Dipole array antenna and loop antenna for estimation of direction and polarization in borehole radar

This paper proposes a receiving array antenna that can estimate direction of arrival (DOA) as well as wave polarization in a borehole. The array antenna consists of a loop antenna and vertical dipole array antennas. These antennas are fed by thin coaxial cables concentrated into a centered conducting cylinder in a borehole. A prototype of the antenna was constructed and used in field experiments. In the first experiment, vertically polarized or horizontally polarized waves were incident on the receiving array antenna in a water-filled borehole. According to the experimental data, the array data contains information on wave polarization of the arriving wave as well as the direction of arrival. In the second experiment, the receiving array antenna and a transmitting antenna were set in a single borehole and used for singlehole measurement. Reflected waves were received from fractures and an interface. After signal processing, there are relations between the estimated polarization and characteristics of the reflectors. Also three-dimensional reflection point positions with the array data were demonstrated.