Directional borehole radar with dipole antenna array using optical modulators

In this paper, we describe a directional borehole radar comprising a dipole antenna array with an optical modulator capable of determining the position of targets in three dimensions (3-D). Optical modulators using a Mach-Zehnder interferometer are used to transform electrical signals into optical signals at the feeding points of the dipole antennas. The advantages of using these modulators are that we can easily arrange the dipole antennas in a borehole, and that we can expect a good agreement between the experimental data and a theoretical model representing the array. We have made a prototype borehole radar system with five dipole antennas for the reception. In order to model the antennas, we used the method of moment (MoM), utilizing a modified Green's function for dipole antennas in multiple cylindrical layers. The Green's function is evaluated analytically by numerical integration. Cross-hole and single-hole measurements were carried out in granite at the Kamaishi mine (Iwate, Japan), and we obtained good agreement between the experimental data and the MoM results. After applying superresolution techniques to the data received by the array, we estimated the 3-D scattering position of a geological interface in granite. The results were in fairly good agreement with borehole scanner images.