The detection of non-metallic anti-personnel landmines with ground penetrating radar (GPR) is complicated by low dielectric constrasts with the surrounding background medium. Previous studies have shown that the addition of water can improve dielectric contrasts but also increases loss so that target detectability is not necessarily improved. Previous studies have also shown that the addition of liquid nitrogen to wet soils can reduce background medium loss and restore target visibility. In this paper, further waveguide studies of target detection through a controlled depth of nitrogen penetration are reported, and it is shown that scattering from known depth targets can be significantly enhanced if an optimal amount of nitrogen is added. The procedure can also be generalized to unknown depth targets if measurements are taken as gradually increasing amounts of liquid nitrogen are added. Both analytical models and waveguide experiments are presented to illustrate these ideas. Finally, initial tests of the soil modification techniques developed through waveguide experiments are reported with a dielectric rod GPR system; results indicate that these methods should be applicable to general GPR sensors.