alleyboy,
I think all of the information could be found in the clerk of the courts offices. If you know the owner of the land, you can search for the deed, and determine if mineral rights were reserved in transfer of title. If a lease was filed in regards to the property, I would think a search of public records using the owners name would turn up such a document, as they keep records of all recorded conveyances. Upon review, you could most likely determine the depth restrictions contained in the language of any lease. Good luck in your efforts.
Shelby