Like you guys, I do airtesting for comparison purposes and to check ID's on targets (especially trash items). The halo situation is very intersting. Some smart folks think it's actually electrical contact with the soil around the item. One thing is for sure, it would HAVE to be a a metal that can oxidize and join with the soil. Gold, for example, shouldn't be able to get a halo because, as far as I know, it doesn't oxidize in the soil. Silver and iron, though, do. So far, I've never seen a detector ground test deeper than it will airtest on the same item. I'm definitely open to debate and proof though!