Acoustic double refraction in low-porosity rocks

Abstract

Anisotropy in physical properties of rocks can arise from preferred mineral orientation, mineral layering, nonhydrostatic stress, and anisotropic crack distribution. For instance, all of the following cause acoustic double refraction: preferential orientation of olivine grains in dunites, alternating layers in laboratory-sized samples of such mineral pairs as olivine-feldspar, wollastonite-diopside, and garnet-pyroxene, alternating layers of basalt flows and lunar breccias, anisotropy in crack distribution of most granites, and anistropy in crack distribution induced by uniaxial stress. We discuss, both experimentally and theoretically, shear-wave propagation in these rock types and indicate how the laboratory data may be applied to the interpretation of the anisotropy observed in the Earth's crust and upper mantle. We discuss the possibility of elastic anisotropy in the Moon.