Influence of HE11 mode on direct wave in singlehole borehole radar

In this paper, we investigate influence of radar sonde eccentricity on a direct wave between a transmitting and a receiving antenna in singlehole borehole radar measurement. According to our calculation, at high frequencies above 200 MHz, guided waves play a vital role in the direct wave. We found that the most important guided wave is HE₁₁ mode, which is excited only when the antenna is eccentered in a borehole. We show that this causes artificial noise in the moving average subtraction, which is a common signal processing method used to remove the direct wave. In field experiments in granite, we conducted a special experiment, in which location and rotation of the radar sonde was controlled mechanically. We found the excitation of the HE₁₁ mode in high frequencies, when the sonde is eccentered by 1.7 cm in a borehole. These are also predicted in the theoretical analysis.