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

Abstrucr4he field scattered from a subsurface object illuminated by a transient pulse is determined using the T-matrix method. Although the method itsell' 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 puls(l: 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 invc rse Fourier transform yields the temporal scattered field. The scatterer ia, modeled as a dielectric ellipsoid, which is presented :is an approximiation to a cylindrical tunnel. Results are compared to those obtained from a field experiment in which a deeply buried tunnel embedded in granite was illuminated by a dipole source.