Experimental velocity anisotropy in crystalline basement rocks of the midcontinental USA

Determination of the in-situ stress orientations in the subsurface is key to understanding crustal behavior. For example, in Oklahoma and Kansas a surge in seismic activity occurred between 2010 and 2019 with the vast majority of hypocenters located in the crystalline basement. This prompted significant interest in characterizing the stress state in this region through indirect geophysical methods, such as shear-wave anisotropy, which is a technique used to identify the principal stress directions through identification of seismic anisotropy. The interpretation of apparent anisotropy from regional-scale seismic measurements can be somewhat limited due to assumptions regarding the physical mechanism for the observed S-wave velocity polarizations and the difficulty in separating the intrinsic anisotropy from other factors. In this work we have investigated the intrinsic velocity anisotropy of crystalline basement rocks from Oklahoma and Kansas using direct laboratory velocity measurement techniques. Two sets of tests were conducted to measure the horizontal and vertical velocities of each rock sample. Tests were conducted under hydrostatic conditions so that the intrinsic rock properties would be the dominant factor in the observed velocity anisotropy. Stereologic techniques were used to quantify the microstructural variation and relate it to both the laboratory and field observations. The results indicate that there is a non-trivial degree of velocity anisotropy in both the horizontal and vertical directions, varying for rocks from different locations. Microstructural observations of fractures show that horizontal fractures orientations dominate the samples, coinciding with the strike-slip regime of the region. However, velocity polarization and fracture orientations do not always align well. The results 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.