Produção Científica
Apresentação
| Weighted AB semblance using very fast simulated annealing We present a modification applied to a work presentend by Ebrahimi et. al, 2017, that shows the weighted AB semblance method which deals with the AVO phenomenon in velocity spectra. This method is based on the use of two weighting functions applied to the AB semblance coherence measure. The first weighting function, uses the ratio between the first and the second singular value of the time window. The second weighting function is based on the position of the seismic wavelet in the time window. Each weighting function depends on two empirical coefficients, determined by the analysis of a matrix which contains the measure called ECM (Energy Concentration af Matrix). In this work, we made a modification in the calculus of these coefficients values. We use the method Very Fast Simulated Annealing to get the best coefficient values that generates a spectrum with the best resolution. We applied the method in both synthetic and real CMP gathers. Results show that the VFSA made easier to obtain the coefficient values. These values allow to generate velocity spectra with excelent resolution. |
Apresentação
| Verificação experimental do modelo de efetivo de Hudson-Crampin para meios anisotrópicos fissurados cujo o background apresenta anisotropia VTI A modelagem física de meios fissurados/ fraturados utilizando experimentos laboratoriais em escala reduzida reduzida tem funcionado como uma excelente alternativa para a compreensão do efeito da anisotropia na caracterização do reservatório de hidrocarbonetos fissurado/fraturado. O objetivo principal deste trabalho foi verificar experimentalmente asprevisão do modelo de meio efetivo para meios fissurados Hudson e Crampim. Para isso, foram realizadas medições ultrassônicas em amostras anisotrópicas sintéticas com diferentes densidade de fissuras distribuídas em quatro razões de aspectos. Dezesseis amostras (quatro amostra para cada razão de aspecto) fissuradas e uma amostra de referência com ansiostropia VTI e sem fissuras. As razões de aspectos variam entre Isso resulta em amostras com valores de razão de aspecto variando de 0.08 a 0.52. Todas as amostras fissuradas foram simuladas por espaços vazios na forma de discos numa matriz poro elástica com baixa anisotropia VTI homogênea feita com areia e cimento. Em relação às previsões de velocidade realizadas pelo modelos teórico, Crampin apresenta um melhor ajuste para VP e VS se propagando perpendicular ao plano de fissuras para as condições secas e saturadas. Estas previsões se tornam mais evidentes no caso em que a densidade de fissuras e razões de aspectos são baixas. A partir dos valores de velocidade, nossa comparação também foi realizada em termos dos parâmetros ε, γ e δ. |
Apresentação
| Estimativa automática de tempo de trânsiotode ondas compressionais: aplicação na determinação do parâmetro de Thomsen O trabalho tem como objetivo estimar o parâmetro elástico ε, que é conhecido como “anisotropia da onda P”. Para estimar este parâmetro, é utilizado um algoritmo que fornece o tempo de chegada da onda P a partir da teoria STA/LTA. As amostras utilizadas no trabalho foram feitas com uma mistura de cimento e areia, tem formato cilíndrico, com diâmetro de 38 milímetros alturas que variam entre 51 a 61 milímetros e inclusões cilíndricas, as amostras simulam um meio VTI de anisotropia fraca. |
Apresentação
| Least Squares Imaging using l1 and Student residuals This paper studies the Least Squares Migration (LSM) procedure as an optimization problem. Particularly, we study the behavior of this imaging procedure upon the use of robust loss functions. Our results demonstrate that LSM can work even under heavy noise if a suitable loss function is employed. Both the ℓ1 norm and the Student's t-norm showed themselves more stable with regard to outliers than the ℓ2 norm conventionally used in LSM. In the optimization step, we employ the hybrid deterministic-stochastic L-BFGS algorithm that exploits the structure of the objective function to reduce the computational burden of this imaging procedure. The quality of the achieved results show no deterioration over the conventional L-BFGS algorithm. |
Apresentação
| AVO analysis using an alternative formulation for estimation of seismic properties of hydrocarbon fluids: A study on gas condensate The equations proposed by Batzle and Wang (1992), referred as BW formulation, have been largely used by the Geoscience community since the early 90’s. These equations can be applied for the determination of fluid properties in a porous medium and the computation of the compressional and shear wave velocities within the fluid. This information is valuable for the application of Gasmmann fluid substitution and seismic reservoir characterization studies. Carvalho and Moraes (2018) compile an alternative formulation based on the equations proposed by Dranchunk and Abou-Kassem (1975) and Sutton and Hamman (2009) (referred as DASH formulation), which yields superior fit to experimental data of acoustic velocity in natural gas fluids collected from literature (Lemmon et al. 2018; Younglove et al. 1993; Plantier et al. 2005; Ahmadi et al. 2017). The present work expands this study, analyzing the differences in AVO analysis in a 2-layer model for fluid properties calculated with the use of BW and DASH formulations |
Apresentação
| 1D CMP inversion of MCSEM data to create a 3D geoelectrical model This paper describes the application of CMP 1D inversion of 3D MCSEM data to generate a 3D resistivity model. The method extends the CMP inversion that has been successfully applied to 2D MCSEM data. The results indicate that the method can be useful as a quick generator of approximate starting models for true 3D inversion. |
Apresentação
| Pore topology and rock physical properties of microporosity in Lower Cretaceous carbonate sediments: insights from X-ray CT imaging Micropores (i.e., pores <30 µm diameter) are volumetrically the most important pore type in many Cretaceous sediments, comprising >50%, and often >80%, of the total pore volume. There is considerable uncertainty as to the capacity of microporosity to store and flow hydrocarbon, and little data to assess the importance of pore shape, size and connectivity on recovery efficiency. This partly reflects the limitations of technology to image and characterise micropore topology. This study focuses upon a large, multiscale dataset of X-ray CT images of 23 samples from Brazil, the Arabian Plate (both Lower Cretaceous) and the North Sea (Upper Cretaceous). The samples include skeletal peloidal pack–grainstones, four of which are pervasively cemented, skeletal wackestones and mudstones/Chalk. This has allowed a unique overview of micropore topology in three dimensions. Interparticle microporosity occurs between grains and/or within grains that have undergone boring and micritisation. Micropores have been diagenetically altered by recrystallization (neomorphism) of primary micrite, which has increased crystal size and decreased micropore volume and connectivity. Conversely, solution enhancement of micrite has enhanced micropore volumes and connectivity. |
Apresentação
| 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
| 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
| 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. |
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