3D Seismic Interpretation of Stratigraphic Horizons and Structures in Time Domain Zürich Nordost
Nagra (2024): 3D Seismic Interpretation of Stratigraphic Horizons and Structures in Time Domain Zürich Nordost. Nagra Arbeitsbericht NAB 23-19.
A 3D seismic evaluation has been carried out for the Jura Ost (JO), Nördlich Lägern (NL) and Zürich Nordost (ZNO) siting regions in the course of the repository site selection process. The aim is to identify and map the most undisturbed area for each of these three siting regions. It is important to highlight that this report focuses on seismic-scale observations. The development of geological concepts and the abstraction to sub-seismicscale interpretation is part of a multi-scale and multi-disciplinary integration and is captured in NTB 24-17 (Nagra 2024d).
While 3D seismic data greatly improve the imaging of the subsurface, the challenge of separating seismic reflections associated with geological features and seismic artefacts represents the key pitfall in the interpretation results, especially for onshore data with complex and heterogeneous shallow geology. Interpreting 3D seismic data, therefore, is a complex process that involves multiple steps. These can include some looping or repetition of the workflow steps, and require a deep understanding of geophysics, geology and seismic data processing.
In addition, for the purpose of site selection, emphasis was placed on the comparability of the seismic interpretation results. Despite being located in the Molasse Basin and being ~20 km apart, the Jura Ost, Nördlich Lägern and Zürich Nordost siting regions are demonstrably characterised by a different subsurface architecture. Therefore, to ensure a comparable and balanced evaluation, not only was seismic data reprocessing designed such that all the processing input parameters and workflows were consistent across the three seismic surveys, but also an interpretation approach with rigorous criteria was also established and implemented in the interpretation of the seismic surveys covering the three siting regions. Consequently, strong emphasis has been placed on the methodology section, which is repeated in the three seismic interpretation reports for the respective siting regions (NAB 23-17, NAB 23-18 and NAB 23-19), where the applied workflow is illustrated based on site-specific examples.
The resulting horizon-fault framework in the ZNO siting region includes the interpretation of six key horizon markers:
(i) 2 Kim 100 - Top Malm
(ii) 2 Kim 700 - Top Villigen
(iii) 9 Aal 300 - Top Opalinus
(iv) 7 Toa 100 - Top Lias
(v) 237 Lad 200 - Intra Bänkerjoch
(vi) 251.9 Per 100 - Near Base Mesozoic Unconformity
as well as 3D fault interpretation in the form of fault sticks/planes.
The six horizons were selected to represent key surfaces for the definition of the host rock (Opalinus Clay) as well as the upper and lower confining geological units while ensuring interpretability (i.e. imaging continuity) to reduce the picking uncertainty across the structurally complex zones.
Confidence in the interpretation results was established by an in-depth analysis of the uncertainties associated with the interpretation of seismic data. Uncertainties associated with data quality were mitigated by the use and comparison of all the seismic processing vintages, while the impact of human bias was assessed by superimposing all the available interpretations from different interpreters. Different seismic processing datasets interpreted by different interpreters resulted in a technically robust, consistent and comparable image of the three investigated siting regions.
The evaluation performed shows that the ZNO siting region can be subdivided into three main structural domains: (i) Neuhausen-Wildensbuch Structural Domain (NWSD) (ii) Niderholz-Marthalen Structural Domain (NMSD), and (iii) the Central Panel (CP, including the Trüllikon Anticline).
The different appearance of the seismic-scale structures observed in the ZNO siting region, compared to Jura Ost (JO) and Nördlich Lägern (NL), is simply the reflection of a very different tectonic configuration. The positioning of the ZNO siting region, with respect to the Permo-Carboniferous Konstanz - Frick Trough, implies a more intense influence of basement faulting on the Mesozoic succession compared to the JO and NL siting regions. Major regional tectonic events are translated into the re-activation and/or generation of high-angle faults, predominantly in a strike-slip motion.
A divergence from a previous interpretation (Nagra 2019) is critically observed for the Central Panel structural domain. Differences arise from the recognised challenge of interpreting seismically subtle faults. Detailed analysis and comparison demonstrated that the NAB 18-36 (Nagra 2019) mapping under-interpreted the seismic-scale deformation in this important area of the ZNO siting region.
A seismically fault-free area cannot be recognised in the ZNO-97/16 merged seismic survey. Nevertheless, a (relative) undisturbed area with an extent of 16.5 km2 is considered and coincides with the Central Panel, where subtle faults are widely distributed and interpreted to be vertically and laterally highly segmented.