Characterization of anisotropic elastic wave behavior in media with parallel fractures and aligned cracks

Abstract

Ultrasonic experiments were performed to investigate the anisotropic properties of an isotropic background medium with a system of long closely spaced parallel fractures. Immersion experiments at frequencies from 0.1‐0.7 MHz were performed on Plexiglas that was cut into thin sheets and then pressed together. Measurements of phase and group velocity were obtained for compressional and shear waves both normal to and parallel to the “fractures” and for quasicompressional and quasishear waves at a variety of oblique angles. The data were analyzed to determine the anisotropy parameters due to the fracturing, based on modeling the fractures as a set of infinite, linear, although not necessarily rotationally symmetric, slip interfaces [M. Schoenberg, J. Acoust. Soc. Am. Suppl. 1 82, S88 (1987)]. For the most part, the experiments fit the theory quite well even though losses that can occur in the fractures have been ignored in this first attempt to match theory and experiment. The anisotropy attributable to the fracture system depends on the geometry of the fractures, the roughness, the static closing stress across the fractures, and the nature of the infilling medium, if any. From a knowledge of the anisotropic elastic moduli of the fractured medium and the isotropic background moduli, the average compliance matrix may be deduced. In turn, this gives the average behavior of each fracture plane.