Abstract
The U.K. government has a policy to dispose of higher activity radioactive waste in a geological disposal facility (GDF), which will have multiple protective barriers to keep the waste isolated and to ensure no harmful quantities of radioactivity are able to reach the surface. Currently no specific GDF site in the United Kingdom has been chosen but, once it has, the site is likely to be investigated using seismic methods. In this study, we explore whether 3-D full-waveform inversion (FWI) of seismic data can be used to map changes in physical properties caused by the construction of the site, specifically tunnel-induced fracturing. We have built a synthetic model for a GDF located in granite at 1000 m depth below the surface, since granite is one of the candidate host rocks due to its high strength and low permeability and the GDF could be located at such a depth. We use an effective medium model to predict changes in P-wave velocity associated with tunnel-induced fracturing, within the spatial limits of an excavated disturbed zone (EdZ), modelled here as an increase in fracture density around the tunnel. We then generate synthetic seismic data using a number of different experimental geometries to investigate how they affect the performance of FWI in recovering subsurface P-wave velocity structure. We find that the location and velocity of the EdZ are recovered well, especially when data recorded on tunnel receivers are included in the inversion. Our findings show that 3-D FWI could be a useful tool for characterizing the subsurface and changes in fracture properties caused during construction, and make a suite of suggestions on how to proceed once a potential GDF site has been identified and the geological setting is known.
Highlights
Over the last 70 yr, a large legacy of radioactive waste has accumulated in the United Kingdom
We look at trends in the RMS misfit for three locations: outside, at the edge and in the centre of the excavated disturbed zone (EdZ) (Fig. 6)
The largest variations in recovered velocity are seen for depths below the tunnel receivers (>1050 m), especially at 1062.5-m depth (Fig. 8). Both surveys detect a velocity anomaly associated with EdZ, for the surface survey the shape of the low velocity region could be interpreted as two separate anomalies, due to the disposal tunnel creating a poorly resolved region (Fig. 8b)
Summary
Over the last 70 yr, a large legacy of radioactive waste has accumulated in the United Kingdom. A significant amount of higher activity waste (HAW) has been accrued (NDA 2015) and needs to be securely isolated from the surface biosphere. No site has been selected, but several potential host rocks have been identified including ‘soft’ rocks (e.g. clays and mudstones), ‘hard’ rocks (e.g. granite) and halite. Granite is considered a potentially suitable host rock for radioactive waste disposal because it has high bulk strength and low ground water permeability, and several countries have already undertaken geophysical investigations to site GDFs in granitic rocks, for example Finland (Cosma & Heikkinen 1996; Saksa et al 2007; Schmelzbach et al 2007; Cosma et al 2008) and Sweden (Juhlin et al 2002; Bergman et al 2006; Juhlin et al 2010). Tunnelling in granite produces two distinct regions under stress: the region closest to the tunnel, the excavated damage zone (EDZ), which is subject to irreversible damage; and the radial region, the excavated disturbed zone (EdZ), where the changes are elastic and recoverable
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