Diffraction pattern by an empty rectangular cylinder in a dielectric

The capability of frequency-swept cross-borehole radar to detect an empty r ectangular cylinder embedded in a. dielectric medium is simulated numerical ly by employing the boundary element method. The frequency loci providing t he strongest double dips in the received signal pattern are plotted as func tions of the observation distance and the cross-sectional width. It is foun d that, regardless of the shape of thf rectangular cross-section, the stron gest double dips become double nulls in the near-field region.