Compressional waves guided between parallel fractures

We have performed experiments on the laboratory scale to image seismic wavefronts of compressional waves that are guided between parallel fractures. The wavefronts are detected using water-coupled 1MHz piezoelectric transducers. The stiffness of the fractures is controlled by applying radial stress in a hydraulic stress apparatus that leaves the observation face of the fractured sample open to measurement. The two fractures form a waveguide in isotropic homogeneous aluminum and act as parallel displacement-discontinuity boundary conditions separated by a distance dE2l: The compressional wave is a leaky guided mode that sheds energy into the medium beyond the fractures. However, the fractures provide sufficiently strong energy confinement inside the central waveguide for a sample length L of approximately LE15l to observe the guided wave. Fractures with low fracture specific stifffness produce strong energy confinement in the guided compressional-mode wave. r 2001 Published by Elsevier Science Ltd.