1 INTRODUCTION
We show that shear waves are especially sensitive to crack formation when the wave particle motion is perpendicular to the fracture plane, because these shear waves couple into interface waves that propagate along a fracture. Long before catastrophic failure, when a macroscopic fracture is formed, the energy in shear wave signals shows a dramatic frequency shift. This frequency shift is a signature of the partitioning of energy out of a bulk wave and into interface waves. Because this signature is observed prior to failure, it suggests the presence of an incipient interface wave that is supported by the network of oriented but disconnected micro-cracks.
Seismic wave propagation through rock masses approaching failure is affected by the formation of micro-cracks, and appears as a modification in the modulus of the rock which affects seismic velocities and introduces anisotropy (Rothman, 1975; O'Connell & Budianski, 1974; Jaeger & Cook, 1979; Crampin, 1981; Zheng, 1989; Sayers, 1990; Hudson, 1991). However, more specific information about the approach to failure can be obtained by taking advantage of the tendency for microcracks to predominantly orient along the plane of principal stress. Seismic waves propagating along the plane of failure would lead to the inception of interface waves that would propagate along the plane of weakness and that would be sensitive to changes in the network of oriented but discontinuous microcracks. The characteristics of interface waves that propagate along fractures are now well established (Pyrak-Nolte & Cook, 1987; Nagy, 1991; Pyrak- Nolte et al., 1992; Gu, 1994; Ekern et al., 1995). It has also been shown that these fractures are sensitive to fracture properties, making them sensitive probes of the physical condition of the fracture (Pyrak-Nolte et al., 1992; Roy & Pyrak-Nolte, 1995; Ekern et al., 1995). For instance, the velocity of interface waves depends on the specific stiffness of the fracture. For a rock undergoing failure, where the micro-cracks are orient along a plane, the stiffness of this plane would decrease as failure is approached, and the frequency content of an interface wave would be expected to shift to lower frequencies. In this paper, we investigate the seismic signature of the inception of interface waves during the growth of microcracks from Brazil-mode failure.
Several cores of Berea sandstone were used to ascertain the seismic precursor to failure. Table 1 gives the dimensions of the cores, BS15 and BS16, presented in this paper. A sketch of the experimental set-up is shown in Figure 1. The sample is placed in a load frame which applies a line load parallel to the long axis of the core, similar to Brazil testing (Jaeger & Cook, 1979) or split cylinder testing. Piezoelectric transducers for sending and receiving seismic waves are attached to the core by a small frame and are coupled to the core with honey. Two polarizations of shear wave (Sj. & Sn) transducers, with a central frequency of 1.0 MHz, were used to monitor the failure process during Brazil-mode fracturing of the cores.
