Directional borehole radar using a dipole antenna array

In this paper, we propose to introduce an electric field sensor using LiNbO₃ optical modulator to an array type directional borehole radar. Advantage of the sensor is accuracy of measurement of fields with no electromagnetic disturbance, no battery and small space to transfer electrical signals to optical signals. If the sensor is used as array elements of the directional borehole radar, we might accurately measure electric fields at several points, and determine directions of arrival waves with the data set. We did experiment that the sensor in a borehole is illuminated from a slot antenna as a source outside the borehole. Transmission between the transmitter and the electric field sensor in a borehole was measured, and we got array data set with the sensor. The array data was analyzed by cross-spectrum analysis, and coherence, time delay and relative amplitude among the array elements was estimated. Experimental results gave good agreement with theoretical results. This is because we could accurately measure electric fields with the optical modulator, and accuracy of the data was enough to determine directions of arrival waves in a borehole. We think that the experimental results show possibility to realize a directional borehole radar using the dipole array with an optical modulator.