The original info I think said air test 12" on quarter. Most soils don't allow detection depth to equal air distance. Some unmineralized salt
beaches one might exceed air distance. But in most soils, expect less depth than the air distance.
Also, there are coils available from NEL that are larger than the 11" DD that may go deeper, but just got mine & haven't tried it yet.
My Omega 8000 is loud on a 5" dime with sensitivity at only 30 so must go significantly deeper. It is stable at sensitivity 55 in back yard, which has power poles. Best wishes, George (MN)