The pi's can go really deep, but you have to ground balance and they pick up iron in the rock strata below. So you pretty much have to have an idea of where it might be at first, and then go over with a detector about 5 or 6 times, figure out the feedback relative to what's around by markers, alignments, auras, etc , and then decide if you want dig. It gets frustrating, but I think it's possible and I know it has been done.
The pi detectors , you are re-ground balancing every 10 feet or so, so it becomes a chore to figure what exactly it's reading. My largest pi coil is 98 inches across, it theoretically will hit a tin can at 6 feet. It takes 2 to operate, about 20 minutes to set up and 15 to take down (yes it's a chore just setting up and carrying all the parts). But , if everything works out on the auras, alignments, markers etc, I would probably try and dig and see what's causing the hit, even a slight one.
Another thing, the magnetometers (spelling?) have been used to find old digging tools in covered mines, as well as impurities in bars. But they will go off on everything else as well that has iron content. Like basalt around my area. Sometimes even cow/horse pee can set off a balanced pi detector. So it's all difficult at best.
I wouldn't say don't use them, I just wouldn't spend 8-20k on unproven equipment. The only person I know claiming to find something with an OKM unit, was selling it right after. Why would he do that if it worked? Makes no sense.
