Shallow tunnel detection using SH-wave diffraction imaging

Abstract

Clandestine tunnels, used for drug or human trafficking and tactical operations, pose a security threat worldwide and remain elusive targets for detection with geophysical methods. P-wave diffraction imaging is an increasingly common technique for detecting subsurface discontinuities that are smaller than the seismic wavelength (such as faults, pinch outs, and small voids) and has been successfully used to detect shallow tunnels. P-wave diffractions from tunnels typically have very low signal-to-noise ratios and are therefore challenging wavefield components for imaging. Mode-specific amplitude characteristics of theoretical diffractions from a shallow tunnel were evaluated using 9C seismic modeling. Results indicate that SH-wave diffraction has the largest amplitude and coherent phase characteristics along the traveltime hyperbola, making it ideal for diffraction imaging. In real data acquired over a 9.2 m deep tunnel, amplitudes of SH-wave diffractions are 20 dB greater than P-wave diffractions. The tunnel signature on the P-wave diffraction section is of low amplitude relative to the background. The SH-wave diffraction section has a high-amplitude signal focused at the horizontal location and a traveltime consistent with the tunnel location, indicating that the SH-wave may be optimal for diffraction imaging to detect shallow tunnels.