Anisotropy of longitudinal and shear wave velocities in rocks under controlled pressure

Reliable interpretation of seismic surveys and borehole loggings requires that the seismic velocities and anisotropies of the bedrock are taken into account. There will be discrepancies in estimated reflector locations if one assumes isotropic velocity. Moreover, improved accuracy in determining reflector locations reduces drilling efforts in ore exploration. Our custom-built ultrasonic triaxial anvil setup permits measuring longitudinal (Vp) (1 MHz center frequency) and shear wave (Vs) (1.1 MHz center frequency with 0° and 90° polarization options) sound velocities in cubic rock samples (sides ~25 mm) under crustal-pressures up to 300 MPa in three directions (X, Y, and Z) simultaneously. Our results of eleven Precambrian rock samples from the FIRE-reflection profile in Central Finland show 3408-7250 m/s (Vs) and 2460-4140 m/s (Vp). The calculated velocity anisotropy values of these samples vary from 22% to 2% with applied pressure (up to 102 MPa).