Data-Driven Optimal Amplitude Variation With Angle and Azimuth Inversion for Brittleness and Fracture Detection

Natural fractures and rock brittleness play an important role in the fracturing and development of shale gas reservoirs. Amplitude variation with angle and azimuth (AVAZ) inversion provides an effective tool for estimating brittleness and fracture properties. However, AVAZ inversion generally employs a linearized reflection coefficient approximation, commonly not applicable for cases with strong contrasts in rock properties and long-offset data. Meanwhile, the large number of model parameters poses great challenges for multiparameter simultaneous inversion. To overcome these limitations, we propose a novel optimal AVAZ inversion approach to estimate the brittleness indicator and fracture density in horizontal transversely isotropic (HTI) media. We first utilize the anisotropic Zoeppritz equation and the convolution model to generate reference AVAZ gathers from well-log data. The singular value decomposition (SVD) method is then employed to obtain optimal basis functions and optimal coefficients that directly link band-limited elastic and fracture reflectivities to observed AVAZ data. This enables the direct estimation of band-limited elastic and fracture reflectivities from observed AVAZ data. Finally, Bayesian poststack seismic inversion, constrained by modified Cauchy prior and low-frequency models, is utilized to invert these band-limited reflectivities for elastic and fracture parameters individually. The feasibility of our method is demonstrated on a synthetic example and a field dataset from the southern Sichuan Basin. Results reveal the ability of our method to produce more satisfactory estimated results of the brittleness indicator and fracture density than the AVAZ simultaneous inversion method, which could aid in the seismic characterization of rock brittleness and natural fractures.