I re-read a bit of Chaplan's "Urban Treasure Hunter" this morning and if I recall correctly, he suggests that most coins are less than 6 inches, unless in an area of heavy sediment activity.
What I'm interested in knowing is to what extents coins are buried by incoming sediment vs. to the extent that they actually "sink" in-place. Some ramifications here:
1) Coins buried by new sediment (driven rain/melt water for instance) may be more common at lower hunting spots. And for every inch of sediment laid on the coin, it may well be that nearby higher/sloped ground has lost a bit. Could be shallower pickings above? But they may be sparser hunting also, since those coins would never have been as deeply/quickly buried, and the odds that someone surface-collected them shortly after being lost would be higher.
2) I read an interesting book "Reaching for the Sun: How Plants Work" (King):
http://www.amazon.com/Reaching-Sun-How-Plants-Work/dp/0521587387. I came away with the feeling that plants, particularly trees, are basically mineral pumps. They suck minerals out of the soil below and re-deposit some of them on the surface (decaying leaves and other organic matter). It would be fascinating to see a sped-up cross-section of coins under trees, over a period of a century. How far down do they go, as the tree pulls out a minute amount of minerals/soil from below and moves them upward? What forces (roots) also work on the coin, in the opposite direction?
3) I've wondered about the extent to which coins "sink" in place. Even without incoming sediment or burial-by-organics, periodic rain storms + loose soil would seem to sink a coin a certain amount.
Would be interesting to perform experiments here. Soils with better drainage would probably sink a coin deeper, whereas tighly compacted soil might cause them to remain closer to the surface over the decades of seasons.
My Garrett GTAx 550 is one of the cheaper detectors with notch-discrimination and fairly reliable target ID. It would be nice to have a detector which is more accurate in judging trash from jewelry, before I bother digging. I'm not sure how the high-end detectors perform. How readily do they distinguish between a crushed aluminum can, bottlecap, pull-tab, and other junk -- vs. jewelry? I'm fairly happy with the depth on my unit, it's mainly target ID I'd like improved.