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Researchers at Oak Ridge National Laboratory in Tennessee tested an acoustic technique that sends signals from bottom to top to identify hidden underground tunnels, with the potential to improve the protection of roads, railways, and other critical infrastructures in risk areas
Researchers at the Oak Ridge National Laboratory in the United States demonstrated a new way to locate hidden underground tunnels by reversing the traditional path of acoustic signals: instead of sending vibrations from the surface downwards, the team generated sound below the target and measured the response in the ground above.
The technique was tested in a field experiment on the laboratory’s own campus, linked to the United States Department of Energy.
The goal was to tackle a long-standing engineering limitation: identifying hidden underground structures that can alter ground stability and create voids under roads, railways, industrial facilities, and other critical infrastructures.
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Why underground tunnels are difficult to detect
The search for underground tunnels often relies on technologies that work from the surface. Among the methods used are seismic surveys, ground-penetrating radar, and electrical resistivity measurements. These tools can be useful, but they do not work the same way in all terrains.
Clay-rich soils, for example, can limit the propagation of certain signals. Complex underground environments also make reading difficult because different layers, materials, and obstacles interfere with the path of the waves.
The problem involves an important physical balance. Higher frequency signals can capture smaller cavities with more detail but quickly lose strength as they advance underground. Lower frequency signals travel longer distances but may miss fine details.
This combination creates blind spots. An already constructed tunnel can remain difficult to locate, especially when it is under areas where stability of the soil matters for the safety of transportation systems and essential facilities.
How the acoustic method was reversed
The ORNL team started with a simple hypothesis, but with great practical impact: if part of the signal is lost when sent from top to bottom, perhaps detection improves when the sound source is below the possible tunnel.
Mike Kass, lead researcher of the study, explained that the idea was to capture the signal dispersion that is normally lost in the conventional method. For this, the researchers adapted a technique used in oil and gas exploration, known as vertical seismic profiling.
In traditional use, sensors are placed inside holes to record energy waves generated on the surface. In the ORNL experiment, the setup was reversed. The acoustic source was placed below the target, while sensors on the surface recorded the resulting vibrations.
This change of direction is the central point of the research. Instead of trying to see the underground only from above, the method provokes an acoustic response from below and analyzes how the sound interacts with the hidden structure.
Subharmonic signal revealed the presence of the tunnel
During the tests, the method produced a distinct subharmonic signal. This type of response has a lower frequency and arises when sound waves bend, or diffract, around the tunnel.
In practice, the presence of the cavity alters the behavior of the sound. The sensors installed on the surface can capture this acoustic signature, revealing that there is an underground structure interfering with the path of the waves.
Charles Finney, senior research and development researcher at ORNL, stated that the geophones detected this signal during the tests. Subsequent measurements indicated that the response appeared consistently only when the tunnel was present and when the sound originated below it.
This detail is relevant because it helps differentiate the tunnel’s signature from noise or natural soil variations. The repetition of the signal under the right conditions reinforces the potential of the technique as a detection mechanism.
Experiment used steel tunnel and surface sensors
To evaluate the method in real conditions, the researchers installed a 40-foot-long steel tunnel, approximately 12 meters, about 10 feet below the surface, around 3 meters.
The team used vertical holes to position an acoustic source at depths of up to 30 feet, approximately 9 meters. On the surface, a set of geophones, sensors sensitive to vibrations, was mounted, capable of recording how the sound traveled through the ground.
The recordings were made before and after the tunnel installation. This direct comparison allowed observation of which changes in the signal were associated with the presence of the underground structure.
The experiment was not limited to indicating the existence of the tunnel. The team also observed that the subharmonic signal appeared only when the sound source was below the structure. This suggests that the technique may offer clues about the depth of the target.
Potential for Infrastructure and Upcoming Tests
The research demonstrates a new mechanism for identifying man-made underground structures. The application can be important in locations where hidden voids beneath the ground pose a risk to the stability of highways, railways, facilities, and operational areas.
There are still development steps ahead. The researchers intend to test the technique’s performance in different soil types, refine the signal analysis, and investigate whether the time and intensity of the acoustic response can generate more detailed underground images.
Besides Mike Kass and Charles Finney, the team included Omar Marcillo, Monica Maceira, and Derek Splitter. The work brought together experts in engineering, acoustics, and seismic research.
The study was supported by the Laboratory Directed Research and Development Seed Money Program, from ORNL, and used resources from the National Transportation Research Center, a user facility of the United States Department of Energy.
The findings were detailed in the technical report “Advancing Tunnel Detection Via Vertical Acoustic Profiling.” ORNL is managed by UT-Battelle for the Office of Science of the United States Department of Energy, focused on basic research in physical sciences.
This article was prepared based on information released by and . The content was supported by AI tools in editorial organization and underwent human review before publication.
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