Suppression of Borehole-Guided Waves Supported by the Connection Cable of a Single-Borehole Monostatic Pulse Radar

Recently, a single-borehole monostatic pulse radar has been developed and operated at a well-suited tunnel test site. The measured B-scan data were severely contaminated by obliquely striped patterns. In this paper, numerical simulations and field experiments are performed to find a proper way of suppressing the unwanted striped patterns. For numerical analysis, the finite-difference time domain is applied to a simplified 2-D model consisting of a water-filled borehole, two different rocks, and a dipole antenna connected to a conducting cable. It can be visualized that a set of oblique lines is generated from the borehole-guided waves reflected at the interface of two different rocks. When the conducting cable is coated by a perfect magnetic conductor (PMC), the oblique lines are nearly eliminated in the simulated B-scan result. In practice, the PMC coating is replaced by a ferrite core loading, and those loading effects on suppressing the borehole-guided waves are measured by operating our single-borehole monostatic pulse radar at a well-suited tunnel test site. Without a ferrite core loading on the connection cable, the borehole-guided waves cause the measured B-scan to be contaminated by the unwanted striped patterns. When a ferrite core is wrapped around the connection cable, the borehole-guided waves are nearly removed in the measured B-scan data. In particular, the unwanted striped patterns can be suppressed over 90% even if the total length of a ferrite core decreases to 0.5 m.