Detection of tunnels in low loss media illuminated by a transient pulse

Abstract

The field scattered from a subsurface object illuminated by a transient pulse is determined using the T-matrix method. Although the method itself is a resonance-region, frequency-domain technique, it is shown that it can be applied to problems in which the pulse has energy that spans several decades in the frequency domain. The illuminating pulse simulates one used in an actual VHF subsurface radar exploration device. The pulse is radiated from an electric dipole in a vertical borehole. The scattered pulse is obtained by first calculating the components in the spectral description of the transmitting antenna current. The T-matrix method is then used to determine the system transfer function, which, in turn, gives the scattered energy in terms of the antenna current. An inverse Fourier transform yields the temporal scattered field. The scatterer is modeled as a dielectric ellipsoid, which is presented as an approximation to a cylindrical tunnel. >