Investigating the dynamic response of rock mass to reservoir drainage at Grimsel test site, Switzerland, as an analogue for glacial retreat

  • Marianna Kinali

Student thesis: Doctoral Thesis

Abstract

An effective solution for the geologic disposal of nuclear waste, with no environmental risk (i.e. avoidance of harmful release of radioactive material), is a fundamental issue for the environment protection, and for the future continued reliance on nuclear power. Although geological disposal is considered as the best option, there are still elements of risk to be addressed, such as glacial retreat, which could impact the safety performance of a geological disposal facility. In this project two consecutive annual cycles of a reservoir in the Swiss Alps are used as a small scale analogue of the glacial retreat cycles, in order to investigate the response of granitic rock (as a host rock to a geologic disposal facility) to significant load changes. Assuming that the reservoir's stress changes cause the fractured and weakened rock slopes to slip, I chose to use microseismic monitoring as a tool to monitor the reservoir induced seismicity. A seismic network was deployed in the tunnels adjacent to the reservoir and recorded continuously ground movement over a 3.5-year period (Nov 2014 - Aug 2018). In order to be able to detect microseismic slips in the acquired real field dataset I explore various algorithms from the literature and develop my own methodology. The two main problems my research focuses on are the length of the dataset (big data issues) and the signal to noise ratio of the events I want to detect (small magnitude events in a varying noisy background). My results show, albeit not all of the seismic signals were possible to locate or characterise, that the reservoir unloading increases the frequency of occurrence of microseismic events for a short time period in the region surrounding the reservoir. It is possible therefore that the construction of a geologic disposal facility will have a similar effect. However, the magnitudes of the induced events are very small and hence unlikely to have a significant effect as part of a safety case for a geologic disposal facility. The contributions of this thesis can be summarised to: (i) using a reservoir as a small-scale test site analogue for exploring the seismic hazard in radioactive deep geologic disposal facilities due to glacial retreat; (ii) sensor deployment design and sensor data cleaning with noise characterisation for microseismic monitoring over several years; (iii) proposal of a new algorithm (NpD) for detecting potential seismic signals under not well-constrained conditions and without requirement of a priori knowledge about the expected signal frequencies and amplitudes; (iv) the NpD detection algorithm and acquired 3.5 years dataset are made freely available; (v) detailed discussion of onset time picking and hypocentre localisation methodologies, where again novelty lies in using, comparing suitability and adjusting a number of well-known approaches for the purposes of my project; (vi) compilation of a seismic catalogue related to the dynamic response of the rock mass to reservoir drainage.
Date of Award1 Jul 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde & Radioactive Waste Management Limited
SupervisorStella Pytharouli (Supervisor) & Rebecca Lunn (Supervisor)

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