Researchers in the US have recently demonstrated a new way to detect hidden underground tunnels by transmitting acoustic signals from beneath the target instead of above it.

Led by Mike Kass, PhD, a researcher at Oak Ridge National Laboratory (ORNL) in Tennessee, the team tested the new technique during a field experiment at the facility’s campus. They were able to identify concealed underground structures that threaten roads, railways, and other critical infrastructure.

Tunnel detection usually relies on signals sent downwards from the surface. Still, for the project, the team flipped the process and transmitted sound upward from below a tunnel to generate a distinctive subharmonic signal.

“Our hypothesis was that if we reversed direction, sending the signal from below a potential tunnel instead of above, we could improve detection by capturing signal scatter that otherwise is lost,” Kass said.

A new detection angle

Tunnels are difficult to locate from the surface. To do so, engineers rely on various sensing technologies, including seismic surveys, ground-penetrating radar, and electrical resistivity. However, these methods can be limited in clay-rich soils and complex underground environments.

Higher-frequency signals sent from the surface can detect small cavities. But they fade quickly as they travel underground. Meanwhile, lower-frequency signals can travel farther, but they often miss smaller details. Now, in a bid to overcome the challenges, the ORNL team reversed the process.

The team found that the distinctive subharmonic signal is a clear sign of a hidden cavity. The lower-frequency acoustic response is created when sound waves bend around the tunnel structure. Surface sensors can then spot the signal and confirm the tunnel’s presence.

According to the team, the inspiration behind the approach was vertical seismic profiling. This technique is widely used in oil and gas exploration. Traditionally, the method involves placing sensors outside boreholes to record energy waves at the surface. However, the ORNL team positioned the sound source underground and measured the resulting vibrations at the surface.

Testing the novel approach

To validate the system in real-life conditions, the team installed a 40-foot steel tunnel about 10 feet below the ground. They then lowered an acoustic source through boreholes to depths of up to 30 feet.

At the surface, they also placed arrays of geophones, which are highly sensitive vibration sensors. These helped them record how sound traveled through the ground before and after the tunnel was installed.

“During testing, the geophones detected a distinct subharmonic signal,” Charles Finney, PhD, a senior R&D researcher at ORNL, said in a statement. “Subsequent measurements showed the signal consistently appeared only when the tunnel was present and only when the sound originated beneath it.”

The findings revealed a detection method that could improve the identification of hidden underground structures. The team believes that it could also help estimate tunnel depth, since the subharmonic signal only appeared when the sound source was placed beneath.

They now plan to further refine the technology by evaluating its performance in different soil types. The researchers will also explore whether signal timing and strength can be used for more detailed subsurface images.

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