Monitoring Infiltration Process in Field Soil Seamlessly Using Array Ground Penetrating Radar.

Ground penetrating radar (GPR), one of the electromagnetic (EM) exploration techniques, is regarded as an effective geophysical method to monitor soil water infiltration process in rapid and non-destructive fashion. The use of multi-offset gather (MOG) is essential in quantifying the location of the wetting front during infiltration. Despite its advantages, application of GPR to quantify infiltration processes in field soils has been limited because collecting MOG data, such as wide-angle reflection-refraction (WARR) or common-mid-point (CMP), in the field is usually time consuming.

Recently, a family of array GPR has been developed. The basic concept is to electrically switch multiple antennas sequentially in milliseconds to scan subsurface processes in a minimal effort. To our knowledge, it has not been fully investigated whether array GPR can be used for monitoring infiltration in the field soil. The main objective of this study was therefore to investigate if MOG obtained by the array GPR could track a wetting front seamlessly during vertical infiltration in the field soil.

An infiltration test was conducted at a test site in Tottori Sand Dune, Japan. An array GPR system was used to acquire time-lapse MOG data seamlessly during the infiltration in the field soil without physically moving antennas unlike conventional GPR. The GPR system used in this study was a step-frequency continuous waveform radar operating over the frequency range from 100 MHz to 3000 MHz, paired with an array consisting of 10 transmitting antennas and 11 receiving antennas, for a total of 110 transmitter-receiver combinations. Collected time-lapse MOG data clearly show the wetting front evolution with time. Reflection signals in MOG data agree well with two-way travel times predicted from dielectric constant observed independently with a soil moisture sensor.