A Hybrid Seismic Inversion Method for VP/VS Ratio and Its Application to Gas Identification

Abstract

The ratio of compressional wave velocity to shear wave velocity (VP/VS ratio) has established itself as one of the most important parameters in identifying gas reservoirs. However, considering that seismic inversion process is highly non-linear and geological conditions encountered may be complex, a direct estimation of VP/VS ratio from pre-stack seismic data remains a challenging task. In this paper, we propose a hybrid seismic inversion method to estimate VP/VS ratio directly. In this method, post- and pre-stack inversions are combined in which the pre-stack inversion for VP/VS ratio is driven by the post-stack inversion results (i.e., VP and density). In particular, the VP/VS ratio is considered as a model parameter and is directly inverted from the pre-stack inversion based on the exact Zoeppritz equation. Moreover, anisotropic Markov random field is employed in order to regularise the inversion process as well as taking care of geological structures (boundaries) information. Aided by the proposed hybrid inversion strategy, the directional weighting coefficients incorporated in the anisotropic Markov random field neighbourhoods are quantitatively calculated by the anisotropic diffusion method. The synthetic test demonstrates the effectiveness of the proposed inversion method. In particular, given low quality of the pre-stack data and high heterogeneity of the target layers in the field data, the proposed inversion method reveals the detailed model of VP/VS ratio that can successfully identify the gas-bearing zones.