Impact of the geological setting of the overburden on long-time series recorded at underground geophysical observatories: case study from the FSU Jena Geodynamic Observatory Moxa (Thuringia, central Germany

To achieve very low ambient noise and thus very good conditions for long-term geophysical observations at a high level of instrumental accuracy in order to decipher also faint signals from Earth and environmental processes, sensors often are installed in the subsurface in galleries or in boreholes. This however, makes it necessary to consider the potential influence of the geological setting and properties of the surrounding rock formations and overburden.Moxa Geodynamic observatory, located in a remote part of the Thuringian slate mountains, approximately 30 km south of Jena, provides an ideal setting to address this topic as it comprises two galleries, which are running perpendicular to each other. As the observatory is built at the toe of a relatively steep slope, coverage of the galleries varies along them. Further, the tectonic structure and hydrological settings of the overburden is rather complex.Instruments sensitive to deformation, which include three laser strain meters measuring nano-strain, borehole tiltmeters and a superconducting gravimeter CD-034, together with other instruments, e.g. a node for the Global Network of Optical Magnetometers for Exotic physics (GNOME), are installed in various positions in the building of the observatory, close to the building, and in the galleries. The laser strainmeters record along three galleries in north-south, east-west and NW-SE directions. Further, information on fluid flow is gained from downhole temperature measurements employing an optical fiber and several groundwater level indicators, some of them installed in shallow boreholes. Additionally, information on environmental parameters is coming from a climate station and on the subsurface tectonic structure from various near surface geophysical data sets. Here, we present first results of an ongoing project which combines actual deformation recordings, structural and drillhole information to decipher how the tectonic structure of the and groundwater movement within the overlying slope on top of the observatory’s galleries may impact on the various instrumental recordings.