Check on Art's thread on dowsing. He has a post on finding depth with rods. ( aarthrj3811 )
It may work better on things from 2 feet deep to deeper. If a target is only in first 12 inches you must move slowly.
Simple depthing: Stand on top of target. Walk away from that spot. When rod gives a response, measure that distance back to the target spot. That distance is the approximate depth assuming there is only one target.
That's the most accurate. You can do the psychic way--counting feet or tens of feet until the rod responds, but any/all psychic work is unreliable and inaccurate.