DSpace Communidade:
http://repositorio.ufpa.br/jspui/handle/2011/2355
2019-04-19T12:36:49ZModelagem eletromagnética 2.5-D de dados geofísicos através do método de diferenças finitas com malhas não-estruturadas
http://repositorio.ufpa.br/jspui/handle/2011/10223
Título: Modelagem eletromagnética 2.5-D de dados geofísicos através do método de diferenças finitas com malhas não-estruturadas
Autor(es): MIRANDA, Diego da Costa
Primeiro(a) Orientador(a): HOWARD JUNIOR, Allen Quentin
Abstract: We present a 2.5D electromagnetic formulation for modelling of the marine controlledsource
electromagnetic (mCSEM) using a Finite Diference frequency domain (FDFD)
method. The formulation is in terms of secondary fields thus removing the source
point singularities. The components of the electromagnetic field are derived from
the solution of the magnetic vector potential and electric scalar potential, evaluated
in the entire problem domain that must be completely discretized for the use of
the FDFD. Finite difference methods result in large sparse matrix equations that
are efficiently solved by sparse matrix algebra preconditioned iterative methods. To
overcome limitations imposed by structured grids in the traditional FDFD method,
the new method is based upon unstructured grids allowing a better delineation of
the geometries. These meshes are completely adaptable to the models we work with,
promoting a smooth design of their structures, and may only be refined locally in
regions of interest. We also present the development of RBF-DQ method, (radial
basis function differential quadrature) which makes use of the technique of functions
approximation by linear combinations of radial basis functions (RBF) and the
technique of differential quadrature (DQ) for approximation of the derivatives. Our
results show that the FDFD method with unstructured grids when applied to geophysical
modeling problems, yield improved quality of modeled data in comparison
with the results obtained by traditional techniques of FDFD method.
Instituição: Universidade Federal do Pará
Tipo: Tese2014-10-23T00:00:00ZInversão de velocidades por otimização global usando a aproximação superfície de reflexão comum com afastamento finito
http://repositorio.ufpa.br/jspui/handle/2011/9069
Título: Inversão de velocidades por otimização global usando a aproximação superfície de reflexão comum com afastamento finito
Autor(es): MESQUITA, Marcelo Jorge Luz
Primeiro(a) Orientador(a): CRUZ, João Carlos Ribeiro
Abstract: The recent geophysical literature has shown the building of an accurate initial model is the more appropriate way to reduce the ill-posedness of the Full Waveform Inversion, providing the necessary convergence of the misfit function toward the global minimum. Optimized models are useful as initial guess for more sophisticated velocity inversion and migration methods. I developed an automatic P-wave velocity inversion methodology using pre-stack two-dimensional seismic data. The proposed inversion strategy is fully automatic, based on the semblance measurements and guided by the paraxial traveltime approximation, so-called Finite-Offset Common-Reflection-Surface. It is performed in two steps, at first using image rays and an a priori known initial velocity model we determine the reflector interfaces in depth from time migrated section. The generated depth macro-model is used as input at the second step, where the parametrization of the velocity model is made layer by layer. Each layer is separated from each other by smoothed interfaces. The inversion strategy is based on the scan of semblance measurements in each common-midpoint gather guided by the Finite-Offset Common-Reflection-Surface traveltime paraxial approximations. For beginning the inversion in the second step, the finite-offset common-midpoint central rays is built by ray tracing from the velocity macro-model obtained in the first step. By using the arithmetic mean of total semblance calculated from the whole common-midpoint gathers as objective function, layer after layer, a global optimization method called Very Fast Simulated Annealing algorithm is applied in order to obtain the convergence of the objective function toward the global maximum. By applying to synthetic and real data, I showed the robustness of the inversion algorithm for yielding an optimized P-wave velocity macro-model from pre-stack seismic data.
Instituição: Universidade Federal do Pará
Tipo: Tese2016-08-25T00:00:00ZInversão da forma de onda orientada ao alvo
http://repositorio.ufpa.br/jspui/handle/2011/9022
Título: Inversão da forma de onda orientada ao alvo
Autor(es): COSTA, Carlos Alexandre Nascimento da
Primeiro(a) Orientador(a): COSTA, Jessé Carvalho
Abstract: We propose a new target-oriented waveform inversion to estimate the physical parameters from a specific target in the subsurface from observed data from deviated-VSP acquisition or surface seismic data. Furthermore, we investigate a strategy to estimate the impulse responses from a local target in the subsurface from deviated-VSP acquisition or surface seismic data as an iterative sparse inversion approach, where the main feature of this strategy is that all multiple scattering in the data is used to enhance the illumination at target level. In these approaches we fit the upgoing wavefields observed at a specific level near the local target with the upgoing wavefields estimated at same depth level through convolution-type representation for the Green’s function. The main feature of the target-oriented waveform inversion is that we just need to know the up- and downgoing wavefields at the depth level above the target area to estimate the physical parameters for the area of interest. We show through numerical tests that the iterative sparse inversion approach does not require dense sources sampling to estimate the impulse responses from a target below a complex overburden, because of all the extra illumination via multiples. The physical parameters above the target area is not necessary to know if we use the data from deviated-VSP geometry of acquisition, but for surface seismic data we need to know a smooth physical parameter above the target area to estimate the up- and downgoing wavefields at depth level nearby the local target. For surface seismic data we used Joint Migration Inversion to estimate the up- and downgoing wavefields at depth level near the target area.
Instituição: Universidade Federal do Pará
Tipo: Tese2016-09-16T00:00:00ZStructural constraints for image-based inversion methods
http://repositorio.ufpa.br/jspui/handle/2011/9021
Título: Structural constraints for image-based inversion methods
Autor(es): MACIEL, Jonathas da Silva
Primeiro(a) Orientador(a): COSTA, Jessé Carvalho
Abstract: This thesis presents two methodologies of structural regularization for Wave-Equation Migration Velocity Analysis and Joint Migration Inversion: cross-gradient regularization and filtering with morphological operators. In Wave-Equation Migration Velocity Analysis, the cross-gradient regularization aims to constrain the velocity contrasts with the reflectivity map by parallelization of the velocity gradient vector and the image gradient vector. We propose a version with cross-gradient of the objective functions: Differential Semblance, Stack Power and Partial Stack Power. We combine the Partial Stack Power with its version of cross-gradient, in order to gradually increase the resolution of the velocity model without compromising the adjustment of the long wavelengths of the velocity model. In Joint Migration Inversion, we propose to apply morphological operators of erosion and dilation in the preconditioning of the velocity model in each iteration. Operators use the reflectivity map to mark the regions with the same value of physical property. They homogenize the geological layer and accentuate the velocity contrast at the edges. Structural constraints do not only reduce the ambiguity in estimating a velocity model, but also make the migration/inversion methods more stable, reducing artifacts, delineating geologically plausible solutions, and accelerating the convergence of the objective function.
Instituição: Universidade Federal do Pará
Tipo: Tese2016-04-22T00:00:00Z