Abstract
During the hydrofracturing experiments of the base rocks at Okabe Town, Shizuoka Prefecture, highly sensitive observations of the Acoustic Emissions (AE) were carried out in order to study the fracturing mechanism. A hydrophone, piezoelectric accelerometers and seismometers were used for the observations. A number of AE was observed by the hydrophone placed at the depth of 10m in the boreholes. The results of the observation revealed the following characteristics of the AE, (1) predominant frequency of the initial motions is about 5kHz and that of maximum amplitude is about 1.5-2kHz, (2) wave-form of the initial motions is very sharp and pulse-like, (3) the AEs are initiated a few seconds after the starting of the increase in water pressure, and (4) number of the AE is closely related to the pressure variation.
These characteristics demonstrate that those AEs are caused by the hydrofracturing of the rocks. It is expected that extensive observations of the AE provide valuable data for the identification of the critical fracturing pressure, and for the basic investigation of the fracturing phenomena.
