Produção Científica
Artigo em Revista
| Time-migration velocity estimation using Fréchet derivatives based on nonlinear kinematic migration/demigration solvers Advanced seismic imaging and inversion are dependent on a velocity model that is sufficiently accurate to render reliable and meaningful results. For that reason, methods for extracting such velocity models from seismic data are always in high demand and are topics of active investigation. Velocity models can be obtained from both the time and depth domains. Relying on the former, time migration is an inexpensive, quick and robust process. In spite of its limitations, especially in the case of complex geologies, time migration can, in many instances (e.g. simple to moderate geological structures), produce image results compatible to the those required for the project at hand. An accurate time-velocity model can be of great use in the construction of an initial depth-velocity model, from which a high-quality depth image can be produced. Based on available explicit and analytical expressions that relate the kinematic attributes (namely, traveltimes and local slopes) of local events in the recording (demigration) and migrated domains, we revisit tomographic methodologies for velocity-model building, with a specific focus on the time domain, and on those that makes use of local slopes, as well as traveltimes, as key attributes for imaging. We also adopt the strategy of estimating local inclinations in the time-migrated domain (where we have less noise and better focus) and use demigration to estimate those inclinations in the recording domain. On the theoretical side, the main contributions of this work are twofold: 1) we base the velocity model estimation on kinematic migration/demigration techniques that are nonlinear (and therefore more accurate than simplistic linear approaches) and 2) the corresponding Fréchet derivatives take into account that the velocity model is laterally heterogeneous. In addition to providing the comprehensive mathematical algorithms involved, three proof-of-concept numerical examples are demonstrated, which confirm the potential of our methodology. |
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Artigo em Revista
| EXTENDING THE USAGE OF GRAPHICS PROCESSING UNITS ON THE CLOUD FOR COST SAVINGS ON SEISMIC DATA REGULARIZATION The usage of graphics processing units is already known as an alternative to traditional multi-core CPU processing, offering faster performance in the order of dozens of times in parallel tasks. Another new computing paradigm is cloud computing usage as a replacement to traditional in-house clusters, enabling seemingly unlimited computation power, no maintenance costs, and cutting-edge technology, dynamically on user demand. Previously those two tools were used to accelerate the estimation of Common Reflection Surface (CRS) traveltime parameters, both in zero-offset and finite-offset domain, delivering very satisfactory results with large time savings from GPU devices alongside cost savings on the cloud. This work extends those results by using GPUs on the cloud to accelerate the Offset Continuation Trajectory (OCT) traveltime parameter estimation. The results have shown that the time and cost savings from GPU devices’ usage are even larger than those seen in the CRS results, being up to fifty times faster and sixty times cheaper. This analysis reaffirms that it is possible to save both time and money when using GPU devices on the cloud and concludes that the larger the data sets are and the more computationally intensive the traveltime operators are, we can see larger improvements. |
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Artigo em Revista
| A statistics-based descriptor for automatic classification of scatterers in seismic sections Discontin uities and small structures induce diffractions on seismic or ground-penetrating radar (GPR) acquisitions. Therefore, diffraction images can be used as a tool to access valuableinformation concerning subsurface scattering features, such as pinch outs, fractures, and edges. Usually, diffraction-imaging methods operate on diffraction events previously detected. Pattern-recognition methods are efficient to detect, image, and characterize diffractions. The use of this kind of approach, though, requires a numerical description of image points on a seismic section or radargram. We have investigated a new descriptor for seismic/GPR data that distinguishes diffractions from reflections. The descriptor consists of a set of statistical measures from diffraction operators sensitive to kinematic and dynamic aspects of an event. We develop experiments in which the proposed descriptor was incorporated into a pattern-recognition routine for diffraction imaging. The obtained method is useful for performing the automatic classification of image points using supervised and unsupervised algorithms, as a complementary step to Kirchhoff imaging. We also develop a new type of filtering, designed to address anomalies on the diffraction operators caused by interfering events. We evaluate the method using synthetic seismic data and real GPR data. Our results indicate that the descriptor correctly discriminates diffractions and shows promising results for low signal-to-noise-ratio situations. |
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Artigo em Revista
| Numerical modeling of flexural stresses in the upper crust of NE Brazil – Implications for stress regimes in intraplate settings We model the contribution of the stresses generated by the topography and density heterogeneities in the stress field of NE Brazil and the interplay between these and regional stresses. Our model consists of a horizontally layered elastic slab (the lithosphere) that floats above an inviscid fluid (the asthenosphere), where the surface load is generated by the topography and bathymetry, and the internal loads are calculated from the Bouguer anomalies. The deviatoric flexural stresses are calculated in the Fourier domain. Our results showed local flexural stresses with magnitudes comparable to those of the plate-wide stresses, and mainly controlled by the long-wavelength Bouguer anomalies. Scenarios for the total stress field were calculated by superposing different values for an E-W-oriented regional compression. We predict the stress axes orientations and stress regimes for the equatorial margin and the Pernambuco Shear Zone (PESZ) region. However, we highlight the importance of spreading stresses, not considered here, in rotating the maximum horizontal compression (SHmax) predictions. The prediction of stress regime and stress axes orientation in PESZ using low to zero regional compression suggest that a decoupling of the shallow stresses from the plate-wide stresses might occur in this region. Another possible explanation found was that the spreading stresses might counterbalance the regional stresses. We conclude that the superposition of principally flexural and plate-wide stresses explains reasonably the observed stress regimes and SHmax orientations in a great part of NE Brazil. Furthermore, flexural stresses are playing an important role in the reactivation of the PESZ, possibly controlling the stress field. |
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Artigo em Revista
| Geophysical evidence for doming during the Pan-African/Brasiliano orogeny in the SeridĂł belt, Borborema Province, Brazil The Neoproterozoic SeridĂł belt located in the Borborema Province, NE Brazil, was deformed and metamorphosed in the Pan-African/Brasiliano orogeny. The basement rocks of the belt were submitted to intense partial melting to form dome-like structures as documented in the Santa Luzia anatetic dome. The aeromagnetic map of the SeridĂł belt shows a long-wavelength anomaly with elongated sigmoidal shape, whose limits coincide with shear zones. Comparison with gravity data shows that both anomalies share common sources, which are felsicrocks more magnetic and less dense than the surrounding crust. Correlation with geology shows that the geophysical anomalies are associated with magnetite-rich migmatites and granites. We modelled the geophysical datasets using magnetic susceptibility and density values from measurements in cores of a stratigraphic borehole. The results are consistent with a migmatite-dome system formed in the south-central part of the belt. This large-scale crustal structure is composed of elongated internal domes, which were geophysically modelled as four anomalous bodies, that resulted from partial melting of rocks mostly of the basement. The anomalies define an elongated migmatite-dome system with the main axis in the NNE-SSW direction that progressively bends to EW approaching to the Patos shear zone, thus indicating that this shear zone had a key role in shaping the domic structure. The migmatitic domes of the SeridĂł belt compose a fault-related dome system, formed in association with the broad strike-slip shear zone array of the Patos-SeridĂł system, which evolved through the crustal extrusion resulting from the continental collisions that amalgamated West Gondwana in the Ediacara. |
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Artigo em Revista
| Parallel Source Scanning Algorithm using GPUs The use of methods using waveform stacking are nowadays more common in microseismic monitoring applications because they avoid manual or automatic phase picking. The Source Scanning Algorithm (SSA) is a widely known technique in which the source location is estimated using a brightness function obtained from stacking the normalized absolute amplitude seismograms recorded at several stations. The SSA has the advantage of the straightforwardness of its implementation but has the inconvenience of being computationally costly even for small-scale experiments. Our approach is then to parallelize the sequential SSA using graphics processing units (GPUs), and we named this parallel version pSSA. We have parallelized the Stacking step of the SSA Algorithm because this is by far the most computationally demanding Step. This can be done efficiently because of the spatial independence of the data. In our test cases we performed sequential and parallel computations of the SSA and pSSA in two different platforms. Additionally, we compared the performance of pSSA with a parallel implementation using OpenMP. We demonstrate that pSSA has produced speedups up to 125× as compared to the sequential version. We implemented a client–server architecture to receive and process the data. This architecture can treat with various simultaneous clients and also with out-of-order data packets. This allows for re-sending lost or corrupted data. We anticipate that pSSA has the impact of allowing SSA like algorithm to be used in microseismic experiment design and the use of on-site real-time denoising techniques, as well as the potential of being used in traffic light warning systems for fluid injection operations. |
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Artigo em Revista
| Karstification and fluid flow in carbonate units controlled by propagation and linkage of mesoscale fractures, JandaĂra Formation, Brazil This study examines how centimeter- to meter-long fracture sets propagate, link and form m-long structures, which are then karstified in the vadose (aerated) zone in the upper parts of exposed layered carbonate units. We characterize the fracture patterns, including both mesoscale extensional joints, veins, stylolites and macroscale faults, within the Rosario pavement, which is a 1100-m-long, 340-m-wide outcrop in the JandaĂra Formation, Potiguar Basin in semiarid Brazil. The work compares the directions of the mesoscale structures with the strikes of surface collapse dolines and cave conduits at depths of 10–20 m. The main results indicate that the background (diffuse) deformation consists mostly of bed-perpendicular, stratabound N-S-striking veins and extensional (opening mode-I) fractures and E-W- to ENE-WSW-striking stylolites, which are consistent with the same stress field. Both sets of structures are laterally continuous and are commonly traceable across the pavement. Linkage of stylolites and stratabound fractures formed via throughgoing fractures that affected several layers, as deep as 15 m. The throughgoing fractures were favorable structures for fluid pathway formation and karstification in the epigenetic environment. We conclude that sets of mesoscale structures fit with the orientations of the collapse dolines and subhorizontal cave passages, and induced hydraulic anisotropy within the vadose level. These findings indicate that the fractures, veins and stylolites grow, link and form localized vertical conduits for water percolation and karst development. These findings can constrain future models and numerical simulations of karst conduits in investigations of groundwater and oil reservoirs. |
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Artigo em Revista
| Can Euler deconvolution outline three-dimensional magnetic sources? Severe limitations of the standard Euler deconvolution to outline source shapes have been pointed out. However, Euler deconvolution has been widely employed on field data to outline interfaces, as faults and thrust zones. We investigate the limitations of the 3D Euler deconvolution–derived estimates of source dip and volume with the use of reduced-to-the-pole synthetic and field anomalies. The synthetic anomalies are generated by two types of source bodies: (1) uniformly magnetized prisms, presenting either smooth or rough interfaces, and (2) bodies presenting smooth delimiting interfaces but strong internal variation of magnetization intensity. The dip of the first type of body might be estimated from the Euler econvolution solution cluster if the ratio between the depth to the top and vertical extent is relatively high (>1/4). For the second type of body, besides dip, the source volume can be approximately delimited from the solution cluster envelope, regardless of the referred ratio. We apply Euler deconvolution to two field anomalies which are caused by a curved-shape thrust zone and by a banded iron formation. These anomalies are chosen because they share characteristics with the two types of synthetic bodies. For the thrust zone, the obtained Euler deconvolution solutions show spatial distribution allowing to estimate a source dip that is consistent with the surface geology data, even if the above-mentioned ratio is much less than 1/4. Thus, there are other factors, such as a heterogeneous magnetization, which might be controlling the vertical spreading of the Euler deconvolution solutions in the thrust zone. On the other hand, for the iron-ore formation, the solution cluster spreads out occupying a volume, in accordance with the results obtained with the synthetic sources having internal variation of magnetization intensity. As conclusion, although Euler deconvolution–derived solutions cannot offer accurate estimates of source shapes, they might provide a sufficient degree of reliability in the initial estimates of the source dip and volume, which may be useful in a later phase of more accurate modelling. |
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Artigo em Revista
| A new look at the causes of “polarization” horns in electromagnetic well logging So-called polarization horns appear in electromagnetic well logs when the sondes move through the interface between formations of different conductivities. Since the early 1990s, peaks in the logs have been attributed to the influence of surface charges due to the presence of electric field components perpendicular to the interfaces. A new analysis finds that surface charges should not be pinpointed as the immediate cause for the appearance of interface peaks in electromagnetic logs from any tool operating in any frequency range. This is accomplished by calculating the derivatives of the magnetic field components with respect to the sonde’s vertical position as it crosses the interface between two homogeneous isotropic media. The mathematical expressions reveal the components with smooth transitions and the ones with discontinuities in their rates of change across the interface. The analysis is applied to the four components needed to simulate the responses of coaxial and coplanar coil configurations in dipping logs. The results show that only the horizontal field from a horizontal dipole source suffers an influence from the current density field perpendicular to the interface between the two media, which gives rise to surface charges, but even for this component, the nonsmooth transitions are not associated with the perpendicular current. An anisotropic example gives further support to the conclusion that the polarization horns are associated with the discontinuous current density field parallel to the interface rather than with the continuous current across the interface. |
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Artigo em Revista
| GEMM3D: An Edge Finite Element program for 3D modeling of electromagnetic fields and sensitivities for geophysical applications This paper presents the 3D modeling program GEMM3D (Geophysical ElectroMagnetic Modeling 3D), a Fortran program for modeling frequency domain electromagnetic responses of geophysical sources in three-dimensional models. The program is an implementation of the Edge Finite Element method, in an electric field formulation, to simulate any electromagnetic source of interest, at all frequencies that are used in the geophysical methods in the quasi-static regime. Its current version includes routines for plane waves and simulation of magnetotelluric data; electrical and magnetic dipoles; circular loops. Other sources and configurations may easily be included with the addition of routines for simulating primary (layered earth) fields. The program includes routines for calculating sensitivities using the Adjoint State method, which makes efficient use of direct solvers for factoring once and solving for multiple systems with the same coefficient matrix. The paper details the methods, the implementation, and the usage of the program. Validation tests are presented for different sources. The program is freely available for use in research and teaching. |
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