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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
61 - 80 of 203 results
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Multi-Frequency Backscattering Tomography (MFBT) for Boundary Scattering and for Volume Scattering
Authors R.S. Wu and F.V. AraújoThe Multi-Frequency Backscattering Tomography (MFBT) for boundary scattering based on the Physical Optics approximation is formulated in this paper. It is shown that the reconstruction formulas are the same as the MFBT for volume scattering (Wu and Araujo, 1991) except for a factor of ik/2. The reconstruction algorithm using direct FT makes the method extremely fast compared with other multifrequency migration or inversion methods, even much faster than the single-frequency diffraction tomography. Numerical tests show that for large scatterers or interfaces the MFBT for boundary scatterirlg give better results than that for volume scattering.
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Born Seismic Migration-Inversion
More LessSeismic migration and inversion describe a class of closely related processes sharing common objectives and underlying physical principles. Born inversion represents a feasible approach in reflection seismic data processing, besides it is closely related to classical migration concepts. Born approximation leads to a linear integral equation relating data and scallering potential, but neglect multiple scattering. Applicalion of high-frequency approximation leads to simple relations between Born inversion, (F-K) frequency wavenumber migralion and Kirchhoff migration. We assumeme a model. of the subsurface characterized by weak fluctuation in velocity, adopting a comanl reference velocity and negligible density variations. We deal with common·midpoint stacked data, that can be considered as an approximation of a zero-offset data. In this paper we attempt to review, theoretically, the relationship between wave-equation migration and wave equation inversion, and present some numerical examples of Born seismic migration-inversion.
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A Influęncia de Zonas de Baixa Velocidade na Inversăo Tomográfica Robusta
More LessA tomografia aplicada na sísmica permite que se obtenha a distribuiçăo da velocidade de um meio a partir do tempo de viagem da onda da fonte localizada num poço até receptores dispostos na superfície e/ou outro poço. A relaçăo entre o tempo medido e a velocidade que se quer determinar é estabelecida pela equaçăo de Fermát. Nesse trabalho utilizamos a tomografia de transmissăo, a qual faz uso apenas da onda direta.
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Seismic Waveform Inversion Using Global Optimization Methods
Authors P.L. Stoffa and M.K. SenA nonlinear optimization method based on the principles of natural selection is used to invert plane wave decomposed seismic data. We develop a genetic algorithm (GA) that uses an ensemble of subsurface models to create synthetic plane wave seismograms which are compared to observed seismic data. A normalized cross correlation function (fitness function) is used to compare the model seismograms to the data. The GA then selects the models with bigh fitness values to generate a new generation of trial models through the process of reproduction. We show that our GA works. We also show that its rate of convergence to a population with bigh fitness values can be significantly increased by defining the fitness function as a probability and using a control parameter analogous to the temperature employed in simulated annealing (SA) that decreases with each generation of modell. Further, we show that by repeating the process, we can estimate the mean model, the posterior probability density in model space and the posterior model covariance matrix.
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A Focus on the Elements of Successful Onshore 3D Imaging
Authors W. Ritchie and K. DingwallSuccessful imaging of onshore three-dimensional (3D) seismic data requires the interaction of innovative field design, cost-effective acquisition and. importantly. an approach to data processing which truly comprehends the 3D nature of the data. Much attention was focussed, in the late 1980's, on the key aspects of geometrically imaging 3D seismic: 3D migration, both partial pre-stack and post-stack, and the associated definition of the 3D velocity field. While these elements are essential components of an imaged solution especially for onshore 3D seismic, they are only part of the total equation. Without the complementary support of 3D signal enhancement tools for 3D noise elimination and 3D surface consistent processes, the later geometric imaging processes would fail to produce the desired result. This paper discusses both aspects of a total 3D processing solution with examples to highlight tools for 3D noise elimination, 3D surface consistent coupling compensation as well as 3D velocity and migration and describes a comprehensive sequence for optimum 3D onshore imaged information.
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Wave Equation Multiple Supression Using Constrained Cross-Equalization
Authors D.J. Monk and R.M. LansleyA method for improving the attenuation of water layer multiple energy is suggested. The improvement is achieved using wave equation extrapolation to generate an initial model of the multiple energy , and then constraining the way the time variant wavelet is modified to fit the observed multiple energy. Reconciling the initial multiple model wavelet with the input data is a critical part of this process, and several techniques have been suggested previously by other authors. The approach used here is to fit time, amplitude and phase of the wavelets, by adapting the model trace using a weighted sum of four traces which can each be derived from the initial multiple model trace. Results on real data suggest that attenuation of primary energy is minimized using this technique, without diminishing the level of multiple attenuation.
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Utilizaçăo da Matriz de Transiçăo no Estudo de Espalhamentos Acústicos
By A.D. MattosO fenômeno do espalhamento de ondas em meios acústicos e elásticos encontra importantes aplicaçőes em uma vasta gama de atividades, entre elas a exploraçăo geofísica. Por esse motivo săo elaborados rotineiramente programas de modelamento que lançam măo dos mais diversos métodos.
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Interpretaçăo de Perfis em Carbonatos Fraturados da Bacia do Paraná - Maranhăo
Authors A.A.S. Laranjeira and H.B. EvansEste trabalho é um estudo de interpretaçăo de perfis em carbonatos fraturados de mineralogia complexa. Os poços X e Y, perfurados na Plataforma. do Pará-Maranhăo, registram um pacote de rochas carbonáticas de mais, de 4000 m de espessura, que apresentam fraturas subverticais descontinuas, parcialmente tamponadas por calcita. A litologia é complexa, tanto do ponto de vista da composiçăo mineralógica, tamanho e forma dos grăos, quanto da geometria dos poços. A presença de fraturas diminue o expoente porosidade da equçăo de Archie para até 1,3, faixa considerada. inerente a. rochas fraturadas. A identificaçăo dos tipos litológicos e feita baseada em Evans(1971), Clavier e Rust (1976) e Schlumberger (1989). A saturaçăo de óleo é calculada com a fórmula de Archie aplicada com expoente de porosidade variável, calculado a partir do modele de Rasmus(1983), e a partir do método estatístico de Porter et al (1969). Um estudo anterior de porosidade e saturac;a.o de fluidos nos poços da Plataforma do Pará e encontrado em Beer (1986).
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Um Algoritmo Eficiente para Obtençăo de Filtros de Forma e L-Preditivos
By M.J. PorsaniPara sinais transientes, a exemplo de um pulso sísmico, a premissa usual de que os dados possuem valores de amplitude nula ao início ao final do pulso, é de fato válida. Neste caso a matriz das equaçőes normais, é a matriz banda-estruturada Toeplitz esimétrica, (Robinson, 1967).
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Radiation Pattern of Seismic Sources Via Disc-Source Analysis
Authors P. Carrion and E.E.S. SampaioSeismic exploration industry is largely based on the concept of the explosive point source, whose radiation pattern is spherically isotropic in acoustic homogeneous media.. In elastic media. the radiation pattern of a point source is not uniform over the phase surface and depends on the load direction and on the type of the generated waves. The situation drastically changes when one considers the drill-bit radiation source. The use of drill-bit as a source of energy was advocated a few decades ago, but it 'rang hollow until recently. Nowadays it emerges as one of the most attractive technologies: the method does not require any borehole recording, and therefore no risk to the borehole is involved. What is striking, however, is that the drill-bit source is poorly understood, probably due to the high complexity of its mathematical treatment. This makes difficult an accurate interpretation of continuously reeorded data. In the current study we develop a theory for disk-type sources with a combined load that generates elastodynamic waves, that tackles the drillbit source radiation pattern problem. We derived an explicit expression for the drill-bit source and solved the inhomogeneous wave equations for the Lame's potentials using Fourier-Hankel transform. We show that the radiation pattern of the drill-bit source significantly differs from the point source radiation pattern. This is in agreement with the observed data generated in the drill-bit experiments, demonstrating the viability of disk-type sources,
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Effects of Surface Roughness or Diffusivity on Migration, and Lateral and Vertical Resolution
By R.W. KnappWhen the wavelength of the roughness of a reflector surface is much Iess than the wavelength of the lateral wavelet it confounds the process of migration and produces a vertical effect that appears as a velocity gradient when the data are inverted. Lateral effects and vertical effects become indistinguishable and the problem is irreparable by any processing of the data. The problem is: more acute on high-resolution data than lower-resolution data because surface roughness is a small-scale feature. For it to affect the data, wavelengths must be dimensionally similar to the scale of the roughness. The real data example shown, for instance, had a peak frequency of about 500 Hz and a dominant frequency of about 350 Hz. That is not to say that the effect might not be seen ·on low-frequency data, just that opportunities increase with higher frequencies. An interpreter aware of the effect of surface roughness might know to consider both possible interpretations, but it requires independent information, such as a onedimensional vertical velocity log or other geological knowledge to separate the two. The distinction between the two is geologically significant. Surface roughness and velocity gradients have different geological implications, so distinguishing the two affects the geological interpretation of the region in question.
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Scale Space, Fractals and a Touch of Chaos
By T.J. UlrychThis paper deals with the fundamental issue of scale. The thesis which is presented is that scale is a concept which unifies some of the most current and interesting ideas in geology and geophysics. The introduction of this ooncept in a. formal wa.y into the literature is due to Witkin (1983) who coined the name scale space and applied it in a fascinating manner to the representation and decomposition of signals. Leaney and Ulrych (1988) extended this approach to event tracking and matching of well data. Scale, of course, is what fractals are all about and the paper goes on to deal with some recent results which describe the fractal nature of the reflectivity series and consequently of the underlying sedimentation process. Since fractal structure is a new geometric property which is exhibited by chaotic motions, the paper concludes with a brief look at a canonical chaotic .example and examines the possibility of wavelet estimation using a particular measure in phase space.
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Revisitando a Correçăo de NMO
By O.O. DuarteComo se sabe, a correçăo de "nmo" tem por objetivo eliminar a influęncia da distância fonte-receptor nos dados de reflexăo sísmica, simulando o registro que sería obtido com o receptor muito próximo da fonte. Um ponto fundamental para a análise desta correçăo é que o afastamento fonte-receptor causa, além da compressăo da funçăo refletividade, um aumento da amplitude variável com o tempo. Assim, por uma questăo de simetria, a correçăo de "nmo" deve ser acompanhada por uma reduçăo equivalente das amplitudes.
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Uma Nova Opçăo para a Eliminaçăo de Fatores que Degradam a Resoluçăo Sísmica
Authors A.S. Oliveira and R.J. CamposDe acordo com o modelo convolucional, o sinal sísmico registrado é o resultado da convoluçăo entre varíos fatores, que podem ser classificados segundo a dependęncia ou năo do offset e do tempo de transito. Săo fatores dependentes, por exemplo, as efeitos dos arranjos de fontes e receptores, a refletividade, a absorçăo seletiva de altas frequęncias, etc. Năo săo caracterizados por tal dependencia a assinatura intrínsica dos elementos da fonte, a resposta impulsiva dos receptores e do instrumento. Incorporando as reflexőes na superficie do mar (fantasmas) a uma estimativa dos efeitos dos arranjos de fontes e receptores, e possivel obter, por deconvoluçăo, uma assinatura que contenha as contribuhőes da fonte, dos recepetores, do instrumento, do efeito bolha e etc. Isto e realizado sobre a onda direta que, em aguas profundas e facilmente identificável.
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Influence of a Near-Surface Structure on Seismic Wave Fields and Multiple Suppression
Authors F.C.M. Andrade and V. ČervenýThe hybrid ray-reflectivity method is applied to the numerical modelliog of seismic wave fields in laterally varying layered models containing a thin near-surface low-velocity layer. The computatioos within the laterally varying layered model are performed by the ray method, but the thin near surface layer is attacked locally by the matrix methods. The thin layer need not be homogeneous, it may include arbitrary inner layeriog and it may lightly vary laterally. All multiples within the layer are automatically taken into account. Numerical examples of hybrid ray-reflectivity seismograms for two models of the thin near-surface layer are presented. An inverse algorithm to remove the effects of a thin near-surface layer from seismograms recorded at the Earth's surface is proposed.
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3-D Static Corrections
Authors W.N. Amorim and W. EckhardtThe calculation of static corrections using the arrival times of refracted waves present in reflection seismograms by means of the method of Seismic Refraction Tomography (SRT) for 2D acquisition geometry has succesfully been applied to straight and crooked seismic lines. The redundancy as obtained by present day CDP shooting is used to estimate the variations in thickness and velocity of the weathered layer, which cause static shifts in the observed reflection times and hence distortions in the final stack. This paper deals with the extension of the method to any acquisition geometry, in particular 3D. After a short review of the basic theory we will highlight some aspects for 3D geometry. Then we discuss some implementation considerations after which real data examples will be shown. We conclude with observing that the method results in an improvement of the quality of the stacked section, especially for the 3D case where the conventionally obtained static corrections by means of refraction seismics do not account for the spatial variations in thickness and velocity of the weathered layer. This implies a pleasant 'fringe' benefit of the method, vz. that the shallow refraction survey may be omitted.
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Atenuaçăo de Alinhamentos Gerados por Filtros de Mergulho
Authors A.B. Abrăo and H. MalebrancheOs filtros de mergulho (ou velocidade) que săo aplicados nas fases pré ou pós empilhamento, objetivam apenas a atenuaçăo dos ruídos coerentes que mascaram os dados sísmicos. Entretanto, alteram o espectro dos ruídos incoerentes induzindo coeréncia local e modificando o carácter dos dados. A consequente aparęncia mixada dos refletores e a presença de alinhamentos onde antes só havia ruído aleatório, diminui a qualidade dos dados e dificulta a interpretaçăo. Essa é uma razăo pela qual esses filtros săo apenas parcialmente aplicados.
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Filtro de Feixes Gaussianos
By R.C. PestanaA transformaçăo de sismogramas de fonte pontual em sismogramas de feixes gaussianos pode ser de grande valor no estudo de meios que possuem a grande vantagem de se propagarem em uma única direçăo e permitem, ainda, uma iluminaçăo mais localizada dos objetos a serem investigados. Já as ondas planas, as quais se propagam também em uma única direçăo e necessitam de um suporte espacial infinito, fornecem uma resposta mais global.
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Geophysical Monitoring Steamflood Advance
By J.M. SoutoA new analytical technique was developed to monitor the changes in oil water saturation during thermal recovery of tar sands. During fracture assisted steamflood recovery, the reservoir is heated and depleted of most of its oil by the injected fluids. Changes in temperature and oil water saturation produce a significant change in resistivity at both the microscopic scale (grain size, pore flushing), and at the macroscopic scale (reservoir size. production pattern). This geophysicial technique uses geothermal technology to detect the steamflood advance from the surface by using a large electromagnetic signal generator and a permanent network of receiving stations to detect and measure the curvature of the secondary electric: field induced in the alteration zone produced by the the steamflood injection. The analytical technique allows interpretation of the observed anomaly in terms of thickness and lateral extent of the invaded zone. In addition, the technique was designed so that the reciever grid network is reoccupied later, producing more resistivity "snapshots", allowing measurements of the changes in distribution of injected fluids, temperature and oil water saturation during various phases of steamflood recovery of heavy oil.
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Assinatura da Fonte e seu Espectro Angular por Inversăo de Sismogramas de Ondas Planas
By S.S. SantosEm um experimento sismica, maritima ou terrestre, a assinatura da fonte é pouco conhecida ou desconhecida o que dificulta a soluçăo de problemas geofisicos onde o conhecimento da assinatura da fonte e necessario. Este trabalho sugere um novo metodo para estimativa de assinatura da fonte baseado na combinaçăo CD/ERT (continuaçăo descendente/extrapolaçăo reversa no tempo) onde fonte e receptores săo colocados na superficie a metodo e iterativo e consiste em computardor a cada passo, os valores iniciais em t=T (onde T é metade do comprimento do sismograma) e a assinatura da fonte. Uma propriedade importante do metodo CD/ERT e que năo requer a hipótese de fase mínima. O número de iteraçőes depende da precisăo desejada e o algoritmo é ilustrado para vanas assinaturas com diferentes amplitudes e fases. Este método também é usado para a reconstruçăo do padrăo de radiaçăo da fonte e pode ser aplicado a dados sísmicos reais desde que o modelo de velocidade seja previamente conhecido.
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