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 ..