Numerical Simulation of the Wave-Guide Effect of the Near-Surface Thin Layer on Radar Wave Propagation

Abstract

This paper assesses ground-penetrating radar (GPR) moveout survey performance in different near-surface geological stratigraphy and different antenna orientations using two-dimensional (2D) finite difference time domain (FDTD) numerical simulations of field data. We first treat the simple cases of radar pulses propagating along (a) the interface between two half-spaces (air∕ice); and (b) an ice thin-layer wave-guide (air∕ice∕water) between two half-spaces. We then simulate four more complex cases combining two radiation polarizations (TM and TE), and two geological settings: a sandy∕gravelly half-space overlain by a silty∕clayey layer, and a silty∕clayey half-space overlain by a sandy∕gravelly layer. Both cases are represented through different dielectric constants. The results show that, first, more EM energy is radiated as an air wave for the TM mode, and more EM energy will be sent into the ground when the TE mode is used, regardless of stratigraphic sequence. Second, where a gravelly sandy half-space overlain by a silty∕clayey layer, more EM energy will be trapped in the silty∕clayey layer as the guided ground wave. Third, when the TE mode is used there is much less air radiation for the case of silt overlying gravel than that of silty∕clayey half-space overlain by a gravel∕sand layer. Fourth, for the stratigraphic sequence of a sandy∕gravelly half-space overlain by a silty∕clayey layer, the TE mode fundamentally contains only a ground wave, and the TM mode essentially contains only air wave energy. This implies that for this case a far more complete separation of the air wave and the ground wave can be reached. Fifth, dispersion of phase and group velocities of the guided ground wave will be well developed for the TE mode. These simulations imply that antenna polarization mode is an important factor when using moveout surveys to study subsurface electromagnetic properties.