I do agree with what he is saying though, and even at closer distances there will be some averaging.. shallow targets will hide the deeper ones, whether its iron or a disced out piece of trash, the less disc you use the better chance you have at hearing any subtle difference... but generally ones that are close enough will average a bit of both
its the main why I like iron mask set at lowest setting and ferrous audio in fact.. nothing masked out that is non ferrous at all, while still not hearing high tone falses from iron, and most nails and other iron are quiet..
But yep the only true way to get it all is to dig every single piece of trash and iron out of the way.. I prefer just to get as much as I can.. save that insanity for when I have no more sites with those targets
its easily provable, I hunted inside a small shallow foundation many many times, it was only about 12 by 8 feet.. , one day decided to just dig some nails cause there were some nice old ones in there, and low and behold I took a shovel of dirt out stuck in the x1 probe and to my surprise I got a blast in the coin range and pulled up a large cent.. the bottom to the clay in the foundation was no more than 8 inches deep.
So I scraped off a small part a little by little, in this small area I got 4 or 5 more large cents many buttons and other targets and a 1740 Bavarian silver coin.. all of this hidden by the nails that were shallower.. most places its not possible to do that though... sure cant do it in a park.. at least if you want to keep hunting it
had another similar small foundation with same results.. lots of stuff hiding we cant hear.. I often wonder just all of what we put our coils over and never will find.. I bet we been over some mighty fine stuff..