Muon imaging and monitoring at the Mont Terri underground rock laboratory, Switzerland.
Résumé
Imaging geological objects by quantifying the screening effect they have on the natural flux of cosmic muons is a powerful method that complements standard geophysical tools in the understanding of the Earth's subsurface. Radiographies of the average density of geological bodies along the muon paths can be retrieved from muon measurements, allowing to image large volumes of rock from a single observation point. Furthermore, continuous muon measurements allow us to monitor density variations in a geological object, by tracking the time changes in the outgoing flux of muons. The Mont Terri underground rock laboratory is located in the Jura Mountains range, at a depth of around 300 meters below the Earth's surface in north-western Switzerland. Its structural setting is dominated by an anticline of Triassic-Jurassic formations, whose spatial distribution is finely characterized given the numerous boreholes and outcrops studied inside and outside the tunnel. In this work we analyze 5 muon imaging experiences performed from inside the rock laboratory from 2013 to 2017. From this dataset we obtain corresponding muon radiographies of the average density between the tunnel and the surface of the Mont Terri site at extensive directions. Moreover, at one site the muon detector continuously recorded data for a whole year. We use these data to study the density variations due to changes on the rock's fluid saturation over time in different regions of the mountain. Finally, we perform a joint interpretation with complementary geological, meteorological and geophysical data from the site.