NWCindy, I assume you are in the NW (northwest)?
You have to realize the limitations presented by the soil in your area. A detector may detect a coin at 12" in air - or in sand with no mineralization (stories I've heard from back East).
However, mineralized soil is a "target" to the detector. The more of it, the more potential depth is reduced. When I lived in the Portland area, maximum depth was about 6"-7", on a dime, with a 8/9" coil for a VLF detector, owing to the mineralization level.
A larger coil will let you cover more ground, or detect a larger object deeper, but the dime detection is limited by the soil and target's size. Put another way, 8-9" is the optimum sized coil for a dime sized object.
Edited 1 time(s). Last edit at 04/06/2019 11:49AM by glabelle.