Plane Wave Arrival Time Studies with a Dipole Array Antenna in a Borehole

In this study we investigated the influence of a borehole on the estimation of wave arrival direction using a dipole array antenna in a directional borehole radar system. According to computer simulation, if a plane wave is incident at certain elevation angles, there is little arrival time difference among the dipole array signals. In this case, the estimation of arrival direction fails. When the elevation angle of wave incidence is very steep, the estimated direction of arrival becomes opposite to the true direction (i.e., off by 180 deg.). We conducted a cross-hole measurement to verify these phenomena. We observed little arrival time difference among the dipole array signals at a certain elevation angle we deemed the critical angle. For a wave incident on the dipole array at very steep angles, field experiments found that the estimated direction of arrival was almost opposite to the true direction like the computer simulation.