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5th International Congress of the Brazilian Geophysical Society
- Conference date: 28 Sep 1997 - 03 Oct 1997
- Location: São Paulo, Brazil
- Published: 07 November 1997
381 - 400 of 416 results
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Migraçăo 3-D no Tempo Usando a Aproximaçăo Paraxial dos Raios
Authors N.P. Queiroz and W. Söllnero trabalho aqui apresentado visa a migraçăo 3-D no tempo, em amplitudes verdadeiras, de dados da seçăo afastamento nulo (zero-offset), usando a aproximaçăo de segunda ordem do tempo de trânsito de difraçăo, o espalhamento geométrico é removido através do processo de migraçăo e aplicaçăo de uma funçăo peso aos dados da seçăo afastamento nulo. O processo de migraçăo no tempo usa a funçăo difraçăo determinada através da medida de nove parâmetres da funçăo tempo de trânsito de reflexăo, e portanto sem o conhecimento do modelo de velocidades.
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Interval Velocity Determination by Time Migration Decomposition: A Synthetic Data Application
Authors Luis Montes, Walter Söllner and Peter HubralUsing the paraxial ray theory in second-order approximation of the traveltimes, we present a method to determine the interval velocities of a seismic system. The seismic system consists of a stack of isotropic and homogeneous layers separated by smooth, second-order interfaces. Measuring the traveltimes of some specially selected reflected rays, recorded on the upper border of the seismic system, the zero-offset difraction time function is determined. The interval velocities are calculated by downward continuation of the zero-offset diffraction time function applying an iterative process layer by layer. This process is controlled by using a principle of focusing.
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3-D depth migration: Various methods and how they perform on real data
By Irshad Mufti3-D depth migration is one of the most active areas of research and development and it has been approached from different angles. An accurate algorithm which work" in the space-frequency domain was introduced by Blacquiere et al. (1989). Unfortunately, it is computationally very expensive and not suitable for solving real-life 3-D migration problems. This led to the development of an alternative scheme (Hale, 1990) which sacrifices some accuracy but is much more efficient. It is commonly referred to as the Hale-McClellan method. An alternative procedure known as reverse-time migration works in the space-time domain. It ;05 based on the full wave equation and yields by far the most accurate results, but it requires extremely large computer resources. However, a series of new developments (Mufti et al., 1996) have made this algorithm a very practical tool even for migrating large exploration-scale 3-D real data. We shall provide a comparative evaluation of the results obtained by this method with the corresponding resultsfrom Hale's algorithm. This will include a detailed investigation of the impulse response as well as migration of both synthetic and real poststack 3-D seismic data. We will not present any results based on prestack migration, but we will discuss a number of serious problems with the current technology developed for prestack migration as well as some food for thought.
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Deep Seismic Reflection Profiling of the European Mosaic of Terranes: Accreation and Collapse
By R. MeissnerFor more than 100 years it has been accepted that the tectonic history of Western and Central Europe started with the Variscan orogeny in the Paleozoic. In Northern and Eastern Europe, Caledonian and older orogenies and even cratons are found, while in the South, younger tectonic features predominate. For a long time, the mechanism of the growing European mosaic, its tectonic framework, the architecture of crust and mantle, and the time sequence of collisions, accretion and collapse, remained a mystery.
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2D Imaging and Depth Tomography by Plane Wave Decomposition
Authors Dan Kosloff, Zvi Koren, Ronit Strahilevitz and Uri I. ZackhemDepth imaging and depth velocity analysis are performed in the variable angle domain. First the input is sorted into common receiver gathers. The gathers are converted into p gathers by a slant stack transformation. Migration is carried out by forming p sections and downward continuing each section separately. Velocity updating is done by grid tomography which detects nonflatness in the migrated p gathers and updates the velocity section accordingly.
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Multiple Supression in Common Offset and Stacked Sections Via Linear Radon Transform Filtering
More LessWe discuss the application of linear Radon transform (LRT) filtering for the supression of complex geometry related multiples in common offset and stacked sections. Common offset sections have to be considered for amplitude versus offset (AVO) analysis. Complex geometries give rise to events of non-hyperbolical moveouts that cannot be properly modeled by parabolic Radon transforms (PRT) and thus are not sufficiently attenuated via PRT filtering. Moreover, wave-equation based methods are still unable to deal with peg-leg multiples. On the other hand, in structurally complex areas, multiples and primaries are in general characterized by . different dips in the stack or in common offset sections allowing discrimination via LRT. For stacked sections, when more radical procedures such as internal muting are mandatory, the LRT filtering on a limited offset range, may be worthwhile.
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Deconvoluçăo de Múltiplas com Filtros Wiener-Levinson Multicanais
More LessAs múltiplas de longo período associadas ŕ reflexăo no fundo do mar chegam como eventos distintos e sua remoçăo ou atenuaçăo representam um problema importante no processamento de dados sismicos. A técnica de deconvoluçăo preditiva para s upressăo de reflexőes nniltiplas e um método estatistico que baseia-se no carater periódico destes eventos. Entretanto, a periodicidade só é preservada no caso de incidęncia vertical e seçăo de afastamento nulo, ou seja, as múltiplas năo săo periódicas para afastamentos diferentes de zero. Neste trabalho foi analisado o efeito da aplicaçăo de filtros de prediçăo multicanais no dominío t-x. Na filtragem multicanal, as características do filtro levam em conta a coeręncia lateral dos eventos refletidos presentes nos canais adjacentes. Esta propriedade dos filtros multicanais pode ser explorada de forma a permitir uma atenuaçăo mais efetiva das reflexőes múltiplas. A teoria utilizada para computaçăo dos operadores discretos multicanais conceitualmente é a mesma teoria de Wiener-Levinson empregada para obtençăo dos filtros preditivos monocanais(Barros e Porsani, 1996). Matematicamente representa a extensăo da álgebra escalar para álgebra matricial, conforme apresentamos a seguir.
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Surface-Related Multiple Removal - Applications to the Gulf of Mexico
Removal of free surface multiples from seismic reflection data is an essential pre-processing step before pre-stack migration. Any effective multiple removal method should require no a priori information about the subsurface and should leave all relevant primary events in the data unaffected.In this abstract, a surface-related multiple removal method is applied to two distinct configurations that are representative for the Gulf of Mexico: a shallow marine (subsalt) field data set and a deepwater data set. Good results demonstrate he effectiveness of surface-related multiple removal methods:only multiple energy is removed from the data whereas relatively weak primary reflections become better interpretable.
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Análise de Velocidades e Empilhamento eMP para Traços Interpolados
Authors Milton José Porsani and Reinaldo Nunes de PaivaA importância da interpolaçăo de traços sísmicos tem sido descrita por muitos autores na última década (Jakubowicz, 1994; Mannin and Spitz, 1995; Spitz, 1991, 1996; Gulunay and Chambers, 1996, Ferreira, Porsani and Ulrych, 1996, Porsani, 1997). Os problemas de aliasing espacial podem afetar a resoluçăo lateral das técnicas de filtragens e migraçăo impondo severas restriçőes no processamento sísmico. Um métoda de interpolaçăo de traços sísmicos que é eficaz mesmo na presença de eventos em aliasing foi proposto por Spitz (1991).
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AVO Processing Calibration
More LessOver the years AVO technology has been deployed in both efficient and inefficient ways in different sedimentary environments, normally functioning as a lithology indicator and sometimes as a direct hydrocarbon indicator. Several factors may be regarded as resposible for the unsuccessful applications. However, the most basic one, the properness of the processing sequence to the problem at hand, is sometimes underestimated. The ambiguity of the AVO technique and the presence of coherent noise in the data are other major factors. In this study, I developed a method to check the consistency of the AVO measurements produced by different processing sequences on a noise contaminated data. An extense reprocessing work has been employed in a marine seismic dataset, where an amplitude anomaly is related to the presence of high porosity sands saturated by light hydrocarbons. The sandstone reservoir is encased in marine shales. The anomaly is localized and is characterized by a decrease in density and Poison's ratio in the reservoir zone. The AVO signature is modeled and the corresponding normalized reflection coefficient curve is used to check the amplitude responses produced by different processing sequences. As a result of such calibration procedure the best processing sequence can be selected and it can be used for all seismic lines in the same area.
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The effects of vector attenuation on AVO of off-shore reflections
More LessIn off-shore seismic exploration, the waves transmitted at the ocean bottom have a particular characteristic. Assuming that water is lossless, their attenuation vectors are perpendicular to the ocean bottom interface. This fact affects the AVO of reflection events generated at the lower layers.
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AVO Crossplotting: Achieving New and Unbiased Hydrocarbon Indicators
More LessAVO crossplotting has been widely used in the past few years as a way of deriving improved hydrocarbon indicators from seismic data. By crossplotting the standard AVO attributes of intercept and gradient, it is possible to obtain an optimum combination of the two (the fluid factor), which is insensitive to the AVO effect of brine-saturated shales and sands. Any AVO anomaly can then be attributed to hydrocarbons or lithologic factors.However, the background shale trend observed in such crossplots is primarily due to a statistical artifact. In the presence of noise, the standard least-squares regression that is used to estimate intercept and gradient has a tendency to correlate these two attributes. This statistical trend has the same pattern as expected from background shales. Fortunately, since there is some signal in seismic data, the background trend does contain some lithologic information. This information is very limited though, and the fluid factor obtained from this mostly statistical trend is in fact very close to a far offset stack. Although far offset amplitudes are good hydrocarbon indicators, crossplotting is a very complicated way to compute a far offset stack. On the other hand, noise does not introduce a statistical correlation between stack and gradient. Since stack and gradient exhibit the same kind of trend for background shales as intercept and gradient, it is very appealing to crossplot them. Any trend observed in this crossplot would have to be related to lithology and not to a statistical artifact. Unfortunately, when doing this we find that the background trend is generally buried under background noise. However, since this new crossplot is statistically unbiased, it is possible to postulate a background trend, leading to a new definition for the fluid factor. This gives rise to unbiased hydrocarbon indicators that are not limited to far offset stacks.
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True Amplitude MZO and AVO: Application to real data
More LessWe discuss the application of true-amplitude migration to zero offset (MZO) to amplitude versus offset (AVO) studies on a real marine seismic data. The true-amplitude MZO is achieved by means of a cascaded application of standard normal moveout correction (NMO) plus a constant-velocity, true-amplitude dip moveout (DMO). This process is expected to be precise as long as we have only mild lateral velocity variations. The impact of the compensation of geometrical spreading losses from large offsets to short ones on AVO analysis is discussed. The reliable results obtained for dipping events makes this process interesting for AVO analysis in areas of relatively small structural complexity.
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Investigaçőes do Uso de Expressőes Linearizadas do Coeficiente de Reflexăo para Extraçăo de Parâmetres Elásticos
More LessA variaçăo do coeficiente de reflexăo com o ângulo de incidęncia contém informaçőes a respeito das propriedades elésticas dos meios. As fórmulas exatas para os coeficientes de reflexăo săo dadas pelas equaçőes de Zoeppritz (1919). Diversas aproximaçőes para estas expressőes podem ser encontradas na literatura, as quais tern por objetivo facilitar a compreensăo da influęncia das propriedades elásticas. Contudo, estas aproximaçőes sofrem limitaçőes em seu intervalo de validade, tais como: pequenas variaçőes nos parâmetres elásticos através da interface e/ou ângulos de incidęncia próximos da vertical. A medida que nos afastamos destas condiçőes, as aproximaçőes afastam-se da forma exata.
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Frontiers of Deep Seismic Reflection Profiling
By L.D. BrownOver the past two decades multichannel reflection profiling has grown to become the lead methodology in many national and international programs to explore the deep structure of the continental lithosphere. Deep reflection surveys by groups such as COCORP (US), LITHOPROBE (Canada), DEKORP (Germany), BIRPS (Great Britain) and ECORS (France) have made a number of major discoveries about the complexity of the lower crust and upper mantle, including delineation of crustal-scale faults, wide-spread lamination in the lower crust, magma "bright spots" in the mid-crust, systematic variations in Moho character and distinct structures in the mantle lithosphere.
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Estudo de uma Metodologia para Calibraçăo de AVO a Diferentes Litologias e Fluidos
Authors Alcides Aggio, Albano Bastos and Ariovaldo da CostaApresentamos neste artigo o resultado da análise de AVO ( Amplitude versus Offset) dos poços A e B, com produçăo em arenito, com gás e carbonato com óleo, respectivamente. Esta análise procurou basear-se nas medidas obtidas no Laboratório de Física de Rochas da PETROBRAS/CENPES, bern como nos dados de perfis de poços. Foi encontrada e implementada para a modelagem do poço B uma melhor estimativa de vs, quando comparada ŕ literatura (equaçăo de Krief, 1990), baseada em dados de carbonato fornecidos pelo laboratório. O AVO também foi calibrado através do cross-plot dos atributos PeG, Verm & Hilterman (1994), Castanha (1985), mostrando uma boa potencialidade de análise e definiçăo de topo e base do reservatório, bern como a possibilidade de rastreamento litológico nos dados sísmicos, possibilitando a estimativa da razăo vp/vs em regiőes onde năo existem poços.
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Estudo Integrado de Modelagem, Inversăo, AVO e Física de Rochas: Um Pitfall Clássico
More LessUrn procedimento combinando informaçőes de processamento sísmico, SEYEL (seçăo de velocidade), modelagem e inversao AYO (Amplitude Variation with Offset) foi aplicado em dados sísmicos 2D levantados em área sem poços de aguas profundas, para apoio no estudo preditivo de urn corpo turbidítico, identificado nas linhas sísmicas como uma anomalia de amplitude. O trabalho subdividiu-se em duas fases, antes e após a perfuraçăo. Na prime ira fase, foram descartados, por inconsistęncia com dados reais, alguns modelos litológicos possíveis de explicar a anomalia, através de modelagem sísmica. Ainda foram estimados parâmetres petrofísicos que favoreciam a ocorręncia de hidrocarbonetos. Na segunda fase, em bora prejudicada pela baixíssima qualidade do perfil de ondas S, pode-se constatar que a anomalia observada no SEYEL sendo mais devida a urn baixo de densidade do que a velocidade propriamente, como gostariamos. A referida anomalia de amplitude é um caso bern conhecido de insucesso do AYO, i.e, urn pacote espesso de areia saturada com água, com baixo conteúdo de argila e com alta porosidade, imerso em folhelhos. O insucesso, em termos de resposta para hicrocarbonetos, é parcial, pois o gás pode mostrar resposta AYO a nivel quantitativo (crossplots) maior que a água. Podemos estar ganhando urn referencial futuro para calibracao. Finalmente a irnportânicia deste trabalho é ser um convite para a integraçăo de dad os sísmicos petrofísicos, como metodologia a ser aplicada em bacias já exploradas e, neste caso, usar também a geoestatfstica para tratar os dados em linguagem de risco.
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Cancelamento de Ruído Harmônico no Traço Sísmico
More LessDuas técnicas de cancelamento de ruído harmônico no trace sísmico săo apresentadas. Cada uma delas se destina a operar sobre interferęncias harmônicas de diferentes natureza: estacionária e nao-estacionária, onde a amplitude e a fase de cada harmônica podem variar ao longo do traço. O cancelamento promovido por qualquer dos dois métodos se apresenta mais eficiente que a filtragem notch convencional, pois o ruído é estimado (amplitude e fase de cada harmônica) e subtraído do traço. Um procedimento automático para o cancelamento de ruído harmônico integrando estes dois métodos é, entăo, sugerido como uma eta+pa do préprocessamento+H62 de dados sísmicos. Resultados para dados sintéticos săo apresentados a fim de demonstrar a eficięncia do pro cedimento proposto.
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Deconvoluçăo de Dados Sísmicos com Filtros de Fase Mista
Authors Reinaldo Nunes de Paiva and Milton José PorsaniO objetivo principal da deconvoluçăo da assinatura da fonte é o aumento da resoluçăo temporal dos dados sísmicos. Uma limitaçăo da deconvoluçăo Wiener-Levinson é o fato de que o filtro, computado a partir dos coeficientes estimados da funçăo de autocorrelaçăo do pulso básico, é sempre de fase mínima (Robinson, 1980; Berkhout, 1977).
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Deconvoluçăo de Ondas Quadradas através do Filtro de Wiener
Authors Roberto J.V. Sacasa, Lourenildo W.B. Leite and André J.N. AndradeA descriçăo e aplicaçăo do filtro de Wiener representa uma técnica de grande utili dade para o tratamento de dados sísmicos no dominio do tempo. Princípios da teoria das comunicaçőes permite a formulaçăo matematica e a obtençăo da soluçăo destes filtros ótimos. O filtro de Wiener é um processo regido por uma distribuiçăo tempo invariante estacionário.
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