This is just too cool--
Mythbusters was on while we dined today, and they tested whether skin-headed drums (about 4' tall and 18-24" wide, they looked like--we came in late) buried 15' in the ground could be used as listening posts and detect underground digging. Apparently there is a very old Chinese writing that talked about ancient Chinese cities protecting themselves from an army underground via this technique. Well, if there's any quartz in the areas being mined, someone with good hearing *could* hear the digging sounds from 50' or more away from the drum hole!
There's no record of it being used, but it's probable--it worked in two of three digging situations. (Maybe the writer was doing fantasy, but if so, great fantasy--and since it was a quartz deposit, it sounds completely possible to me.)