Researchers from the Oak Ridge National Laboratory (ORNL), part of the U.S. Department of Energy, demonstrated a new method for detecting hidden tunnels using acoustic signals generated underground.
The technique reverses the traditional detection approach, which typically sends signals from the surface to the subsurface. In this case, scientists transmitted sound upward from boreholes, allowing for the identification of underground structures that might go unnoticed with conventional methods.
Acoustic signals reveal hidden tunnels
For decades, tunnel detection has relied on seismic surveys, ground-penetrating radar, and electrical resistivity.
However, these technologies can present limitations in clay soils, complex underground environments, or areas where signals lose resolution at greater depths.
The ORNL team proposed that by sending the signal from beneath a potential tunnel, it would be possible to capture scattering patterns that are normally lost when waves are emitted from the surface.
According to Mike Kass, the study’s lead researcher, the change in direction improved detection by observing how the signal interacted with the buried structure.
The method generated a distinctive subharmonic signal, a low-frequency response produced when sound waves diffract or bend around a tunnel. This signal was captured by surface sensors, revealing the presence of the underground cavity.
The trial was conducted on the ORNL campus
To test the system in real-world conditions, researchers installed a 12-meter-long steel tunnel approximately 3 meters deep on the ORNL campus. Then, through vertical boreholes, they placed an acoustic source 9 meters underground.
On the surface, they placed an array of geophones—sensors sensitive to vibrations—to record how sound propagated through the ground before and after the tunnel was installed.
This comparison allowed them to verify that the subharmonic signal appeared consistently only when the tunnel was present and only when the sound originated from below the structure.
Charles Finney, a senior researcher at ORNL, explained that subsequent measurements confirmed the repeatability of the phenomenon, reinforcing the method’s potential as a detection tool.
Inverted vertical acoustic profiling
The research adapted a procedure used in oil and gas exploration known as vertical seismic profiling. In its traditional configuration, sensors located in wells record waves generated from the surface.
ORNL reversed this logic by placing the sound source beneath the target tunnel and measuring the resulting vibrations on the surface. This modification allowed for the detection of signals related to the presence of buried man-made structures.
In addition to confirming the existence of a tunnel, the technique could provide clues about its depth, as the subharmonic signal was observed only when the acoustic source was located below the cavity.
Applications in critical infrastructure
The new method could help identify hidden underground structures that pose risks to roads, railways, strategic facilities, and other critical infrastructure assets.
Undetected tunnels or cavities can alter ground stability and create operational or security threats.
The team plans to continue testing in different soil types, refine signal analysis, and investigate how acoustic timing and intensity could allow for more detailed subsurface imaging.
These advances could strengthen monitoring, security, and interpretation capabilities for acoustic events in underground environments, especially in critical situations where it is necessary to distinguish signals associated with tunnels, cavities, or underground explosions.
Source and photo: https://www.ornl.gov/
