That would be great!
Old mine workings were a huge problem with an exploration drill program I worked on. Tried to convince the company to some small scale seismic, but no go. Spent a lot of time digitizing old mine maps in 3D to plan our drill route. Surprisingly, it was fairly accurate. We hit one void withing a few feet of my prediction.
So yes, if this can see deeper than ground penetrating radar, that'd be great!
As far as your last question, beats me. If I come across anything I'll let you know. Diurnal changes in water depth in wells and the like.