Nagra has been drilling in the Zürich Nordost region for close to a year. The Trüllikon borehole was completed in the spring, and drilling in Marthalen is expected to take only a few more weeks. Whether additional boreholes will be drilled in the Zürcher Weinland has not been decided yet. “The deep boreholes in Trüllikon and Marthalen as well as an earlier one in Benken have already provided us with important new data, and we have not yet decided whether it will be necessary to drill additional boreholes in Zürich Nordost”, explains Tim Vietor, member of Nagra's Executive Board.
The topic under discussion is a borehole in Rheinau. East of Rheinau, at the margin of the geological siting region, Nagra suspects the presence of so-called “steeply-dipping faults” deep underground. Nagra has obtained evidence of such faults from earlier investigations, so-called 3D seismic measurements, that can be compared to ultrasound investigations. “Steeply-dipping” means that the faults have a vertical trend.
By drilling a borehole, Nagra can investigate whether and how these faults run through the Opalinus Clay and whether and how they have impacted the properties of the Opalinus Clay in this region. Nagra plans to eventually construct a deep geological repository in the Opalinus Clay.
Whether a borehole in Rheinau or another location can clarify the impact of such faults has not yet been determined. Before the Rheinau drill site can be constructed, the Canton stipulates archaeological investigation of the area. Depending on the findings, these can be time-consuming, and, to allow Nagra to drill the borehole without delay, the archaeological investigations will already be conducted over the next few weeks. Archaeologists use a small excavator to dig ditches that allow them to explore the topmost ground layers and the history concealed in them.