Al, if you go get a nugget machine like some here are suggesting (to reach these crazy depths you are talking about): yes, there is no doubt that such nugget machines "go the deepest". They can reach an individual coin to nearly 2 ft. deep! And a toaster-sized object to 5 or more foot, and so forth. At first glance, that sounds like your dream machine, right? WRONG! You will be continually bedevilled by every single staple, push pin, birdshot pellet, nail, pinhead, etc.... They will all sound off "like the bells of notre dame". In any area of human habitation/history, you simply have no idea how much of this type stuff carpets the ground.
So if, as you say in your OP, your suspected objects are in "pots" or "wooden boxes", and .... assuming you truly are after those big-ticket items (and don't wish to be bothered by pesky individual coins, nails, pins, staples, etc..), then do as luvsdux says: Get a two-box unit. You can get a TM 808 very reasonably in the used market on Ebay. It will not pick up any object smaller than a soda can (or perhaps a fist sized object if you're tuned very finely/closely). It will find a jar sized item to perhaps 2 ft. deep, and a toaster sized item to 4 ft. And a volkswagon to 10 ft, and so forth. Yes it's true that standard coin and/or nugget machines can do as deeply, but as I say, if you're in an area of even modern junk, you will find yourself digging endless "small stuff". Even if you try to mentally tell yourself "I'll ignore all shallow or small stuff", you'll be perpetually bedevilled trying to second guess all the signals. Much better off to get a 2-box unit.