Estimating infiltration front depth using time-lapse multioffset gathers obtained from ground-penetrating-radar antenna array
Non-destructive and non-invasive visualization and quantification of dynamic subsurface hydrological processes are needed. Using a ground penetrating radar (GPR) antenna array, time-lapse common-offset gather (COG) and common mid-point (CMP) data can be collected by fixing the antenna at a given location when scanning subsurface. This study aims to determine wetting front depths continuously during a field infiltration experiment by estimating electromagnetic (EM) wave velocities at given elapsed times using ground penetrating radar (GPR) antenna array data. A surface GPR antenna array system, consisting of 10 transmitters (Tx) and 11 receivers (Rx), that can scan each Tx-Rx combination in 10 s at a millisecond scale was used to acquire all 110 Rx-Tx combinations in approximately 1.5 s. The field infiltration experiment was conducted at an experimental field near the Tottori Sand Dunes in Japan. Using the estimated EM wave velocity from the CMP data, the depth to the wetting front was computed every minute. The estimated wetting front arrival time agreed with the time at which a sudden increase in the moisture sensor output was observed at a depth from 20 cm and below. This study demonstrated that time-lapsed CMP data collected with the GPR antenna array system could be used to estimate EM wave velocities continuously during the infiltration. The GPR antenna array was capable of accurate and quantitative tracking of the wetting front.