Navegando por Assunto "Sismologia"

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Acesso aberto (Open Access)
Dispersão das ondas de Rayleigh na Plataforma Sulamericana
(Universidade Federal do Pará, 1989-05-10) SANTA ROSA, Antonio Nuno de Castro; LEITE, Lourenildo Williame Barbosa; http://lattes.cnpq.br/8588738536047617
This work represents a dispersion study of the vertical component of the Rayleigh wave with trajectories on the South American platform. The records used are from stations located in the Brazilian territory such as the ones from Rio de Janeiro (RDJ), Caicó (CAI), Brasília (BDF) and Belém (BEB). These are the only seismological stations in Brazil with long period sensors that can be useful for the dispersion studies in the interval 4 to 50 seconds. The earthquakes used are localized along the eastern part of Andean mountain belt and inside the South American platform, with tipically continental trajectories. 34 events were selected using the following pratical criteria: the localization, the magnitude mb, the depth, and that they occured in the period of January 1978 to June 1987. The dispersion study done here means the determination of group velocities and spectral amplitudes, corresponding to the fundamental and first higher modes. Normally the modes of second order or more are rarely observed. Two types of measurements were made: (i) group velocity versus period and (ii) spectral amplitude versus period. Dispersion studies are important for determining the structure of the crust and upper mantle which are directly related to gelogical phenomena. In this work, regionalization is defined as the identification of different forms of dispersion curves and their relation to the epicenter-station trajectory on the South American platform. We also correlate them geologicaly, as described in item 4.3 of this work. The distribution of epicenters occurs from the extreme South of Argentina to the extreme North of Venezuela, with the objective of iniciating with this work a sistematic study of the sub-Andean area in our institution. Three distinct types of curves were observed in 27 trajectories and grouped in three families (1, 2 and 3). Their forms were compared with the regional geological features of South American platform. The multiple filter technique was used to obtain the dispersion curves (Dziewonski et al 1969). This filter has the property of separating the modes through their group velocities for each frequency selected and also recovers the espectral amplitudes characteristic for each harmonic (Herrmann, 1973). The theorectical development of the filter properties as well as the limitations and aplicabilities are treated by Dziewonski et al (1972). As part of this work there was the implementation, adaptions and the layout of fluxograms of the multiple filter software, as well as the digitalization procedure for data processing and non-automatic interpretation of the results.
Acesso aberto (Open Access)
Inversão de dados de sísmica de refração profunda a partir da curva tempo-distância
(Universidade Federal do Pará, 1990) CRUZ, João Carlos Ribeiro; LEITE, Lourenildo Williame Barbosa; http://lattes.cnpq.br/8588738536047617
The aim of this thesis is to obtain crustal model through the inversion of deep seismic refraction data, considering lateraly homogenous horizontal plain layers over a half-space. The direct model is given by analitic expression for the travel-time curve, as a function that depends on the source-station distance and on the array of parameters, formed by velocity and thickness of each layer. The expression is obtained from the trajectory of the seismic ray by Snell's Law. The calculation of the arrivel time for seismic refraction by this method, takes into account a model with velocities increasing with depth. The occurrence of low velocity layers (LVL) are solved as a model reparametrization, taking into account the fact that top boundary of the low velocity layer is only a reflector, and not a refractor of seismic waves. The inversion method is used to solve for the possible solutions, and also to perform an analysis about the ambiguity of the problem. The search region of probable solutions is constrainted by high and lower limits of each parameter considered, and by high limits of each critical distance, calculated using the array of parameters. The inversion process used is an optimization technique for curve fitting corresponding to a direct search in the parameter space, called COMPLEX. This technique presents the advantage of using any objective function, and as being practical in obtaining diferent solutions for the problem. As the travel-time curve is a multi-function, the algorithm was adaptaded to minimize several objective funtions simultaneously, with constraints. The inversion process is formulated to obtain a representative group of solutions of the problem. Afterwards, the analysis of ambiguity is made by Q-mode factor analysis, through which is possible to find the commom properties of the group of solutions. Tests with synthetic and real data were made having as initial aproximation to the inversion process, the velocity and thickness values calculated by the straightforward visual interpretation of the seismograms. For the synthetics, it was used seismograms calculated by the refletivity method, with diferent models. For test with real data, it was used seismograms colected by the Lithospheric Seismic Profile in Britain (LISPB), in the northern region of Britain. It was verified in all tests that geometry of the model has most importance for the ambiguity of the problem, while the physical parameters present only smaller changes into the group of solutions obtained.
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.