There are several reasons why ground-penetrating radar is hard to make work reliably in practice. Soil and dirt are generally moist and full of conductive elements, which act to scatter and absorb an extremely large portion of the radar energy. This seriously limits the depth the radar can reasonably operate. This is compounded by the fact that you want imaging radars to have a high operating frequency so that your range resolution (see range profile) is sufficiently small. Unfortunately, the higher the frequency, the more power is absorbed by the ground. Not good. Mines and other objects you wish to detect scatter the energy, however so do rocks, plant matter, and various other subterranean objects that aren't the target. This forces the engineer to design algorithms which can discriminate between a known types of scatterers such as mines and rocks. Dirt is a highly dispersive media, unlike air, in that different radio frequencies travel faster inside it than others. This acts to distort the radar waveform in an unknown way as it travels, again requiring the engineer to devise clever equalization techniques to un-mangle the recieved signal.

Oh, and engineers feel really wierd pointing radar antennas at the ground for some reason ..