Velocity Anisotropy in Crystalline Basement Rocks of the US Midcontinent

Understanding observations of the subsurface and its behavior over time requires quantifying inherited geologic structures, the intrinsic material properties, and the in-situ conditions. 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 structures, material and state of stress in the region. Velocity anisotropy can be a powerful tool for determining the in-situ stress orientations in the subsurface. Interpretation of apparent anisotropy from regional-scale seismic measurements can be hampered due to assumptions regarding the physical mechanism for the observed velocities. 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 the orientation of structural features, fracture orientations, and mineral alignment. In this work we investigated the intrinsic velocity anisotropy of crystalline basement rocks through a field experiment and a series of direct laboratory velocity measurements. In the field, we measured the variation of P-wave velocity with respect to azimuthal direction in a basement rock outcrop located near Mill Creek, Oklahoma. Observed velocity variations correlated with the local fracture pattern and two locally mapped faults. We then performed experiments on samples, from both Oklahoma and Kansas, 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 experimental results were then compared with well log and seismically measured anisotropy. Overall, our results document velocity anisotropy at a variety of scales in the midcontinent crystalline basement. Observed anisotropy was dependent on local structures, the presence of oriented microfractures, and the scale at which velocity anisotropy was measured. Our analyses indicate a clear intrinsic anisotropy in the crystalline basement rocks of the midcontinent and show that such characterization must be conducted prior to interpreting velocity polarization data at regional scales.