Produção Científica



Artigo em Revista
09/10/2010

Anisotropic complex Pade hybrid finite-difference depth migration.
Standard real-valued finite-difference (FD) and Fourier finite-difference (FFD) migrations cannot handle evanescent waves correctly, which can lead to numerical instabilities in the presence of strong velocity variations. A possible solution to these problems is the complex Padé approximation, which avoids problems with evanescent waves by rotating the branch cut of the complex square root. We have applied this approximation to the acoustic wave equation for vertical transversely isotropic media to derive more stable FD and hybrid FD/FFD migrations for such media. Our analysis of the dispersion relation of the new method indicates that it should provide more stable migration results with fewer artifacts and higher accuracy at steep dips. Our studies lead to the conclusion that the rotation angle of the branch cut that should yield the most stable image is 60° for FD migration, as confirmed by numerical impulse responses and work with synthetic data.
Artigo em Revista
09/10/2010

Including Lateral Velocity Variations into True-amplitude Common-Shot Wave-equation Migration.
In heterogeneous media, standard one-way wave equations describe only the kinematic part of one-way wave propagation correctly. For a correct description of amplitudes, the one-way wave equations must be modified. In media with vertical velocity variations only, the resulting true-amplitude one-way wave equations can be solved analytically. In media with lateral velocity variations, these equations are much harder to solve and require sophisticated numerical techniques. We present an approach to circumvent these problems by implementing approximate solutions based on the one-dimensional analytic amplitude modifications. We use these approximations to show how to modify conventional migration methods such as split-step and Fourier finite-difference migrations in such a way that they more accurately handle migration amplitudes. Simple synthetic data examples in media with a constant vertical gradient demonstrate that the correction achieves the recovery of true migration amplitudes. Applications to the SEG/EAGE salt model and the Marmousi data show that the technique improves amplitude recovery in the migrated images in more realistic situations.

Artigo em Revista
09/10/2010

Obliquity-correction imaging condition for reverse time migration.
The quality of seismic images obtained by reverse time migration (RTM) strongly depends on the imaging condition. We propose a new imaging condition that is motivated by stationary phase analysis of the classical crosscorrelation imaging condition. Its implementation requires the Poynting vector of the source and receiver wavefields at the imaging point. An obliquity correction is added to compensate for the reflector dip effect on amplitudes of RTM. Numerical experiments show that using an imaging condition with obliquity compensation improves reverse time migration by reducing backscattering artifacts and improving illumination compensation.
Artigo em Revista
09/10/2010

The role of fluids in triggering earthquakes: observations from reservoir induced seismicity in Brazil.
We relocate precisely micro-earthquakes induced by the Açu reservoir in Brazil and observe seismicity migration consistent with pore-pressure diffusion on a single fault zone. Fluids are believed to play a major role in triggering tectonic earthquakes; reservoir induced seismicity provides a natural laboratory in which to investigate the spatio-temporal evolution and triggering of earthquakes caused by fluid and pore-pressure diffusion. Between 1994 and 1997, 267 earthquakes (ML≤ 2.1) were recorded and located beneath the Açu reservoir. The seismicity increased several months following annual water level peaks, implying that pore-pressure diffusion is the principal triggering mechanism. The small station spacing and very low-attenuation, Precambrian basement, rock enabled starting earthquake locations with uncertainties of only a few hundred metres. We relocate 155 earthquakes from the largest cluster at Açu using waveform cross-correlation to obtain groups of similar events. We use these groups to improve the pick accuracy (to subsample accuracy in 200 sample per second data), and then invert for more accurate hypocentral locations. Our uncertainties are on the order of 10 m, and our locations are more tightly clustered. We observe temporal migration of the earthquakes, both along strike, and to increasing depth. We observe a seismicity migration rate between 15 and 58 m d–1. The rate is highest during the time of peak seismicity rate, and there is some suggestion that the rate decreases with increasing depth. Peak depth in seismicity is reached 175 d after the water peak, that is 192 d after the water low, and the maximum depth then decreases at a similar rate to the rate of increase. Our observations are consistent with triggering by pore-pressure diffusion within a heterogeneous fault zone with an average hydraulic diffusivity of ∼0.06 m2 s–1 and fracture permeability of ∼6 × 10−16 m2.
Artigo em Revista
09/10/2010

Detecção de camadas delgadas usando sísmica de reflexão.
A melhoria da resolução vertical no dado sísmico tem se constituído em um dos maiores desafios para o método de reflexão sísmica. O grande problema está relacionado à maior perda das amplitudes das altas freqüências, quando comparada às baixas, durante a propagação da onda. Isto faz com que na composição final do pulso sísmico a contribuição das baixas freqüências seja maior do que a das altas. Em princípio, isto não deveria se constituir em obstáculo para a recuperação plena do espectro, na medida em que o processamento sísmico dispõe de várias ferramentas para a equalização de todas as freqüências presentes no volume sísmico. O que impede que essas ações logrem êxito é a presença de um ruído com comportamento randômico, o que faz com que exista uma freqüência limite, freqüência crítica, a partir da qual não é possível recuperar o sinal de forma a efetivamente contribuir para o encurtamento do pulso no tempo. Tentativas de contornar este problema, a partir da atenuação do ruído aleatório, têm se mostrado insuficientes. A alternativa a estas tentativas é o aumento da multiplicidade de empilhamento. Como a perda de amplitude com a freqüência é uma relação exponencial, é necessária multiplicidade extremamente elevada para que o ganho seja efetivo. A capacidade de utilização de altíssimas multiplicidades é exatamente a grande virtude da técnica CRS (Common Reflection Surface). Neste trabalho propomos a combinação do método CRS com o chamado balanceamento espectral (spectral whitening), para a recuperação de altas freqüências, com o objetivo de aumentar ganhos na resolução vertical e, como conseqüência, propiciar melhor discriminação de reservatórios delgados. Os primeiros testes em dados sintéticos e reais apresentados neste trabalho são bastante encorajadores e permitem concluir que a estratégia proposta tem bom potencial de aplicação prática.
Artigo em Revista
09/10/2010

Intraplate earthquake swarm in Belo Jardim, NE Brazil: reactivation of a major Neoproterozoic shear zone (Pernambuco Lineament).
Intraplate earthquakes in stable continental areas have been explained basically by reactivation of pre-existing zones of weakness, stress concentration, or both. Zones of weakness are usually identified as sites of the last major orogeny, provinces of recent alkaline intrusions, or stretched crust in ancient rifts. However, it is difficult to identify specific zones of weakness and intraplate fault zones are not always easily correlated with known geological features. Although Northeastern Brazil is one of the most seismically active areas in the country (magnitudes 5 roughly every 5 yr), with hypocentral depths shallower than ∼10 km and seismic zones as long as 30–40 km, no clear relationship with the known surface geology can be usually established with confidence, and a clear identification of zones of weakness has not yet been possible. Here we present the first clear case of seismic activity occurring as reactivation of an old structure in Brazil: the Pernambuco Lineament, a major Neoproterozoic shear zone. The 2004 earthquake swarm of Belo Jardim (magnitudes up to 3.1) and the recurrent activities in the nearby towns of São Caetano and Caruaru (magnitudes up to 4.0 and 3.8), show that the Pernambuco Lineament is a weak zone. A local seismic network showed that the Belo Jardim swarm of 2004 November occurred by normal faulting on a North dipping, E–W oriented fault plane in close agreement with the E–W trending structures within the Pernambuco Lineament. The Belo Jardim activity was concentrated in a 1.5 km (E–W) by 2 km (downdip) fault area, and average depth of 4.5 km. The nearby Caruaru activity occurs as both strike-slip and normal faulting, also consistent with local structures of the Pernambuco Lineament. The focal mechanisms of Belo Jardim, Caruaru and S. Caetano, indicate E–W compressional and N–S extensional principal stresses. The NS extension of this stress field is larger than that predicted by numerical models such as those of Coblentz & Richardson and we propose that additional factors such as flexural stresses from the nearby Sergipe-Alagoas marginal basin could also affect the current stress field in the Pernambuco Lineament.
Artigo em Revista
09/10/2010

Applications of Information Theory, Genetic Algorithms, and Neural Models to Predict Oil Flow.
This work introduces a new information-theoretic methodology for choosing variables and their time lags in a prediction setting, particularly when neural networks are used in non-linear modeling. The first contribution of this work is the Cross Entropy Function (XEF) proposed to select input variables and their lags in order to compose the input vector of black-box prediction models. The proposed XEF method is more appropriate than the usually applied Cross Correlation Function (XCF) when the relationship among the input and output signals comes from a non-linear dynamic system. The second contribution is a method that minimizes the Joint Conditional Entropy (JCE) between the input and output variables by means of a Genetic Algorithm (GA). The aim is to take into account the dependence among the input variables when selecting the most appropriate set of inputs for a prediction problem. In short, theses methods can be used to assist the selection of input training data that have the necessary information to predict the target data. The proposed methods are applied to a petroleum engineering problem; predicting oil production. Experimental results obtained with a real-world dataset are presented demonstrating the feasibility and effectiveness of the method.
Artigo em Revista
09/10/2010

Evidence of hydraulic connectivity across deformation bands from field pumping tests: two examples from Tucano Basin, NE Brazil.
It is assumed that deformation bands may compartmentalize aquifers or hydrocarbon reservoirs because these low-permeability structures may behave as barriers to fluid flow. To address the question whether there is, at a reservoir scale, hydraulic connectivity across a damage zone dominated by cataclastic deformation bands, we present results of two pumping tests carried out in a fluvial-deltaic phreatic sandstone aquifer from the Ilhas Group at Tucano Basin (NE Brazil) where intense concentration of deformation bands occurs. Both test sites are associated with macroscopic damage zones that are approximately 1 km in length and 15 m thick. In situ permeability measurements show values of 2000 mD for host rock and 0.1 mD for deformation bands. GPR profiles reveal good continuity of the primary sedimentary structures with almost no deformation band vertical offsets (less than 10 cm). The well locations for the pumping tests were chosen so that the damage zone is located between pumping and monitoring wells. Pumping tests in both cases revealed hydraulic connectivity across the damage zone since the observed stationary drawdown at monitoring wells was a considerable fraction of the drawdown observed in the pumped well. In one experiment using eight monitoring wells the drawdown cone is evolving through the damage zone instead of contouring it. Local deviations in the natural groundwater flow allow to say that the damage zone dimensions are quite large both in horizontal and vertical directions compared to the distances among the wells. The interpretation of all experimental results is that deformation bands do not fully compartmentalize the aquifer. Generalization of this result to hydrocarbon reservoirs has to take into account capillary effects which are not present in the studied case.
Artigo em Revista
09/10/2010

Ground-roll attenuation using a 2D time-derivative filter.
We present a new filtering method for the attenuation of ground-roll. The method is based on the application of a bi-dimensional filter for obtaining the time-derivative of the seismograms. Before convolving the filter with the input data matrix, the normal moveout correction is applied to the seismograms with the purpose of flattening the reflections. The method can locally attenuate the amplitude of data of low frequency (in the ground-roll and stretch normal moveout region) and enhance flat events (reflections). The filtered seismograms can reveal horizontal or sub-horizontal reflections while vertical or sub-vertical events, associated with ground-roll, are attenuated. A regular set of samples around each neighbourhood data sample of the seismogram is used to estimate the time-derivative. A numerical approximation of the derivative is
computed by taking the difference between the interpolated values calculated in both the positive and the negative neighbourhood of the desired position. The coefficients of the 2D time-derivative filter are obtained by taking the difference between two filters that interpolate at positive and negative times. Numerical results that use real seismic data show that the proposed method is effective and can reveal reflections masked by the ground-roll. Another benefit of the method is that the stretch mute, normally applied after the normal moveout correction, is unnecessary. The new filtering approach provides results of outstanding quality when compared to results obtained from the conventional FK filtering method.
Artigo em Revista
09/10/2010

Time evolution of the wave equation using rapid expansion method.
Forward modeling of seismic data and reverse time migration are based on the time evolution of wavefields. For the case of spatially varying velocity, we have worked on two approaches to evaluate the time evolution of seismic wavefields. An exact solution for the constant-velocity acoustic wave equation can be used to simulate the pressure response at any time. For a spatially varying velocity, a one-step method can be developed where no intermediate time responses are required. Using this approach, we have solved for the pressure response at intermediate times and have developed a recursive solution. The solution has a very high degree of accuracy and can be reduced to various finite-difference time-derivative methods, depending on the approximations used. Although the two approaches are closely related, each has advantages, depending on the problem being solved.
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