Underground acoustic signals reveal hidden tunnels

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

Tested in a field experiment on the laboratory's campus, the approach detected tunnels by transmitting sound upward from boreholes. The directional shift addresses a blind spot in tunnel detection and could help identify concealed underground features that pose risks to transportation and critical infrastructure by altering ground stability 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, or bend, around a tunnel. Surface sensors detected that signal, revealing the tunnel's presence.

Surface-level signals fall short

Once a tunnel is constructed, it can be difficult to locate from the surface. To detect these underground structures, researchers have used sensing technologies such as seismic surveys, ground-penetrating radar, and electrical resistivity, but these methods can be limited, especially in clay-rich soils or complex subsurface environments. Sent from above, higher-frequency signals can detect small cavities but fade quickly underground, while lower-frequency signals travel farther but often miss finer details.

Sending the signal from underground

ORNL's research team adapted a method commonly used in oil and gas exploration called vertical seismic profiling, in which sensors placed inside boreholes record energy waves generated at the surface. The team reversed the configuration by placing the sound source below the target tunnel and measuring the resulting vibrations above ground.

To test the approach under real-world conditions, a 40-foot-long steel tunnel was installed roughly 10 feet below the surface. Through vertical boreholes, the team placed an acoustic sound source as deep as 30 feet below ground. At the surface, researchers placed an array of geophones, or sensitive vibration sensors, to record how the sound traveled through the ground before and after the tunnel was installed, allowing for direct comparison.

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

Looking ahead to different soil types

The findings demonstrate a new detection mechanism that could improve the ability to identify man-made underground structures. Beyond confirming a tunnel's presence, the technique may also provide clues about tunnel depth, since the subharmonic signal appeared only when the sound source was placed below the tunnel.

The research team plans to further explore the method's capabilities by testing different soil types, refining signal analysis techniques and investigating how timing and signal strength could enable more detailed imaging.

In addition to Kass, ORNL's research team included Charles Finney, Omar Marcillo, Monica Maceira, and Derek Splitter. The project brought together expertise from engineering, acoustics, and seismic research, highlighting the cross-cutting collaboration enabled by America's national laboratories. This research was supported through ORNL's Laboratory Directed Research and Development Seed Money Program and utilized resources at the National Transportation Research Center, a DOE user facility. The findings are detailed in the DOE technical report, "Advancing Tunnel Detection Via Vertical Acoustic Profiling."

UT-Battelle manages ORNL for the Department of Energy's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science.