Produção Científica
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. |
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| 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. |
Artigo em Revista
| Filtering and frequency interpretations of Singular Spectrum Analysis New filtering and spectral interpretations of Singular Spectrum Analysis(SSA)are provided. It is shown that the variables reconstructed fromd iagonal averaging of reduced-rank approximations to the trajec-tory matrix can be obtained from a noncausal convolution filte rwith zero-phase characteristics. There-constructed variables are readily constructed using a two-pass filtering algorithm that is well known in the signal processing literature. When the number of rows in the trajectory matrix is much larger than number of columns, many results reported in the signal processing literature can be used to derive the properties of the resulting filters and their spectra. New features of there constructed series are revealed using these results. Two examples are used to illustrate the results derived in this paper. |
Artigo em Revista
| Otimização global para resolver problemas inversos em eletrorresistividade com flexibilidade na escolha dos viÌnculos Inversion in DC-resistivity is an ill-posed inverse problem because different realizations of the same model might satisfy approximately the same data fitting criterium. It is therefore necessary to use constraints to obtain unique and / or stable solutions to small perturbations in the measurements. However, in general, the introduction of constraints has been restricted to cases of differentiable constraints, which can be treated with local optimization algorithms. 1D and 2D modeling in DC-resistivity is computationally inexpensive, allowing the use of global optimization methods (GOMs) to solve 1.5D and 2D inverse problems with flexibility in constraint incorporation. Changes in the cost function, either in the constraints or data fitting criteria, can be easily performed, since each term of the cost function is properly normalized to allow the approximate invariance of the Lagrange multipliers. GOMs have the potential to support a computational environment suitable for quantitative interpretation in which the comparison of solutions incorporating different constraints is one way of inferring characteristics of the actual distribution of the underground resistivity. In this work, we developed: (i) comparison of the performances of the Simulated Annealing (SA), Genetic Algorithm (GA) and Particle Swarm optimization (PSO) methods to solve the 1.5D inverse problem in DC resistivity using synthetic and field data; (ii) an inversion approach based on particle swarm optimization (PSO) to solve the 2D DC-resistivity inverse problem; (iii) exploration of several constraints in the variation of log-resistivity, including spatial continuity in both L1 andL2 norms, total variation and sparsity constraints using discrete cosine and Daubechies bases. In addition, we explore the minimum inertia constraint, including the case of using the Earth’s surface as the target axis, to impose the concentration of resistive or conductive materials along target axes. The main results of the comparison for the 1.5D case are: a) all methods reproduce quite well the resistivity distribution of synthetic models, b) PSO and GA are very robust to changes in the cost function and SA is comparatively much more sensitive, c) PSO first and GA second present the best computational performances, requiring smaller number of forwarding modeling than SA, and d) GA shows the best performance with respect to the final attained value of the cost function and its standard deviation, whilst SA has the worst performance in this aspect. Equally important for both 1.5 and 2D cases, from the stopping criteria of the PSO algorithm results not only the best solution but also a cluster of suboptimal quasi-solutions from which uncertainty analyses can be performed. As a result, the interpreter has freedom to perform a quantitative interpretation process based on a feedback trial-and-error inversion approach, in a similar manner he/she has when using a friendly forward |
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| Evaluation of model performances in reproducing measures of thermal conductivity of crystalline rocks We evaluate the performances of the Krischer-Esdorn (KE), Hashin-Shtrikman (HS), classic Maxwell (CM), Maxwell-Wiener (MW), and geometric mean (GM) models in reproducing 1,105 measurements of thermal conductivity of crystalline rocks collected in Borborema Province (NE-Brazil). Percent volumes of quartz, K-feldspar, plagioclase, andmafic minerals were also measured. Rock samples were divided into the IOG (igneous and ortho-derived) and MET (metasedimentary) groups. IOG-group (939 samples) covered most the lithologies of the Streckeisen diagram and MET-group (166 samples) covered low-to-medium metamorphic grade lithologies. Reproducing rock conductivities was treated as an inverse problem, where conductivity measurements and constituent mineral volumes are the known quantities while the constituent mineral effective conductivities and model parameters are the unknowns. To identify the model better reproducing the measurements, model performances were compared by using the percentage of number of samples whose estimated conductivities are close to the measured conductivities within the tolerance level of 15%. For all models, the performances are relatively inferior for the MET-group. In the IOG-group, the KE- and HS-model performances are relatively superior. In the MET-group, model performances are very contrasting but the KE-model is again superior. The KE-model thus presents the best performance in reproducing thermal conductivities of crystalline rocks. |
Artigo em Revista
| Retrieval of Body-Wave Reflections Using Ambient Noise Interferometry Using a Small-Scale Experiment We report the retrieval of body-wave reflections from noise records using a small-scale experiment over a mature oil field. The reflections are obtained by cross-correlation and stacking of the data. We used the stacked correlograms to create virtual source-to-receiver common shot gathers and are able to obtain body-wave reflections. Surface waves that obliterate the body-waves in our noise correlations were attenuated following a standard procedure from active source seismics. Further different strategies were employed to cross-correlate and stack the data: classical geometrical normalized cross-correlation (CCGN), phase cross-correlation (PCC), linear stacking**** and phase weighted stacking (PWS). PCC and PWS are based on the instantaneous phase coherence of analytic signals. The four approaches are independent and reveal the reflections; nevertheless, the combination of PWS and CCGN provided the best results. Our analysis is based on 2145 cross-correlations of 600 s data segments. We also compare the resulted virtual shot gathers with an active 2D seismic line near the passive experiment. It is shown that our ambient noise analysis reproduces reflections which are present in the active seismic data. |
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| Neogene–Quaternary fault reactivation influences coastal basin sedimentation and landform in the continental margin of NE Brazil We investigate the role of reactivation of Precambrian basement fabric in the tectono-sedimentary and geomorphological evolution of the ParaÃba Basin, continental margin of northeastern Brazil, during the Cretaceous, Neogene, and Quaternary. This basin represents part of the last bridge between South America and Africa before the last breakup stage of the South Atlantic rifting in the early Cretaceous. The ParaÃba Basin infill is composed of siliciclastic and carbonate Cretaceous units, as well as aeolian, fluvial and marine Quaternary units. We used shuttle radar imagery, aeromagnetic, wellbore and field data. The reduced-to-the-pole magnetic map (RTP) indicates the continuity of the steeply dipping Precambrian basement shear zones beneath the ParaÃba Basin. The combined analysis of surface and subsurface data shows that NE–SW and E–W-striking shear zones were subjected to brittle reactivation in the Aptian–Middle Albian during the basin opening and again in the Neogene–Quaternary, forming a system of horsts and grabens along the basin; some of these structures such as the E–W-oriented Pernambuco shear zone present modern-day seismicity. N–S- and mainly NW–SE-striking transfer faults cut across Aptian–Middle Albian to Neogene–Quaternary strata. These four main fault directions control main river channels and alluvial valleys up to 2 km wide. Topographic breaks up to 50 m were created by late reactivation of rift faults, which mark the boundary between horsts and grabens along the basin. In addition, structural data evidence syn-tectonic faulting with vertical offsets up to 80 m in the Cretaceous and up to 70 m in the Neogene–Quaternary. We conclude that shear zones across the study area are long-lived structures that have behaved as weakness zones. Their neotectonic brittle reactivation has controlled sediment deposition and landform development, which continued through the Neogene–Quaternary. |
Artigo em Revista
| Reverse time migration using phase cross-correlation Additional information regarding the continuity and resolution of selected seismic reflectors in reverse time migration (RTM) images can be beneficial for seismic interpretation. We have developed and evaluated new imaging conditions for RTM based on the phase coherence between the forward- and backward-propagated wavefields. These imaging conditions make use of the instantaneous phase and envelope of the analytical signals of the source and receiver wavefields, in addition to their real parts. Once the analytical wavefields are available, these imaging conditions can be calculated simultaneously with conventional conditions at little or no extra cost. The availability of these fields at each image point enables several alternative ways to define imaging conditions. We explore, in addition to pure phase crosscorrelation (PC), two approaches of amplitude-weighted PC. Our numerical experiments, imaging synthetic and field data sets, indicate that these new imaging conditions provide additional images that can highlight some weak reflectors by locally improving the resolution of RTM images. In our examples, this happens particularly in the deep portions of the seismic images. In addition, reflection events produced at discontinuities are enhanced as sharp signals, suggesting that the proposed imaging conditions can help to delineate stratigraphic and structural features that are harder to see in conventional images. These properties of the PC imaging conditions make them an interesting tool to provide additional information that can aid seismic interpretation in complex structural settings. |
Artigo em Revista
| Extending the useful angle range for elastic inversion through the amplitude-versus-angle full-waveform inversion method We have developed the amplitude versus angle full-waveform inversion (AVA-FWI) method. This method considers the complete seismic response of the layered medium, and so it is capable of correctly handling seismic amplitudes from prestack data with a wide angle range. This capability is very important because a reliable estimate of the elastic parameters and the density requires an incidence angle that goes beyond 30°. Our method inputs seismic traces from prestack time-migrated gathers ordered by angle of incidence and works under the local 1D assumption. AVA-FWI is a nonlinear inversion based on forward modeling by the reflectivity method, which substantially increases its computational cost with respect to conventional AVA inversion. To address this problem, we developed an efficient routine for angle gather modeling and a new method for differential seismogram generation that greatly reduces the amount of computation involved in this task. The AVA-FWI method was applied to synthetic data and to a geophysical reservoir characterization case study using the North Viking Graben open data set. |
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