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Singlehole borehole radar measurement with a thin directional borehole radar sonde

This study demonstrates 3-D imaging of a fault using a thin (5.4 cm diameter) directional borehole radar sonde. In order to make the thin sonde, we needed to reduce interference between the centered conducting cylinder (CCC) and the antenna elements in the coaxial-fed circular dipole array in a borehole (CFCAB). Ferrite beads were loaded in the CCC for reduction of the interference. We conducted singlehole measurements with the thin sonde in an air-filled borehole. In this experiment, according to the power spectrum of a direct wave, we found that the ferrite loading decreased the resonant frequency of the CCC, widening the operating frequency bandwidth. The image constructed using the directional borehole radar agrees well with the information obtained from the boring core samples and observations in the gallery.