Directional borehole radar antenna calibration

A directional borehole radar system was developed and tested to assess its directional accuracy. The radar probe employs an Adcock dipole array as its receiving antenna. It is well known that the directional accuracy of the radar probe deteriorates significantly with increased antenna eccentricity. A simple solution of mechanical centralizers is impractical and increases the risk of the probe getting stuck deep inside the borehole. In this paper, we propose an antenna calibration solution that corrects the effects of eccentricity. After a brief discussion of the effects of eccentricity and a theoretical concept of antenna calibration, the calibration method and its implementation in hardware are discussed. Finally simulation and experimental results, which validate the proposed calibration solution, are presented. At an extreme eccentricity, the estimated azimuthal angle had mean error of more than 60°. After calibration the error is reduced to less than 5°.