Northern Switzerland is covered by a dense grid of 2D seismic lines (Fig. 2‑2) acquired over several exploration campaigns from the early 1980s by Nagra and third parties (mainly the hydrocarbon exploration industry). In SGT Stage 2 the existing seismic data were reprocessed in a consistent way, and further complementary 2D seismic data were acquired to fill gaps in and around the siting regions. These lines of data, with a typical line spacing of 1 – 3 km, provided a robust understanding of the regional tectonic framework and elements as well as the depth and geometry of key Mesozoic units including the Opalinus Clay (Madritsch et al. 2013, Nagra 2014a, 2014b, Meier et al. 2014).

In conjunction with SGT Stage 3, Nagra acquired 3D seismic surveys in all three siting regions between 2015 and 2017 to provide a continuous and more comprehensive image of the subsurface necessary for site-scale geological characterisation. Part of the Zürich Nordost (ZNO) siting region had previously been covered by a 3D survey, acquired by Nagra in 1997 in connection with the demonstration of disposal feasibility for a HLW repository (Nagra 2001). To include areas with shallower Opalinus Clay potentially suited for a L/ILW repository, the existing survey in ZNO was extended to the north. The processing of the seismic data included data from both the original and the new acquisition, resulting in a merged continuous 3D dataset. The area covered by 3D seismic data in the three siting regions amounts to a total of 240 km². The seismic processing is documented in Nagra (2024s) and the final interpretation in Nagra (2024a) for Jura Ost (JO), Nagra (2024b) for Nördlich Lägern (NL) and Nagra (2024c) for Zürich Nordost (ZNO).

The aims of the new surveys were 3D mapping of key geological horizons to provide detailed information on depth, thickness and inclination of relevant geological units, the distribution and geometry of faults, the identification of lateral facies changes and the reconstruction of the tectonic history. The survey design was informed by existing geological knowledge from Stage 2 of the Sectoral Plan, primarily 2D seismic data, outcrop information and a few deep boreholes (Madritsch et al. 2013, Nagra 2014b). The surveys primarily targeted the host rock and the confining units in the central areas with flat-lying little disturbed strata, but also provided infor­mation on the larger-scale tectonic elements in the surrounding regions. The processing of the seismic field data was optimised to support the comparison of the three siting regions for the purpose of the site selection (Nagra 2024s). The seismic imaging in the potential disposal zones in all the siting regions is of high quality, achieving the survey aims and providing the data necessary for site selection and the demonstration of post-closure safety. This, together with the systematic interpretation approach applying rigorous criteria (Nagra 2024a, 2024b, 2024c) ensures a high comparability of the interpretation results between the regions despite some differences in topography, geological setting and acquisition method.