THE ROLE OF ACOUSTIC-EMISSION IN THE STUDY OF ROCK FRACTURE

The development of faults and shear fracture systems over a broad rang e of temperature and pressure and for a variety of rock types involves the growth and interaction of microcracks. Acoustic emission (AE), wh ich is produced by rapid microcrack growth, is a ubiquitous phenomenon associated with brittle fracture and has provided a wealth of informa tion regarding the failure process in rock. This paper reviews the suc cesses and limitations of AE studies as applied to the fracture proces s in rock with emphasis on our ability to predict rock failure. Applic ation of laboratory AE studies to larger scale problems related to the understanding of earthquake processes is also discussed. In this cont ext, laboratory studies can be divided into the following categories. 1) Simple counting of the number of AE events prior to sample failure shows a correlation between AE rate and inelastic strain mte. Addition al sorting of events by amplitude has shown that AE events obey the po wer law frequency-magnitude relation observed for earthquakes. These c umulative event count techniques are being used in conjunction with da mage mechanics models to determine how damage accumulates during loadi ng and to predict failure. 2) A second area of research involves the l ocation of hypocenters of AE source events. This technique requires pr ecise arrival time data of AE signals recorded over an array of sensor s that are essentially a miniature seismic net. Analysis of the spatia l and temporal variation of event hypocenters has improved our underst anding of the progression of microcrack growth and clustering leading to rock failure. Recently, fracture nucleation and growth have been st udied under conditions of quasi-static fault propagation by controllin g stress to maintain constant AE rate. 3) A third area of study involv es the analysis of full waveform data as recorded at receiver sites. O ne aspect of this research has been to determine fault plane solutions of AE source events from first motion data. These studies show that i n addition to pure tensile and double couple events, a significant num ber of mom complex event types occur in the period leading to fault nu cleation. 4) P and S wave velocities (including spatial variations) an d attenuation have been obtained by artificially generating acoustic p ulses which am modified during passage through the sample.