Exploration Underground Structure by AE Observation in the Hatchobaru Geothermal Area, Japan

Abstract

We have been carrying out AE (acoustic emission) observation since 1995 in the Hatchobaru geothermal area, Kyushu, Japan. The AE observation started using borehole-in a 3-station network and 9-station network completed in 1997. Over a thousand AE and several AE swarms were observed a year by this network.

We tried to estimate the underground structure such as location of faults and fracture-rich zones by calculating the hypocenter, determining the AE source mechanism and AE attenuation. Comparing the AE data with detailed gravity and resistivity data, we find that most of the hypocenters of AE swarms are located in the low-gravity anomaly zone. The strikes of faults estimated by AE mechanical analysis agree with isogal and isoresistivity contours.

AE data are effective for estimating high permeable zone. Some high permeability wells were located near estimated faults in the AE swarm areas. Particularly in low resistivity zones, it is sometimes difficult to locate faults from magnetotelluric data but can be rather effective form AE data.