Analysis of a direct wave in directional borehole radar with a circular dipole array antenna

摘要

In this paper, we discuss influence of radar sonde on a direct wave in singlehole directional borehole radar using a circular dipole array. The numerical simulation shows that the guided wave, HE11 mode, is excited at the higher frequencies than 200 MHz only when the radar sonde is eccentered in a water-filled borehole. Since this guided mode is a nonaxially symmetric wave, the guided wave is observed by the circular dipole array antenna with phase difference. Because of this characteristic, the direct wave containing the HE11 mode may be confused with the reflected waves from subsurface objects such as faults. We conducted field experiments, and set the circular dipole array in a water-filled borehole. In these experiments, we confirmed that eccentricity of the radar sonde excites the HE11 mode, which is observed at the circular array antenna with phase difference.