It's been awhile since I've sat in on geology, but I seem to recall that they can use short-range seismic signals (by firing a shotgun shell into the ground). I'm not sure as to the resolution -- one could imagine getting better resolution firing multiple shells simultaneously over a certain area. My $.02
"39 explosions for the purpose of geological exploration (trying to find new natural gas deposits by studying seismic waves produced by small nuclear explosions)"
