Produção Científica

Artigo em Revista

Hypocentral relocation using clustering-along-planes constraints: implications for fault geometry
Hypocentre location is an ill-posed inverse problem even assuming that the velocity model
is known, because different sets of hypocentre locations may satisfy the fitting criterion.
We present a regularized hypocentre inversion in which the constraints of spatial proximity
of the hypocentres to target planes are used. This constraint introduces the geological bias
that earthquakes might occur along fault planes. Here, the target planes may be either (1)
planes specified by the interpreter or (2) planes fitting groups of events. We assume also that
initial estimates of hypocentres and origin times are available. Then, the initial hypocentre
estimates, origin times and target planes are used as input to an inversion problem to relocate
the hypocentres so that the maximum-possible clustering of events along the given planes is
attained, matching the observed traveltimes. We use L1 norm for data fitting, L2 norm for
the plane proximity criterion and a polytope algorithm to minimize the functional. Results
from synthetic and real data indicate that the plane proximity constraint allows for hypocentre
relocation presenting a high degree of clustering along planes. The real-data example is an
intraplate earthquake sequence in NE Brazil. Our methodology defined the geometry and
strike of fault segments close to known geology and focal mechanism data. In addition, the
new method indicates that the fault is characterized by a splay geometry in its southern end
and that more than three fault segments are necessary to explain the hypocentre distribution.
Artigo em Revista

Evidences of buried loads in the base of the crust of Borborema Plateau (NE Brazil) from Bouguer admittance estimates
In the Borborema Province (BP) e northeastern Brazil e two important Cenozoic events occurred at the surface: the Macau magmatism and the Borborema Plateau epeirogenesis. To obtain appropriated-scale geophysical data to explain the deep origins of these two events, different gravimetric/elevation databases were integrated with new surveys. Bouguer admittance estimates reveal that isostatic condition of the BP, especially in the Borborema Plateau, can be explained using elastic models to the lithosphere only if surface and buried loadings are combined. If the buried load is applied in the base of the crust, the ratio between buried and surface weights is circa 15 for a lithosphere with effective elastic thickness around 15 km and crust thickness around 33 km. From an nterpretative viewpoint of the buried load, it is assumed that the lower crust under the Borborema Plateau might have an anomalous high value of
density. Magmatic underplating might explain this fact as well as the observed surface magmatism and epeirogenesis. Crustal thickening of about 4 km under the Borborema Plateau and intracrustal seismic
velocity discontinuity with high Vp/Vs ratio are geophysical facts consistent with magmatic underplating. However, the surface magmatism presents low volume and mainly alkaline composition e facts that are not entirely consistent with the hypothesis of magmatic underplating. Regardless the validity of this hypothesis, Cenozoic-to-present events in BP might be somewhat associated with imbalances in lithosphere-asthenospheric mantle and/or crust-lithospheric mantle systems. The existence of free-air anomalies showing no null integral over area and of an expressive positive geoid anomaly are geophysical evidences of these imbalances. Possibly, the Borborema Plateau is still suffering epeirogenesis.
Post-depositional deformation found in Barreiras Formation strata, Late Quaternay fault reactivations, and AFT thermochronology analysis suggesting the existence of a cooling stage between 20 and 0 Ma might be geologic evidences of the continued action of peirogenesis until the present. In addition, the relatively high level of the present intraplate seismicity recorded in several regions of the BP is another unequivocal geophysical evidence that the crust of the province is still submitted to accommodation processes.
Artigo em Revista

A fast modified parabolic radon transform.
We propose a fast and efficient frequency-domain implementation of a modified parabolic Radon transform (modified PRT) based on a singular value decomposition (SVD) with applications to multiple removal. The problem is transformed into a complex linear system involving a single operator after merging the curvature-frequency parameters into a new variable. A complex SVD is applied to this operator and the forward transform is computed by means of a complex back-substitution that is frequency independent. The new transform offers a wider curvature range at signal frequencies than the other PRT implementations, allowing the mapping in the transform domain of low-frequency events with important residual moveouts (long period multiples). The method is capable of resolving multiple energy from primaries when they interfere in a small time interval, a situation where most frequency-domain methods fail to discriminate the different wave types. Additionally, the method resists better to amplitude variations with offset (AVO) effects in the data than does the iteratively reweighted least-squares (IRLS) method.The proposed method was successfully applied to a deep-water seismic line in the Gulf of Mexico to attenuate water-bottom multiples and subsequent peg-legs originating from multiple paths in the water column. Combining the suggested method with the surface-related multiple elimination (SRME) has led to the best attenuation results in removing residual multiple energy in the stack. ©2011 Society of Exploration Geophysicists
Artigo em Revista

Total variation regularization for depth-to-basement estimate: Part 1 — Mathematical details and applications
We have developed an inversion approach that estimates the basement relief of a fault-bounded sedimentary basin. The sedimentary pack is approximated by a grid of 3D or 2D vertical prisms juxtaposed in the horizontal directions of a right-handed coordinate system. The prisms' thicknesses represent the depths to the basement and are the parameters to be estimated from the gravity data. To obtain depth-to-basement estimates, we introduce the total variation (TV) regularization as a stabilizing function. This approach lets us estimate a nonsmooth basement relief because it does not penalize sharp features of the solution. We have deduced a compact matrix form of the gradient vector and the Hessian matrix of the approximation to the TV function that allows a regularized Gauss-Newton minimization approach. Because the Hessian matrix of the approximation to the TV function is ill conditioned, we have modified this Hessian matrix to improve its condition and to accelerate the convergence of the Gauss-Newton algorithm. Tests conducted with synthetic data show that the inversion method can delineate discontinuous basements presenting large slips or sequences of small-slip step faults. Tests on field data from the Almada Basin, Brazil, and from the San Jacinto Graben, California, U.S.A., confirm the potential of the method in detecting and locating in-depth normal faults in the basement relief of a sedimentary basin. ©2011 Society of Exploration Geophysicists
Artigo em Revista

Total variation regularization for depth-to-basement estimate: Part 2 — Physicogeologic meaning and comparisons with previous inversion methods
We applied the mathematical basis of the total variation (TV) regularization to analyze the physicogeologic meaning of the TV method and compared it with previous gravity inversion methods (weighted smoothness and entropic Regularization) to estimate discontinuous basements. In the second part, we analyze the physicogeologic meaning of the TV method and compare it with previous gravity inversion methods (weighted smoothness and entropic regularization) to estimate discontinuous basements. Presenting a mathematical review of these methods, we show that minimizing the TV stabilizing function favors discontinuous solutions because a smooth solution, to honor the data, must oscillate, and the presence of these oscillations increases the value of the TV stabilizing function. These three methods are applied to synthetic data produced by a simulated 2D graben bordered by step faults. TV regularization and weighted smoothness are also applied to the real anomaly of Steptoe Valley, Nevada, U.S.A. In all applications, the three methods perform similarly. TV regularization, however, has the advantage, compared with weighted smoothness, of requiring no a priori information about the maximum depth of the basin. As compared with entropic regularization, TV regularization is much simpler to use because it requires, in general, the tuning of just one regularization parameter. ©2011 Society of Exploration Geophysicists
Artigo em Revista

Partitioned least-squares operator for large-scale geophysical inversion Geophysics 75, R121, 2010
Least-squares (LS) problems are encountered in many geophysical estimation and data analysis problems where a large number of observations (data) are combined to determine a model (some aspect of the earth structure). Examples of least squares in seismic exploration include several data processing algorithms, theoretically accurate LS migration, inversion for reservoir parameters, and background velocity estimation. A frequently encountered problem is that the volume of data in 3D is so large that the matrices required for the LS solution cannot be stored within the memory of a single computer. A new technique is described for parallel computation of the LS operator that is based on a partitioned-matrix algorithm. The classical LS method for solution of block-Toeplitz systems of normal equation (NE) to the general case of block-Hermitian and non-Toeplitz systems of NE. is generalized. Specifically, a solution of a block-Hermitian system of NE is shown that may be obtained recursively by linearly combining the solutions of lesser order that are related to the forward and backward subsystems of equations. This results in an efficient parallel algorithm in which each partitioned system can be evaluated independently. The application of the algorithm to the problem of 3D plane wave transformation is demonstrated. ©2010 Society of Exploration Geophysicists
Artigo em Revista

Simultaneous 3D depth-to-basement and density-contrast estimates using gravity data and depth control at few points
We have developed a gravity-inversion method for simultaneously estimating the 3D basement relief of a sedimentary basin and the parameters defining a presumed parabolic decay of the density contrast with depth in a sedimentary pack, assuming prior knowledge about the basement depth at a few points. The sedimentary pack is approximated by a grid of 3D vertical prisms juxtaposed in both horizontal directions of a right-handed coordinate system. The prisms' thicknesses represent the depths to the basement and are the parameters to be estimated from the gravity data. To estimate the parameters defining the parabolic decay of the density contrast with depth and to produce stable depth-to-basement estimates, we imposed smoothness on the basement depths and proximity between estimated and known depths at boreholes. We applied our method to synthetic data from a simulated complex 3D basement relief with two sedimentary sections having distinct parabolic laws describing the density-contrast variation with depth. The results provide good estimates of the true parameters of the parabolic law of density-contrast decay with depth and of the basement relief. Inverting the gravity data from the onshore and part of the shallow offshore Almada Basin on Brazil's northeastern coast shows good correlation with known structural features.
Artigo em Revista

Métodos estocásticos para modelagem de escoamento estacionário e transiente em meios porosos.
Uma maneira de incorporar a incerteza das medidas de campo e a variabilidade espacial nas propriedades hidráulicas de aquíferos e estabelecer distribuições de probabilidade para os parâmetros físicos do meio. As propriedades estatísticas do potencial hidráulico são então determinadas pela solução numérica das equações
diferenciais estocásticas que regem o regime de escoamento no aquífero. Neste trabalho descrevemos a utilização de dois métodos de elementos finitos estocásticos (o método de Monte Carlo e o método da Colocação) para estimar a média e a variância do potencial hidráulico para fluxo saturado em meio estatisticamente heterogêneo,
supondo que o coeficiente de transmissividade hidráulica é descrito por um processo lognormal. Um dos fatores decisivos na precisão numérica dos métodos é o comprimento de correlação associado à transmissividade. Discutimos também algumas configurações de baterias de extração que foram recentemente propostas para uma exploração adequada do aquífero Recôncavo na bacia do rio Capivara (Bahia, Brasil), comparando dois modelos clássicos e introduzindo a aleatoriedade da transmissividade em um dos arranjos de poços que foram propostos.
Artigo em Revista

Influence of Sea Water Resistivity on MCSEM Data.
O Marine Controlled Source ElectroMagnetic (MCSEM) é um método geofísico para a detecção de camadas resistivas contidas abaixo do assoalhooceânico. Neste trabalho nós modelamos os dados do MCSEM em um ambiente unidimensional, incluindo variações na resistividade da água do mar, na forma de camadas de resistividades uniformes. Estas variações na resistividade da água podem surgir devido à influência de correntes marinhas, gradientes de temperatura ou mudança na salinidade da água. Nós estudamos o efeito destas variações nos dados do método MCSEM. Nossos resultados mostram que a interpretação pode ser fortemente influenciada, principalmente quando são analisados os dados normalizados. Vimos que as mudanças na resistividade da água têm um efeito sobre os dados similar àquele de mudanças na profundidade da lâmina d'água, sendo que ambos influenciam na atenuação da chamada "air-wave".
Artigo em Revista

Estudo de formas implementacionais da equação da onda imagem para remigração na profundidade.
Neste trabalho estudamos teórica e numericamente novas formas implementacionais da equação da onda imagem para remigração em profundidade. Trata-se de uma equação diferencial parcial de segunda ordem semelhante à equação da onda acústica. Determinamos, além da consistência e da estabilidade, as condições de dispersão e dissipação de seis esquemas de diferenças finitas para esta equação em diversas formas obtidas por mudanças de variáveis. Estas condições não podem ser satisfeitas simultaneamente, ie., um resultado preciso não é de fácil obtenção. Testes numéricos confirmam os resultados teóricos da estabilidade para três esquemas, mas falham nos outros. Os esquemas avançados na variável da velocidade e avançados ou atrasados em profundidade foram os que tiveram os resultados mais proveitosos, pois estes esquemas satisfazem as previsões teóricas quanto à sua estabilidade e podem ser utilizados para realizar a propagação da onda imagem, sendo o primeiro para velocidades crescentes e o segundo para velocidades decrescentes.
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