Can vouch for gimp. If you have Amber client to draw gridlines on your maps, even better. Clip a pic of your caves, clip a pic of the surface, layer them, align then, and compare. i successfully prospected 3 separate nodes which were each ~10 tiles from a cave wall using about 5 doses each.
Be sure to prospect close to caves you're close to. If your prospecting doesn't return the red circle/ore pointer, move a good thirty tiles in a direction. When you get a ping you can be more granular in your search