Produção Científica

Artigo em Revista

Colored and linear inversions to relative acoustic impedance
Acoustic impedance (AI) is a widely used seismic attribute in stratigraphic interpretation. Because of the frequency-band-limited nature of seismic data, seismic amplitude inversion cannot determine AI itself, but it can only provide an estimate of its variations, the relative AI (RAI). We have revisited and compared two alternative methods to transform stacked seismic data into RAI. One is colored inversion (CI), which requires well-log information, and the other is linear inversion (LI), which requires knowledge of the seismic source wavelet. We start by formulating the two approaches in a theoretically comparable manner. This allows us to conclude that both procedures are theoretically equivalent. We proceed to check whether the use of the CI results as the initial solution for LI can improve the RAI estimation. In our experiments, combining CI and LI cannot provide superior RAI results to those produced by each approach applied individually. Then, we analyze the LI performance with two distinct solvers for the associated linear system. Moreover, we investigate the sensitivity of both methods regarding the frequency content present in synthetic data. The numerical tests using the Marmousi2 model demonstrate that the CI and LI techniques can provide an RAI estimate of similar accuracy. A field-data example confirms the analysis using synthetic-data experiments. Our investigations confirm the theoretical and practical similarities of CI and LI regardless of the numerical strategy used in LI. An important result of our tests is that an increase in the low-frequency gap in the data leads to slightly deteriorated CI quality. In this case, LI required more iterations for the conjugate-gradient least-squares solver, but the final results were not much affected. Both methodologies provided interesting RAI profiles compared with well-log data, at low computational cost and with a simple parameterization.
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 ofmathematical and computational problems, such as manual or automatic,
stack ormigration, 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 (fromwell logs), geometrical, and physical parameters. In addition, all geophysics problems are to be considered three dimensional spatially, as twodimensional imaging suffers from structural side effects. In the conventionalmethod, the steps of velocity analysis for each common-mid-point are as follows: (1) normalmoveout
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 hasmultiple 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

Constraint nip-tomographic inversion of strong sparse seismic data
This work is a result of specific numerical experimentsmotivated by real cases of processing strong sparse seismic data, as an application of techniques based on the common-reflection-surface (CRS) stack technology aiming at estimating a smooth velocity depth distribution. The paper is primarily limited to numerical tests with a depth velocity model that attends closely the paraxial theory validated by the seismic ray hypotheses. A complete modeling of a seismic surveywas performed, and the common-shot sections were submitted to random muting of traces, to noise addition, and afterwards followed by reconstruction of the section by trace interpolation. The interpolation was controlled by the 2D spectral non-aliasing condition, where the t − x spectral amplitude content was limited to the two main Fourier quadrants f − k. It was admitted that most information was based on
primary compressional (P) wave content; therefore, multiples and the P − S conversion were considered as noise. The trace interpolation used the stack attributes of the original gather (conventional stack) with sparse
data to construct supergather sections (for the supergather stack). The velocity distribution in depth uses the principle of interpreting the inversion data as normal incidence point (NPI) information. The applied inversion algorithm is NIP-tomographic, classified as curve fitting, non-linear, multi-parametric, that uses the wave front kinematic and dynamic CRS attributes as data-driven constraints to estimate a consistent depth velocity distribution. As a general conclusion, we emphasized also interpolation, inclusive of sparse data, as a step for spectral analysis, consequently in filtering, stacking, and tomography to obtain a velocity distribution for further use in the estimation for velocity distribution, imaging, geological interpretation and sedimentary basin modeling.
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 (ρDCCA) 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.
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