The amplitude-variation-with-offset (AVO) inversion is an important technology in estimating elastic parameters which are sensitive to reservoirs and fluid. The fluid factor based on the Biot-Gassmann equation is very popular in the fluid discrimination study. However, in the deep reservoir, seismic data with low signal-to-noise ratio often lack the information of large angle of incidence. Therefore, the three-term AVO inversion will not be stable. The main purpose of this study is to help improve the stability of fluid factor inversion for deep reservoir. Under the guidance of the theory of multi-porous media theory, we derive a new two term AVO approximate equation in terms of Gassmann fluid term and shear modulus. The new equation has the nearly same accuracy as the three term AVO equation. The analysis of information contained in reflections demonstrated that the information content didn’t lose, so we can utilize the two term AVO approximation equation to invert the fluid factor to discriminate the fluid saturated in deep reservoir. The L1 norm regularization and smooth model constraints were performed in the inversion method. Tests on synthetic data and field data application show that both fluid factor and shear modulus can be estimated reasonably. Therefore, it can be used for fluid discrimination in deep reservoir.
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