Micro-Gravity Monitoring and Repeated GPS Survey at Hatchobaru Geothermal Field, Central Kyushu, Japan

Abstract

Hatchobaru geothermal field is located 5km northwest of Kuju volcano, central Kyushu, and it shows a fractured type of geothermal reservoir. We have been conducted GPS and micro-gravity monitoring to detect the ground deformations and mass fluid movement caused by the production and reinjection.
Increases in gravity were observed in the reinjection zone and part of the production zone just after the commencement of No. 2 unit. After that, in the production area, a rapid decrease of gravity (up to 200 gal) was observed. Since 1998 the gravity change became steady. A contour map of recent gravity changes show that the areas of gravity changes became very small. And these areas are limited in the part of the production and reinjection zone. We suppose that the recharge from surrounding area became gradually stable and reached a new equilibrium state.
Changes of baseline up to 64 mm, were detected from August 2000 to October 2003. We tried to estimate the location of pressure source applying a point source model on the changes in length of baselines. The location of a pressure source is estimated to be Komatsuike sub fault and approximately 750m in depth. This pressure source is located the upper zone of reservoir. There is possibility of detecting the pressure change of reservoir. As a result of observations, the sufficient accuracy proved to be achieved in this case of sort baseline observation.