smhog,
Are you digging a pond or a well?
There are significant risks associated with your approach. A spring, given enough pressure, will reverse itself and what you thought was a source of water may become a bottomless sink. Free water becomes "freed" water. Even without a spring, a rockbed often leaks causing the owner great grief when the pond has to be drained and packed and start all over again. My advice is to avoid the rockbed completely. Digging through that will be expensive, needless, and may leave you with a pond that does not hold water.
Pulling no punches here, but I really hate to see you get into a situation where many others have been with very bad results.
In addition, you simply do not need 15 foot depth in a 1/2 acre pond.
Dig your pond to fit your needs, forget about any springs, and in fact if you run across one, cover it with 2 feet of clay, unless it is a roaring source of water.
