Laboratory observation of stress and saturation effects on shear wave anisotropy

Abstract Shear-wave splitting, also known as shear-wave birefringence, has been widely used to evaluate subsurface anisotropy. Yet, the effects of stress and fluid saturation on S-wave anisotropy are still not well understood. A laboratory study was conducted on a Bentheim sandstone sample subjected to different stress and saturation conditions. The S-wave stress-induced anisotropy of the sample was measured using a laser Doppler interferometer (LDI), which allows estimating the particle displacement vector at the central point on the surface of the sample. Based on the derived hodograms, the polarization of the fast S-wave and the degree of the S-wave anisotropy were inferred. In the dry sample, a shift in the polarization direction toward the direction of the applied stress and an increase in the anisotropy degree in some cases were observed, most likely due to the preferential opening of cracks in the direction along the applied stress. For the water-saturated case, no significant change in the S-wave anisotropy was observed.