Produção Científica



Apresentação
07/11/2019

Influence of the Pore System Properties on P-wave Attenuation in Carbonates
Biot's theory of poroelasticity provides a theoretical basis for phenomena understanding of the attenuation due to the elastic wave propagation. Carbonate rocks have complex pore system with a variety of pore types and a wide range of pore size. Many studies indicate that pore size, pore shape and geometry, and amount of micropores are important factors influencing P-wave attenuation in rock pores, and can explain the variation of attenuation at the same porosity. Therefore, the effects of the pore structure and its properties on elastic wave attenuation cannot be neglected. In this work, the petrophysical and physical properties of carbonates data set were obtained from literature and the P-wave attenuation was estimated through the Dvorkin-Mavko model to investigate the influence of petrophysical, mineralogy, geometrical parameters and porosity scale in P-wave propagation and attenuation. The results allowed to correlate the pore geometry properties of the medium such as DomSize and PoA to attenuation effects and contributed to the understanding of the attenuation in seismic studies of carbonates and complexities of the pore system.
Apresentação
07/11/2019

Reverse time migration with causal imaging condition using an improved method to calculate the analytical wavefield
In this work the analytical wavefield is computed by just solving the wave equation once, differently of conventional methods that need to solve the wave equation twice: once for the source and another for the Hilbert transformed source. Our proposed method can improve the computation of wavefield separation and can bring the causal imaging condition into practice. For time extrapolation, we are using the rapid expansion method to compute the wavefield and its first order time derivative and then compute the analytical wavefield. This method is unconditionally stable and free of numerical noise. By computing the analytical wavefield, we can, therefore, separate the wavefield into down- and up-going components for each time step in an explicit way. For RTM applications, we can now employ the causal imaging condition and through a synthetic example, we could demonstrate the effectiveness of this new imaging condition without applying a Laplacian filter. The RTM result shows that it can successfully remove the low-frequency noise which is common in the typical cross-correlation imaging condition.
Apresentação
07/11/2019

Wavefield separation methods for gradient filtering in time-domain full-waveform inversion
In this work, we review multiscale approach based on the direction of wavefield propagation when applied to full-waveform inversion (FWI). This strategy states that low-wavenumber model updates can be obtained by selective correlation of source and residuals wavefields in gradient vector construction. By enhancing this component the convergence to global minimum is improved and cycle-skipping artifacts are avoided.

Using synthetic data, the numerical implementation of this hierarchical strategy is performed by adapting three different wavefield separation methodologies: implicit and explicit separation in the vertical wavenumber domain and via Poynting vector. We state explicit equations for normalizing these components, in order to ensure a descent direction for the model update, and for computing gradient components when one of these methods is used. For the inversion algorithm, we adopt a scheme in which the step length is estimated via quadratic interpolation, being adjusted at each iteration as solution approaches the global minimum. Alternative non-quadratic functionals for objective function are also implemented, showing the applicability of the method in those cases.
Apresentação
07/11/2019

Complex resistivity measurements on plugs from Corvina oil field, Campos basin, Brazil
Petrophysical parameters have been measured for 12 plugs from two wells of Corvina oil field, Campos basin, Brazil. Their spectral complex resistivity have been measured in the frequency interval 10-3Hz ÔÇô 105Hz, as phase and amplitude. Two models have been used for fitting the experimental data, the Dias model and a new composed Dias/Cole-Cole model. The reasons for introducing this second model was the discrepancies for an intermediate frequency interval when the Dias model was used, presumably due to samples imperfections. In this way, the data fitting problems were solved. The results provide by the two models were used to estimate the values of permeability, successfully in both cases. This analyses also determined the frequency interval to be observed when complex resistivity measurements are taken looking for permeability determination.
Apresentação
07/11/2019

Singular value decomposition and multichannel predictive deconvolution applied to multiple attenuation of Jequitinhonha basin
In this work, it is proposed the use of singular value decomposition and multichannel predictive deconvolution to multiple attenuation. Each method kept its modus operandi; however, in order to improve the results, the deconvolution was applied only on the short offsets, whilst the SVD was applied to the whole data. The results obtained by using this method on the 2-D marine seismic data from an acquisition on Jequitinhonha basin were satisfactory, since the multiple reflections were attenuated, and the amplitude of the reflectors was conserved.
Apresentação
07/11/2019

DCCA e An├ílise Espectral: Aplica├ž├úo em Dados de Po├žos da Bacia de Jequitinhonha
Advances in the understanding of lithosphere are obtained through the analysis of geological and geophysical data, which will help in the exploration of its resources, reducing the risks and making it more efficient. Much of this information, which is in the form of spatial series, such as the well log data, in our case collected in the Jequitinhonha Basin, located in the southeastern region of the state of Bahia, Brazil. The importance of interpretation of well log data can provide information about subsurface geological continuity and even the type of rock layer. We performed the kind of analysis for the identification of correlations present in a signal (autocorrelation) or in two distinct signals (cross -correlation), using the approach of DCCA or Detrended Cross Correlation Analysis. As an extension of our previous work, the present one detailed the application of the DCCA method to well profiles of the Jequitinhonha Basin, inferring that a certain anomaly present in two wells is due to a layer of calcilutites. In addition, Spectral Analysis was used in the same data, so that the spectral coefficient from this method was compared with what was obtained through DCCA. The discrepancies obtained, depending on the analyzed well log, ranged from 1% to 15% for most of the results.
Apresentação
07/11/2019

On the problem of low-frequency static correction
This paper considers factor models used in the static and waveform correction. Such models lead to a system of linear equations, which allows us to perform a detailed analysis of the obtained solutions. In particular, it is possible to understand the low-frequency static correction problem and its connection with the structure of the system of seismic observations. At the same time, it is shown how this problem can be solved at the algorithmic level to optimize the use of a priori information. The latter is particularly important for the analysis of dynamic problems. In the study of linear systems two approaches are realized: an iterative process to estimate the parameters (factors) of the model, as well as analysis of heuristic conditions to guarantee uniqueness and stability of their determination.
Apresentação
07/11/2019

Basin Modeling: Pressure Prediction for the Marine Jequitinhonha
This paper aims at the composition of a seismicstratigraphical framework for part of Jequitinhonha basin (marine east part of the State of Bahia), for pressure prediction in gas and oil exploration in sedimentary basins. The knowledge of the velocity distributions can be based on seismic sections, petrophysical information and empirical model, and it is based on the knowledge of the compressional (vp) and shear (vs) velocities, and densities (¤ü). We presented details of the theoretical model, and an example to show how the pressure varies in the subsurface, where we highlight that pressure prediction does not necessarily increase linearly, but in a complex way that requires specific numerical formulas to be able to see important details. The model poses the vertical gravity load as pressure agent on the geological formations, and does not take into account the effects of curvatures, faulting and diagenesis, and lateral tectonic events. An accurate prediction needs a 3D model for a significantly complete practical application.
Apresentação
07/11/2019

Using a new approach of construction synthetic orthorhombic media: Application on the estimative of Thomsen's parameters
The goal of this work is to make artificial anisotropic synthetic samples with orthorhombic symmetry with different crack densities and relate these densities with Thomsen's parameters. For this purpose, we made four core samples with bedding planes and different number of parallel cracks. The first sample was made without fratcures and the others with 3, 4, and 6 fartcures, respectively. Based on the pulse transmission technique that propagates P- and S-waves through each sample to determine the waves' traveltimes and calculate their velocity in each direction for each sample. Those velocities were used to calculate Thomsen's parameters for orthorhombic media based on Tsvankin's notation.
Apresentação
07/11/2019

Verifica├ž├úo experimental do modelo efetivo de Hudson/Liu para meios fortemente fraturados
The knowledge about properties of subsurface, including cracking characteristics, are extremely important in the context of seismic exploration of anisotropic reservoir. One way of studying the waves behavior for different geological contexts is through the use of effective medium theories. A large number of effective theories have been proposed to study the influence of the geological medium by seismic wave investigation. These models, however, need a calibration in order to define their limitations. This process can be performed through physical modelling. This present work was done an attempt to calibrate a theoretical model (heavily fractured model from Hudson/Liu) simulating a medium containing crack planes. For this to be done, eight physical synthetic fractured sandstones were built. The anisotropic Thomsen parameter ╬│ were estimated through ultrasonic procedure. The results show that the theoretical model for low crack density has a good fit with experimental values of S-wave velocities as well as ╬│ parameter for crack densities up to 8%.
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