Time-lapse crosshole radar profiling of infiltration process in a vadose zone

Ground-penetrating radar (GPR) is an effective tool for imaging spatial distribution of hydrogeologic parameters. An artificial groundwater recharge test was conducted in Nagaoka City in Japan, and time-lapse crosshole GPR data were collected to monitor the infiltration process in a vadose zone. In the experiment, zero-offset profiling (ZOP) was utilized to rapidly scan the inter-borehole region. The infiltration process is clearly observed as a variation of EM wave velocities, which can be transformed into dielectric constants and further converted to water contents. GPR responses in a vadose zone are largely controlled by variations in water saturation. An increase in traveltimes can be attributed to an increase in water saturation, and a wetting front moves downward with an average velocity of about 2.7 m/h in the test zone. Interpretation of experimental results was guided by a finite- difference time-domain (FDTD) method for two-dimensional cylindrical coordinates to simulate radargrams associated with the advance of a wetting front during the filtration process. To accurately determine the water content profile, correct velocity analysis requires identification of first-arriving critically refracted waves from the traveltime profile. The standard ZOP for which all first arrivals are assumed to be direct waves, results in 4.5 - 16 % underestimation of water content in a transition zone.