Analysis of Eccentered Dipole Antenna for Borehole Radar

Abstract

A dipole antenna in a borehole may be placed away from the center of the borehole, and this eccentricity produces additional complicated electromagnetic fields, which may influence borehole radar signals. In this paper, we analyze an eccentric dipole antenna in a borehole for borehole radars. Our approach is an extended version of the pseudoanalytical formulation that was previously applied for analyzing an induction logging tool and a dielectric logging tool. The proposed equations are suitable for modeling the finite dipole antenna, utilizing a method-of-moments. The eccentricity of the radar sonde has two consequences. First, it modifies the antenna characteristics such as its impedance, and second, it changes the electromagnetic field inside the borehole, owing to the change of the structure of the cylindrical layers. These two changes are evaluated by the proposed algorithm and two types of experiments. The first one is a measurement of the input impedance of a monopole antenna which is in cylindrical layers on a ground plane in air. The second one is about the transmission between two antennas, one of which is eccentered in cylindrical layers. This experiment was done in granite at a field test site and on a ground plane in air. In the field experiments, we controlled the position of the radar sonde in a borehole and made the antenna eccentered artificially. We found that a 4- cm displacement resulted in changes of 4 dB in amplitude and 4 ns in time delay. Comparing the experimental and calculated data obtained by the present method shows the validity of the proposed analytical formulation.