Easy instructions
Bigger coil=deeper
LF Coil=silver
MF Coil=general purpose
HF=Gold
DD coil bad soil
concentric coil you must overlap your swings
Large DD coil cover more area.
On a personal note: dig all deep signals 30 and up. A silver dime at 7 inches will ring 30-32(like zinc or copper penny) a shallow one will ring in at 38-40.