TECHNICAL | R&D
The broad features described by the data are clear and 4500 4000
agree well with expectations, e.g., the muon rate drops rapidly as you enter the tunnel and rises as you leave the tunnel. In areas where there are clear open shafts to the surface the detected muon rate rises dramatically (with typically 10 standard deviations of significance). The survey also identified a further area of interest at
3500 3000
Measured rate Expected rate Inferred rate
2500 (44yd) 40 (66yd) 60 (88yd) Position [m/yd] 80 (110yd) 100 (131yd) 120
a location approximately 80m (88yd) from the Langley Mill portal. Close inspection of the data around this point (see Figure 3) indicated an increase in the muon rate, measured in a number of data points that describes a profile suggesting a hidden shaft – the presence of which was subsequently confirmed by Network Rail to the Geoptic team. It should be stressed that the team had no awareness of this feature prior to the survey. Following the identification of this hidden shaft,
Geoptic were contracted to return to the tunnel in 2019. At this point, a more detailed survey of the hidden shaft took place. Using muon tomography, the exact location, extent (i.e., ‘footprint’ on the tunnel roof) and effective overburden (opacity) were measured. The shaft was confirmed to be approximately 3m (10ft) in diameter and almost a full height voided shaft.
Above, figure 3:
Zoom-in on Alfreton Old Tunnel suspected hidden shaft data
In 2018, Geoptic was offered the opportunity of
performing the first demonstration of application of muon tomography for imaging of railway tunnels. Specifically, the company was asked to perform a portal-to-portal survey of a disused railway tunnel – the 700m-long (765yd) Alfreton Old Tunnel, between Alfreton and Langley Mill in Nottinghamshire, England. The tunnel was built from the portals as well as
using at least three full-height vertical shafts, which are visible both from within the Alfreton tunnel and from the surface (see Figure 1). In a data-taking campaign that lasted 12 days, a
detailed portal-to-portal scan of the tunnel was carried out. Instrumentation capable of detecting and recording both the passage of a muon and its approximate direction was deployed in a van, which traveled to along the tunnel, stopping at more than 150 predetermined muon scanning points, located at intervals of approx. 10.9yd (reduced to approx. 5.4yd around objects of interest). Data recording at each location lasted typically 20-30 minutes (depending on the depth of overburden, as measurements were made looking upward to catch muons penetrating downward through the ground). The results from this data-taking campaign
(Thompson, 2020) are depicted in Figure 2, which shows the rate of muons detected – ‘measured rate’ – at each point along the tunnel as well as the ‘expected rate’ and ‘inferred rate’, respectively. The ‘expected rate’ is the rate expected from pre-existing geological and topological information, undisturbed by such rail tunnel construction activities; the ‘inferred rate’ is calculated using redundant data from the muon detection equipment, sampled from directions other than measuring vertically upwards.
38 | Summer 2023
OTHER APPLICATIONS As noted earlier, muon tomography has been used to image high-profile objects, such as pyramids and volcanoes. It is fair to say that, until relatively recently, use of the method has been largely the preserve of academia. However, over the last few years a surge in interest along with a multitude of commercial projects has taken place, including muon tomographic imaging in mining, archaeological buildings and blast furnaces (IAEA TechDoc Series, 2022). Muon tomographic systems have also been installed
on tunnel boring machines (TBMs) for metro line creation. Variations in muon flux can provide warning of oncoming dangers, such as obstructive man- made structures and hidden voids and, consequently, reduce the risk of collapse and damage to existing infrastructure (Chevalier et al., 2019). The method also has significant potential
for application to the nuclear waste sector – to permanently store radioactive material, such as spent nuclear fuel, deep underground in ‘geological repositories’ located in highly competent, suitable rock. In this deployment scenario, long-term monitoring of the radioactive material is essential in two aspects: safety (ensuring the processes around the storage areas are safe); and, safeguarding (ensuring the stored waste remains in place and is not diverted for illegal use). Once moved from interim to long-term storage in the geological repositories, canisters of radioactive material are expected permanently to remain in place. At depths where final disposal sites are likely to
be located, the muon flux is significantly attenuated deeper underground compared to levels of flux at the surface. However, this is mitigated with the possibility of performing long image times – where detectors have longer exposure to the muon flux – hence making
Rate [counts/30mins]
Possible shaft (~80m/88yd)
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