Measuring the advance of a wetting front using cross-borehole GPR

Two infiltration experiments were conducted to monitor an advancing wetting front at 2.25 m below ground surface with cross-borehole ground penetrating radar (XBGPR). The focus of the experiment was to understand how XBGPR responds to dielectric permittivities that vary on a scale that is smaller than the antenna length. To test this response, a sharp wetting front was formed by applying water evenly over a 5 m by 5 m area at a rate of 5x10-4 cm/s through porous hoses. The center of XBGPR antennae were placed at a depth of 2.25 m in a pair of vertical, PVC lined access tubes located within the irrigated area. The velocity of the first arrival was converted to moisture content using a standard calibration. The measured water content increased linearly with time during the advance of the wetting front. Through comparison with modeled results of flow in unsaturated media, we demonstrate how water contents are "averaged" along the antennae.