There was an attempt, on at least one machine that I recall, back in the late 1980s or early 1990s, that had an option of "surface blanking". Designed for the intention that some types of hunting (typically turf), has newer targets @ shallow, and older targets @ deeper . Conditions where coins tend to be stratified commensurate by age. And a lot of hunters would simply mentally pass shallow signals, in their quest for older deeper whispers in turf. Hence: Why not make the detector have this an automated feature , right ? It's merely a matter of having an intensity meter, where those louder signals are simply filtered into a reject category, eh ? Genius idea,
right ? 
Designed by George Payne.
But it was unpopular, d/t the "devils in the details". Here's why :
Assume for the moment that there's a shallow memorial penny or soda can or whatever. Naturally, it's a loud signal, right ? But here's the problem: As you're swinging along, and your coil catches the edge/fringe of such a target, it would give a whisper signal. So the user would think "aha !". Yet, of course, when you go criss-cross it (and now you're over the loud center), then PRESTO, it disappears. So you move on to the next target. Only to have the same phenomenon keep repeating over and over. And people found themselves trying to "pinpoint" the flutters of the sides of targets, blah blah blah. It simply didn't work as intended, in actual field conditions.
There was also VERY Poor (almost deceptive) advertising that accompanies that (Marketing dept. genius) : When you saw the magazine ads for this machine, it gave the impression that you were "seeing through" that top 3 or 4" . Ie.: defeating masking. As if that top 3 or 4" magically wasn't there. This was not true at all. You simply nulled out over the larger object AND anything that was underneath it, at the same time.
Thus it's better to let your ears do the work. If you get a machine with full tones and sounds, you will, like still-looking-52 said, be able to use the modulation to tell deep vs shallow. Pretty soon, you'll do it without even thinking.
The exception would be monotone machines. Ie.: the beep-or-no-beep type machines, with no in-between states. Like: machines with audio-depth boosters (where they try to make the "weak signals louder. While that seems like a noble idea (so you're less inclined to miss a deep whisper) yet : You can kiss goodbye any ability to tell deep vs shallow. Best just to train your mind and ears.
