Physical Limitations on Detecting Tunnels Using Underground-Focusing Spotlight Synthetic Aperture Radar

This paper examines the feasibility of underground-focusing spotlight synthetic aperture radar (UF-SL-SAR) systems for tunnel detection applications. A general formulation is reviewed for generating UF-SL-SAR imaging by using multiple frequencies across a wide band and by focusing in space to subsurface points using well-known ray refraction at the nominal ground surface. A full-wave finite-difference frequency-domain model is used to consider wave propagation in realistic soil with loss- and frequency-dependent dielectric constant and a randomly rough ground surface, both of which serve to obscure and distort the returned tunnel target signal. Imaging results are presented for two representative soil scenarios: dry sand and moist clay loam. Considering the ground surface ray refraction for focusing greatly improves the SAR image relative to conventional SAR focusing at the ground surface. Using UF-SL-SAR, a small shallow tunnel is reasonably imaged for the sand case, despite the roughness of the ground interface. However, for higher conductivity moist clay loam, the clutter from the rough surface overwhelms the significantly attenuated target signal, which must propagate through the lossy intervening soil. It is demonstrated that, despite ideal focusing, the tunnel is successfully imaged only for the sand case.