Long term deformation and seismic observations at the Mont Terri rock laboratory

The Mont Terri rock laboratory, located in the Swiss Jura Mountains, is dedicated to research on argillaceous rocks. Since its founding in 1996, the objective is the hydrogeological, geochemical, and geotechnical characterisation of Opalinus Clay in the context of nuclear waste repositories. More recently, the work has broadened to additional fields, covering potential uses of the deep geological subsurface such as geological storage of carbon dioxide and geothermal energy. With the excellent infrastructure, a comprehensive database, and the broad scientific and technological expertise, knowledge is enhanced e.g. through the advancement and comparison of approaches as well as the development and testing of novel investigation methods. These, as well as studies on feasibility and risk assessment, are of benefit also for underground laboratories in general and in situ explorations in different rock types worldwide. Due to the long-term commitment and the available gallery space of the research facility, elaborate as well as decade-long experiments can be implemented.

In order to detect, quantify, and understand short- and long-term deformations in the Mont Terri rock laboratory, quasi continuous time series are established employing various monitoring techniques. The latter complement each other in regard to their spatial dimensions, operational frequency optima, and their point or integral information. The approach combines

• a 50 m long uniaxial hydrostatic levelling system (HLS, Type “PSI”, positioned along a gallery wall, measuring principle: electrical plate capacitors),
• four mini-arrays of very-broad-band triaxial seismometers, installed in the rock laboratory (one under the HLS) as well as outside the rock laboratory at the surface,
• and an array of high resolution, biaxial platform tiltmeters, with instruments situated close to the HLS and in various parts of the rock laboratory, integrated in other in situ experiments.

The observed signals and their analysis differ in space and time. They range from the detection of local nanoseismic as well as large tele seismic events, to the determination of earth tides, and to the identification of seasonal trends versus other long term geodetic movements. Besides the mutual comparison of the three deformation measurements, the time series provide valuable input for numerous scientific questions such as the stability of the rock laboratory as a whole or in its parts, the influence of excavation, ventilation, or fluid injection on rock matrix and faults. Long data series of ambient parameters, essential for interpretation of the deformation records, such as temperature, pressure, and humidity, are recorded by sensors integrated in the above listed instruments and are also of interest in further experiments performed by the Mont Terri Consortium.