Here’s a funny toy: a remote-controlled car with a built-in metal detector. Drive it over a piece of metal and it’ll go BEEP and light up. It doesn’t have anything like serious ground penetration, but still, a cool toy.
There are several reasons that metal detecting has not been made into a mechanised remote sensing technique. I guess the main one is that only archaeologists would have any use for such a machine, and we don’t have the money to make it worthwhile to develop and market it. Also, while building a mechanised detector and find mapper would be easy, it would be considerably more tricky to equip it with a sensitive digging mechanism that could get finds out of the ground and bag them without damaging them.
Thanks to Hans for the tip.