Global joint inversion for the estimation of acoustic and shear impedances from AVO derived P- and S-wave reflectivity data
Although seismic inversion of post-stack data, estimating an acoustic impedance model, has become an almost routine procedure, the results can be of limited value due to the overwhelming problem of nonuniqueness in many cases of lithology and fluid discrimination. Variations in acoustic impedance may be due to any combination of a wide range of factors: lithology, porosity, fluid content and saturation, or pore pressure. In the last decade or so, there has been an increased interest in pre-stack inversion. This is because prestack inversion can generate not only compressional but also shear information about a rock. Therefore the use of two elastic parameters estimated from pre-stack inversion can have a significant impact in reducing the ambiguity in many exploration and development situations. For example, when gas is introduced into the pore space of a compressible brine-saturated sand, VP drops, and VS is slightly increased. These different behaviours of P- and S-waves when gas is present in the pore spaces make pre-stack inversion useful as a direct hydrocarbon indicator.