Sorry to keep kibitzing on this. I should point out, my experience is anecdotal, and perhaps 10 years out of date at this point. Most of the work done was on the "wet" side of Washington state, where ground water levels are pretty much at the surface most days (at least it was whenever it was me that had to be the shovel monkey). Workers using these devices have told us that the ground water, or soil moisture content, tends to play hob with the propagation of the electrical signals. I think there are some acoustical devices that work, and are somewhat more reliable in such conditions, but I have no experience with those. Buried cable runs are now mandated by our local utility company to be backfilled with location tape attached to the conduit and run along the trench at 12-24" of backfill cover.