Dissertações em Geofísica (Mestrado) - CPGF/IG

URI Permanente para esta coleçãohttps://repositorio.ufpa.br/handle/2011/4993

O Mestrado Acadêmico pertente a o Programa de Pós-Graduação em Geofísica (CPGF) do Instituto de Geociências (IG) da Universidade Federal do Pará (UFPA).

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Agora exibindo 1 - 11 de 11

Acesso aberto (Open Access)
Análise de resolução em tomografia interpoços em 3-D
(Universidade Federal do Pará, 2000-12-15) SOUZA, Syme Regina de Deus; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
The joint inversion of multiple crosswell surveys traveltime data allows the reconstruction of high resolution 3-D slowness models suitable for monitoring enhanced oil recovery processes and detailed reservoir characterization. However, the non-uniform volumetric ray coverage unavoidable in crosswell surveys requires additional information to the tomographic system in order to obtain stable solutions. The 3-D tomographic problem is analyzed discretizing the models in 3-D prismatic meshes and using the singular value decomposition. The projection angle of target models in the subspace orthogonal to the effective null space of the tomographic matrix is an useful criteria for designing mesh and data acquisition geometry in order to better conditioning the tomographic reconstruction. The approach can also be used during the linear iterations to redefine the mesh or evaluate the requirement of additional prior information to the tomographic system.
Acesso aberto (Open Access)
Análise de sensibilidade para estereotomografia em meios elípticos e anelípticos
(Universidade Federal do Pará, 2005-12) BARBOSA, Brenda Silvana de Souza; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
Stereotomography is extended to general anisotropic models and implemented for elliptical and anelliptical anisotropy. The elliptical and anelliptical models present only three parameters. This makes them less sensitive to the ambiguity due to limited coverage of surface seismic experiments than transversaly isotropic or orthorhombic models. The corresponding approximations of the slowness surface restrict the validity of the present approach to qP events and mild anisotropy. Numerical experiments show the potential and the limitations of stereotomography in estimating macro-velocity models suitable for imaging in the presence of anisotropy as well as the importance of transmission events from multiple-offset VSP experiments for the success of the approach.
Acesso aberto (Open Access)
Avaliação de algoritmos para conversão de modelos de velocidade de tempo para profundidade
(Universidade Federal do Pará, 2013-03-26) VALENTE, Leandro da Silva Sadala; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
Even today, time migration is the imaging process substantially employed in the oil industry. Such popularity is due to its high degree of efficiency and robustness, and its ability to focus reflectors in various geological environments. However, in areas of high geological complexity, time migration fails so that depth migration and a depth velocity field are indispensable. This field is usually obtained through tomographic iterative processes starting from an initial velocity field. The conversion of velocity fields from time to depth is a quick way of obtaining a geologically more consistent initial velocity field for such processes. Some algorithms for time-to-depth conversion based on newly developed image-ray tracing are reviewed and an alternative algorithm based on propagation of the image-wavefront is proposed. The algorithms are applied to two-dimensional synthetic data and evaluated according to their efficiency and accuracy, highlighting their advantages, disadvantages and limitations in obtaining depth velocity fields.
Acesso aberto (Open Access)
Implementações alternativas de tomografia do tempo de trânsito utilizando a equação da onda
(Universidade Federal do Pará, 2014-04-23) CARDOSO FILHO, Josafat Lopes; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
Wave equation tomography is a robust methodology for velocity analysis when strong velocity variations occurs. This approach has been successfully applied for reservoir monitoring and characterization using crosswell data. The choice of the objective functions, preconditioners and regularizing functionals controls the robustness, eciency and the quality of the velocity reconstruction. This dissertation investigates each of these design parameters and its consequences for the performance of the wave equation tomography using synthetic crosswell data generated from smoothly and strongly heterogeneous velocity models. Two proposals for the objective functions are used in this work; the first is sensitive to phase dierences and the other is proposal to be less sensitive to the source pulse. Both do not require velocity picking performed well in the numerical experiments. A preconditioning strategy adapted from the imaging processing literature produced a noticiable improvement the convergence rate of the algorithm by eliminating artifacts caused by limited aperture, random noise and artifacts produced by sources and receivers. A regularizing functional penalizing deviations from velocity information available near the wells additionally contributes to recover a velocity tomogram with higher resolution and consistent with the synthetic model. Wave equation tomography is a robust methodology for velocity analysis when strong velocity variations occurs.
Acesso aberto (Open Access)
Medidas de coerência para análise de velocidade na migração em tempo
(Universidade Federal do Pará, 2011) MACIEL, Jonathas da Silva; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
Iterative methods for migration velocity analysis depend on objective functions to measure the flatness of reflection events in common image gathers (CIG). Time migration is a simple imaging method to evaluate these objective functions. Using time migration we studied the influence of objective functions on the results of migration velocity analysis. We propose two new objective functions for migration velocity analysis: Extended Differential Semblance and the product of Classical Semblance times the Extended Differential Semblance. Numerical experiments using the Marmousoft data show the effectiveness of the new objective functions to estimate velocity models producing at events in common image gathers.
Acesso aberto (Open Access)
Migração FD 3D em profundidade usando aproximação de Padé complexa
(Universidade Federal do Pará, 2011-06-21) COSTA, Carlos Alexandre Nascimento da; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
Conventional implementations of 3D finite-difference (FD) migration use splitting techniques to accelerate performace and save computational cost. However, such techniques are plagued with numerical anisotropy that jeopardizes correct positioning of dipping reflectors in the directions not used for the splitted operators. We implement 3D downward continuation migration without splitting in the space coordinates using a complex Padé approximation and implicit finite differences. In this way, the numerical anisotropy is eliminated at the expense of a computationally more intensive solution of a large banded linear system. We compare the performance of the iterative stabilized biconjugate gradient (Bi-CGSTAB) and the multifrontal massively parallel direct solver (MUMPS). It turns out that the use of the complex Padé approximation provides an effective preconditioner for the Bi-CGSTAB, reducing the number of iterations relative to the real Padé expansion of the FD operator. As a consequence, the iterative Bi-CGSTABmethod ismore efficient than the directMUMPSmethodwhen solving for a single termin the Padé expansion. Forwide angle approximations direct methods are required. These algorithms are validated and the properties evaluated computing themigration impulse response in the SEG/EAGE salt model.
Acesso aberto (Open Access)
Migração FD e FFD com aproximações de grande abertura angular
(Universidade Federal do Pará, 2007) AMAZONAS, Daniela Rêgo; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
Seismic Migration by downward continuation using the unidirectional wave equation approximations has two shortcomings: imaging steep dip reflectors and handling evanescent waves. Complex Padé approximations allow a better treatment of evanescent modes stabilizing the finite difference migration, and does not require special treatment for domain boundaries. Imaging of steep dip reflectors can be improved using several terms in the Padé expansion. This dissertation discuss the implementation and evaluation of complex Padé approximation for finite difference migration and Fourier finite difference migration. The study of the dispersion relation and impulsive response associated to the migration operator is used to select the number of terms and coefficients in the Padé expansion which assures stability for a prescribed maximum reflector dip. The implementations are validated in the Marmousi and SEG/EAGE salt model datasets, and compared to other wave equation migration methods. The results of FD and FFD complex Padé migrations can handle steeper dips, and present a much lower signal to noise ratio than their real valued counterparts.
Acesso aberto (Open Access)
Migração FFD 3D em profundidade usando aproximação de Padé complexa
(Universidade Federal do Pará, 2011) CAMPOS, Itamara do Socorro da Silveira; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
Fourier finite-difference (FFD) migration implementations use splitting techniques to accelerate performace and save computational cost. However, such techniques introduce numerical anisotropy which leads to mispositioning of dipping reflectors along directions not used for splitting the migration operator. We implement 3D FFD continuation migration without splitting in the frequency-space domain using the complex Padé approximation and implicit finite differences. This approach eliminates numerical anisotropy at the expense of a computationally more intensive solution of a large banded linear system. Numerical experiments in homogeneous and heterogeneous models show that splitting techniques produce noticiable positioning erros for models with strong lateral velocity variation. We compare the performance of the iterative stabilized biconjugate gradient (BICGSTAB) and the multifrontal massively parallel direct solver (MUMPS). It turns out that the use of the complex Padé approximation provides an effective preconditioner for the BICGSTAB, reducing the number of iterations relative to the real Padé expansion. The iterative BICGSTAB method is more efficient than the direct MUMPS method when solving for a single term in the Padé expansion. For wide angle approximations more terms are required to represent the migration operator, in this case direct methods are required. The algorithm is validated and the properties evaluated computing the migration impulse response in the SEG/EAGE salt model.
Acesso aberto (Open Access)
Migração usando equação da onda unidirecional de abertura arbitrária
(Universidade Federal do Pará, 2007) COSTA, Lúcio Alan Pires da; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
The arbitrarily wide angle wave equations are applied to post-stack migration. This class of wavefield continuation operators are based on the concept of semi-space stiffness, Dirichlet to Neumann transformations and its discretization using finite elements. The specification of the continuation operator requires auxiliar variables. The number of these variable increase with the maximum angular aperture. The algorithm is implemented in space-frequency domain which allows straightforward parallelization. Based on numerical experiments using the dispersion relation and the impulse response of these operators we prescribe the number of auxiliar variables and downward continuation step. The migration of the zero offset SEG-EAGE data using the wide angle continuation operators shows their efectiveness for imaging steep dips in a complex velocity model.
Acesso aberto (Open Access)
Modelagem acústica por diferenças finitas e elementos finitos em 2-D e 2,5-D
(Universidade Federal do Pará, 2004-10-26) SILVA NETO, Francisco de Assis; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
Acoustic modeling provides useful synthetic data for evaluating seismic processing and imaging in complex geological settings. High order finite difference (FD) and finite elements (FE) was implemented and evaluated in homogeneous and inhomogeneous model. The FD algorithm are estended to 2,5-D for variable density models. Seismic modeling of oil reservoirs targets somewhat similar to those occuring at Paleozoic Basins in the Amazon are presented. Long period multiples produced between the free-surface and the Cretaceous- Paleozoic interface, the low resolution of the seismic waves near the reservoir and the week reflections at the interface between the reservoir rocks and the cap rock are the main features of the synthetics which presents a challenge to seismic imaging.
Acesso aberto (Open Access)
Regularização em estereotomografia
(Universidade Federal do Pará, 2009) MELO, Luiz André Veloso; COSTA, Jessé Carvalho; http://lattes.cnpq.br/7294174204296739
Obtaining an accurate velocity model is an essential part of imaging complex structures. In complex environments, conventional methods do not produce satisfactory results. Slope tomography is an effective tool for improving the velocity estimate. This method uses the slowness components and traveltimes of picked reection or difraction events for velocity model building. On the other hand, the unavoidable data incompleteness requires additional information to assure stability of inversion. One natural constraint for raybased tomography is a smooth velocity model. This study proposes to evaluate smoothness regularizations to slope tomography that require the evaluation of partial derivatives of the velocity model with respect to the spatial coordinates. One of evaluated regularizations is a new kind of smoothness constraint based on the reection angle. I evaluate results measuring data mist, velocity model results and scattering points recovered after inversion on synthetic data. In numerical tests the new constraint leads to geologically consistent models.

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Navegando Dissertações em Geofísica (Mestrado) - CPGF/IG por Orientadores "COSTA, Jessé Carvalho"