Cylindrical air layer to reduce the influence of the direct wave in singlehole borehole radar

This paper proposes a method to improve detectability of a reflected wave from an object near a borehole in singlehole borehole radar measurement. If a reflected wave arrives at almost the same time as a strong direct wave, the direct wave often makes it difficult to detect the reflected wave in the time domain. We calculated borehole radar signals with the Method of Moments and modeled the medium around the antennas in a borehole with five cylindrical layers. Replacing part of a water layer with an extra air cylindrical layer increases detectability of the reflected wave. In a field experiment in soil, we inserted an air tube, which consists of two vinyl chloride pipes, between the radar probe and the wall of the borehole. The experiment confirmed the air tube insertion improved detectability of the reflected wave.