CROSS-POLARIZATION BOREHOLE RADAR MEASUREMENTS WITH A SLOT ANTENNA

In this paper we discuss co-polarization and cross-polarization reflection measurements in borehole radar. Most conventional borehole radars use axially-oriented electric dipole antennas for transmission and reception, resulting in a response only to the co-polarized signal component of the reflected wave. However, more information can be obtained if the cross-polarized component of the reflected wave is also measured. Cross-polarization borehole radar measurements are presented in which an axially-oriented slot antenna is used as the receiving antenna and an electric dipole is used as the transmitting antenna. The slot antenna was excited by a short DC pulse (<10 ns in duration) to obtain broadband pulse-echo operation. The physical configuration of this antenna is quite suitable for borehole applications, because the slot antenna aperture can be made an integral part of a metallic cylinder. Field measurements using two prototype radar sonde arrangements containing co-polarized (vertical) and cross-polarized antennas were carried out in granite rock. The measured signals showed that detection of the short-range reflections can be significantly improved with cross-polarization measurements since direct coupling between the transmitting and receiving antennas is considerably reduced. Comparisons of the reflected signals produced by the co- and cross-polarization measurements exhibit responses which show that polarimetric radar measurements in a borehole can be practical and beneficial.