do modelo, afim de reduzir a flutuaçăo dos dados, devida a erros induzidos pela instabilidade do método de inversao.
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Simultaneous statics and velocity estimation for data from structurally complex areas
More LessWhere subsurface structure is complex, reflection data from land seismic surveys often suffer triply - from statics variations, the presence of various types of noise, and imperfect moveout correction due to crossing events and rapidly varying non-hyperbolic moveout. For such data, velocity analysis, already difficult due to the non-hyperbolic moveout and crossing events, is further complicated by anomalies due to variations in the near-surface. Conversely, accurate statics estimation requires accuracy in the velocity model. Because the velocity and statics problems are intertwined for data from structurally complex areas, it is appropriate to treat the two problems as one. Modem practice for data from structurally complex areas is to base velocity analysis on iterative use of prestack depth migration in conjunction with some process such as depthfocussing analysis. We propose to incorporate into this iterative velocity analysis a statics-estimation procedure that also utilizes prestack migration. When applied to staticscontaminated data, the prestack depth migration will correct the data for non-hyperbolic moveout, dipping, and crossing events, provided that the velocity model is sufficiently accurate. These prestack-migrated data are then stacked, and the stacked traces are input to a multi-offset modeling that generates an unmigrated reference trace for each of the traces in the unstacked data. Cross-correlation of each data trace with its associated reference trace gives a time shift for that data trace. The derived shifts are then used in a conventional statics-estimation procedure under the familiar assumption of surface-consistency. Tests with statics-contaminated Marmousi model data show that even where the initial velocity model is poorly known, the migration-based method produces acceptable i~itial- stage statics estimates. After applying the method Iteratively in conjunction with a migration velocity analysis, alternating between statics and velocity estimation, both velocity and statics solutions are improved, producing a good statics solution in the Marmousi data after just two iterations. Further iterations can then be directed solely towards improvement of the velocity model.
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3-D Post-Stack Random Noise Reduction applied on 3-D seismic data from Tierra del Fuego, Argentina
More Less3-D Fxdecon algorithms have become available over the past couple of years, and their use has greatly improved our ability to attenuate random noise on 3-D surveys. The Canadon Piedras 3-D survey, recorded on the island of Tierra del Fuego in 1994, provides a particularly good example of the application of 3-D Fxdecon on real seismic data. Prior to application of this process, stacked data were of very poor quality over most of the survey. Application of 3-D Fxdecon improved the data dramatically. Subtraction fFxdecon data from the input shows that only random noise was attenuated.
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Seismic Differentiation - A Tool for Boosting Higher Frequencies
Authors S. Pokhriya and A. NautiyaAbsorption deconvolution designed on the basis of absorption parameter Q is used conventionally to compensate the apparent loss of higher frequencies in the earth. Determining Q from seismic data may be cumbersome and expensive, and sometimes erroneous. This paper describes an alternative approach to enhance the higher frequency content of the data by differentiating the trace itself. Though this process is theoretically known to introduce frequency-dependent phase distortion, it has been seen that either this problem is not very severe on real data or can greatly be overcome by introducing a minor constant static shift to data. This technique, while requiring no apriori information about Q., can result in enhancement of higher frequencies matching with Q-compensated sections. VSP corridor stack were also generated to visually judge the increase in high frequency content.
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Subsalt Imaging over the Mahogany Salt Sill
Authors D.W. Ratcliff and D.J. WeberExploring for hydrocarbons below salt has been, and will continue to be, a high risk/high potential business endeavor. Recent subsalt discoveries have generated a tremendous interest in subsalt exploration, both in the Gulf of Mexico as well as in other basins around the world. The geophysical technology which has played the most significant role in the success of subsalt exploration is, by far, Depth Imaging. Depth imaging technology (poststack and prestack) has allowed explorationists to identify and map structural and stratigraphic features below salt that trap significant hydrocarbon accumulations. In this paper, we discuss subsalt imaging strategies and technology applications (3-D Time and Depth Migration) that can assist the subsalt explorationist in prospect evaluation, risk assessment, well placement and field development, using the Mahogany Salt Sill Area as a calibration point. In addition to utilizing a specific "subsalt" 3-D acquisition geometry, the collective processing strategy will incorporate 2-D time imaging, 2-D poststack depth migration, 2-D prestack depth migration, wide-line profiling, 3-D time imaging, 3-D poststack depth migration.
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Effects of Binning Parameters on Lateral and Vertical Resolution of P-S Converted Waves
Authors J. Sierra, R.J. Michelena and E. AtaTolerances allowed in the ratio of the P-wave velocity, Vp, to the S-wave velocity, Vs (VpNS) ratio and trace dispersion within a common conversion point gather(CCP) dictate the final quality of stacked P-S converted wave sections. These parameters can affect the positioning of reflection events, the signal-to noise ratio, and the lateral and vertical resolution of subsurface images obtained from P-S converted wave data. Using synthetic and field P-S converted wave data, we evaluate the effects of varying the VpNs ratio and the dispersion within a CCP on the final quality of the stacked sections. The results show that well focused images, in comparison to corresponding P-wave images, may be obtained by the CCP gathering method. The correct positioning of the events, the IN ratio, and the resolution of the data can be preserved by careful selection of the VpNs ratio and the tolerance in trace dispersion around a central point within a CCP.
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