The ground you detect sound similar to the ground I detect, I do get a little more depth than you but not much over 5 1/2 to 6 inches all due to a thick layer of magnetite in my ground that runs any where from 4-6 inches down below the surface
if you drop a 2 inch rare earth magnet on the ground I detect it will come back looking like a fuzzy tennis ball, it makes for some vary tough detecting conditions and in a lot of cases a PI is your only option to battle the Magnetite and salt in my ground and even then sometimes it is a crap shoot with a PI