THREE-DIMENSIONAL TIME-LAPSE INFILTRATION MONITORING USING MULTI-CHANNEL GROUND PENETRATING RADAR

Time-lapse ground-penetrating radar (GPR) measurements are a powerful tool for imaging dynamic hydrologic conditions in the subsurface due to the strong relationship between GPR wave velocity and water content. An experiment was performed to illustrate the capabilities of multi-channel GPR to image the dynamic nature of infiltration events on an appropriate time scale. The infiltration experiment was conducted in a tank of homogenous sand 0.6 m deep. Sixteen channels of data were collected in two lines separated by 5 cm at 13 representative times. After data were collected for background conditions, water was pumped through an irrigation grid with dimensions 104 x 65 cm at a constant flux of 0.08 cm/min for 120 minutes. The GPR data were collected at varying time intervals throughout the infiltration experiment. The sand tank has 16 drains that allowed the first outflow of water to be observed from one drain at 115 minutes into the experiment. The three-dimensional, multi-offset GPR data indicate heterogeneous flow within the homogenous sand as diffractions within the data indicate flow fingering. The results illustrate that one-dimensional flow in the unsaturated subsurface is a poor assumption for this experiment.