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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
321 - 330 of 330 results
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Migración en profundidad 2D preapilamiento con la ecuación de onda transforamada linealmente
By V. CelisEn este trabajo se presenta la extensión a migración en profundidad de la metodología basada en la ecuación de la onda bajo una transformación lineal en su versión de preapilamiento 2D. Perfiles migrados son obtenidos del proceso y los mismos son sumados por receptor común para obtener la sección cero "offset" migrada preapilamiento. El método no tiene restricciones en cuanto al ángulo de buzamiento de las reflexiones y es capaz de manejar fuertes variaciones laterales de velocidad. El algoritmo es probado con datos sintéticos y reales.
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Evaluación de Algoritmos de Migración en Tiempo y Profundidad, Pre-apilamiento y Post-amilamiento
Authors C.E. Moreno, E. Del Pino, H. Cerquone and E. RoaCon datos sintéticos generados a partir de un modelo que simula rasgos de una zona estructuralmente compleja, se evaluan distintos algoritmos de migración, en tiempo y profundidad, post-apilamiento y pre-apilamiento, de cinco paquetes comerciales disponibles en el mercado. Dentro de las características que se pueden observar en los datos generados tenemos que la banda de frecuencia espaciales y temporales permiten definir bien las estructuras; la relación seńal-ruido es suficientemente amplia como para resolver facilmente el objetivo; ademas el campo de velocidades de propagación en el medio es bien conocido, las mismas del modelo con el que se generaron los datos; por lo tanto, si no se obtiene una imagen bien enfocada del subsuelo, el principal motivo será por limitaciones del algoritmo de migración que se utilice. Los datos sintéticos fueron generados por trazado de rayos, a partir de un modelo que simula condiciones geológicas complejas con un campo de velocidades muy sencillo: dos capas de velocidad constante. EI modelo, se encuentra basado en el logotipo de la operadora venezolana CORPOVEN, SA (Filial de PDVSA). Sobre este objetivo, se simuló una adquisición sismica 2D para generar una sección de incidencia normal y registros de disparo común. EI resultado final es la comparación de las secciones migradas con los modelos originales y el análisis de los distintos ruidos y artefactos producidos por los operadores de migración.
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Demonstration of Migration Velocity Analysis Tomography on a Complex Structure
Authors R. Roundtree and C. StorkWe demonstrate an approach called MVA (Migration Velocity Analysis) tomography for determining an accurate velocity model for prestack depth migration. As a direct inversion method, this approach produces an accurate velocity model in significantly less time than iterative migration approaches. Moreover, as a tomographic technique that uses raypaths, this approach may resolve complex velocity models that cause problems for focusing analysis methods. In this paper we will illustrate via synthetic models why the MVA Tomographic Inversion technique provides improved results over the more common method of Focussing Analysis. Additionally, we will demonstrate on a geologically complex overthrust area how you can iterate and improve the migration velocity field through incorporating geologic constraints to the Tomographic Inversion.
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Relating Horizontal Resolution and Bandwidth after Migration
Authors D.A. Ebrom, S.A. Markley and J.A. McDonaldBefore migrations, horizontal resolution in seismic data is a function of both frequency and depth to target. After migration, the dependence of horizontal resolution on depth is removed, resulting from the collapse of the Fresnel zone to a smaller region. The horizontal extent of this smaller region Is predicted by plane wave theory to be controlled by the predomtnant frequency and the migration aperture. An essential point is that the presence of high frequencies in the wavelet (above the noise level) is important not just to the vertical resolution (which is relatively unaffected by migration) but also to the post-migration horizontal resolution. In order to experimentally verify this point, we collected physical-model seismic data over a model that had a variety of horizontal scale lengths. The data, which originally contained high frequencies, was low-pass filtered and the time-slices from high frequency and low frequency 3D seismic volumes were compared. The resolution losses are well explained by the predictions of plane wave theory.
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Two Methods for Subsurface Velocity and Structure Determination Based on RefiecHon Tomography
Authors D. Kosloff, E. Machet, Y. Falkowitz and Z. KorenPrestack depth migration has become a powerful tool for subsurface structure and velocity determination. We present two tomographic approaches that modify an initial model to produce an updated model which yields flatter prestack migration gathers. The first approach updates a model which consists of closed polygons and is suitable for situations where such a model can be interpreted from the data. The second approach is grid based and can allow a detachment between structure and velocity. Both approaches are derived from the tomographic principle. The spatial discretization yields, in both cases, a set of overdetermined linear equations which are solved by weighted least squares.
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Pattern Recognition Using Cluster Analysis Applied to Automated First Break Picking
By G. LiebhardtCluster analysis is a well-known statistical tool which is often applied when classification problems arise. The identification of first break signals may be interpreted as a specific classification or recognition problem. A "learning" classificator type is used to generate the initial database, which includes the manual choice of first breaks within some reference data and the calculation of probability densities describing the attribute space. Because the database may be updated during the automatic picking process, the system may adapt itself to changing signal and noise conditions.
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Imaging Alternatives Around Salt Bodies in the Gulf of Mexico
Authors S.Y.C. Lee, I.R. Mufti, G.A. Zelewski and N.J. House-FinchMobil has applied an arsenal of specific imaging techniques to a variety of salt related imaging problems in the Gulf of Mexico. The key to success in any imaging problem is close interaction between interpreters, velocity model builders, algorithm developers and computer hardware experts. What is presented is an array of different problems and associated technologies ranging from conventional 20 time processing to full 3D prestack depth migration as needed to solve specific geologic problems. These advanced technologies would not be possible without access to the latest fast thinking computers as well as an advanced computer network linking together the interpreter model builders, processors, mass storage devices and algorithm developers.
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An Iterative Procedure Employing Quasilinearization for Parameter Estimation in Acoustic Media
Authors E. Fernandez, P. Gauzellino and J.E. SantosAn iterative algorithm using quasilinenrization techniques to solve a parameter estimation problem for the acoustic wave equation in a single espace variable with absorbing boundary conditions depending on the parameter is described. The problem is formulated as the minimization of a quadratic cost functional. The derivative of the functional with respect to the parameter is obtained using the solution of a differentialequation similar to that of the direct problem. ' The non convexity of the cost functional is also adressed. Numerical examples are presented to illustrate the application of the algorithm.
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Removal of surface-related multiples from marine seismic data: A field data example
Authors R.G. van Borselen, J.T. Fokkema and P.M. van den BergRemoval of the effects of the free surface from seismic reflection data is an essential pre-processing step before pre-stack migration. The problem can be formulated by means of Rayleigh's reciprocity theorem which leads to an integral equation of the second kind for the desired pressure field, that does not include these free-surface effects. In order to solve the integral equation an estimate for the source wavelet is required. To obtain an accurate estimate for the source wavelet the minimum energy criterion is employed. The energy minimization problem is solved using a modified conjugate gradient method. As a result, a causal estimate for the source wavelet is obtained. The method is applied to a marine field data set and demonstrates the effectiveness of the procedure.
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Considerations on the Applications of the Parabolic Radon Transform
More LessDespite the wide use of the Parabolic Radon Transform in the context of multiple attenuation, some of its properties have not been discussed yet. These properties determine the best way of dealing with the Parabolic Radon Transform (PRT) in what concerns to both efficiency and CPU saving. Some common procedures in seismic processing like mute and NMO stretch have to be considered to avoid misuse of the PRT. This paper offers a discussion of important issues to deal with PRT. Some conclusions are based on theoretical predictions of the transform of an event with a hyperbolic moveout, taken here as representative of an ordinary situation in practice.
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