Seismic Network Development for Monitoring an Engineered Geothermal System in Finland

Date: 4/26/2019

Time: 11:30 AM

Room: Cascade I

We introduce here the development of a 3-tier seismic monitoring network around an EGS project a few km from the center of Helsinki, Finland. This network telemetered seismicity data back to a central processing system in near realtime for regulatory and industrial purposes. We describe the network’s design, special features, and use in meeting government-set Traffic Light System limits. As described in a subsequent talk, this system was to keep a ~ 6 km deep, 18,160 cubic meter hydraulic stimulation from producing a TLS limit M2.1 earthquake.

Somewhat paradoxically, in Finland’s seismically quiet south, special instruments and installations are needed to observe both hazardous and industrially useful seismicity. The paradox arises from the fact that each day there are hundreds of manufactured events each day – from construction of a metro to housing, shopping, and roadways. The resulting Gutenberg-Richter statistics suggested that the network’s sensitivity needed to be between -1 < M < 0 to be useful to the TLS system.

This sensitivity can be reached with borehole seismometers. A depth of 350 m was selected for a 1st tier, 8-station net of 4.5 Hz 3-C sensors. Data from this net revealed an even more overwhelming stream of construction noise. Moreover, the detection threshold at 350 m was noticeably higher than expected. Consequently, 4 new ones were added, 3 were place at greater depths: 666 m, 1133 m, and 1148 m. A 2nd tier, 12-level, array of seismometers was installed between 2200-2650 m in a borehole ~10 m from the stimulation well. The network was completed with a 3rd -tier of 17 ground-motion stations in buildings and outcrops around the stimulation site.

This monitoring system met its goals, automatically detecting and locating 5,806 ~ -1 < M < 2 events over 49 days of pumping a net volume of 18,160 m3 of drinking quality water.

Presenting Author: Peter E. A. Malin

Authors