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Monitoring structural integrity in submerged tunnels presents a formidable engineering challenge. Traditional methods like borehole drilling and sonar scans require disruptive shutdowns, offer low resolution, and struggle with dynamic environments where water-saturated sediments exert uneven, shifting pressures. For Shanghai's 3km-long Outer Ring Tunnel beneath the Huangpu River—a vital artery in China's largest city—these limitations posed significant risks.

Now, researchers at Shanghai Jiao Tong University have pioneered a groundbreaking solution using cosmic-ray muons. When high-energy particles from space collide with Earth’s atmosphere, they generate muons that penetrate deep underground. By measuring how these particles attenuate through materials, scientists can map density variations—a technique called muography.

"We took this ‘old-school’ technique and pioneered its use in a completely new scenario: dynamically monitoring low-density, watery sediment build-up above a submerged, operational tunnel," explains physicist Kim Siang Khaw. The team developed a portable dual-layer scintillator detector that suppresses background noise while capturing muons across wide angles. Crucially, it operates unobtrusively within active infrastructure.

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During a maintenance window, the team rolled the detector through the entire tunnel, mapping sediment profiles by comparing muon flux with predictive models. Unexpectedly, they discovered their system could also track river tides in real-time.

"The most surprising discovery was a clear anti-correlation between muon flux and the tidal height of the Huangpu River," Khaw notes. "We’ve effectively shown a dual-purpose technology for sediment and tidal monitoring."

The results confirmed no dangerous sediment obstructions while demonstrating unprecedented non-invasive oversight capabilities. This approach eliminates costly shutdowns and could transform infrastructure management in megacities, with potential applications in dam safety, pipeline monitoring, and volcanic activity tracking. By repurposing cosmic phenomena as diagnostic tools, engineers gain X-ray vision for Earth’s hidden layers.

Source: Physics World