Seismic monitoring network design in the Lower Rhine Embayment using pre-existing dense deployments

Geothermal energy offers great potential for supplying heat to the large district heating networks common in central Europe and is a viable substitute for the heat fed into the district heating by coal-fired power plants that will soon be decommissioned. In the Lower Rhine Embayment, efforts are ongoing to explore the region's geothermal potential. To facilitate any future drilling and testing activities, we contribute in a seismotectonic prescreening of the study area. However, because of various anthropogenic noise sources, including open pit mining, industrial activity and densely populated urban areas, the region has an elevated noise level, influencing the seismological data quality. Furthermore, soft sediments dipping towards the northeast cause local site amplification effects. The thickness of these sediments varies greatly from almost non-existent up to 600 m. Thus, noise levels and general data quality differ strongly across the region. To overcome these restrictions, we have gradually optimized and focused our seismological monitoring in the region over the last couple of years.

As an addon to existing, permanent monitoring stations from Earthquake Observatory Bensberg (FDSN-network code BQ), Royal Observatory of Belgium (BE), the Royal Netherlands Meteorological Institute (NL), and the Geological Survey of North Rhine-Westphalia (NH), we first deployed a dense network (ZB) operating from 2021 to 2022 and consisting of 48 broadband and short period stations to investigate background seismicity and the spatially resolved noise level using probabilistic power spectral densities (PPSD). The PPSDs show the correlation of the noise level with the sediment thickness and reveal high seismic noise, especially in the north-eastern region. The seismicity is moderate and concentrated in the western part.

Following the ZB network, we currently operate a research network (YV) of eight broadband stations. The station locations were optimized using the information gained from the ZB network to deploy the best quality stations with a focus on the area where most natural seismicity occurs. One broadband sensor was deployed in a 100 m deep well, about 30 meters below the softer sediments in the center of the region to remove the effect of site amplification and anthropogenic noise as much as possible.

To prepare upcoming drilling activities, additional five stations are deployed surrounding the potential drill site at the Weisweiler power plant. We interpolate I95 values to find potential station locations with a low noise level, based on the noise levels observed on the ZB network and taking into account the existing YV stations. Different network geometries resulting from the potential locations are investigated for optimum accuracy and magnitude of completeness up to the anticipated drilling depth of about 3 km.

The optimal model promises a magnitude of completeness of down to 0.5 in the target region at the Weisweiler power plant at a depth of 3 km, showing that all potential events with a magnitude > 0.5 will be detectable. We present the used workflow, show modeling results and the evolution of data quality over the last years.