Joint AVO inversion based on the adjoint-state method and multiscale inversion strategy

ABSTRACT Amplitude variation with offset (AVO) inversion, capable of deriving information on subsurface elastic parameters and fluid properties, represents a pivotal technique in reservoir prediction. Compared with PP-wave AVO inversion, the joint inversion incorporates PS-wave seismic data, enhancing the accuracy and reliability of inversion results, thus providing a more comprehensive portrayal of subsurface geologic formations. PP-wave and joint inversions can be framed as least-squares optimization problems, with the adjoint-state method serving as an effective tool for computing gradients in optimization problems. A joint inversion method was proposed based on the adjoint-state method and a multiscale inversion strategy for the inversion of three key parameters: fluid factor, shear modulus, and density. Initially, the objective function for joint inversion was constructed based on the Russell approximation, and the adjoint-state equation was derived using the Lagrangian function to compute the gradient of the objective function. Combined with the limited-memory Broyden–Fletcher–Goldfarb–Shanno algorithm, efficient unconstrained optimization is achieved. To further enhance inversion accuracy, a multiscale inversion strategy was introduced, which progressively optimized model parameters from coarse to fine scales by using data at different frequencies in stages, thereby improving the accuracy and lateral continuity of the inversion results. Synthetic data tests demonstrate that although the PP-wave inversion based on the adjoint-state method can invert elastic parameters, the joint inversion results were more accurate. The incorporation of the multiscale inversion strategy further enhanced the stability and accuracy of the inversion results, particularly demonstrating strong adaptability in complex geologic conditions. Finally, the proposed method was applied to field seismic data, with the inversion results accurately characterizing subsurface media, thereby validating the effectiveness and practicality of the method.