3D Seismic Interpretation of Stratigraphic Horizons and Structures in Time Domain Nördlich Lägern
Nagra (2024): 3D Seismic Interpretation of Stratigraphic Horizons and Structures in Time Domain Nördlich Lägern. Nagra Arbeitsbericht NAB 23-18.
A 3D seismic evaluation was 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 subseismic scale 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. Therefore, interpreting 3D seismic data 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 the final 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 the 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 established and implemented in the interpretation of the three seismic surveys covering the three siting regions. Consequently, importance was given to the methodology section, which is repeated in the three seismic interpretation reports for the respective siting regions (NAB 23-17, NAB 23-18, NAB 23-19), where the applied workflow is illustrated based on site-specific examples.
The resulting horizon-fault framework in the NL siting region included the interpretation of six key horizon markers:
(i) 149.2 Kim 100 – Top Malm
(ii) 149.2 Kim 700 – Top Villigen
(iii) 168.2 Baj 400 – Top Clay Unit
(iv) 201.4 Rha 100 – Top Klettgau
(v) 237 Lad 400 – Top Muschelkalk
(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 subsurface image of the three investigated siting regions.
The evaluation performed shows that the NL siting region can be subdivided into four main structural domains, from south to north: (i) the Southern Fold-and-Thrust Zone (SFTZ, i.e. Baden-Irchel-Herdern Lineament), (ii) the Undisturbed Central Panel (UCP), (iii) the Weiach-Glattfelden-Eglisau Structure (WGE) and (iv) the Northern Fold-and-Thrust Zone (NFTZ, i.e. Siglistorf Anticline). The Undisturbed Central Panel (UPC) represents a ~25 km2 seismically undisturbed area between the deformation zones. In this area, the Opalinus Clay is consistently interpreted not to act as detachment level but rather as a decoupling layer between the structures in the lower and the upper competent panels and to accommodate deformation by localised non-coaxial deformation zones (i.e. shear planes) at meso- and microscales (i.e. at core scale) without reactivation / formation of seismic-scale structures.
Within a regional tectonic framework, several structural evolution scenarios for the NL siting region were proposed by several authors (Diebold et al. 1991, Naef et al. 1995, Madritsch et al. 2013, Malz et al. 2016, Bond et al. 2023, Schöpfer et al. 2023, Zambrano et al. 2024, Bond et al. 2023). The main discussion points focus on the relative timing of the major deformation zones (SFTZ, NFTZ and WGE), their kinematics and the role and interaction of the Mesozoic faults with deep-seated grains (thin- vs thick-skinned models).
While the alternative scenarios describe different tectonic evolutions of the NL siting region and the larger area of Northern Switzerland as discussed in NTB 24-17 (Nagra 2024d), none of those evolutionary tectonic scenarios propose either a different structural configuration or a different areal definition for the UCP in the NL siting region.