VELOCITY ANISOTROPY IN CRYSTALLINE BASEMENT ROCKS OF THE US MIDCONTINENT

Understanding observations of the subsurface and its behavior over time requires quantifying both the in-situ conditions and the intrinsic material properties. In Oklahoma and Kansas, a surge in seismic activity occurred between 2010 and 2019 with the vast majority of hypocenters located in the Precambrian crystalline basement. This surge in seismicity drove significant interest in characterizing the material and state of stress in the region. Velocity anisotropy, particularly shear-wave splitting, is a powerful tool for determining the in-situ stress orientations in the subsurface. The interpretation of apparent anisotropy from regional-scale seismic measurements can be hampered due to assumptions regarding the physical mechanism for the observed S-wave velocity polarizations. For the crystalline basement, rocks are often assumed as isotropic and thus observed anisotropy is attributed solely to the stress orientations. However, factors other than the stress field are capable of generating velocity anisotropy, including fracture orientations and mineral alignment. In this work we investigated the intrinsic velocity anisotropy of crystalline basement rocks from Oklahoma and Kansas using direct laboratory velocity measurements. Experimental samples were taken from both outcrops and recovered core. Two sets of tests were conducted to measure the horizontal and vertical velocities of each rock sample. Stereologic techniques were then used to quantify the microstructural variation and relate it to both the laboratory and field observations. Our results were then compared with well log and seismically measured anisotropy to show that the agreement between the velocity polarizations and stress orientations depends upon 1) whether the stresses are aligned with anisotropic structural features such as faults; 2) the degree of deformation in the basement that can induce velocity anisotropy; and 3) the scale at which velocity anisotropy is measured in the basement. Our analyses indicate a clear intrinsic anisotropy in the basement rocks of Oklahoma and Kansas and our work highlights the need for a suite of other measurements (i.e. borehole breakouts, stress inversion, or others) to aid in determining the stress orientations, aside from relying solely upon shear-wave polarization to determine subsurface relative stress orientations.