An analytical model for studies of soil modification effects on ground penetrating radar

Due to the similar dielectric constants of buried nonmetallic targets and dry soils, it is often difficult to detect and identify nonmetallic targets with ground penetrating radar. The addition of properly chosen chemical agents to modify soil properties can potentially provide improved detection. Previous studies using waveguide experiments have shown that the addition of water improves dielectric contrasts but also increases loss so that target detectability is not necessarily improved. The addition of liquid nitrogen to wet soils can reduce background medium loss and restore target visibility, and waveguide studies of target detection through controlled depth of nitrogen penetration have shown that scattering can be significantly enhanced if an optimal amount of nitrogen is added. A simple physical optics (PO) model for scattering from a three-dimensional target buried below a half space is presented, and it is shown that the radar cross section of the target depends on the dielectric contrast with and attenuation in the background medium. The model is validated through comparison with a method of moments code and found to yield accurate predictions for near normal incidence geometries. Analytical studies of target detection with two concepts of soil modification are then described: obtaining an "optimal" homogeneous soil water content and the addition of a large quantity of water along with an optimal amount of liquid nitrogen. Finally, initial tests of these soil modification techniques with a dielectric rod antenna ground penetrating radar are performed and demonstrate that the addition of liquid nitrogen to excessively wet soils can reduce loss and enhance target visibility.