Elevation and Gravity Changes at Geothemal Fields on the Reykjanes Peninsula, Sw Iceland

Abstract

On the outer part of the Reykjanes Peninsula in SW Iceland are three high temperature geothermal fields, Svartsengi, Eldvorp and Reykjanes. These have been monitored with precise elevation and gravity measurements since 1976. In Svartsengi a power plant has been in operation since 1975 with an annual 8 Mt (109 kg) production. The Eldvorp geothermal field west of Svartsengi has not been utilised, and is connected to the Svartsengi field. The third field is on the tip on the Reykjanes Peninsula where the Mid-Atlantic Ridge comes onshore. A geothermal field with minor productivity but great potential. At Svartsengi, land has subsided over an area of 100 km2, which is much more extensive than the borefield, where the subsidence is highest, i.e. 237 mm from 1976 to 1999. This corresponds to 10 mm/year. Subsidence rate was highest right after the exploitation started in Svartsengi in 1975 or 14 mm/year, but decreased to 7 mm/year during the time period from 1987 to 1992. From 1992 to 1999 the subsidence rate has increased again to 14 mm/year. The subsidence bowl in Svartsengi has a NNA-SSW direction, extending to the Eldvorp geothermal field where the subsidence has been 4-10 mm/year. At the Reykjanes geothermal field the subsidence rate has been around 6 mm/year since 1986. The subsidence at Svartsengi varies linearly with observed pressure decrease as observed at 900 meter depth in boreholes, indicating that the subsidence is mainly due to compaction of pores space in the rock matrix. Gravity measurements on the outer part of the Reykjanes Peninsula show little changes. Those measurements have mainly been focused around the Svartsengi field, where the average maximum gravity reduction, after corrections for subsidence, is 5 µgal/year. Calculations using Gauss law for the integrated gravity change indicates that the geothermal field in Svartsengi has 70% natural recharge