Drilling permits for offshore sampling shall authorize drilling to a maximum depth of 500 feet below the ocean floor: Provided, That drilling may be continued to such additional depth, not to exceed 1,000 feet below the ocean floor, as is required to obtain a sample of consolidated rock: Provided further, That notice of intent to drill beyond 500 feet shall first be given the observer or supervisor. No hole drilled more than 500 feet below the ocean floor shall be drilled more than 20 feet into consolidated rock.