ORNL Researchers Detect Tunnels by Sending Sound Upward

Title Image: On ORNL's campus in Oak Ridge, Tennessee, researchers constructed an underground tunnel during a field experiment to test a new detection method. Credit: ORNL, US Dept. of Energy

For decades, engineers have searched for underground tunnels by sending signals downward from the surface, an approach that can miss what lies below. Researchers at the US Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) have reversed that approach, using acoustic signals generated below ground to reveal hidden tunnels.

In a field experiment on the lab's Tennessee campus, the method detected a tunnel by transmitting sound upward from boreholes. The shift addresses a blind spot in tunnel detection and could help identify concealed structures that threaten transportation and other infrastructure by destabilizing ground or creating hidden voids beneath roads, rail lines, and facilities.

“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,” said ORNL's Mike Kass, lead researcher on the study. The method produced a distinct subharmonic signal—a lower-frequency response created when sound waves diffract around a tunnel—which surface sensors detected.

Once built, tunnels can be hard to locate from the surface. Existing sensing methods such as seismic surveys, ground-penetrating radar, and electrical resistivity have limits, especially in clay-rich or complex soils: higher-frequency surface signals detect small cavities but fade quickly, while lower-frequency signals travel farther but miss finer details.

To address this, ORNL adapted vertical seismic profiling, a technique from oil and gas exploration in which borehole sensors record surface-generated energy waves, by reversing the configuration—placing the sound source below the target and measuring vibrations above ground. To test it, the team installed a 40 ft (12.2 m) steel tunnel about 10 ft (3 m) down, then placed an acoustic source as deep as 30 ft (9.1 m) below through vertical boreholes. Surface geophones recorded ground vibrations before and after installation for comparison.

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

The findings point to a new detection mechanism, and the subharmonic signal's dependence on source placement may also offer clues about tunnel depth. The team next plans to test different soil types, refine signal analysis, and explore how timing and signal strength could enable more detailed imaging.

The work was supported through ORNL's Laboratory Directed Research and Development Seed Money Program and used resources at the National Transportation Research Center, a DOE user facility. Findings are detailed in the DOE technical report available at https://www.osti.gov/servlets/purl/3012495.