The technique of amplitude variation with offset (AVO) allows geoscientists to extract fluid and lithology information from the analysis of prestack seismic amplitudes. Various AVO parameterizations exist, all of which involve the sum of three weighted elastic-constant terms. In present-day AVO approaches, the weighting terms involve either knowledge of the incidence angle only, or knowledge of both the incidence angle and the in situ VP/VS ratio. We have used the theory of poroelasticity to derive a generalized AVO approximation that provides the estimation of fluid, rigidity, and density parameters. We have combined two previously independent AVO formulations, thus reducing, instead of adding to, the total number of formulations. This new approach requires knowledge of a third parameter to compute the weights: the dry-rock VP/VS ratio. We have derived a new equation and applied it to model and real data sets. The new formulation has allowed us to estimate fluid properties of the reservoir in a more direct manner than previous formulations.
This paper draws together basic rock physics, AVO, and seismic amplitude inversion to discuss how fluid discrimination can be performed using pre-stack seismic data. From both Biot and Gassmann theories for porous, fluid-saturated rocks, a general formula is first derived for fluid-property discrimination given that both the P and S impedances are available. In essence, an AVO inversion is transformed into the elastic properties of the pore space. This formula provides a more sensitive discriminator of the pore-fluid saturant than the acoustic impedance and is especially applicable in hard-rock environments. The formulation can be expressed with either the Lamé constants and density, or the bulk and shear moduli and density. Numerical and well-log examples illustrate the applicability of this approach. The combination of an AVO inversion and the parameters of the formula are then discussed to show how this technique can be implemented using pre-stack seismic data. Finally, a shallow gas-sand example from Alberta and a well-log example from Eastern Canada are shown to illustrate the techniques.
Single-trace post-stack seismic inversion is a processing rechnique whose aim is to extract acoustic impedance information from stacked seismic data. In principle, the process is straightforward, and involves the assumptions that the seismic trace can be modelled usin the convolutional and vertical incidence reflection coef lcient B equations. In practice, there are many separate methods used to perform post-stack inversion. These methods can be conveniently grouped into three basic categories: 1) classical Recursive methods; 2) sparse-spike methods; and 3) model-based methods.
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