Ultrasonic Velocity Anisotropy in Crystalline Basement Rocks of the Central United States

Quantifying the in-situ state of stress remains a significant priority in various fields of subsurface research as knowledge of the stress state is a key factor in mitigating risks from various subsurface applications. Both direct and indirect techniques have been developed to determine the stress state. A common method for indirect interpretation of the principal stresses is the use of velocity anisotropy measured in the field. Ideally the maximum stress direction in a plane will be parallel or subparallel to the dominant fracture direction and concomitantly the maximum velocity measured. However, velocity anisotropy may be induced by factors aside from the principal stress directions, so its use is suspect without a reasonable understanding of the subsurface geology velocity behavior. The surge in seismic activity in the past decade in Oklahoma-Kansas region has generated interest in quantifying the stress states in the crystalline basement. To make use of the observed velocity anisotropy in the region, the inherent anisotropy of the basement rocks requires quantification.

This work provides new insight into the inherent anisotropy of the basement rocks of the Oklahoma-Kansas region. Experimental methods were utilized to measure the ultrasonic velocities of five basement rock types oriented vertically and horizontally to the surface at hydrostatic conditions to eliminate the effect of stress-induced anisotropy. The inherent anisotropy measured was also compared with the stress-induced anisotropy determined by applying a differential load to one rock type and measuring the velocity changes. Basement rocks tested were also analyzed through stereologic techniques to correlate microstructural observations with the velocities measured. Our results indicate variable but distinct inherent anisotropy exists in certain basement rocks. We observed horizontal velocity anisotropies of ~15% or higher in some samples, which can correlate with microfracture density and orientation. Our results indicate that care must be taken when interpreting the principal stress directions and changes in subsurface stress via certain geophysical observations.

Funding support was provided by the Carbon Storage office of the Department of Energy (DOE-FE00031687) with cost share from Southern Company.