A Seismic Network to Monitor the 2020 EGS Stimulation in the Espoo/Helsinki Area, Southern Finland

Abstract

We present the deployment of a seismic network in the Helsinki capital area of Finland that was installed to monitor the response to the second stimulation phase of an ∼6-kilometer-deep enhanced geothermal system in 2020. The network consists of a dozen permanent broadband stations and more than 100, predominantly short-period, temporary stations. This 2020 deployment is characterized by a mix of single stations and arrays with diverse configurations. It covers a larger area and exhibits a smaller azimuthal gap compared with the network that monitored the first stimulation in 2018. We surveyed the outcropping rocks at one of the large array sites to study surface expressions of shear or weakness zones that are possibly connected to the stimulated volume at depth. We link the relatively large number of macroseismic reports received during the stimulation to an increased public awareness of the project together with an increased sensitivity because the second stimulation occurred during the local COVID-19 mobility restrictions. The spatial distribution of the reports seems to be controlled by the radiation pattern of the induced earthquakes and hence by the stress state in the reservoir. The continuous records contain strong energy at high frequencies above 50 Hz that is attributed to anthropogenic processes in the densely populated urban area. However, the exceptionally low attenuation of the bedrock yields good signal-to-noise ratio seismograms of the induced small events, the largest of which was magnitude ML 1.2. The signal quality of the obtained noise correlation functions is similarly very good. The data set has been collected to underpin a wide range of seismic analysis techniques for complementary scientific studies of the evolving reservoir processes and the induced event properties. These scientific studies should inform the legislation and educate the public for transparent decision making around geothermal power generation.

Limited earthquake interaction during a geothermal hydraulic stimulation in Helsinki, Finland

“…The obtained mechanisms are similar to those obtained by Rintamäki et al (2021) for the two largest events from the same stimulation using FOCMEC software and local and regional polarity data (strike/dip/rake = 140°/58°/26°). On average, calculated focal mechanisms are rotated 20° clockwise in fault strike, whereas dip and rake are the same (within the uncertainties obtained by sampling of input data) when compared to focal mechanisms obtained by Rintamäki et al (2021). The observed rotation is well-explained by a relatively weak control on the strike and rake of mechanisms due to the limited number of stations at larger epicentral distances.…”

“…A total of 2,875 m 3 of water were injected into the open-hole section of the OTN-2 well in the depth interval 4,856-5,765 m b.s.l. (Figures 1b and 1c) (Rintamäki et al, 2021) to establish communication between two wells. Similar to the 2018 stimulation campaign performed in well OTN-3, the 2020 stimulation was flow-rate controlled.…”

“…In this study we analyze the induced seismicity associated with a follow-up stimulation campaign performed in May 2020 (Rintamäki et al, 2021). We first develop a high-resolution seismicity catalog and then analyze the seismic activity in response to the injection operations in 2020 and compare it to the previous massive stimulation campaign in June-August 2018 (e.g., Kwiatek et al, 2019).…”

Numerical simulations of seismo-acoustic nuisance patterns from an induced M1.8 earthquake in the Helsinki, southern Finland, metropolitan area

“…The seismic response to the stimulations has been monitored by a 2 km deep borehole string, 12 shallow borehole stations (Kwiatek et al, 2019), and a surface station network consisting of about 100 sensors organized as single stations and in mini arrays (Hillers et al, 2020;Rintamäki et al, 2021). Most stations were installed within 20 km distance from the wellhead.…”

“…Compared to the ground shaking that is potentially linked to stimulations, the excitation of annoying audible effects typically appears to be of minor (Megies and Wassermann, 2014) or no concern (Majer et al, 2012). However, the 2018 and 2020 deep geothermal stimulation experiments in the Helsinki metropolitan area led combined to more than 300 macroseismic reports of felt and heard earthquake effects (Ader et al, 2019;Hillers et al, 2020;Rintamäki et al, 2021;Lamb et al, 2021). This demonstrates that persistent earthquake noise has the potential to negatively affect the public attitude towards stimulation activities (Stauffacher et al, 2015).…”