Norman gives a good answer, except that I think he meant that "caches will NOT be over a foot deep, d/t the owner wanting to get back to it".
There is somehow a perception that buried caches are necessarily deeeeep. But think of it: if you were some guy in the 1800s, who wanted to stash something in your basement (or wherever) for safekeeping, what the heck difference does 1 ft. verses 3 ft. or 5 ft. make, when the only purpose in burying it, is to be un-seen from the surface by others? I mean, the surface (to passerbys) will look exactly the same, no matter 1, vs. 3 vs. 5 ft, etc.... And also, assuming they plan to occasionally add to, or take from their stash (much like you would do to a bank account, for instance), why would they make it any more difficult for later retrieval than necessary? All that was necessary was/is to have an invisible cover, period. So caches are found all the time at a mere foot or whatever deep.
But back to the question: I would start by looking for individual coin losses. Just dig the coin-size signals, and skip the hub-cap over-load signals for now. Then when you've covered for fumble-fingers type losses, then go back through digging the big signals.
If you were strictly looking for cache-sized items (jar sized and larger), you could consider a 2-box unit, which will not even give a signal, unless the target is can/jar sized or larger (a perfect "discriminator" for those pesky coin, nail, etc... sized items).