MoM analysis of circular array resonance to design directional borehole radar

This paper investigates that resonance of a circular array influences estimation of azimuth of arrival waves for directional borehole radar with Method of Moment (MoM). In the developed radar system, receving dipole array antennas utilyzes optical modulators. Using the array antennas, we will be able to measure electric fields on a circle at several points in a borehole with no electromagnetical disturbance by feed lines, because received signals of the antenna is transmitted by an optical fiber. The dipole antenna array in a borehole can be modeled with MOM. Generally, large number of array elements can improve resolution of directions of arrival waves. However, in dipole array antenna in a borehole, the MOM analysis shows that increase of element number does not always improve the resolution in some condition. This is caused by resonance among antenna elements closely arranged in a borehole. In order to verify the phenomena, we carried out a field experiments. Two boreholes, which are drilled in granite, were used at field experiments site in Osaka, Japan. One dipole antenna was set for transmitter in one borehole. In the other borehole, the seven dipole antennas with the optical modulators on a circle were set for receivers. We measured transmission between the transmitter and the receivers in frequency domain with a network analyzer. As the results, we concluded the first order resonance of the circular array influence estimation of azimuth of arrival waves with the dipole antenna array in a borehole.