From Tunnel Overburden Reconstruction to Tidal Measurements via atmospheric muon
The HAWL and HAWLU projects demonstrate the capabilities of dynamic muography by successfully mapping mountainous terrain with a portable, vehicle-mounted detector and measuring tidal water-level variations in an undersea tunnel with meter-scale precision using a compact, upgraded system.
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Deep beneath the Earth's surface, a constant rain of invisible particles falls from the sky. These are cosmic-ray muons, subatomic fragments born when high-energy particles from space collide with our atmosphere. Unlike most matter, muons are incredibly tough travelers; they can punch through hundreds of meters of rock and water before stopping. This unique ability makes them natural probes for looking inside things that are otherwise impossible to see. For decades, scientists have used muons to peer into the hearts of volcanoes and ancient pyramids, mapping their internal density by counting how many muons make it through. If a structure is dense, fewer muons get through; if it is hollow or less dense, more arrive. While this technique has long been used to study static objects, a new study from South Korea demonstrates how to use these particles to watch the world change in real time, tracking both the movement of a vehicle through a mountain and the rhythmic rise and fall of the ocean.
A team of researchers from Chung-Ang University, led by Jiwon Seo, developed a portable system called HAWL to test these ideas in two very different ways. First, they wanted to see if they could map the ground above a road while driving through tunnels. They built a detector using plastic scintillators, which are blocks of material that flash with light when a muon passes through, and connected them to sensitive sensors. They mounted this device in the trunk of a car and drove along the Seoul–Yangyang Expressway, passing through forty-seven tunnels. As the car moved, the detector counted the muons arriving from above. When the car was inside a tunnel, the muons had to travel through the mountain rock overhead to reach the sensor. By analyzing the rate at which muons arrived, the team could calculate the thickness of the mountain above them. They found that their system could measure tunnel lengths with an accuracy of 6.0 percent and determine the weight of the rock above, ranging from 8 to 400 meters of water equivalent. This means a single detector moving at highway speeds could effectively map the shape of a mountain range in real time.
The second part of the study took place in a different setting: the Boryeong Undersea Tunnel, a 6.9-kilometer passage running beneath the sea. Here, the researchers did not move the detector. Instead, they left it fixed in place for thirty-one days to watch how the ocean tides affected the muon count. The idea was simple but profound: as the tide rises, the layer of seawater above the tunnel gets thicker, blocking more muons. As the tide falls, the water layer thins, and more muons get through. The team observed this exact pattern. Over the course of their measurement, they recorded a clear, rhythmic drop in the number of muons whenever the water level rose. They calculated that for every meter the tide went up, the muon count dropped by 1.72 percent. This small but consistent change proved that their detector was sensitive enough to weigh the ocean itself from deep underground.
To ensure these results were real and not just a variation of the weather, the scientists checked for other factors that could influence the count. They looked at atmospheric temperature and pressure, which are known to change the number of muons reaching the ground. However, they found no significant connection between these weather changes and their data. They also used a mathematical technique to analyze the timing of the fluctuations, which revealed the distinct 12.4-hour cycle of the tides, confirming that the signal came from the ocean and not random noise. To further validate their findings, they drove the detector through the same undersea tunnel ten times, comparing the moving scans against a computer model of the tunnel's engineering. The data from the moving scans matched the model perfectly, proving the detector was stable and accurate.
This work shows that muon detection is evolving from a tool for static mapping into a dynamic monitoring system. The researchers demonstrated that a portable device can scan vast areas of terrain while in motion, and that a fixed device can track slow, natural changes in the environment with meter-scale precision. These capabilities open the door to new ways of watching our planet. The same technology that measured the tides and the mountains could potentially be used to monitor other shifting hazards, such as the swelling of a volcano before an eruption, the shifting ground around a sinkhole, or the rapid rise of water during a tsunami. By turning invisible particles into a live feed of the Earth's interior, this research offers a new way to understand the dynamic world beneath our feet.
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