Geophysical detection of clandestine tunnels is a complex problem that has met with limited success. Multiple methods have been applied, spanning several decades, but a reliable solution has yet to be found. We evaluated shallow seismic data collected at a tunnel test site representative of geologic settings found along the southwestern U.S. border. Our results demonstrated the capability of using P-wave diffraction and surface-wave backscatter techniques to detect a purpose-built subterranean tunnel. Near-surface seismic data were also collected at multiple sites in Afghanistan to detect and locate subsurface anomalies, including data collected over the escape tunnel discovered in 2011 at the Sarposa Prison in Kandahar, Afghanistan, which allowed more than 480 prisoners to escape, and data from another shallow tunnel recently discovered at an undisclosed location. The final example from Afghanistan was the first time surface-based geophysical methods have detected a tunnel whose presence and location was not previously known. Seismic results directly led to the discovery of the tunnel. Interpreted tunnel locations for all examples were within less than 2 m of the actual location. Seismic surface-wave backscatter and body-wave diffraction methods showed promise for efficient data acquisition and processing for locating purposefully hidden tunnels within unconsolidated sediments.

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