Panel (a) is a map view of the Mont Terri lab with location of the DR-D experiment in Gallery 18. An arrow is indicating the view direction for (b). The geological formations around the rock laboratory are indicated by colouration along tunnels and galleries (Graebling et al. 2022). (b) 3-D horizontal view onto the rock laboratory along Gallery 18. The curved planes in light and dark brown indicate the positions of the lower and upper boundaries of the upper sandy facies of Opalinus Clay. The position of the experimental plane of the DR-D experiment is indicated. An inset shows the location of the Mont Terri laboratory in North-Western Switzerland (red dot). (c) Detailed 3-D perspective view onto the experimental plane with boreholes (BDR-D1 and BDR-D2) and seismic receiver line on the floor of Gallery 18 (dots) indicated.