INTRODUCTION
The development of faults and shear fracture systems over a broad range of temperature and pressure and for a variety of rock types involves the growth and interaction of microcracks. As a result, acoustic emission (AE) is a ubiquitous phenomenon associated with brittle fracture and has provided a wealth of information regarding the failure process in rock. This papereviews the successes and limitations of AE studies as applied to the fracture process in rock with emphasis on our ability to predict failure. We will also comment on how laboratory AE studies can be applied to larger scale problems regarding the understanding of earthquake processes.
LABORATORY AE STUDIES
AE techniques have been applied to a broad range of laboratory fracture studies. Some of the important recent advances are discussed here. Additional areas to be considered include such topics as the observation of fault propagation from 3D AE hypocentral locations and determination of dilatancy and fluid penetration through the use of acoustic tomography. AE counts, inelastic Ssrain and Omori's law
The simplest detection system, and one that can easily be constructed, involves attaching a piezoelectric transducer to the sample assembly and counting acoustic emissions. Since this technique gives equal weight to all events regardless of size, a common modification involves recording peak amplitude and ringdown count (which indicates the duration of the wave packet) to obtain a rough estimate of the energy radiated by the event. With just a single transducer, however, corrections for attenuation and radiation pattern of individual events cannot be made and energy estimates of individual events may not be accurate. In any case, good correlations have been reported between inelastic strain rate and AE rate. The correlation between inelastic strain and AE can be used to relate earthquake aftershock sequences to transient creep behavior in rock. Aftershock sequences have been found to decay with time according to an empirical relation
[Equation available in full paper] (1)
known as Omori's law. Here K, c and p are constants, and v is the acoustic emission rate. Omori's law has been used [1] to estimate the probability of damaging aftershocks following large earthquakes. Lockner and Byerlee [2] conducted creep tests on granite and sandstone samples at 100 MPa confining pressure and stress levels from 45 to 95% of failure strength. They found a decreasing decay rate for AE activity with increasing stress level. However, they showed that during transient creep, AE rate obeyed
