Abstract:
Laboratory measurements of elastic interface waves were made using a synthetic fracture in an aluminum specimen. Elastic interface waves were observed for shear wave polarization perpendicular to the fracture plane but not for shear wave polarization parallel to the fracture plane. Group velocities of the elastic interface waves were measured as a function of stress on the sample. The group velocities of both interface waves increased with increasing stress. The specific stiffness of the synthetic fracture in aluminum is predicted using the measured group velocities of both interface waves.
Fractures play a key role in underground structures and processes. Locating and characterizing fractures by seismic techniques is therefore of great importance to mine stability, production of energy sources in fractured reservoirs, waste isolation, and the study of earthquakes. Though fractures in the Earth are widespread, fundamental issues about their properties must still be addressed. An example is the existence of elastic interface waves along a fracture. A Stoneley wave (Stoneley, 1924) is an elastic interface wave that can travel along a welded interface, i.e., an interface across which stresses and displacements are both continuous. However, a fracture is a non-welded interface, i.e., an interface across which stresses are continuous but displacements are discontinuous. The discontinuity in fracture displacement has been measured quasi-statically in the laboratory by several investigators for normal and shear displacements (Goodman, 1976; Bandis et al., 1983; Swan, 1983; Brown & Scholz, 1985; Yoshioka & Scholz, 1989).
The displacement discontinuity model considers a non-welded interface for wave propagation across a fracture. From this purely elastic model, transmission and reflection coefficients, and group velocities are derived which depend on the frequency of the excitation signal and the ratio of fracture specific stiffness (stress per length) to the seismic impedance of the half-spaces. The displacement discontinuity model has been found to reproduce the effects of fractures on wave propagation for both synthetic fractures (Myer et al., 1985; Pyrak-Nolte et al., 1990a) and natural fractures in rock (Pyrak-Nolte et al, 1990b). In this paper, the theory for existence of elastic interface waves propagating along a fracture will be presented and compared with direct observations on a synthetic fracture in the laboratory.
