The direct ground wave method of ground penetrating radar (GPR) has been suggested as a cost-effective means of estimating field-scale soil moisture variability for irrigation and water resource management. Knowing the sampling depth of the GPR direct ground wave (GW) is very important because it is critical to know the depth when measuring soil moisture in the field. Few studies have addressed this particular aspect of the GPR method. Numerical simulation of GPR electromagnetic waves using GPRMAX2D was performed for two-layer soil models to estimate the direct GW sampling depth for soil moisture. Dry over wet soil layers and wet over dry soil layers were modeled by using appropriate dielectric permittivity values for each layer. Model runs were conducted for a gradually decreasing upper layer thickness. The GW sampling depth was estimated as the upper dry or wet layer thickness when the modeled GW velocity decreased or increased by 5% as affected by the lower wet or dry layer, respectively. It was found from this modeling exercise that the GW sampling depth changed with the antenna frequency as well as the moisture content of the upper layer. A very strong linear relationship (r2 = 0.98) was found between the wavelength and the sampling depth of the GPR direct GW.

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