Effects of Development on Geothermal Systems Deduced from Gravity and Thermal Measurements: Japanese Case Studies

Abstract

Repeated gravity measurements have been conducted at four geothermal fields (Hatchobaru, Takigami, Yamakawa and Oguni) and one erupting volcano (Kuju volcano) in Kyushu in order to monitor the underground geothermal fluid flow system. Characteristic common features of gravity changes were detected both in geothermal fields and in erupting volcano. Gravity decreased rapidly just after the commencement of production of geothermal fluid or phreatic eruption and then decreased gradually. Such a pattern of gravity change shows that the underground fluid flow is reaching to a new equilibrium state. Repeated thermal measurements were started recently in geothermal fields, although they have been conducted at Kuju volcano after the October 1995 eruption. Simultaneous measurements of thermal activity and gravity give more effective information about the changes in the geothermal system.