How about this as a suggestion - a resistivity mapper.
Basically, you measure current between two spikes placed in the ground. They should be spaced about 500mm apart and insulated except for the tips so the electrical contact area stays constant regardless of how far it penetrates the Earth. The probes are mounted on an insulated frame that can be moved along in 500mm steps. At each position, the current and therefore the resistance is measured and logged on a computer or in a memory device. When a large enough area has been probed, the results are plotted as a map with different shades or colors used to indicate the resistance at that position. It should show underlying geology and man made constructions even when they are hidden from view at the surface.
You could expand it with GPS to get exact positions and experiment with interpolation when the basic unit was working.
Brian.