Produção Científica



Artigo em Revista
04/07/2013

Exploring self-affine properties in seismograms
Self-affine properties have been observed in a large variety of rough profiles and time series from natural data sets. In this work, seismograms used for oil prospecting, which contain information of distinct subsurface features collected by seismic waves reflected or scattered at their interfaces, are taken into consideration. It is expected that any self-affine property, measured by the Hurst exponent H, depends on the depth. For each seismic trace, H is evaluated locally within a moving window, which is chosen narrow enough to reveal space dependency but also wide enough to display scale invariance. With the use of color code diagrams, it is possible to draw two-dimensional diagrams that show the local dependence of H for the analyzed seismogram. The reliability of the method is tested by the investigation of seismograms that contain ground roll components, as well as multiple reflections. The effect of different kinds of filter in the scaling properties is also investigated. In this case, comparisons are drawn among the diagrams obtained from original seismograms and those subjected to appropriate filter to eliminate spurious components.
Artigo em Revista
04/07/2013

Amplitudes e padr√Ķes de polariza√ß√£o de pulsos em meios anisotr√≥picos
Extrair informa√ß√Ķes litol√≥gicas da subsuperf√≠cie atrav√©s de dados s√≠smicos constitui-se num grande desafio √† prospec√ß√£o s√≠smica, pois a hip√≥tese de estratifica√ß√Ķes formadas por camadas isotr√≥picas se mostra insuficiente para representar o comportamento do campo el√°stico em levantamentos com grandes afastamentos entre fonte e receptor, geofones multicomponentes, medidas de VSP tridimensional, entre outros. Sob este panorama, a prospec√ß√£o s√≠smica passa a considerar modelos anisotr√≥picos de subsuperf√≠cie para, por exemplo, caracterizar reservat√≥rios. O objetivo deste texto √© apresentar um formalismo para modelar o espalhamento de pulsos a partir de ondas planas incidentes em interfaces planas horizontais que separam meios anisotr√≥picos. Este espalhamento √© obtido primeiramente, atrav√©s da formula√ß√£o expl√≠cita dos campos de deforma√ß√£o e tra√ß√£o como fun√ß√£o das matrizes propagadoras, de polariza√ß√£o e de imped√Ęncia do meio. Em seguida, este formalismo √© usado para a obten√ß√£o das matrizes dos coeficientes de reflex√£o e transmiss√£o atrav√©s de uma interface plana horizontal para posteriormente, ser generalizado para o espalhamento atrav√©s de m√ļltiplas camadas. Finalmente, inserem-se ao campo da onda incidente as amplitudes de um pulso anal√≠tico para calcular o espalhamento do pulso atrav√©s de estratifica√ß√Ķes.
Artigo em Revista
16/01/2013

Solu√ß√Ķes de Problemas envolvendo Equa√ß√Ķes Diferenciais Sujeitas a Incertezas
Este trabalho objetiva analisar, atrav√©s de alguns exemplos, a influ√™ncia de se considerar aleatoriedades na solu√ß√£o de equa√ß√Ķes diferenciais com dados e/ou par√Ęmetros aleat√≥rios. Um comparativo das m√©dias das solu√ß√Ķes das equa√ß√Ķes estoc√°sticas com as solu√ß√Ķes das equa√ß√Ķes determin√≠sticas simplificadas, nas quais substitu√≠mos os par√Ęmetros aleat√≥rios por suas m√©dias, √© apresentado. Estes
exemplos mostram que a m√©dia da solu√ß√£o, que normalmente √© uma informa√ß√£o relevante em aplica√ß√Ķes, pode ser qualitativamente diferente da aproxima√ß√£o obtida pela solu√ß√£o de uma equa√ß√£o diferencial determin√≠stica na qual substitu√≠mos os par√Ęmetros aleat√≥rios por suas m√©dias.
Artigo em Revista
16/01/2013

A space‚Äďtime multiscale method for computing statistical moments in strongly heterogeneous poroelastic media of evolving scales
A new multiscale procedure is proposed to compute flow in compressible heterogeneous porous media with geology characterized by power-law covariance structure. At the fine scale, the deformable medium is modeled by the partially coupled formulation of poroelasticity with Young’s modulus and permeability treated
as stationary random fields represented by their Karhunen‚ÄďLo√®ve decompositions. The framework underlying the multiscale procedure is based on mapping these random parameters to an auxiliary domain and
constructing a family of equivalent stochastic processes at different length scales characterized by the same ensemble mean and covariance function. The poromechanical variables inherit a space‚Äďtime version of the scaling relations of the random input parameters which allows for constructing a set of multiscale solutions of the same governing equations posed at different space and time scales. A notable feature of the multiscale method proposed herein is the feasibility of solving both the poroelastic model and the Fredholm integral equation for the eigenpairs of the Karhunen‚ÄďLo√®ve expansion in an auxiliary domain with much lower computational effort and then derive the long term behavior at a coarser scale from a straightforward rescaling of the auxiliary solution. Within the framework of the finite element approximation, in conjunction with
the Monte Carlo algorithm, numerical simulations of fluid withdrawal and injection problems in a heterogeneous poroelastic reservoir are performed to illustrate the potential of the method in drastically reducing the computational burden in the computation of the statistical moments of the poromechanical unknowns in large-scale simulations.
Artigo em Revista
16/01/2013

A Numerical Comparison Between Quasi-MonteCarlo and Sparse Grid Stochastic Collocation Methods
Quasi-Monte Carlo methods and stochastic collocation methods based on sparse grids have become popular with solving stochastic partial differential equations.These methods use deterministic points for multi-dimensional integration or interpolation without suffering from the curse of dimensionality. It is not evident which method is best, specially on random models of physical phenomena. We numerically study the error of quasi-Monte Carlo and sparse gridmethods in the context of groundwater flow in heterogeneous media. In particular, we consider the dependence of the variance error on the stochastic dimension and the number of samples/collocation points for steady flow problems in which the hydraulic conductivity is a lognormal process. The suitability of each technique is identified in terms of computational cost and error tolerance.
Artigo em Revista
16/01/2013

Effect of Element Distortion on the Numerical Dispersion of Spectral Element Methods
Spectral element methods are well established in the field of wave propagation,in particular because they inherit the flexibility of finite element methods and have low numerical dispersion error. The latter is experimentally acknowledged, but has been theoretically shown only in limited cases, such as Cartesian meshes. It is well known that a finite element mesh can contain distorted elements that generate numerical errors for very large distortions. In the present work, we study the effect of element distortion on the numerical dispersion error and determine the distortion range in which an accurate solution is obtained for a given error tolerance. We also discuss a double-grid calculation of the spectral element matrices that preserves accuracy in deformed geometries.
Dissertação de Mestrado
10/01/2013

Filtragem adaptativa SVD de volumes sísmicos 3D para realçar refletores e estruturas geológicas.
Washington Oliveira Martins. Filtragem adaptativa SVD de volumes sísmicos 3D para realçar refletores e estruturas geológicas. 2012. Dissertação (Mestrado em Geofísica) - Universidade Federal da Bahia, . Orientador: Milton José Porsani.
Tese de Doutorado
10/01/2013

Dorian Caraballo Ledesma. Deconvolução de dados sísmicos de reflexão utilizando mudança de fase do filtro de Wiener-Levinson. 2011.
Dorian Caraballo Ledesma. Deconvolução de dados sísmicos de reflexão utilizando mudança de fase do filtro de Wiener-Levinson. 2011. Tese (Doutorado em Geofísica) - Universidade Federal da Bahia, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior. Orientador: Milton José Porsani.
Dissertação de Mestrado
10/01/2013

Processamento de Dados S√≠smicos com Grandes Afastamentos: Dados Sint√©ticos e Linha S√≠smica do Campo de Tenerife, Col√īmbia.
Francisco Ortega Gamboa. Processamento de Dados S√≠smicos com Grandes Afastamentos: Dados Sint√©ticos e Linha S√≠smica do Campo de Tenerife, Col√īmbia. 2012. Disserta√ß√£o (Mestrado em Geof√≠sica) - Universidade Federal da Bahia, Coordena√ß√£o de Aperfei√ßoamento de Pessoal de N√≠vel Superior. Orientador: Amin Bassrei.
Apresentação
03/09/2012

GêBR: a free seismic processing interface
There are many programs for processing seismic data that are freely available and widespread, for example Seismic Un*x, Madagascar, FreeUSP, and SEPlib, among others. All these packages consist of packages of command-line-oriented programs that are designed to be used in sequence; the data conceptually flow in a pipeline through one program after another. Each program is generally controlled by its own set of command-line options. To take full advantage of such a toolkit, the user must have considerable knowledge beyond general geophysical expertise: shell scripting, process submission and management, and batch queue processing, to name a few. While these skills are useful, they should not be a requirement for seismic data processing.
A suitable graphical user interface could take care of these computational details, allowing the user to focus on the central problem of processing seismic data. This is particularly important during training courses, where the limited duration of the does not leave time for learning skills that are not essential to the material being taught. A graphical user interface may also boost the uptake of a new program, by making it more accessible to users and allowing its easy integration with other programs available within the same interface. These principles have guided the development of GêBR, a graphical user interface to control commandline programs for seismic processing. It permits users to build complex processing flows from predefined modules known as menus. Menus describing new programs can be easily added to the interface, extending its capabilities. GêBR is also designed to be simple, in the sense that a couple of hours is enough to introduce the core features of the interface, to allow the user to start working with the seismic data.
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