Produção Científica



Apresentação
03/09/2012

GêBR: a free seismic processing interface
There are many programs for processing seismic data that are freely available and widespread, for example Seismic Un*x, Madagascar, FreeUSP, and SEPlib, among others. All these packages consist of packages of command-line-oriented programs that are designed to be used in sequence; the data conceptually flow in a pipeline through one program after another. Each program is generally controlled by its own set of command-line options. To take full advantage of such a toolkit, the user must have considerable knowledge beyond general geophysical expertise: shell scripting, process submission and management, and batch queue processing, to name a few. While these skills are useful, they should not be a requirement for seismic data processing.
A suitable graphical user interface could take care of these computational details, allowing the user to focus on the central problem of processing seismic data. This is particularly important during training courses, where the limited duration of the does not leave time for learning skills that are not essential to the material being taught. A graphical user interface may also boost the uptake of a new program, by making it more accessible to users and allowing its easy integration with other programs available within the same interface. These principles have guided the development of GêBR, a graphical user interface to control commandline programs for seismic processing. It permits users to build complex processing flows from predefined modules known as menus. Menus describing new programs can be easily added to the interface, extending its capabilities. GêBR is also designed to be simple, in the sense that a couple of hours is enough to introduce the core features of the interface, to allow the user to start working with the seismic data.
Apresentação
03/09/2012

Minimum-delay seismic trace decomposition and SVD filtering for seismic reflection enhancement
Spiking deconvolution corrects for the effect of the seismic wavelet, assumed to be minimum delay, by applying an inverse filter to the seismic trace to get an estimate of reflectivity. In order to compensate for propagation and absorption effects one may use time-varying deconvolution where a different inverse filter is computed and applied for each output sample position. We modify this procedure by estimating a minimum-delay wavelet for each time-sample position of the seismic trace. This gives a decomposition of the seismic trace as a sum of minimum-delay wavelets, each multiplied by a reflectivity coefficient. The reflectivity estimation is a single-trace process which is sensitive to non-white noise, and it does not take into account lateral continuity of reflections. We therefore have developed a new data processing strategy by combining it with adaptive SVD filtering. The SVD filtering process is applied to the data in two steps. First, in a sliding spatial window on NMO-corrected CMP or common shot gathers. Next, after local dip estimation and correction, on local patches in common-offset panels. After the SVD filtering, we apply the new reflectivity estimation procedure. The SVD filtering removes noise and improves lateral continuity while the reflectivity estimation increases the high-frequency content in the data and improves vertical resolution. The new data processing strategy was successfully applied to land seismic data from North-east in Brazil. Improvements in data quality are evident in prestack data panels, velocity analysis and the stacked section.
Apresentação
10/08/2012

Migration velocity analysis using residual diffraction moveout in the pre/post-stack depth domain
Diffraction events contain direct information on the medium velocity. In this work, we develop 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 arbitrary 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 velocity rays from incorrectly migrated diffraction curves. These velocity rays are constructed from a ray-tracing like approach applied to the image-wave equation for velocity continuation. After picking the diffraction events in the migrated domain, the method has a very low computational cost, and the diffraction points are located automatically. We demonstrate the feasibility of our methods using a synthetic data example.
Apresentação
10/08/2012

Experimental relations between stress and fracture properties on synthetic anisotropic media
Elastic anisotropy due to aligned cracks has been the subject of many seismic physical modeling experiments. In earlier investigations, different experimental approaches have taken into account the size, shape and density of cracks. In this paper we present a physical study of the aspect ratio as a function of applied uniaxial stress. We carried out pulse transmission measurements of P- and S-wave velocities in a reference model without inclusions and in a model with penny-shaped neoprene inclusions. The reference model is an anisotropic matrix that consists of stacked plexiglass plates. Rubber discs were used as inclusions in that anisotropic matrix leading to secondary anisotropy. We recorded ultrasonic seismic data using P-wave transducers with central frequency 120 kHz and S-wave transducers with 90 kHz. We compressed the physical models using pressures ranging from 3 to 15.8 MPa. Full crack closure occurs at stress 14.6 MPa normal to model faces. Our analysis indicates three different regimens for the behavior of the inclusions. These results suggest a different dependence of the crack aspect ratio on uniaxial stress at the low state of stress than usually described in the literature. Though our results are not extensive, they show that simple experimental approaches might provide valuable insight into the behavior of cracked rocks at reservoir stress levels.
Apresentação
10/08/2012

Estimation of fracture orientation through elastic ultrasonic waves
Estimate the preferential fracture orientation based on an analysis of cross-correlated S-wave seismograms and Thomsen parameter g is the main goal of this work. For this purpose, we analyzed ultrasonic measurements of elastic 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 has three regions each with a different fracture orientation. We used a rotation of the S-wave polarizations for a cross-correlation analysis of the orientations, and S-wave measurements to evaluate
the weak anisotropic parameter g . The shear-wave source had a dominant frequency of 90 kHz. This frequency corresponds to long wavelengths compared to the crack aperture, ensuring effective-media behavior. Integrating the results from crosscorrelation
with anisotropic parameter analysis, we were able to estimate the fracture orientation in our anisotropic cracked physical model. The g parameter has shown good agreement
with the cross-correlation analysis and, beyond that, provided additional information about the crack orientation that crosscorrelation alone did not fully resolve. Moreover, our results show that the shear waves are strongly influenced by crack orientation.
Apresentação
10/08/2012

Poststack Depth Migration Velocity Analysis Using Residual Diffraction Moveout
"DEDICATED - Case Studies in Diffraction Imaging and Interpretation"
Diffraction events contain direct information on the medium velocity. In this work, we develop 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 arbitrary initial velocity model as input. It provides an average velocity model and diffraction locations in the depth domain as a result. The algorithm is based on the focusing of remigration velocity rays from incorrectly migrated diffraction curves. These velocity rays are constructed from a ray-tracing like approach applied to the image-wave equation for velocity continuation. After picking the diffraction events in the migrated domain, the method has a very low computational cost, and the diffraction points are located automatically. We demonstrate the feasibility of our methods using one synthetic data example.
Apresentação
10/08/2012

Fracture Characterization from Elastic Waves - An Ultrasonic Experimental Approach
The main goal of this work was to estimate the preferential fracture orientation of a cracked medium based on cross-correlated S-wave seismograms analysis and Thomsen parameters. For this purpose, we analysed ultrasonic measurements of elastic (P and S) waves in a physical-modelling experiment with an artificially anisotropic cracked model. The solid matrix consisted of epoxy-resin, and small rubber-strip pieces simulate weakly-filled cracks. The anisotropic cracked model has three regions with three different fracture orientations. We used rotation of S1 and S2 polarizations for a cross-correlation analysis of the orientations, and P and S-wave measurements to evaluate the anisotropic parameters gamma and epsilon. The shear-wave source has a
dominant frequency of 90 kHz, which corresponds to low wavelengths as compared to to the crack aperture, ensuring effective-media behaviour. Integrating the results from crosscorrelation with anisotropic parameters analysis, we were able to estimate fracture orientation in anisotropic cracked model. The anisotropy parameter gamma showed good agreement with the cross-correlation analysis and, beyond that, provided additional information about the crack orientation that cross-correlation alone did not resolve.
Apresentação
10/08/2012

Lithostatic stress driving changes on fracture aspect ratio - Ultrasonic experiment and results,
Elastic anisotropy due to aligned cracks has been the subject of many seismic physical modeling experiments. In this paper we present a physical study of the aspect ratio as a function of applied uniaxial stress. We carried out pulse transmission measurements of P- and S-wave velocties in a reference model without inclusions and in a model with penny-shaped neoprene inclusions. The reference model is an anisotropic matrix that consists of stacked plexiglass plates. Rubber discs were used as inclusions in that
anisotropic matrix leading to secondary anisotropy. We recorded ultrasonic seismic data using P-wave transducers with central frequency 120 kHz and S-wave transducers with 90 kHz. We compressed the physical models using pressures ranging from 3 to 15.8 MPa. Full crack closure occurs at stress 14.6 MPa normal to model faces. Our analysis indicates three different regimens for the behavior of the inclusions. These results suggest a different %dependence of the crack aspect ratio on uniaxial tress at the low
state of stress than usually described in the literature. Though our results are not extensive, they show that simple experimental approaches might provide valuable insight into the behavior of cracked rocks at reservoir stress levels.
Artigo em Revista
09/08/2012

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
09/08/2012

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.
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