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Santos L.K.,Federal University of Pará | de Figueiredo J.J.S.,Federal University of Pará | de Figueiredo J.J.S.,National Institute for Petroleum Geophysics INCT GP | Omoboya B.,University of Houston | And 4 more authors.
Journal of Applied Geophysics | Year: 2015

Shear-wave propagation through anisotropic fractured or cracked media can provide valuable information about these fracture swarms and their orientations. The main goal of this work is to recover information about fracture orientation based on the shear waveforms (S-waveforms). For this study, we carried out ultrasonic S-wave measurements in a synthetic physical model made of epoxy resin (isotropic matrix proxy), with small cylindrical rubber strips as inclusions (artificial cracks) inserted in it to simulate a homogeneous anisotropic medium. In these experiments, we used low, intermediate, and high frequency shear-wave sources, with frequencies 90, 431, and 840. kHz. We integrated and interpreted the resulting S-wave seismograms, cross-correlation panels and anisotropic parameter-analysis curves. We were able to estimate the crack orientation in single-orientation fracture zones. The high frequency peaks associated with scattered S-waves provided interpretable information about the fracture orientations when the propagation direction was parallel to the fracture plane. The analysis was possible utilizing results from frequency-versus-polarization-angle curves. Moreover, we applied a bandpass filtering process to the intermediate and high frequency seismograms in order to obtain low frequency seismograms. A spectral analysis using frequency-wavenumber (F-K) spectra supports this filtering process. The results obtained using an analysis of cross-correlograms and the Thomsen parameter γ extracted from filtered high-frequency data were quite similar to those obtained using a low-frequency source. This highlighted the possibility of using less expensive high-frequency sources to recover information about the fracture set. © 2015.

Barreto A.C.R.,Federal University of Pará | Gomes E.N.S.,Federal University of Pará | Gomes E.N.S.,National Institute for Petroleum Geophysics INCT GP | Macambira R.N.A.,Federal University of Pará | And 2 more authors.
Journal of Geophysics and Engineering | Year: 2013

We investigate a linear inversion scheme to determine the local anisotropy around a well using multi-azimuthal walkaway VSP data. The input data consist of the vertical components of the slowness vector and the polarization vector of direct and reflected P-wave measurements. The inversion assumes that weak anisotropy can be modelled by first-order perturbations around an isotropic reference medium. This inversion scheme is not restricted to symmetrical classes of anisotropy and the well local orientation. Based on numerical simulations, we show that the inversion is sensitive to the number and orientation of the acquired azimuths. Using conservative assumptions regarding the noise level, phase velocity can be well estimated for a limited angular opening around the well local orientation. © 2013 Sinopec Geophysical Research Institute.

Costa C.A.N.,Federal University of Pará | Campos I.S.,Federal University of Pará | Costa J.C.,Federal University of Pará | Costa J.C.,National Institute for Petroleum Geophysics INCT GP | And 5 more authors.
Journal of Geophysics and Engineering | Year: 2013

Conventional implementations of 3D finite-difference (FD) migration use splitting techniques to accelerate performance and save computational cost. However, such techniques are plagued with numerical anisotropy that jeopardises the correct positioning of dipping reflectors in the directions not used for the operator splitting. We implement 3D downward continuation FD migration without splitting using a complex Padé approximation. In this way, the numerical anisotropy is eliminated at the expense of a computationally more intensive solution of a large-band linear system. We compare the performance of the iterative stabilized biconjugate gradient (BICGSTAB) and that of the multifrontal massively parallel direct solver (MUMPS). It turns out that the use of the complex Padé approximation not only stabilizes the solution, but also acts as an effective preconditioner for the BICGSTAB algorithm, reducing the number of iterations as compared to the implementation using the real Padé expansion. As a consequence, the iterative BICGSTAB method is more efficient than the direct MUMPS method when solving a single term in the Padé expansion. The results of both algorithms, here evaluated by computing the migration impulse response in the SEG/EAGE salt model, are of comparable quality. © 2013 Sinopec Geophysical Research Institute.

de Oliveira A.G.,Federal University of Pará | Gomes E.N.S.,Federal University of Pará | Gomes E.N.S.,National Institute for Petroleum Geophysics INCT GP
Revista Brasileira de Geofisica | Year: 2013

The presence of multiple reflections is common in marine surveys due to the air-water interface. Multiples have significant energy and can mask deep reflectors, leading to the misinterpretation of seismic cross-sections. In this study, surface-related multiple elimination (SRME), predictive deconvolution in the τ - p domain and Radon and f - k filtering are used to eliminate surface multiples in real 2D marine data. These methods are applied in different combinations, and the results are analyzed with the aim of determining an optimal seismic processing flow for the removal of surface multiples.

Pila M.F.,PGS Investigacao Petrolifera Ltda | Pila M.F.,University of Campinas | Schleicher J.,University of Campinas | Schleicher J.,National Institute for Petroleum Geophysics INCT GP | And 4 more authors.
Journal of Applied Geophysics | Year: 2014

Based on the chaining of diffraction-stack migration and isochron-stack demigration, we derive a general true-amplitude Kirchhoff-type single-stack operator for 3D and 2.5D redatuming. It consists of performing a single weighted stack along adequately chosen stacking surfaces or lines. The corresponding traveltimes and weight functions can be calculated using quantities obtained from dynamic ray tracing. The operator simplifies when specified for zero-offset data. For simple types of media, we derive analytic expressions for the stacking lines and weight functions and demonstrate their functionality with numerical examples. © 2014 Elsevier B.V.

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