The sign doesn't say anything about digging holes...I say, leave the metal detector at home, and grid the whole area and start shoveling and screening. You'll get more coins that way, everything, and be in compliance with the intent of the sign. If anyone gives you any guff, just tell them that you are looking for artifacts. Archeologists do it all the time, with the gov's blessing. Just be sure to wear the appropriate attire and hold a mason's trowel and brush when they walk up.