Improved Discrimination of Subsurface Targets Using a Polarization-Sensitive Directional Borehole Radar

We propose a polarization-sensitive directional borehole radar with a dipole array and loop antenna in a borehole (DALAB), along with an algorithm for radar signal processing. This radar enables us to discriminate between a vertical conducting cylinder and inhomogeneity in soil in 3-D space. The antenna configuration difference, antenna coupling, and borehole effects were calibrated using the method of moments (MoM) in the proposed algorithm. According to computer simulation, the magnitude of the polarization ratio (MPR) of the incident wave may be used for discrimination between signals from targets and background reflections. A prototype of the borehole radar with the DALAB was constructed and tested in laboratory and field experiments. In the laboratory, we confirmed that the DALAB can estimate the polarization state of an incident wave despite the presence of a water layer. In field testing, we confirmed that the MPR was nearly 0 dB, as theoretically estimated when the DALAB received electromagnetic waves radiated by a tilted dipole antenna. We also carried out single-hole measurements with the DALAB and found that the MPR of a reflected wave from a vertical conducting cylinder was more than 20 dB, whereas that from soil inhomogeneity was less than about 20 dB. This implies that we may discriminate between a conducting cylinder and soil inhomogeneity and determine target locations in three dimensions using the DALAB and the MPR.