Ultrasonic shear wave birefringence as a test of homogeneous elastic anisotropy

Abstract

Ultrasonic shear wave birefringence is used as a positive indicator of elastic anisotropy. Polarized shear waves are generated by conversion of a compressional P wave to a shear S wave at a free surface (Jamieson and Haskins, 1963). The P-S conversion technique is preferred over the direct use of AC-cut quartz shear wave transducers because the wave amplitudes are considerably greater in the conversion technique. Even though the P/S wave amplitude ratio for the P-S conversion transducers is 1/80, the P wave has sufficient amplitude for recognition. The rotation of the sample allows identification and measurement of the travel times of two orthogonally polarized shear waves. Relative difference in the two shear wave travel times positively establishes homogeneous anisotropy, even though the sample may be nearly isotropic in the P wave mode. Thus P wave velocity and travel path distance need not be known in order to detect elastic anisotropy. Shear wave birefringence is not restricted to any particular scale, and a minimum of one propagation direction may be used to establish anisotropy. A suite of 20 rock samples of various lithologies was measured according to this technique. All samples were found to be anisotropic. These results point out that elastic anisotropy is probably the rule rather than the exception in describing the elastic behavior of rock materials of hand sample size. It is therefore of interest to determine at which scale, if any, elastic anisotropy is no longer the rule but the exception. The use of shear wave birefringence may aid in answering this question.