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.

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Record sections from three‐dimensional acoustic models often contain diffracted events not predictable by classical raypath theory. Several observed and calculated record sections from models of typical geologic structures such as synclines, anticlines, and faults verify this diffraction phenomenon. A careful interpretation of the character and moveout of these diffracted events is required to delineate certain portions of the geologic structures. A far‐field approximation of the retarded potential equation is suitable for direct time‐domain evaluation and is used to synthesize the calculated sections. The excellent comparisons between the calculated and observed record sections suggest that the mathematical modeling technique can be a useful tool for enhancing field interpretations.

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