R&D | TECHNICAL


Australia (George, 1955). Later, in the 1960s, Nobel Prize winner Luis Alvarez and colleagues employed muon tomography to search for hidden chambers in the second pyramid of Chephren in Giza, Egypt (Alvarez, 1970). Modern methodology typically involves creating a


‘digital twin’ of a structure and then computer simulations of muons penetrating these structures – with and without assumed voids – are performed. The simulated results are compared with collected field data. Since the research by Alvarez and his team, a


number of pyramids have been analyzed using muon tomography. In 2017, teams of Japanese and French scientists famously reported the discovery of a large void with a minimum length of 33yd in the Great Pyramid of Giza (Morishima, 2017). In total, three independent teams of scientists used different types of muon detectors, and their own simulations and data analysis techniques, to ensure that there could be no misinterpretation of the data. All three teams saw increases in the muon rate in their detectors in a particular direction, which could be interpreted as a hitherto unknown large void above the Queen’s chamber. In another novel application of muon tomography,


in 2001, Professor Hiroyuki Tanaka from the University of Tokyo led a team that used the technique to image the active volcano of Mount Asama, in Japan (Tanaka, 2001). A muon detector system, placed at the base of the volcano, gathered data on the muon flux passing through the volcano, their trajectory being determined using two segmented detectors, positioned a short distance (~5ft) apart. Subsequently, in 2013, the team performed the first ever visualization of an erupting volcano using muon tomography, providing information on the movement of the top of a magma column before and after two eruptions (Tanaka, 2013). Geoptic specializes in civil infrastructure


investigations via the novel and unique imaging technique known as muon tomography.


One application of muon tomography, more recently,


and pioneered by Geoptic, is its use in identifying and characterizing hidden voids in the overburden of railway tunnels. The same instrumentation can also be employed to perform long-term monitoring of the overburden and, in particular, is able to correlate any density changes with possible water ingress from rainfall.


MUON TOMOGRAPHY IN UK RAIL SECTOR The Victorian era saw a boom in railway construction. Many of the UK’s railway tunnels, bridges, and viaducts were built from the mid-1800s to the early 1900s. Vertical shafts were used in tunnel construction, helping remove spoil and transport materials down to the workers below. It was not uncommon for several such vertical shafts


to be used during a tunnel project but their subsequent fate was rather arbitrary, it seems – some were left open to the surface, others infilled (to some extent), while yet others were capped off at the tunnel lining and the surface but otherwise left empty as so-called ‘hidden shafts’. Even partially-filled shafts can undergo additional degradation of the shaft lining and/or differential settlement over the extended periods. All of this leads to a need for as complete an


inventory as possible of all such hidden shafts on the UK rail network, not least for regular maintenance checks but, where necessary, for corrective actions to take place. In the absence of muon tomography, approaches to


void identification include rather crude methods, such as drilling into the tunnel wall to assess the integrity of the material behind the lining. This carries a number of health and safety risks as invasive drilling could aggravate any structural instability around a hidden void. There is, therefore, clear appeal and merit to having a fully non-invasive and non-destructive technique (NDT) for investigation beyond rail tunnel walls, such as muon tomography.


10000 8000 6000 Left, figure 2: 4000 2000 0 0 (219yd) 200 (438yd) Position [m/yd] 400 (657yd) 600 (876yd) 800


Measured rate Expected rate Inferred rate


Data from the Alfreton Old Tunnel survey


Summer 2023 | 37


Rate [counts/30mins]


Open shaft (~220m/240yd)


Open shaft (~420m/460yd)


Open shaft (~620m/678yd)


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