Browsing by Subject "Induced earthquakes"

The ST1 Deep Heat company has been developing an enhanced geothermal system for district heating in Espoo, Southern Finland by St1 Deep Heat Company. Complete project will consist of geothermal doublet which involves two wells expanding up to 6,1 km depth. The stimulation of the first well was conducted in summer of 2018 as a 7-week period during which the fluid permeability of the crystalline rock at the base of the well was increased by pumping water with high pressures into the well. The stimulation process interacts with the ambient stress field of the subsurface and similar processes have been associated with induced seismicity when the stress changes lead into earthquakes as pre-existing faults and cracks are activated and new ones are created. For the monitoring of the seismic activity associated with the project a seismic network was installed in the Espoo/Helsinki area. The network consisted of 12 borehole installed 3-component seismometers provided by ST1 Deep Heat Company and from 5 semi-permanent surface 3-component broadband seismometers and in total 100 geophones installed by the Institute of Seismology, University of Helsinki (later ISUH). The geophones were installed in 3 large 25 instrument arrays, 3 smaller 4- instrument arrays and as single stations. Data from suitable seismic stations from the Finnish Seismological network was also used in the study. The aim of the study was to map possible differences and trends in the determined event magnitudes between different types of seismic stations used in the seismic network during the stimulation. The effect of different filter choices on amplitude measurement is also studied and discussed for some of the seismic events involved in the study. The results are also compared to the traffic light system warning thresholds of the EGS project and automatically calculated magnitudes of the monitoring system. The Finnish local magnitude formula is also discussed considering near-source events. Reliability and behaviour of the seismic arrays used during the stimulation is studied and discussed. Azimuthal effects noted during the data processing are illustrated. According to the results there appears to be clear differences in estimated magnitudes between the different station types used in the network and differences and trends between the seismic arrays and between individual instruments of single arrays. In total 21 induced events were chosen based on their automatically determined magnitudes, favouring events with large as possible magnitudes. Magnitude range for the events was from ML 1.8 to ML 1,0. The events were picked for P- and S-wave arrivals and for amplitudes estimated from vertical component S wave arrivals. The locations and magnitudes of the events were determined using the currently used practices of the ISUH. Main tools used in the study are the Geotool software for waveform analysis, an ISUH program used for earthquake locating and GNU Octave programs written by the author used in the data procession.