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Arbeitsbericht NAB 14-88

Simulation of Layout Determining Fault Networks based on 2D-seismic Interpretations: Implications for Subsurface Space Reserves in Geological Siting Regions in Northern Switzerland

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In the course of the first stage of the "Sachplan geologisches Tiefenlager" (SGT) Nagra pro-posed five geological siting regions in northern Switzerland to potentially host a repository for radioactive waste, namely Südranden (SR), Zürich Nordost (ZNO), Nördlich Lägern (NL), Jura Ost (JO) and Jura-Südfuss (JS) (Fig. 1.1). All five of the siting regions are considered for disposal of low and intermediate level waste (L/ILW, respectively in German: SMA1), while a repository for high level waste (HLW, respectively in German: HAA2) was only considered suitable in the siting regions ZNO, NL and JO (Nagra 2008a).

In stage 2 of the SGT a minimum of two siting regions for SMA and HAA are to be nominated for further field exploration. For this purpose, the siting regions and the potential disposal perimeters within them are compared against each other in the course of the comparison of safety aspects, in German: "Sicherheitstechnischer Vergleich" (SV, Nagra 2014). The SV considers a wide variety of the siting region characteristics /indicators, including the indicator "Platzangebot untertags". It addresses the subsurface space reserves available for repository layout options in the siting regions. This report is dedicated to a study exploring the potential impact of tectonic faults (in the following referred to as "layout determining faults", LDF) on these subsurface space reserves from an engineering perspective. Its main goals can be summarized as follows:

  • Develop understanding of how LDFs potentially affect the space reserves for repository layout in any given area
  • Develop a methodology to simulate the LDF networks and condition these simulations to the  tectonic situation in the different geological siting regions
  • Explore the resulting subsurface space reserves in the different siting regions when applying the simulated LDF networks

Accordingly, the study consists of three parts. In the first part the general roles of various fault characteristics on space reserves in a given area are explored using generic discrete fracture network (DFN) models. In the second part fault networks conditioned to each geological siting region are set up. With the exception of the ZNO siting regions, the siting regions are not covered by 3D-seismic data. Hence, knowledge regarding the location of LDFs is limited. However, interpretations from a dense grid of 2D-seismic lines are available in all the siting regions that revealed the location of some potential LDFs (Madritsch et al. 2013). The seismic interpretations are analysed to simulate site-specific fault networks. In the third part of the study the given siting regions and disposal perimeters within them are analysed in terms of space reserves, applying the simulated fault networks. While this analysis does not attempt to simulate a precise tectonic fault model for each siting region and hence determine the effective space reserves, it does allow a quantitative assessment of the probable limitation of space reserves due to presence of as-yet undetected LDFs.

The report first outlines the concept of LDFs in the context of the SGT and defines the space requirements for SMA and HAA repositories considered during this analysis (Chapter 2). The available data relevant for the simulation of LDF networks is described in Chapter 3. Chapter 4 outlines the fault characteristics considered in the LDF network models and the parameterisation of these characteristics. The actual methodology underlying the simulation of discrete fracture networks, subsurface space reserves queries, data analysis and site region conditioning is described in Chapter 5. Following a discussion addressing model simplifications, assumptions and limitations (Chapter 6) the results of the three study parts outlined above are presented in Chapter 7. The report concludes with a summary giving a brief comparison of the geological siting regions in northern Switzerland.

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