DEM Analysis of the P-Wave Anisotropy Response to the Microstructure of Sedimentary Rock Under Biaxial Compression.

This study presents a method to characterize anisotropic microcracks and discontinuous bedding in rock using the discrete element method (DEM). We focus on the influence of microstructure on P-wave velocity anisotropy under both isotropic and anisotropic stress states. The methodology is applied to three experimentally calibrated models: a homogeneous model, a bedding model, and a crack model. Thomsen's parameters are utilized for analyzing P-wave velocity anisotropy. The results indicate that the response of particle motion to seismic waves, cracks, and bedding structures are key inducers of anisotropy in sedimentary rocks. Under isotropic stress, particle motion patterns being different in diverse directions lead to a weak anisotropy. Interestingly, in a homogeneous model, horizontal P-wave velocity slightly exceeds the perpendicular direction velocity because of that motion. Preset crack closure modifies the anisotropy, with bedding direction influencing the final orientation of the symmetry axis. Consistency between preexisting crack and bedding directions leads to a decrease in the anisotropy coefficient with increased confining stress, while an initial decrease followed by an increase occurs otherwise. Under anisotropic stress, bedding and cracks aligned perpendicular to the loading direction more effectively increase anisotropy compared to other orientations. A consistent increase in anisotropy is observed when induced cracks exceed 40% of the total crack number at peak stress due to anisotropic cracks enlarging the difference of P-wave velocity. An anisotropy reversal is noted when bedding orientation does not align with principal stress. Additionally, increased lateral compression reduces the anisotropy parameter ε in sedimentary rock with perpendicular bedding. Highlights: A DEM method to characterize anisotropic microcracks and discontinuous bedding in sedimentary rocks is developed. Under isotropic stress, a homogeneous rock model still presents a weak anisotropy due to particle motion pattern in diverse directions. Under anisotropic stress, bedding and cracks aligned perpendicular to the loading direction more effectively increase anisotropy compared to other orientations. A consistent increase in anisotropy is observed when induced cracks exceed 40% of the total crack number at peak stress. [ABSTRACT FROM AUTHOR]