Lithological control on shear-wave velocity anisotropy in core samples from the Taiwan Chelungpu Fault Drilling Project

Abstract

The present study provides laboratory constrains on the in situ source of shear wave birefringence in siltstone and sandstone lithologies from the Taiwan Chelungpu Fault Drilling Project (TCDP). Bench top measurements at ambient pressure and temperature are performed across the end faces of vertically drilled samples while the polarization angle of the source and receiver is rotated from 0 to 360° in azimuth. Shear wave birefringence in siltstone is low (∼3.5%) and controlled by low angle (∼30°) bedding dip. The sandstone samples exhibit moderate (∼15%) shear wave anisotropy due to a network of vertical microcracks with normal about the bedding strike direction. As related studies infer that the microcracks responsible for shear wave birefringence in sandstone are also open at depth, our results suggest that anisotropy in borehole measurements can simply be accounted for by the observed siltstone and sandstone microstructures, i.e., without invoking larger scale objects, nor randomly oriented cracks/fractures subjected to triaxial stress state.