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With seismic exploration advancing to the deep Earth, the seismic response of fractured strata has currently become a hot research topic. However, characteristics of the amplitude variation with angle (AVA) of saturated fractured shale still remain unclear. Furthermore, the direct relationships between the AVA response and fracture system parameters have not received much attention. This study is aimed at analyzing the effect of fracture density on AVA responses of fractured shale. For this purpose, we propose a method for modeling saturated fractured shale and analyzing AVA responses of PP- and split PS-waves. First, we introduce Gurevich’s fluid theory into the fractured-shale modeling and establish the relationship between Thomsen’s weak anisotropy parameters and fracture density and fluid properties. Second, we perform forward simulation considering an isotropic overburden and a fractured stratum. The results show that differences in AVA responses of the fractured-shale model and the isotropic model increase with increasing fracture densities. At small to intermediate incidence angles, the reflection coefficients of split PS-waves increase, whereas those of PP-wave decrease. The reflection coefficients of the two models differ dramatically at incidence angles larger than 55°. Furthermore, when the fracture density is large, polarity reversal occurs only in PS2-wave AVA gathers.

PP- and Split PS-Wave AVA Responses of Fractured Shale