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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
41 - 60 of 203 results
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Substituiçăo de Camadas com as Integrais de Kirchhoff e Rayleigh
Authors W.P. Gouveia Jr. and A.L.R. RosaO procedimiento de substituiçăo de camadas tem como objetivo retirar os efeitos associados ŕ variaçăo lateral de velocidade junto ŕ superfície, nas reflexőes provenientes das camadas mais profundas. É um procedimento importante, principalmente no processamento marítimo, onde a topografia do fundo do marapresenta-se irregular em muitas situaçőes. Os dados resultantes podem ser submetidos, com melhor desempenho, ŕs técnicas convencionais de empilhamento e de migraçăo em tempo.
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Exact Analytical Solution for the Seismic Tunneling Problem
By K. SchielAn exact analytical solution for the tunneling problem is presented. The canonical configuration consists of two acoustic half-spaces in contact including a source. Classical solutions are based on numerical schemes or on other well known methods like the Cagniard-de Hoop method, the WKBJ method, or the steepest descent method. In common with the Cagniard-de Hoop method and as opposed to the other methods mentioned above, the one presented here doesn't use approximations and is explicit in the space-time domain. Furthennore uniqueness of the solution is established which is not the case with the Cagniard-de Hoop method or any other classical solution known. The Green's functions and the seismograms calculated with the method presented here diverge significantly from that obtained with the Cagniard-de Hoop method.
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Reflection and Transmission of Elastic Plane Waves at the Interface between Anisotropic Media
Authors J.S. Protázio and M. SchoenbergAnisotropy affects the reflection and transmission of elastic waves significantly as soon an the energy propagates over a wide angular aperture. Plane wave reflection and transmission coefficients at an interface between anisotropic elastic media can be expressed explidtly in terms of 'iIupedance matrices", as long as the plane of the interface is a mirror plane of symmetry of both media. For the two dimensional case, plane strain 'quasi-compressional' (qP) and 'quasi-transverse' (qS) waves (associated displacements in the plane of propagation) are uncoupled from the anti-plane strain SH waves (associated displacements perpendicular to the plane of propagation) and the plane of propagation' also must be a mirror plane of symmetry of both media, implying that both media are at least orthorhombic. This case includes the important problem of reflectivity and transmissivity between isotropic and transversely isotropic media. The ultimate aim of amplitude vs. offset (AVO) analysis in this case is the estimation of the density ratio and the elastic constants in each medium on which the reflection and transmission coefficients depend. However, these constants are not equally well resolved. For all models that could conceivably represent the anisotropy in real rock masses, even the anisotropy of the reflecting medium can not be estimated, since each anisotropic medium has its 'quasi-equivalent' isotropic medium which gives reflection coefficients close to those of the anisotropic medium, ovr a wide range of incidence angles.
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Tempos de Trânsito de Ondas Quase Cisalhantes em Meios Heterogęneos Anisotrópicos: Precisăo do Metodo de Perturbaçăo
More LessAté 1977 (Bamford e Crampin, 1977), as observaçőes de meios anisotrópicos se restringiam praticamente a alguns km no topo do manto superior, tanto sob oceanos quanto sob continentes. A partir de 1984, observaçőes de separaçăo das ondas cisalhantes na crosta, fenômeno característico da propagaçăo de ondas elásticas em meios anisotrópicos, se multiplicaram e hoje se reconhece esta característica em quase toda onda cisalhante se propagando nos 10-20 km superiores da crosta (Crampin, 1987). Esta distribuiçăo quase geral de meios com comportamento anisotrópico na Terra justifica o interesse crescente por seu estudo.
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Ray Tracing in Layered Anisotropic Media
Authors J. Costa, M. Schoenberg and D. MillerTwo dimensional ray tracing in layered elastic media is shown to be a simple procedure, whether the Iayers are isotropic or anisotropic, when certain criteria are met, those of 'mild anisotropy'. This implies not that the 'anisotropy parameters' are met, but that the medium behaves so that there is no anomalous polarization, there is no triplication, and any quasi-shear wave is slower than any quasicompressional wave. The two-point scheme uses the fact that all rays from source to receiver, including converted and reflected waves, have a common value of horizontal slowness (Snell's Law). Possible values of horizontal slowness have bounds depending on the specified layers through which the ray passes, and the specified ray type in each layer. Whenever a ray originates from the source with a horizontal slowness within these limits, it will reach the receiver depth, although in general at a point that is horizontally offset from the actual receiver position. When the conditions of mild anisotropy are met, range increases monotonically with horizontal slowness, yielding &. unique, easy to find, horizontal slowness, and its associated ray which arrives at the receiver depth at the correct range. The scheme is analysed in detail for the wave modes in the vertical plane of a transversely isotropic medium or in a plane of symmetry of an orthorhombic medium. In particular, the modes studied are those whose displacements lie in the plane of propagation, the so called qP and qS modes.
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Inversăo em Geofisica de Poço: Um Estudo sobre Ambiguidade
Authors A.B. Buoro and J.B.C. SilvaA interpretaçăo,de formaçőes geológicas complexas a partir de dados de perfilagem de poço pode ser feita utilizando-se toda a informaçăo disponível simultaneamente. Para isto é necessaria solucianar o sistema de equaçőes associado ao modele interpretativo proposto a priori, a que caracteriza um problema inverso em geofisica. Este problema năo tem soluçăo única e é instável uma vez que a quantidade de informaçăo contida nos dados geofisicos é via de regra insuficiente para determinar a complexidade das formaçőes geológicas, o que caracteriza portanto um problema ambíguo.
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True Amplitude Migration
Authors P. Hubral, M. Tygel and J. SchleicherA primary offset reflection of a point source from a smooth reflector within a laterally inhomogeneous velocity earth model is (within the framework of ray theory) defined by parameters pertaining to the reflected ray. The geometrical spreading factor - usually computed along the ray by dynamic ray tracing in a forward modeling approach - can be recovered from traveltime measurements at the surface. As a consequence, offset reflections can be time- and depthmigrated such that the geometrical spreading factor along the offset ray is removed. This leads to a. so-called "true amplitude migration. In this work, true-amplitude timemigrated reflections are obtained by nothing more than a simple Kirchhoff-type weighted diffraction stack, followed, essentially, by a time derivative of the diffraction-stack traces. For small transmission losses of primary offset reflections through intermediate layer boundaries, the true-amplitude time· migrated reflection provides a direct measure of the angle-dependent reflection coefficient at the reflecting lower end of the primary reflected ray. The time-migrated field-can easily be transformed into a depth migrated field with the help of image rays. The theory-described here is similar to the inversion approach derived by Bleistein (1987) based upon fundamental ideas expressed by Beylkin (1985). The similarities and differences will be elaborated.
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3-D Migration to Zero Offset for a Constant Velocity Gradient
Authors W.S. French and W.T. PerkinsDMO processes are designed to move energy to where it would have been recorded by a zero-offset experiment conducted on the datum, For a constant velocity model, the spatial correction is always on a line between the source and the receiver to a position updip from the trace midpoint. and the temporal correction is a shift towards zero time additional to NMO, there being no DMO effects for pure strike shooting. In the case of a non-constant velocity field refraction effects complicate the situation, and the appropriate operator for a given reflection title becomes an areal surface whose characteristics may be counterintuitive; for example, there can be marked "DMO" effects even for strike shooting, with spatial and temporal corrections occurring in a direction opposite to the corrections for dip shooting. In this paper+H182 we derive the MZO (Migration to Zero Offset) operator for a velocity which increases linearly with depth, and we exbibit the surprising and important effects of this operator when applied to data for which the source-receiver orientation is strike to the reflectors.
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Pre-Stack Partial Migration in the Double Radon Domain
Authors J.T. Fokkema and M. VissingaIn this paper we propose an alternative approach for the NMO, DMO, CMP stacking and post-stack migration route. Our approach is based on a high-frequency analysis of the data after a double Radon transform with respect to the true field coordinates. 'It will be shown that the spatial complexity of the data leads to a well-defined topological subdivision in this space. The migrated result is obtained by a single inverse Radon transform. applied in the different subdomains.
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Uma Técnica Mista para Substituiçăo de camada
Authors W.P. Gouveia Jr. and P.L.M. OsórioA presença de interfaces de topografias irregulares próximas ŕ superficie caracterixa uma das situaçőes em que processamentos adicionais săo necessários para se obter urna imagem confiável do subsolo de interesse exploratório. Tais topografias, que ocorrem com alguma frequęncia no levantamento marítimo, onde o fundo do mar apresenta esta característica irregular, impőem um forte contraste de velocidades a propagaçăo do campo de ondas sísmico. Consequentemente, as reflexőes registradas em superficie provenientes de camadas mais profundas perdem suas caracteristicas hiperbólicas, seja em grupos de tiro comum como tambem em grupos de ponto medio comum.
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Aplicaçăo de um Método de Inversăo năo Linear ŕ Eliminaçăo de Reflexőes Múltiplas
More LessUm novo método năo-linear de inversăo de dados sísmicos, ora em desenvolvimento, é abordado. O método aplica-se a meos multidimensionais e tem sido explorado como um procedimento de supressăo de reflexőes múltiplas. Neste trabalho mostramos, de forma sucinta, o seu funcionamento e o aplicamos a um dado analítico que representa um meio unidimensional constituido por uma superfície livre e um único refletor, com ondas planas propagando-se em incidęncia normal.
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Ray-Born Synthetic Seismograms in Complex Structures Containing Scatterers
Authors A.C.D. Miranda and V. ČervenýThe method based on a hybrid combination of the ray theory with the Born approximation can be used to compute synthetic seismograms in complex laterally varying layered structures containing small sca.tterers. The scatterers can be combined to form objects of a complex shape. The wave field in the background laterally varying layered strudure is computed by the ray metbod and the scattered wave field by the Born approximation. A computer program package designed for such hybrid ray-Born computations in 2-D models is briefly described and numerical applications are presented. The ray-Born numerical modelling of seismic wave fields considerably extends the possibilities of the ray modelling.
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Método de Fourier com Aproximaçăo de Alta Ordem no Tempo e Transformada Rápida de Hartley no Espaço
By R. OliveiraA principal característica do M, é o calculo das derivadas espaciais através do usa da propriedade da derivada da Transformada de Fourier, alcançando assim, alta precisăo. Já a derivada temporal é calculada através da aproximaçăo par operador de diferenças finitas (DF) de segúnda ordem (Kosloff, et al. 1982, 1984, Oliveira, 1988). Este trabalho, propőem usar a propriedade da derivada da Transformada de Hartley, para calculo das derivadas espaciais (Saatcilar, 1990) e operadores de DF de alta ordem, para a derivada temporal (Dablain, 1986).
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Correction of Wavelet's Shape in Depth Migration
By S.V. GoldinWhile propagation through complicated media seismic signal passes such singularities as points of focusing and caustics having changed its shape with accordance to Hilbert transformation. Is this distortion compensated at reverse wave field continuation? If it is not, does one have possibility to apply a filter-corrector under the condition that geometry of the reflector is unknown? The positive answer for the last question is given in the paper. First, it is shown that reversed wave field continuation automaticaly compensates influence of the singularities that appear due to reflector's geometry.This fact is independent on the kind of pattern used for migration. Second, knowing the velocity model of overburden it is easily to construct a filter-correctcr which compensate influence of other singularities. The result is derived from analysis of the migration integral and Kirchhoff's approximation of reflection wave with help of the method o:f discontinuities recently developed by thla author.
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Is Geotomography Doomed?
Authors P. Carrion, G. Boehm, A. Vesnaver and I. PettenatiGeotomography has emerged from medical tomography but unlike medical tomography it largely does not work well. Why it happens? The answer is trivial: seismic experiments are different from medical and do not suit tomographic procedure. What can be done? To reject tomography as a tool for velocity reconstruction or to adopt it with modifications? Geotomography is probably a unique tool for velocity reconstruction. Conventional velocity analysis is too simplistic and is limited to layered media which do not bear much interest. In this presentation we introduce a possibility to significantly improve tomography and to make use of it in real situations related to typical seismic experiments. We achieve it with the help of the so-called compensation operator which reduces the dependence of results upon angular coverage and thus adjusts it to real world.
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Tomographic Inversion of Travel Time Data for Estimating VelocityStructure and Discontinuities
Authors J.A. Madrid and J.C.V. TraslosherosWe present a method to invert travel time data for determining the velocity structure and geometry of discontinuities in 2-D laterally varying media. The algorithm is based in the construction of a matrix of partial derivatives of travel times where the parameters may be either the velocity or the depth, or both, and SVD. The partial derivatives are computed using previous formulae for derivatives with respect to the velocity developed by Madrid (1986), and a ray tracing technique based on the partition of a medium in triangular regions wherein the gradient of velocity is constant. This kind of parameterization allows us to implement specific constrictions on the variations of depth parameters that are adequate in the search of discontinuities. The method is tested with synthetic data in the presence of noise. A simple real example is included.
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Técnicas Estatísticas para Prever Cobre Usando Perfilagem Geofisica de Poços
Authors D.A. Santos and O.P. VermaMétodos geofisicos podem ser utilizados para detectar minerais metáIicos devido as suas diferentes propriedades físicas. Isto ocorre diretamente, quando estes minerais possuem quantidade suficiente para afetar as propriedades eIetricas. Elemento metálico como o cobre, geralmente se encontra em pequena quantidade e năo é suficiente para ser dectado diretamente por metodos geofísicos. No entanto, ele esta associado a algum tipo de feiçăo geologica ou elemento que é dectável facilmente. Desta forma, técnicas geofisicas tem sido utilizadas para delinear alteraçőes associadas e/ou feiçőes favoráveis a mineralizaçăo (Urbank and Bailey, 1988). Este trabalho baseia-se em estabelecer uma correlaçăo entre a mineralizaçăo de cobre através de seus teores geoquímicos com parâmetros em perfilagem de poços. Para isto utilizou-se os metodos elétricos: polarizaçăo induzida (IP), resistividade aparente (Pa), condutividade apaxente (Oa), logarítmo da. Pa. (Log(pa)), potencial espontâneo (SP), resistęncia (Re); e raias gama (i). As possíveis correlaçăo săo feitas quantitativamente por análise R modal e qualitativamente através de exame visual. Fizemos ainda, a regressăo múltipla destes dados e usamos os coeficientes obtidos para prever furos de uma mesma área local. Neste trabalho foram utilizados dados obtidos em poços localizados na área de Saloba (furos 68 e 69) na regiăo de Carajás-PA.
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Tomografia Sísmica de Tempos de Percurso com e sem Vínculo
Authors D.J. Carneiro and P. CarrionMétodos tomográficos sísmicos podem ser divididos em duas categorias, que săo: tomografia sísmica de difraçăo (Devaney, 1984; Slaney Et al., 1984; Wu and Toksoz, 1987), e a tomografia sísmica de tempo de percurso. Subclassifica-se a tomografia sísmica de tempo de percurso em: tomografia sísmica. de tempo de percurso sem vínculos e tomografia sísmica de tempo de percurso com vinculos.
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3-D Inversion of Seismic Reflection Data Using Dynamic Ray Tracing
Authors M.A. Castro and V. ČervenýPossible applications of the dynamic ray tracing in the 3-D Born and Kirchhoff' inversion of seismic reflection data are discussed. It is snown that the most important quantities in the inversion integrals, such as the Beylkin determinant and the amplitudes of the ray theory Green functions, can be determined using the dynamic ray tracing procedure. The dynamic ray tracing can be simply performed along known rays in a very general, laterally varying layered background medium. As the cost of the dynamic ray tracing is merely a small percentage of the cost of the standart two-point ray tracing, the authors believe that the inversion algorithms which use dynamic ray tracing, will be useful in the inversion. They will find valuable applications particularly in complex 3-D structures.
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Reflection Tomography: Practical Aspects and Test Cases
By M. Silva A.Seismic tomography has proven to be a powerful tool in velocity determination. Originally, the method was applied to transmission data, and more recently has been adapted to reflection data by Bishop et al. (1985). In newly explored areas, where reflection data is the only data available, and where the geology and structure are complex, reflection tomography is the best velocity determination method since other conventional velocity analysis methods fail. In this paper, we show the basic principles and practical aspects of this method as well as some test cases that prove its efficiency.
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