Produção Científica
Apresentação
| An efficient fully spectral method for constant-Q seismic-wave propagation We present a new modeling approach for wave propagation in anelastic media, based on a fractional spatial differential operator. The problem is solved with the Fourier pseudo-spectral method in the spatial domain and the REM (rapid expansion method) in the time domain, which, unlike the finite-difference scheme, does not require an intermediate storage of the solution and is more accurate. To show the accuracy of the scheme, an analytical solution in a homogeneous anelastic medium is computed and compared with the numerical solution. We present an example of propagation at a reservoir scale and show the efficiency of the algorithm against the conventional finite-difference scheme. The new method, being spectral in time and space simultaneously, offers a highly accurate and efficient solution for wave propagation in attenuating media. |
Apresentação
| Subsalt structure prediction based on the fast 3d salt tectogenesis modeling There are presented some results of testing a computational program developed for prediction of the geological structures and the stress-strain state in exploration and characterization of the hydrocarbon deposits in the areas of salt tectonics. |
Apresentação
| An Improved Method to Calculate the Analytical Wavefield for Causal Imaging Condition 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 reverse time migration (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. Moreover, our method has shown to be efficient in term of computation time in comparison with the conventional RTM method using the cross-correlation imaging condition. The RTM result also shows that it can successfully remove the low-frequency noise which is common in the typical cross-correlation imaging condition. |
Apresentação
| On the temperature dependence of elastic velocities in a synthetic porous VTI media In this work we performed elastic velocities (P and S) measurements as function of temperature in a porous synthetic anisotropic sample constructed in laboratory using a new technique developed by Santos et al (2016). In order to calculate the Thomsen's parameters gamma and epsilon, measurements were made in two different directions of propagation: perpendicular and parallel to the crack planes. For temperatures ranging from 25° C to 175° C the P-wave velocities decreased 7.8 % and S-wave velocities 7.1 % on average. The anisotropic parameters, γ and ε remained practically constant along of this range of temperature. |
Apresentação
| Modelo hÃbrido de predição de velocidade cisalhante utilizando dados de poço Empirical observations are common alternatives used to provide a better understanding of how the rock properties are related among them, and even predict those that are difficult to measure in situ. In this work, we present a hybrid approach to estimate shear-wave velocity in brine-saturated composite lithologies modified from Greenberg-Castagna method. Combining local linear regressions with a fluid substitution analysis (Gassmman equation) we obtained good estimates of Vs when lithology and saturation information are available. We applied our methodology on a set of well log data from the North Sea region. |
Apresentação
| Analysis of Eshelby-Cheng's and Hudson on synthetic cracked models Physical modeling of cracked/fractured media through downscaled laboratory experiments has worked as a great alternative for understanding the effect of anisotropy in the hydrocarbon reservoir, and in the crustal and mantle seismology. The main goal of this work was to experimentally verify the predictions of effective elastic parameters in anisotropic cracked media predicted by Hudson and Eshelby-Cheng's effective cracked models. For this proposal, we carried out ultrasonic measurements on synthetic anisotropic samples with low crack density and different aspect ratios. Six samples were prepared with 5% of crack density. These samples, with three-different aspect ratio cracks (0.13, 0.17 and 0.26), were simulated by penny-shape rubber inclusions in a homogeneous isotropic matrix made with epoxy resin. Moreover, an isotropic sample for reference was constructed with epoxy resin only. Among all samples, three presented only one aspect ratio type (samples with single crack aspect ratio), while other three showed three types of different aspect ratio (mixed crack aspect ratio samples). Regarding predictions performed by the theoretical models, Eshelby-Cheng shows a better fit when compared to the experimental results for samples with single and mix crack aspect ratio. Our comparisons were also performed in terms of γ parameter (Thomsen parameters). |
Apresentação
| Seismic data inversion with curvelet denoising preconditioning Seismic inversion methods are highly sensitive to noise present in the data set. The need to enhance the signal-to-noise ratio (SNR) motivates many researchers do develop increasingly sophisticated denoising methods and combine them into other techniques. While some methodologies operate on a single scale, the curvelet transform established itself as multi-scale transform useful to decompose the seismic signals into multi-resolution elements. In this study, we evaluate the benefits of curvelet denoising as a preconditioning method for poststack seismic data in a 2D acoustic inversion process using a Bayesian framework. Our tests on a synthetic data set simulated in the Marmousi model, and a real data set from the Campos Basin offshore Brazil have shown that the curvelet thresholding method can be successfully applied for random noise elimination. Even the use of a hard global threshold might allow improvements in the deepest parts. Future work will have to show whether alternatives that ensure a more robust way of selecting the coefficients can take into account the wavelength change with depth. |
Apresentação
| Mechanical Hysteresis of Carbonate Outcrop Samples Reservoir rocks characterization consists of a critical stage on petroleum reserves evaluation in which determining the relationships between petrophysical and mechanical rock properties is required to guarantee the accuracy of data. Mechanical properties affect both drilling programs and hydrocarbon exploitation. Hydrostatic compression tests on rocks, if conducted at stresses below failure, typically exhibit both non-linearity and hysteresis in the stress–strain curve. This behavior can be associated with the presence of cracks and pores. In this study, porosity and bulk volume deformation were analyzed and the different behavior under loading/ unloading conditions reveals the grain/pore interaction influence on these properties. Experiments were conducted increasing hydrostatic pressure from 400 psi to 1000 psi, registering loading and unloading data, on carbonate samples of American outcrops. The dissipated energy related to frictional sliding and adhesion effect, during confinement cycle, was estimated by the area of hysteresis loop in the effective pressure versus bulk volume crossplot. |
Apresentação
| A comparison of iterative methods performance to exact and pseudo adjoint operators in least-squares migration Least-squares migration is often used to attenuate migration artifacts that arise in conventional migration due to, for instance, data incompleteness and limited recording aperture. It uses iterative methods to obtain a model that best fits the data, and it requires a forward/adjoint operator pair to do so. These methods use the gradient of the cost function to estimate search directions and update the current model. The gradient is obtained by migrating the residuals between estimated and observed data at each iteration. Migration operators are regarded as adjoint of forward operators but only those which pass the dot-product test are exact adjoints. These operators estimate more accurate search directions, enhancing the convergence rates of iterative methods. We test the performance of adjoint and pseudo-adjoint operators in LSM based on three different iterative methods (steepest descent, conjugate gradients and limited-memory BFGS) in order to assess their sensitivity to the adjointness of the migration operator. We then compare the performance of each method. |
Apresentação
| Seismic trace analysis using minimum phase and singular value decomposition methods. Application to ground-roll attenuation The spiking deconvolution aim is to correct the effect of the wavelet in seismic trace by apply an inverse filter, assumed to be minimum delay, and to get an estimate of reflectivity. In order to compensate propagation and absorption effects one way is to use a time-variant deconvolution where a different inverse filter is computed and applied for each output sample position. We modify this procedure by estimating a minimum-delay wavelet for each time-sample position of the seismic trace. This gives a decomposition of the seismic trace as a sum of minimum-delay wavelets, each multiplied by a reflectivity coefficient. We done the SVD decomposition through moving windows in the matrix which contains the minimum phase wavelets in each column. The seismic trace can be represented as a sum of the eigenimages of the wavelet matrix multiplied by the reflective function. In this way we can obtain an estimate of the reflectivity function through the inversion of this system. This seismic signal decomposition method has a range of applications in the data processing and interpretation of the seismic signal. We use this method to ground-roll attenuation present in land seismic data. Improvements in data quality are evident in prestack data panels and velocity analysis. |
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