Assessment of Soil Moisture Content Measured by Borehole GPR and TDR Under Transient Irrigation and Drainage

Abstract

Borehole ground penetrating radar (GPR) was assessed as a method to measure the temporal and spatial variability of soil moisture under uniform wetting and drying conditions on a sandy loam soil. Zero Offset Gather (ZOG) and Multiple Offset Gather (MOG) surveys were conducted before and during the uniform irrigation and the subsequent drainage experiments using the PulseEKKO 100 borehole system with [Formula: see text] antennas in horizontal access tubes. Time domain reflectometry (TDR) data were collected from 10 vertical probes installed at [Formula: see text] increments from [Formula: see text] below the ground surface. The TDR data were used as standard measures of soil moisture to compare with the GPR estimated soil moisture. Electromagnetic wave velocity along the survey profile at about [Formula: see text] below the ground surface was estimated using ZOG and MOG data by picking the arrival time of the first event. Volumetric soil moisture was calculated using a standard empirical relationship between velocity and water content for each ZOG location. MOG data were used to develop horizontal soil moisture distribution maps at about [Formula: see text] below the soil surface by performing borehole tomogram analyses. Velocity tomograms were developed assuming straight ray paths between the source and receiver antennas and the velocity tomograms were converted to soil moisture maps. High repeatability of soil moisture measurement was found with borehole GPR method since the observed high moisture zones were consistent with time throughout both irrigation and drainage. The radius of influence of the borehole GPR measurements was about [Formula: see text] as determined theoretically and by comparing the GPR and TDR soil moisture contents.