EFFECT OF GRAIN-SIZE ON FRACTAL STRUCTURE OF ACOUSTIC-EMISSION HYPOCENTER DISTRIBUTION IN GRANITIC ROCK

Triaxial compression tests were carried out on Inada and Oshima granodiorites under constant loading rate conditions at a confining pressure of 50 MPa. Acoustic emissions (AE) were monitored and recorded using a multichannel system, and the spatial distributions of AE in the two samples were examined and compared as the experimental conditions were identical. Approximately 3000 and 1000 events were located in Oshima (fine-grained) and Inada (coarse-grained) granodiorites respectively, and the fractal dimensions of their spatial distributions were estimated by Grassberger's method. The AE hypocenter distribution of the fine-grained granodiorite showed a single fractal structure with a fractal dimension of 2.7, whereas that of the coarse-grained granodiorite showed a band-limited fractal structure. The fractal dimension of the spatial distribution of Inada granodiorite changed from 2.0 to 2.4 when the length scale r became nearly equal to the average grain size (5 mm) of the rock. These results imply that the spatial distribution of newly developed microcracks, which are the sources of AE and the scatterers of elastic wave, can be influenced by the grain of the rock.