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Coimbra T.A.,University of Campinas | Novais A.,University of Campinas | Novais A.,National Institute of Petroleum Geophysics INCT GP | Schleicher J.,University of Campinas | Schleicher J.,National Institute of Petroleum Geophysics INCT GP
Geophysics | Year: 2015

The offset-continuation operation (OCO) is a seismic configuration transform designed to simulate a seismic section, as if obtained with a certain source-receiver offset using the data measured with another offset. Based on this operation, we have introduced the OCO stack, which is a multiparameter stacking technique that transforms 2D/2.5D prestack multicoverage data into a stacked common-offset (CO) section. Similarly to common-midpoint and common-reflection-surface stacks, the OCO stack does not rely on an a priori velocity model but provided velocity information itself. Because OCO is dependent on the velocity model used in the process, the method can be combined with trial-stacking techniques for a set of models, thus allowing for the extraction of velocity information. The algorithm consists of data stacking along so-called OCO trajectories, which approximate the commonreflection- point trajectory, i.e., the position of a reflection event in the multicoverage data as a function of source-receiver offset in dependence on the medium velocity and the local event slope. These trajectories are the ray-theoretical solutions to the OCO image-wave equation, which describes the continuous transformation of a CO reflection event from one offset to another. Stacking along trial OCO trajectories for different values of average velocity and local event slope allows us to determine horizon-based optimal parameter pairs and a final stacked section at arbitrary offset. Synthetic examples demonstrate that the OCO stack works as predicted, almost completely removing random noise added to the data and successfully recovering the reflection events. © 2016 Society of Exploration Geophysicists.

Santos L.,University of Campinas | Santos L.,National Institute of Petroleum Geophysics INCT GP | Schleicher J.,University of Campinas | Schleicher J.,National Institute of Petroleum Geophysics INCT GP | And 3 more authors.
Geophysics | Year: 2011

Present-day techniques to estimate the traveltime parameters of the common-reflection-surface (CRS) stack are tedious, time-consuming, and expensive processes based on local coherence analyses along a large number of trial surfaces. With the 2D CRS method, faster and cheaper determination is possible. The complete set of CRS parameters can be extracted from seismic data by an application of modern local-slope-extraction techniques. The necessary information about the CRS parameters is contained in the slopes of the common-midpoint section at the central point and one or several common-offset sections in its vicinity. We studied two procedures for the CRS parameter extraction technique. Their difference lies in the way the common-offset parameters are determined. One technique requires slope-derivative information (a possible source of instability); the other uses slope information at two different locations and less data redundancy. Testing on a synthetic data example proved that the procedures are sufficiently robust to allow for high-quality extraction of all CRS parameters from the extracted slope fields. In this way, the CRS parameter extraction can be sped up by several orders of magnitude as compared to the conventional procedure based on coherence analysis along trial surfaces. © 2011 Society of Exploration Geophysicists.

Da Silva Neto F.A.,Federal University of Pará | Da Silva Neto F.A.,National Institute of Petroleum Geophysics INCT GP | Costa J.C.,Federal University of Pará | Costa J.C.,National Institute of Petroleum Geophysics INCT GP | And 4 more authors.
Geophysics | Year: 2011

Reverse-time migration (RTM) in 2.5D offers an alternative to improve resolution and amplitude when imaging 2D seismic data. Wave propagation in 2.5D assumes translational invariance of the velocity model. Under this assumption, we implement a finite-difference (FD) modeling algorithm in the mixed time-space/wavenumber domain to simulate the velocity and pressure fields for acoustic wave propagation and apply it in RTM. The 2.5D FD algorithm is truly parallel, allowing an efficient implementation in clusters. Storage and computing time requirements are strongly reduced compared to a full 3D FD simulation of the wave propagation. This feature makes 2.5D RTM much more efficient than 3D RTM, while achieving improved modeling of 3D geometrical spreading and phase properties of the seismic waveform in comparison to 2D RTM. Together with an imaging condition that compensates for uneven illumination and/or the obliquity factor, this allows recover of amplitudes proportional to the earth's reflectivity. Numerical experiments using synthetic data demonstrate the better resolution and improved amplitude recovery of 2.5D RTM relative to 2D RTM. © 2011 Society of Exploration Geophysicists.

Costa J.C.,Federal University of Pará | Costa J.C.,National Institute of Petroleum Geophysics INCT GP | Schleicher J.,University of Campinas | Schleicher J.,National Institute of Petroleum Geophysics INCT GP
Journal of Geophysics and Engineering | Year: 2011

Path-integral imaging forms an image with no knowledge of the velocity model by summing over the migrated images obtained for a set of migration velocity models. Double path-integral imaging migration extracts the stationary velocities, i.e. those velocities at which common-image gathers align horizontally, as a byproduct. An application of the technique to a real data set demonstrates that quantitative information about the time migration velocity model can be determined by double path-integral migration velocity analysis. Migrated images using interpolations with different regularizations of the extracted velocities prove the high quality of the resulting time-migration velocity information. The so-obtained velocity model can then be used as a starting model for subsequent velocity analysis tools like migration tomography or other tomographic methods. © 2011 Nanjing Geophysical Research Institute.

Amazonas D.,Federal University of Pará | Aleixo R.,CGGveritas | Schleicher J.,University of Campinas | Schleicher J.,National Institute of Petroleum Geophysics INCT GP | And 2 more authors.
Geophysics | Year: 2010

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. © 2010 Society of Exploration Geophysicists.

De Figueiredo J.J.S.,Federal University of Pará | De Figueiredo J.J.S.,National Institute of Petroleum Geophysics INCT GP | Schleicher J.,National Institute of Petroleum Geophysics INCT GP | Schleicher J.,University of Campinas | And 5 more authors.
Geophysical Journal International | Year: 2013

To understand their influence on elastic wave propagation, anisotropic cracked media have been widely investigated in many theoretical and experimental studies. In this work, we report on laboratory ultrasound measurements carried out to investigate the effect of source frequency on the elastic parameters (wave velocities and the Thomsen parameter γ) and shear wave attenuation) of fractured anisotropic media. Under controlled conditions, we prepared anisotropic model samples containing penny-shaped rubber inclusions in a solid epoxy resin matrix with crack densities ranging from 0 to 6.2 per cent. Two of the three cracked samples have 10 layers and one has 17 layers. The number of uniform rubber inclusions per layer ranges from 0 to 100. S-wave splitting measurements have shown that scattering effects are more prominent in samples where the seismic wavelength to crack aperture ratio ranges from 1.6 to 1.64 than in others where the ratio varied from 2.72 to 2.85. The sample with the largest cracks showed a magnitude of scattering attenuation three times higher compared with another sample that had small inclusions. Our S-wave ultrasound results demonstrate that elastic scattering, scattering and anelastic attenuation, velocity dispersion and crack size interfere directly in shear wave splitting in a source-frequency dependent manner, resulting in an increase of scattering attenuation and a reduction of shear wave anisotropy with increasing frequency. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.

Oliveira F.D.S.,Federal University of Pará | De Figueiredo J.J.S.,Federal University of Pará | De Figueiredo J.J.S.,National Institute of Petroleum Geophysics INCT GP | Freitas L.,Geoprocessados
Acta Geophysica | Year: 2015

A redatuming operation is used to simulate the acquisition of data in new levels, avoiding distortions produced by near-surface irregularities related to either geometric or material property heterogeneities. In this work, the application of the true-amplitude Kirchhoff redatuming (TAKR) operator on homogeneous media is compared with conventional Kirchhoff redatuming (KR) operator restricted to the zero-offset case. The TAKR and the KR operators are analytically and numerically compared in order to verify their impacts on the data at a new level. Analyses of amplitude and velocity sensitivity of the TAKR and KR were performed: one concerning the difference between the weight functions and the other related to the velocity variation. The comparisons between operators were performed using numerical examples. The feasibility of the KR and TAKR operators was demonstrated not only kinematically but also dynamically for their purposes. In other words, one preserves amplitude (KR), and the other corrects the amplitude (TAKR). In the end, we applied the operators to a GPR data set. © 2014 Versita Warsaw and Springer-Verlag Wien

De Figueiredo J.J.S.,Federal University of Pará | De Figueiredo J.J.S.,National Institute of Petroleum Geophysics INCT GP | De Figueiredo J.J.S.,University of Campinas | Schleicher J.,National Institute of Petroleum Geophysics INCT GP | And 3 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2012

Elastic-wave propagation in fractured and cracked media depends on the dominant spatial orientation of the discontinuities. Consequently, compressional and shear-wave velocities can give valuable information about the orientation of the cracks. The main goal of this work is to estimate the preferential fracture orientation based on an analysis of cross-correlated S-wave seismograms and Thomsen parameters. For this purpose, we analyzed ultrasonic measurements of elastic (P and S) waves in a physical-modeling experiment with an artificially anisotropic cracked model. The solid matrix of the model consisted of epoxy-resin; small rubber strips simulate cracks with a compliant fill. The anisotropic cracked model consists of three regions, each with a different fracture orientation. We used the rotation of the S-wave polarizations for a cross-correlation analysis of the orientations, and P-and S-wave measurements to evaluate the weak anisotropic parameters and . The shear and compressional wave sources had dominant frequencies of 90kHz and 120kHz. These frequencies correspond to long wavelengths compared to the spacing between layers, indicating a nearly effective-media behavior. Integrating the results from cross-correlation with anisotropic parameter analysis, we were able to estimate the fracture orientation in our anisotropic cracked physical model. The parameter showed good agreement with the cross-correlation analysis and, beyond that, provided additional information about the crack orientation that cross-correlation alone did not fully resolve. Moreover, our results show that the shear waves are much more strongly influenced by, and can thus contain more information about, crack orientation than compressional waves. © 2012. American Geophysical Union. All Rights Reserved.

Coimbra T.A.,University of Campinas | Coimbra T.A.,National Institute of Petroleum Geophysics INCT GP | de Figueiredo J.J.S.,National Institute of Petroleum Geophysics INCT GP | de Figueiredo J.J.S.,Federal University of Pará | And 6 more authors.
Geophysics | Year: 2013

Diffraction events contain more direct information on the medium velocity than reflection events. We have developed a method for migration velocity improvement and diffraction localization based on a moveout analysis of over- or undermigrated diffraction events in the depth domain. The method uses an initial velocity model as input. It provides an update to the velocity model and diffraction locations in the depth domain as a result. The algorithm is based on the focusing of remigration trajectories from incorrectly migrated diffraction curves. These trajectories are constructed by applying a ray-tracing-like approach to the image-wave equation for velocity continuation. The starting points of the trajectories are obtained from fitting an ellipse or hyperbola to the picked uncollapsed diffraction events in the depth-migrated domain. Focusing of the remigration trajectories points out the approximate location of the associated diffractor, as well as local velocity attributes. Apart from the migration needed at each iteration, the method has a very low computational cost, but relies on the identification and picking of uncollapsed diffractions.We tested the feasibility of the method using synthetic data examples from three simple constant-gradient models and the Sigsbee2B data. Although we were able to build a complete velocity model in this example, we think of our technique as one for local velocity updating of a slightly incorrect model. Our tests showed that, within regions where the assumptions are satisfied, the method can be a powerful tool. © 2013 Society of Exploration Geophysicists.

Santos L.K.,Federal University of Pará | De Figueiredo J.J.S.,Federal University of Pará | De Figueiredo J.J.S.,National Institute of Petroleum Geophysics INCT GP | Da Silva C.B.,Federal University of Pará
Ultrasonics | Year: 2016

For decades, seismic and ultrasonic physical modeling has been used to help the geophysicists to understand the phenomena related to the elastic wave propagation on isotropic and anisotropic media. Most of the published works related to physical modeling use physical similitudes between model and field (geological environment) only in the geometric and, sometimes, in the kinematics sense. The dynamic similitude is approximately or, most of the time, not obeyed due to the difficulty to reproduce, in laboratory, the forces and tensions excited inside the earth when elastic waves propagate. In this work, we use expressions for dynamic similitude related to the ratio between stiffness coefficients or Lamé parameters. The resulting expression for dynamic similitude shows that this type of similitude has multiple solutions in the context of dynamic stress (non-uniqueness problem). However, the regularization of this problem can be reached by controlling porosity and clay content. Ultrasonic measurements (elastic) as well as petrophysical measurements (density, porosity and clay content) in synthetic sandstone rocks show how difficult it is to reproduce experimentally the three physical similarities studied in this work. © 2016 Elsevier B.V. All rights reserved.

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