The GPR direct ground wave method for soil moisture content estimation: Field experiments and modeling

Ground penetrating radar (GPR) is an alternative and potentially efficient method for soil moisture content estimation for relatively large areas; however certain aspects of using the GPR direct ground wave approach are poorly understood. For example, estimation of the direct ground wave penetration (sampling) depth is an important factor to understand in developing efficient water management practices for irrigation and drainage. The fixed offset (FO) GPR method is a potentially rapid and more practical survey method for large scale applications. Proper time zero calibration, optimum antenna offset and alternative means to determine the ground wave travel time require more research when using the FO method.; The soil moisture variability was assessed using the direct ground wave of surface and borehole GPR under transient irrigation and drainage conditions in a sandy loam soil. Soil moisture variability, estimated using the direct ground wave of GPR, was compared to the soil moisture measurements obtained with vertically installed TDR probes. As well, the influence depth of the surface GPR direct ground wave was modeled for layered soils and different frequencies. The direct ground wave penetration depth was found to be in the range of 0.30∼0.50 m experimentally with 450 MHz antennas under transient irrigation and drainage. The GPR ground wave modeling showed that the ground wave influence depth varied from 0.75 m to <0.12 m for the frequency range of 100 to 900 MHz, respectively. The influence depth was found to be directly and strongly proportional to the wavelength.; The optimum antenna offset for the FO method was determined in the field to be 1.5∼2.0 m for 450 MHz. Time zero calibration based on the measured airwave velocity was stable during the measurement time. The ground wave arrival time based on the ground wave peak time was found to be a more practical method for calculating the ground wave travel time for the FO method. It is recommended that a WARR survey be completed first at each field site and then continue using the FO method. The GPR frequency chosen will depend on the desired sampling depth.*; *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Microsoft Office; Windows MediaPlayer or RealPlayer.