Elastic anisotropy of layered rocks: Texture-based theoretical predictions (effective media modeling) versus ultrasonic measurements of gneiss

摘要

We present experimental and theoretical studies on a highly anisotropic layered rock sample characterized by alternating layers of biotite, muscovite, plagioclase and quartz, respectively. We applied two different experimental methods to determine seismic anisotropy at pressures up to 400MPa: (1) measurement of P-and S-wave phase velocities on a cube in three foliation-related orthogonal directions and (2) measurement of P-wave group velocities on a sphere in 132 directions On the basis of the crystallographic preferred orientations (CPOs) of major minerals obtained by time-of-flight neutron diffraction, effective media modeling was performed. The implementation of a nonlinear approximation of the P-wave velocity pressure relation was applied to estimate the mineral matrix properties and the orientation distribution of microcracks. The observed discrepancies (about 10%) between the effective media modeling and ultrasonic velocity data are a consequence of the inhomogeneous structure of the sample and inability to perform long-wave approximation. Small differences between elastic moduli predicted by the different theoretical models were observed. It is shown that the bulk elastic anisotropy of the sample is basically controlled by the CPO of biotite and muscovite and their volume proportions in the layers dominated by phyllosilicate minerals.