Adelson Santos de Oliveira

Seismic pulses obtained from the analysis of direct waves: A comparison to simulated pulses obtained with modeled notionals

This paper discusses a deterministic signature-estimation method for marine seismic data. This method assumes that the direct wave is known and all notionals can be represented as a scaled version of an effective notional.

The impact of true amplitude DMO on amplitude versus offset

SUMMARY We discuss the application of true-amplitude Dip MoveOut (DMO) to amplitude versus offset (AVO) studies on real marine seismic data. This process is expected to be accurate at least to mild lateral velocity variations. Besides the usual kinematics of any DMO process, true amplitude DMO is designed to properly deal with the variations in the geometrical spreading losses with offset. After the application of a true- amplitude DMO, primary reflections in the input CO section are transformed into their corresponding ZO primary reflections so that (a) geometrical spreading losses are automatically transformed and (b) reflection coefficients remain unchanged. The process affects, thus, the desired compensation of the offset-dependent, geometricalspreading, which is needed for a reliable AVO analysis. The performance of the proposed true-amplitude DMO algorithm to correctly compensate geometricalspreading losses at large offsets is examined. For a known gas anomaly the AVO gradient section showed larger absolute values, indicating larger amplitude compensation not predicted by conventional methods. The reliable results obtained by the proposed method have shown to be useful for AVO analysis at least in areas with relatively small structural complexity.

Recuperação de atributos sísmicos utilizando a migração para afastamento nulo

A conventional processing, without a reliable adjustment in order to preserve the seismic amplitudes, could damage the mapping of the petrophysical properties, jeopardizing the correlation between the seismic data and the well profile. A manner to estimate correctly the amplitudes and, therefore, the reflection coefficients is to perform a pre-stack migration in true amplitude, where an amplitude distortion caused by the geometrical spreading throughout the ray path is compensated by the migration calculation. Nevertheless, this process has an expensive cost as well as is dependent from the velocity model. A routine less expensive than the other one and also more stable taking into account the velocity model, is to transform the seismic section obtained from the acquisition in common offsets in simulated section in zero offset with true amplitudes. This transformation is called true amplitude Zero Offset migration (TA MZO). In a media with constant velocity, the stack curve for the MZO and the weight function are reduced in analytic formulas, mitigating the computational effort. This work has two main objects: the first is to verify the TA MZO algorithm efficiency for a constant velocity in a synthetic model to a complex geology based on a Neo-Albian turbidity reservoir, where the assumption of constant velocity is not respected. The second one is to perform quantitative studies as results of the technique described above. Likewise, the study tries to analyze how useful is the methodology to compute the graphics for AVO and AVA analyses, helping the reservoir characterization.

An Approximate Representation of the Fourier Spectra of Irregularly Sampled Multidimensional Functions: A Cost-Effective, Memory-Saving Algorithm

This article discusses how to estimate the Fourier spectra of irregularly sampled multidimensional functions in an approximate way, using current fast Fourier transform (FFT) algorithms. This estimate may be an alternative for more rigorous approaches when the inversion of huge matrices is prohibitively expensive. The approximation results from a Taylor expansion of the Fourier transform kernel, which is a series on the product of wavenumbers and displacements from centers of a grid where a regular, discrete Fourier transform (DFT) is defined. Convergence and efficiency, as well as some shortcuts for implementation, is indicated. The problem of finding a Fourier spectrum of a function given a finite set of irregular measurements is usually associated with the idea of regularization/interpolation. This, in turn, raises the question of representativeness of continuous functions via its discrete measurements. Although this question is central for the very Fourier spectrum estimation, the scope of this article will be restricted to the trigonometric interpolation, postponing representativeness issues. For the sake of simplicity, the proposed approximation is first derived for the one-dimensional (problem where most related aspects are more clearly stated. Then, extensions to higher dimensions are straightforwardly presented.