| Abstract |
Yanaizu-Nishiyama Geothermal Field is located in the western part of Fukushima Prefecture, northeast Japan. In this field, sixteen production wells have been drilled. A 65 MWe power station was built and has been operated commercially by Tohoku Electric Power Co., Inc. since May 1995. Okuaizu Geothermal Co., Ltd. has supplied steam to the power station. Gravity monitoring survey was started in September 1994. The original survey area was about 15km2, covered the production and reinjection areas and contained 83 gravity stations. Annual gravity monitoring surveys were carried out from 1994 to 1997 by Okuaizu Geothermal Co., Ltd. and Tohoku Electric Power Co., Inc. Since 1997, New Energy and Industrial Technology Development Organization (NEDO) has sponsored the survey, and the survey area has been expanded by 30km2. In the new survey area, 138 gravity stations were set, and 10 groundwater-monitoring wells were drilled to study the effect of the change in groundwater level. A gravity station and a weather monitoring system were installed at a monitoring well in the central part of the survey area. Seasonal gravity monitoring surveys have been carried out three times a year since 1998. Between the seasonal monitoring surveys, gravity has been monitored near the groundwater-monitoring wells. At these wells, groundwater levels have been monitored every 30 minutes. At the gravity station with the weather monitoring system, gravity, barometric pressure, air temperature, precipitation and water content in the vadose zone have been monitored together with groundwater level. From examination of gravity and barometric pressure data recorded in the central part of survey area, a precise earth tidal model and correlation between gravity and barometric pressure were established. These models enable us to reduce ìnoiseî in gravity monitoring results and to study the effect of groundwater level change on gravity monitoring. |