Produção Científica
Artigo em Revista
| Automatic seismic velocity analysis based on nonlinear optimization of the semblance function We developed and analyzed a method for automatic velocity picking in the semblance domain as a nonlinear optimization problem that is computationally fast, robust, and a simple model for testing. The obtained results can be considered as an initial model for other data-driven methods. Seismic velocity analysis can be considered the major aim for application in data imaging and post-imaging processes. It falls into several classes of mathematical and computational problems, such as manual or automatic, stack or migration, and nonlinear local or global optimization. In all cases the process needs assistance in terms of a priori information and input-output constraints, that can be geological (from well logs), geometrical, and physical parameters. In addition, all geophysics problems are to be considered three dimensional spatially, as two-dimensional imaging suffers from structural side effects. In the conventional method, the steps of velocity analysis for each common-mid-point are as follows: (1) normal-moveout stack velocities are estimated by means of semblance summation along hyperbolic time trajectories producing a map of S(vrms, t0); (2) manual picking is performed in the semblance map for several stack times t0; and (3) interval velocities, vint, are calculated based on the picked smooth stack velocities, vrms, to construct an earth velocity time model that does not require a reference subsurface model. In conclusion, the present automatic velocity analysis has multiple tasks: (1) diminishing the picking step by considering that the stack velocities are based on an interval velocity model; (2) searching for an interval velocity model that best explains the estimated stack velocities; and (3) automatically searching, subject to geological, physical and mathematical constraints, and editing. |
Artigo em Revista
| Target-level waveform inversion: a prospective application of the convolution-type representation for the acoustic wavefield Nowadays, full-waveform inversion, based on fitting the measured surface data with modelled data, has become the preferred approach to recover detailed physical parameters from the subsurface. However, its application is computationally expensive for large inversion domains. Furthermore, when the subsurface has a complex geological setting, the inversion process requires an appropriate pre-conditioning scheme to retrieve the medium parameters for the desired target area in a reliable manner. One way of dealing with both aspects is by waveform inversion schemes in a target-oriented fashion. Therefore, we propose a prospective application of the convolution-type representation for the acoustic wavefield in the frequency–space domain formulated as a target-oriented waveform inversion method. Our approach aims at matching the observed and modelled upgoing wavefields at a target depth level in the subsurface, where the seismic wavefields, generated by sources distributed above this level, are available. The forward modelling is performed by combining the convolution-type representation for the acoustic wavefield with solving the two-way acoustic wave-equation in the frequency–space domain for the target area. We evaluate the effectiveness of our inversion method by comparing it with the full-domain full-waveform inversion process through some numerical examples using synthetic data from a horizontal well acquisition geometry, where the sources are located at the surface and the receivers are located along a horizontal well at the target level. Our proposed inversion method requires less computational effort and, for this particular acquisition, it has proven to provide more accurate estimates of the target zone below a complex overburden compared to both full-domain full-waveform inversion process and local full-waveform inversion after applying interferometry by multidimensional deconvolution to get local-impulse responses. |
Artigo em Revista
| Multi-frequency electromagnetic method for inductive measurement of ground induced polarization and resistivity A geophysical electromagnetic method to inductively measure the ground electrical resistivity and induced polarization has recently been tested. Its basic characteristics involve three major differences from other methods: the two electrical ground parameters are obtained through measuring magnetic field. For this purpose, a transmitter–receiver (T, R) electromagnetic system is used that operates in the frequency domain and consists of a horizontal loop as the transmitter for the perpendicular loops configuration on the ground surface; the measured function is the (T, R) inductive coupling main variation produced due to the presence of the earth, that is the magnetic field radial component; the measurements are conducted at a large number of frequencies (139 in the more advanced prototype), and the measured function is explored in the frequency interval 0.2 Hz to 1 kHz, a much broader frequency range of the induced polarization effect spectrum, than the one conventionally used in field exploration. Three major aspects are emphasized: (1) the existence of a small ‘main zone’ interior to a half-space, which is responsible for most of the magnetic energy that the receiver measures on the half-space surface. This permits to substitute the entire half-space by the ‘main zone’ and, in a second step, to substitute the ‘main zone’ by an equiv- alent homogeneous half-space with the electrical characteristics of such ‘main zone';(2) the existence of a closed solution for the fields that the (T, R) system generates on the surface of a homogeneous isotropic half-space, which provides exact functions with the two electrical parameters of interest as the variables (the apparent resistivity and relative polarization parameter); (3) the values of the electrical parameters so determined can be attributed to the central point of the ‘main zone’. Three-horizontal layers half-space and a conductive sphere in the free-space are discussed as models. Four field surveys are analysed as examples and show a satisfactory performance of the method for detection of on-shore hydrocarbon reservoirs, description of induced reservoir variations and structural features mapping at depths up to 2.5 km. |
Artigo em Revista
| Effects of torque produced by wake on the maneuverability of a flatfish autonomous underwater vehicle Autonomous underwater vehicles (AUV) are important resources to be used in the oil exploration industry in deep waters as well as a platform for scanning devices used in open sea regions of difficult human access. This work aims to analyze through computer simulations the influence of marine currents on the maneuverability of a flatfish shaped AUV. The 3D realistic scale simulations were performed on the Yemoja supercomputer located at SENAI-CIMATEC and describe the temporal evolution of the torques in the three rotational degrees of freedom - roll,yaw and pitch. The torques were calculated for two different inlet velocities and three angles (yaw) of attack showing a significant gain in the amplitude of these with increasing velocity and pitch being the component with the greatest amplitude of oscillation. |
Artigo em Revista
| Cross-correlation in a turbulent flow: Analysis of the velocity field using the pDCCA coefficient The stochastic process of a turbulent flow in a pipeline provides a time series of the velocity field at any point of the domain by solving numerically the Navier-Stokes equation. The turbulent flow was produced by obstacles near the inlet, injecting eddies into the current. Moving downstream, these eddies evolve to a fully turbulent flow. Many length and time scales are involved in this process. We explore the cross-correlations of the velocity field time series at different points and also at different time scales using the detrended cross-correlation coefficient, pDCCA, designed to analyze the cross-correlations in non-stationary time series. Thus, the results with DCCA allow interpreting how these eddies propagate downstream, and also quantify how adherent the velocity fields are with respect to the pipeline position. |
Artigo em Revista
| Detection of the persistency of the blockages symmetry influence on the multi-scale cross-correlations of the velocity fields in internal turbulent flows in pipelines In this paper we analyze the influence of obstacles symmetry on the development of the turbulent flow of a fluid through a pipeline. The analysis is based on the numerical solutions of the Navier–Stokes equations for the velocity field. The influence of the obstacle symmetry on the turbulence is detected by changing their shape while keeping the blockage ratio constant and calculating velocity field cross-correlations on the time series resulting from the simulation. The Detrended Cross-Correlation coefficient (ÏDCCA) is applied to obtain two-point correlations located at different regions of the channel: at mid-channel and near the walls (above and below). With this cross-correlation coefficient we quantify how far from the obstructions these coefficients become independent on the obstructions shape, establishing a scale for the obstruction symmetry memory loss. |
Artigo em Revista
| A numerical viscoelastic model of ground response assimilating pore-water pressure measurements We consider a simple one-dimensional, viscoelastic model for shear-wave propagation on liquefiable soils. The soil is modelled as a layered medium parametrized by shear modulus and viscosity, which in turn depend on the excess pore-water pressure ratio. We numer ically solve the resulting wave propagation model with the spectral element method, and employ simulated annealing and weighted Gauss-Newton inversion algorithms to minimize the misfit of surface displacement, velocity, and acceleration. This procedure is validated us ing recorded ground motion and pore-water pressure data from the Imperial Valley Wildlife and the Garner Valley downhole arrays. Parameter inversion is also carried out with linear models with constant shear modulus and viscosity, and the proposed model provides better fitness in the presence of strong motion, especially in the 1987 Superstition Hills earthquake. Key words: viscoelastic wave equation, liquefaction, spectral element method, site response. |
Artigo em Revista
| Complex Autoregressive Time–Frequency Analysis - Estimation of time-varying periodic signal components Time–frequency representations of nonstationary signals have a wide range of geophysical applications, including seismics, seismology, volcanology, and astrophysics. In this article, we estimate a complex autoregressive (AR) model from a short time window of the analytic signal. The local power spectrum is the inverse of the spectrum of this AR model. Since the coefficients are complex, the time window can be shorter than for the real AR model, which requires more coefficients. This results in higher time–frequency resolution, as seen in a synthetic data examples with different signal components. The new technique also gives good results when computing the instantaneous average frequency (IAF) of marine seismic data. Applied to digitized and downloaded data from the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Hanford, Washington, the result clearly shows the linear chirp associated with the merger of two black holes. |
Artigo em Revista
| APPLICATION OF TIME-FREQUENCY DECOMPOSITION METHOD IN THE STUDY OF GAS RESERVOIR IN THE SERGIPE-ALAGOAS BASIN The sedimentary basin of Sergipe-Alagoas, located on the Brazilian east bank, presents one of the most complete stratigraphic sections of the Brazilian continental margin. Hydrocarbon exploration activities began more than 50 years ago. The recent discoveries of hydrocarbons (gas and oil of high API grade) in turbiditic reservoirs of deep waters have further awakened the exploratory interest of the basin. Problems related to the processing and interpretation of seismic data have always received great attention from the scientific community. Currently, the use of time-frequency decomposition methods of the seismic signal is of great interest. Spectral decomposition has been widely used in reservoir characterization, such as determination of layer thickness, stratigraphic visualization with seismic attributes and identification of low frequency anomalies associated with the presence of gas. The mechanism causing these anomalies is not yet well known, but they are often attributed to the high attenuation of gas filled reservoirs. The approach used for spectral decomposition combines the maximum entropy method and the Wigner-Ville distribution, based on the idea of the Burg method that uses the prediction error operator to extend the Wigner-Ville kernel sequences by applying the Fourier transform to each extended sequence, thus allowing to obtain the Wigner-Ville distribution of maximum entropy. Keywords: Sergipe-Alagoas Basin,Wigner-Ville distribution, maximum entropy, spectral decomposition, seismic attributes, low frequency anomaly. |
Artigo em Revista
| Processing of large offset data: experimental seismic line from Tenerife Field, Colombia Exploration seismology provides the main source of information about the Earth’s subsurface, which in many cases can be presented as a simple model of horizontal or near-horizontal layers. After the seismic acquisition step, conventional seismic processing of reflection data provides an image of the subsurface by using information about the reflections of these layers. The traveltime from a source to different receivers is adjusted using a hyperbolic function. This expression is used in the case involving an isotropic medium, which is a simplification of nature, whereas geologically complex media are generally anisotropic. A subsurface model that more closely resembles reality is the vertical transverse isotropy, which defines two parameters that are required to correct the traveltimes: the NMO velocity and the anellipticity parameter. In this paper, we reviewed the literature and methodology for velocity analysis of seismic data acquired from anisotropic media. A model with horizontal layers and anisotropic behavior was developed and evaluated. The anisotropic velocity was compared to the isotropic velocity, and the results were analyzed. Finally, the methodology was applied to real seismic data, i.e. an experimental landline from Tenerife Field, Colombia. The results show the importance of the anellipticity parameter in models with anisotropic layers. |
