繰り返し重力測定による地熱貯留層モニタリング

Abstract

After the Great East Japan Earthquake, renewable energies were reconsidered as one of the important electric power sources in Japan. Of the renewable energies, geothermal power generation does not depend on the weather and can ensure stable electric power supplies. There were 17 geothermal power stations in Japan at the end of FY2013, and the present total installed capacity is only half of the capacity of a nuclear power station, although Japan has the third largest geothermal energy potential in the world. In order to guarantee sustainable geothermal power generation, some techniques for geothermal reservoir monitoring are indispensable. Repeated precise gravity measurement is suitable for monitoring reservoirs because it can obtain the planar distribution of temporal gravity changes caused by apparent density changes in the reservoir at low cost. We have conducted repeated precise gravity measurements at some geothermal power station areas not only in Japan, but also in Indonesia and New Zealand, and have detected temporal gravity changes with hybrid measurements that utilize both relative and absolute gravimeters. The main cause of difficulty when using this technique is that the measurement accuracy of our present gravimeters is almost the same as the gravity change caused by the phenomenon we would like to detect. Therefore, we expect that a superconducting gravimeter, which has several thousand times higher measurement accuracy, will lead to a breakthrough for the problem at hand.