Observaton of Ground Deformation at Geothermal Fields by GPS

摘要

It is important to monitor ground deformation at geothermal fields to maintain the stable production and reinjection of geothermal fluids for a long term. We have been conducting the observation of ground deformation with the single frequency GPS receivers at Hatchobaru geothermal field in Kyushu, Japan, to detect the three dimensional ground deformations caused by the production and reinjection of geothermal fluids. As a result, we detected a horizontal deformation (contraction) of about 15mm in the western margin of the field during December 1998 to July 1999. The location of the pressure source is estimated to be 3km in depth and 1.5km west of the production and reinjection zones. The cause of the contraction is considered to be not the production of geothermal fluids but the change in the local tectonic stress field. We did not detect any meaningful deformation in the production and reinjection zones, which means the recent stable mass balance in the geothermal reservoir. Such results show that the portable GPS system with the single frequency GPS receivers is very effective in the case of short baseline observation.