Long term investigations at the Mont Terri rock laboratory of tilt and their near and far field influences

Underground research laboratories provide advantageous conditions to observe a broad range of various rock parameters to characterise rock matrix and geological features, and to enhance knowledge of their dynamic behaviour, all under relatively undisturbed conditions. One of their favorable features is that the overburden protects against large environmental changes, although those influences cannot be mitigated in full. Especially for long term investigations, a holistic observation of ambient environmental parameters is necessary on the local scale and beyond.

The Swiss Mont Terri rock laboratory is situated in the Jura Mountains about 250 m below the surface. Starting in 1996, the international Mont Terri Consortium has conducted about 150 experiments in the native Opalinus Clay. Embedded in several of the ongoing in situ experiments, platform tiltmeters assist in the often interdisciplinary investigations. Two different types of biaxial instruments with resolutions of 0.1 urad and better than nrad are distributed throughout the laboratory, together forming a small, growing array, with the first tiltmeters installed in April 2019.

Tiltmeters observe the direct local deformation, they are exposed to near field but also far field impacts. Known local influences are mainly temperature, air pressure, and humidity. In Mont Terri, all of these parameters are registered directly at the location of the tilt sensors with the same relatively high sampling of once every few seconds. In addition, Mont Terri's comprehensive database imparts valuable complementing information. However, the detected deformation pattern is also influenced on a much larger spatial scale, e.g. far field, extensive changes in weather patterns, earth tides, and teleseismic events.

Therefore, to allow detection, identification, and realistic interpretation of complex signal responses on different spatial scales, it is mandatory to distinguish transient and long term signals, natural and anthropogenic disturbances. Their understanding is essential for the evaluation of stability and the safety of a rock laboratory for the benefit of its personnel and visitors. Obviously, long term, continuous data series require long term commitments. But the efforts pays off, not the least, as decade-long deformation studies contribute to multifaceted technical and scientific aspects of long term behavior of barrier rocks, and these are relevant for the exploitation of the deep geological subsurface such as nuclear waste disposal, geological storage of carbon dioxide, use of geothermal energy, or inter-seasonal thermal energy storage.