DETECTION OF TUNNELS IN LOW-LOSS MEDIA ILLUMINATED BY A TRANSIENT PULSE

The field scattered from a subsurface object illuminated by a transien t pulse is determined using the T-matrix method. Although the method i tself is a resonance-region, frequency-domain technique, it is shown t hat it can be applied to problems in which the pulse has energy that s pans several decades in the frequency domain. The illuminating pulse s imulates 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 cu rrent. An inverse Fourier transform yields the temporal scattered fiel d. The scatterer is modeled as a dielectric ellipsoid, which is presen ted as an approximation to a cylindrical tunnel. Results are compared to those obtained from a field experiment in which a deeply buried tun nel embedded in granite was illuminated by a dipole source.