Hey, 2 Much Trash, yes, that was me about the "VDI" with depth being real good. It is not PERFECT -- at a few inches, a memorial penny will hit a solid, non-varying 83 on my machine, and it won't do that at 8". However, down there at 8" to 10", where the tone begins to tend toward being a quiet whisper, the VDI might read, on successive sweeps "82-77-74-85-80-81-71-87-77-75-83-86-79" or something like that. With a zinc hitting in the mid 70s, a nickel in the upper 50s, and a quarter in the upper 80s, it is clear that the range of VDI that I gave in my example says, very clearly, "this is likely a copper penny or dime type of target, so DIG it." To me, I call this VERY GOOD VDI on a target which sits near the fringe depth capability of the machine. No, it's not a steady 83, but if you average those low to mid 70s to low to mid 80s tones, you end up very close to that 83 number which says "dime/penny." One other trick, is if you get a high, but slightly bouncy VDI, pinpoint the target, and then do quick but short sweeps over the exact center of the pinpointed target, and it tends to steady out, a bit, the bouncy VDIs into a tighter range of numbers.
If you want, I can hit the test garden, and run over my fringe depth targets with, say, ten sweeps, and report the exact results. Keep in mind, though, my test garden is only a month old, so it will not give as accurate of VDI as would be likely with a coin that has sat 50 years in undisturbed soil.
Steve