WASHINGTON, Sept. 16, 2025 – Over 200 underwater bridge tunnels exist for vehicular traffic around the world, providing connectivity between cities. Once constructed, however, these tunnels are difficult to monitor and maintain, often requiring shutdowns or invasive methods that pose structural risks.
Muography — an imaging technique using high-energy particles, called muons, which can traverse hundreds of meters within the earth — can provide a noninvasive approach to examining subterranean infrastructure. In the Journal of Applied Physics, by AIP Publishing, a group of researchers from public and private organizations in Shanghai applied this technique to the Shanghai Outer Ring Tunnel, which runs under the Huangpu River as part of the city's ring expressway.
Because sediment composed of mucky soil and silty clay has a higher density than the water surrounding the tunnel, it is more effective at reducing muon flux than water on its own. When placed within the tunnel, the researchers' portable muon flux detection system is sensitive to these differences, proving useful for identifying locations with high levels of sediment buildup.
"Muons lose energy primarily through ionization, where they electromagnetically interact with and eject electrons from atoms — denser materials lead to a higher energy loss, effectively blocking more muons," said author Kim Siang Khaw. "The granular or clay composition of sediment intensifies this effect."
Using a combination of a spatial scan over the length of the tunnel and a simulation of muons passing through a simplified tunnel model, the researchers mapped the thickness of the sediment. They took 10 minutes of data per location at 50-meter intervals as proof of their technique, but in its actual deployment, they plan to permanently install multiple detectors at fixed points throughout the tunnel, allowing for round-the-clock monitoring.
They intend to extend their studies to several more tunnels in Shanghai and note that other cities can easily adopt the technique into their own infrastructure. All that is needed is basic information about the tunnel's geometry and materials, environmental data, and baseline muon flux measurements.
"No complex models are necessary upfront — the method works with simplified inputs, validated through simulations in this study," said Khaw. "This technique can also identify dangerous underground cavities, such as those formed when a burst pipe washes away soil, creating a hidden collapse hazard."
Muography has been used for archaeological studies, mine exploration, and more, but tracking a system's change over time is a relatively new, real-world application.
"We are now in a truly exciting era for muography," said Khaw. "We hope to collaborate with more researchers to apply these advancements in fundamental science to solving pressing societal challenges."
