Reflexionsseismik zur Erkundung des Opalinustons in der Nordschweiz

The investigations of potential siting areas for a repository for high-level radioactive waste which are concentrated in the crystalline basement of Northern Switzland have been expanded since the late 1980s to include suitable sedimentary units. After extensive desk study evaluation, the Opalinus Clay of the Tabular Jura east of the Aare river was chosen as the most promising sedimentary option. In the area North of Lageren - Zurich Weinland, where the Opalinus Clay is situated in the optimum depth range of 400 to 1000m below surface, approximately 220 km of new high-resolution seismic profiles were recorded and interpreted together with existing seismic lines. Due to thorough field work and data processing, a very good quality of seismic lines was obtained, allowing precise mapping of the marker horizons. The regional seismic character of the Middle Mesozoic units was modelled using borehole data from Weiach and Herdern. From this model the thickness of the Opalinus Clay along the new seismic lines was determined. The results indicate a relatively constant thickness of 95 to 120 metres in the investigation area. By way of interactive interpretation of all available seismic lines, borehole data and surface data from the investigation area, depth maps of the marker horizons have been calculated and geological cross sections constructed along the new seismic lines. Finally a geological model for the region between the Aar river in the west and Lake Constance in the East was designed. Taking the depth map «Base of Opalinus Clay» as an example the four tectonic units of the area and their structural character are explained. Relying on these results, the most promising area lies between the rivers Rhine and Thur and the Cholfirst hill in the Zurich Weinland, which will be examined by a 3D seismic survey and a deep well.