Diffraction pattern due to a trapezoidal air cylinder in a dielectric

As a theoretical model of frequency-swept cross-borehole radars for locatin g deeply buried tunnels with asymmetric cross sections, E-polarized scatter ing by a trapezoidal air cylinder in a dielectric medium is analyzed. The r esults of simulations performed by employing the boundary-element method il lustrate that the null frequencies corresponding to the upper and lower bou ndaries of the cylinder are different. Also, the physical mechanism for the occurrences of nulls, which depend on the signal frequency and the observa tion distance, is presented in detail. (C) 2000 John Wiley & Sons, Inc.