3-D estimation of target positions with borehole radar using E-field sensor array

In this paper, we demonstrate three-dimensional estimation of target positions with borehole radar in single borehole. We developed array type borehole radar having four dipole antennas with optical modulators. Experiment at a model site was done using the developed radar system. In the site, one borehole, in which there is no fluid, and one cave exist. Receiving dipole antennas and a transmitting antenna are set in a single borehole. The 3-D MUSIC algorithm modified for array type radar in a borehole is applied to the measured data to estimate location of the cave. 3-D estimation results from the experiments were reasonable. In the estimation, Method of Moment (MoM) was used to model the antennas in a borehole. Green's function used in MoM was modified to include influence of cylindrical boundary around antennas in a borehole. Finally, we did experiments in granite, where boreholes are filled with fresh water. Cross-hole measurement was carried out to investigate characteristic of the dipole array in a water-filled borehole. Cross-spectrum analysis was adopted to obtained data, and we found that the experimental results give fairly good agreement with the theoretical results. This implies that the experimental results are accurate enough for estimation of azimuth of arrival waves, even if the antennas are in a water-filled borehole.