Background Seismicity Monitoring to Prepare for Large-Scale CO2 Storage Offshore Norway

Prior to planned CO2 injection startup in the Horda platform offshore western Norway, in 2024, the Horda Network project has taken several measures to assess the potential of seismic hazard in the area. A study of the fault-plane solutions in the Horda platform region confirms that the direction of maximum horizontal stress is dominantly north-west-southeast to east-west over the entire area. The relative stress ratio is higher in the southeast near the Norwegian craton and lower in the northwest. Analysis of the catalog of seismicity (in the period of 2001-2021) in the Horda platform region suggests a moderate rate of seismicity with a b-value of similar to 1. The magnitude of completeness is 1.5 (ML). One of the main challenges in monitoring offshore earthquakes in the Norwegian continental shelf (NCS) is the lack of azimuthal coverage when using the onshore permanent seismic stations from the Norwegian National Seismic Network (NNSN), located to the east of offshore events. To improve the azimuthal coverage, we integrated a limited number of offshore geophones from permanent reservoir mon-itoring systems of selected oil and gas fields (Grane and Oseberg on NCS) with the onshore NNSN seismic stations. This integration is challenging because of the level of ambient noise in the offshore geophones. To further improve the detection and loca-tion capability, we deployed a nine-element onshore array of broadband seismometers (HNAR) on Holnsnoy island to the east of the Horda platform. By incorporating array processing methods on HNAR, the signal-to-noise ratio is improved, and several previ-ously uncataloged earthquakes could be detected. Offshore sensors are often subject to correlated noise from seismic interferences and platform or shipping noise sources, so we also incorporated array processing for selected geophones from offshore deploy-ments, which greatly reduced such noise and hence improved the event detection.