Monitoring Saturation In The Vadose Zone After Simulated Rainfall Using Ground Penetrating Radar

Abstract

Ground penetrating radar (GPR) is a proven method for detecting changes in the water table. In this paper, we present data that suggest that GPR can also be used to monitor the percolation of rainfall into the subsurface. Two series of bistatic 500 MHz GPR data were taken over a 4x4 foot grid in the center of a polyethylene tank. For the first series of measurements, the tank was filled with gravel and sand. The GPR measurements were taken both before and after two separate one-inch simulated rainfalls were sprinkled on the top of the tank. For the second series of measurements, the tank was filled with gravel, sand, and a sand-clay mixture. GPR measurements were taken both before and after a single one-inch simulated rainfall. For both experiments, multiple sets of GPR measurements were taken after the simulated rainfalls over a number of hours. As time progressed we can see in both the 2-D sections and 3-D blocks of our data how the water first infiltrates the tank, then accumulates in low permeability zones, and finally migrates to the bottom of the tank. Although we are unable to image the wetting front, we can see anomalies that arise as the water passes through the tank, and we can see water table rise as the water accumulates at the bottom of the tank. The effect of the sand-clay mixture in the tank is to attenuate the GPR signal. We discuss the potential of using GPR measurements to estimate hydraulic parameters under the proper field conditions. We conclude that GPR is sensitive to changes in water content in the vadose zone. Not only is GPR sensitive enough to monitor changes in water table levels, but it is also sensitive enough to monitor the infiltration of water into the subsurface.