Experimental Analysis of Velocity Anisotropy in Intraplate Crystalline Basement Rocks

Remote observations of velocity anisotropy and shear wave splitting in the shallow crust have been used to infer spatial and temporal changes of subsurface conditions such as in-situ stress, pore fluid, and lithology. An important, though often overlooked, factor in these observations is the inherent elastic anisotropy within subsurface rocks. In Oklahoma and southern Kansas, the surge in seismicity between 2009 and 2016 has renewed interest in using velocity anisotropy and shear wave splitting measurements to determine and map subsurface conditions. In this study, we aim to corroborate surface geophysical observations by analyzing the elastic heterogeneity in crystalline basement rocks from Oklahoma and southern Kansas. Using cylindrical and octahedral specimens, compressional and shear wave velocities were measured in core samples returned from boreholes that penetrated the crystalline basement of Oklahoma and Kansas. Three cylindrical samples of each rock were cored orthogonally to one another and velocities were recorded under hydrostatic conditions up to 60 MPa. Vertically-cored octahedral samples of three rocks were confined in a multi-axial compression frame at hydrostatic conditions, where elastic wave velocities perpendicular to each specimen face were recorded up to 60 MPa. Strong velocity anisotropy was observed in all specimens during experiments, though degree of variation was highly dependent on specific lithology. Preliminary observations indicate velocity anisotropy of both compressional and shear waves is lowest in vertically-oriented basement cores. Elastic anisotropy decreases with increasing confining pressure, and is positively correlated with pre-existing damage and grain-size amongst basement rocks.