Electromagnetic Analysis and Experiments for Directional Borehole Radar Using Optical Electric Field Sensors

Abstract: We propose to introduce an electric field sensor using LiNbO3 optical modulator to an array type directional borehole radar. It is great advantage to measure electric field very accurately with no electromagnetic disturbance in use of the electric field sensor. Furthermore, the sensor needs no battery and only small space to transfer electrical signals to optical signals. If the sensors are used as array elements of the directional borehole radar, we might accurately measure electric fields at several points. And we can determine directions of arrival waves with time delays and relative amplitude of signals received at the points. We did experiment that the sensor in a borehole is illuminated with a transmitter outside the borehole. Transmission between the transmitter and the electric field sensor in a borehole was measured, and we obtained array data set with the sensor. The array data was analyzed by cross-spectrum analysis, and time delay and relative amplitude among the array elements was estimated. In order to give theoretical results, we analyzed the dipole array in a borehole by Method of Moment (MoM). In the MoM, effect of scattered fields caused by cylindrical inhomogeneity around a borehole is included. The theoretical results gave good agreement with experimental results. This is because we could accurately measure electric fields with the optical modulator. We think that the experimental results show a possibility to realize directional borehole radar using the dipole array with an optical modulator