京都大学21世紀COEプログラム 活地球圏の変動解明 アジア・オセアニアから世界への発信
Seminar Series of Active Geosphere #24: September 16, 2005
by Jun Nishijima (Special Lecturer)
Faculty of Engineering, Kyushu University
"Micro-gravity monitoring and repeated GPS surveys at some geothermal power plants in Kyushu, Japan"
It is necessary to grasp a condition of the geothermal reservoir to maintain the stable generating of geothermal power plant for a long term. Generally, the periodical leveling is frequently conducted at the developed geothermal area for the maintenance of geothermal reservoir, because ground deformation directly reflects condition of geothermal reservoir. We have been conducted the observations of ground deformation and gravity changes in order to clarify the geothermal fluid movement caused by the production and reinjection.
Repeat gravity measurements have been conducted at four geothermal fields (Takigami, Oguni, Hatchobaru and Yamakawa) in Kyushu, Japan. 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.
Changes of baseline up to 47mm were detected from August 1998 to October 2002. 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 700m in depth.
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 micro gal) was observed. About three years later, the rate of gravity decrease became smaller. Since 1997 the rate of gravity decrease became larger. The gravity changes show there is the zone of gravity decrease around the production zone, especially towards to southern part of production zone. The pattern of gravity changes in the production zone is very similar to that of reservoir pressure, and there is good correlation between gravity change and reservoir pressure.
This result shows that the decrease of gravity in the production zone reflects the net mass loss in the reservoir.