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

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.