Cross-hole georadar monitoring for moisture distribution and migration in soil beneath an infiltration pit: a case study of an artificial groundwater recharge test in Niigata, Japan

Continuous monitoring by time-lapse and repetitive measurements using cross-hole geo-radar was conducted to investigate soil moisture distribution and migration beneath infiltration pit for artificial groundwater recharge. This monitoring enabled us to clarify the infiltration process from the infiltration pit into the vadose zone in a quantitative, nondestructive, and noninvasive way. The infiltration pit was 2.0 x 2.0 m square and 2.3 m deep between 2 boreholes in gravel soil. The groundwater table was at about —10 m. We monitored the veitical distribution of electromagnetic wave traveltime beneath the infiltration pit by repetitive measurements using cross-hole geo-radar profiling with zero-offset gathering. Traveltime was distinctly retarded from the upper layer to the deeper one after ponding of the pit. The downward retardation velocity of the infiltration rate into the soil was estimated at8 x 10-2cm/s. The estimated values for water content and water seepage rate in the soil were almost coincident with the directly measured values. In our case of an infiltration pit test, cross-hole geo-radar monitoring was an efficient, noninvasive method for visualizing the infiltration process and estimating water migration properties of the soil on a macro scale.