A numerical study on using guided GPR waves along metallic cylinders in boreholes for permittivity sounding

We performed a numerical study on using guided ground-penetrating radar (GPR) waves in boreholes for permittivity soundings using finite-difference (FDTD) simulations. The method presented here uses a GPR antenna that is placed next to a borehole in which a metal waveguide is lowered. Electromagnetic (EM) signals that the antenna sends out on the surface, couple to the waveguide and are reflected from the bottom end of the metal waveguide. Analysing the traveltimes yields accurate vertical distributions of the wave velocity, permittivity and water content in specified depth intervals. We performed numerical studies of the field distribution around the waveguide, the influence of the plastic borehole casing, as well as the resolution capabilites of the method in layered media. In this study, as a source, the GPR signal is introduced in the simulation via a 3D model of a real 400 MHz bowtie GPR antenna. We replicated the essential components of the antenna, e.g. the antenna bowties and metal casing, to accurately reproduce the transmitted signal. The guided wave has a skin depth drop in amplitude away from the waveguide of about 4.1 cm, Furthermore a maximum vertical resolution of high-contrast permittivity layers of about 5 cm is possible, and a formula for correcting the effect of the borehole casing on permittivity calculations is derived. We envision that this method and the insight from this study enables more precise soil soundings than other established GPR methods or time-domain reflectometry (TDR).