Description: In autumn 2017 a network of 14 broadband seismic stations was deployed at the Theistareykir high temperature geothermal field (NE Iceland). This experiment was conducted as part of the current efforts to characterize the field's main structures, and possible short and long term stress variations due to the ongoing fluid injection and extraction operations which started in autumn 2017. In this work, we use two years of continuous seismic records (October 2017-October 2019) to compute a 3D shear wave velocity model of the geothermal field and to detect possible crustal stress changes related to the injection and production activities. From phase cross-correlations of the vertical component recordings, we measure the Rayleigh wave group velocity dispersion curves to obtain 2D group velocity maps between 1 and 5 s. Subsequently, we use a neighborhood algorithm to retrieve the 3D shear wave velocity model of Theistareykir. Mainly, two sets of elongated high and low velocity anomalies can be observed oriented in a NW/WNW direction, parallel to the lineaments of the active Tjörnes fracture zone. Velocity reductions west of Ketilfjall and at Baerjafjall could indicate the location of upflow zones of the magmatic reservoir or hydrothermal system. We analyzed the temporal evolution of phase and amplitude of phase auto-correlations using the stretching technique and discuss their behavior in relation to the geothermal field operations. We notice a slightly stronger long-term velocity decrease in the reservoir region compared to outer regions. This could be related to the mass depletion in that area (higher fluid extraction compared to the water reinjection). In summary, our findings show how a monitoring network can be set up to enable a detailed imaging and monitoring of reservoir behavior in general.

Description: Continuous passive seismic monitoring is carried out between September 2017 and December 2021 around the Theistareykir geothermal area located at the intersection between the active Northern Rift Zone and the active Tjörnes Fracture Zone in NE Iceland. This experiment, in addition to an extensive gravimetric monitoring survey, was conducted in the framework of the MicroGraviMoTiS project for a better understanding of the structures and behavior of the local geothermal system under exploitation and for further development of local and regional geothermal resources. 14 broadband stations (Trillium C-120s) recording at 200 Hz comprise the temporary network, that is installed to complement stations of the national seismological network of IMO and stations of Landsvirkjun, the National Power Company of Iceland. The stations were placed in and around the producing zone to primarily retrieve local natural and/or induced seismicity associated to the injection and production operations. The retrieved seismic data is also used for obtaining a representative 1D velocity model of the region, for computing a seismic ambient noise tomography, and for monitoring the system using coda wave interferometry techniques. Funding for this project is provided by the German Federal Ministry for Education and Research (MicroGraviMoTiS , BMBF, grant: 03G0858A), the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences and Landsvirkjun. Waveform data are available from the GEOFON data center, under network code 3P, and are embargoed until December 2025.

Description: IMAGE is a two year seismological experiment realized in Reykjanes Peninsula by Philippe Jousset (GFZ Potsdam), Gylfi P. Hersir (ISOR Iceland). Reykjanes is the south west tip of Iceland, the emergent part of the medio-oceanic Ridge. This area is probe to many earthquakes and is exploited for its high geothermal potential. The deployment is performed to carry out local seismological study, such as seismic tomography (both earthquake based, e.g. Jousset et al., 2016, or ambient noise e.g., Martins et al., 2020). The aim of the seismic experiment is to monitor the seismic activity associated with the rift seismic activity and the seismicity (Blanck et al., 2020) or the induced seismicity. The network comprised 30 onland stations (GIPP) and 24 OBS (Lobsters, DEPAS),. Onland stations were deployed from April 2014 until august 2015 and comprise 20 broadband seismic stations (Nanometrics Trillium Compact 120 s), 10 short-period sensors (Mark sensors 1 Hz) and data loggers (data cube) with acquisition frequency 200 Hz. Sensors were buried 30–40 cm deep in the ground in containers. Data gaps are minimal, just occurred every 3 months when the batteries were exchanged and data downloaded from the cubes. OBS were deployed in August 2014 and recorded for about a year. From this dataset, converted to MSEED using GIPP tools for the onland part, a catalogue of about 2000 earthquakes could be extracted. Waveform data are available from the GEOFON data centre, under network code 4L.

Description: Continuous high-resolution gravimetry is increasingly used to monitor mass distribution changes in volcanic, hydrothermal or other complex geosystems. To quantify the often small target signals, gravity contributions from, e.g. atmospheric mass changes, global and local hydrology should be accounted for. We set up three iGrav superconducting gravity meters for continuous monitoring of the Þeistareykir geothermal field in North Island. Additionally, we installed a set of hydrometeorological sensors at each station for continuous observation of local pressure changes, soil moisture, snow and vertical surface displacement. We show that the contribution of these environmental parameters to the gravity signal does not exceed 10 µGal (1 µGal = 10–8 m s−2), mainly resulting from vertical displacement and snow accumulation. The seasonal gravity contributions (global atmosphere, local and global hydrology) are in the order of ± 2 µGal at each station. Using the environmental observations together with standard gravity corrections for instrumental drift and tidal effects, we comprehensively reduced the iGrav time-series. The gravity residuals were compared to groundwater level changes and geothermal mass flow rates (extraction and injection) of the Þeistareykir power plant. The direct response of the groundwater levels and a time-delayed response of the gravity signal to changes in extraction and injection suggest that the geothermal system is subject to a partially confined aquifer. Our observations indicate that a sustainable “equilibrium” state of the reservoir is reached at extraction flow rates below 240 kg s−1 and injection flow rates below 160 kg s−1. For a first-order approximation of the gravity contributions from extracted and injected masses, we applied a simplified forward gravity model. Comparison to the observed gravity signals suggest that most of the reinjected fluid is drained off through the nearby fracture system.