Shots were fired at a fixed location and constant shallow depth at12‐hrintervals over a period of 15 hr. Shots fired in this way produced a highly repeatable signal. The signals were received at a fixed location 63 miles away on hydrophones at 3000‐ and 9100‐ft depths. The signals exhibit about 40% correlation over a period of12 hrand about 20% correlation over a period of 1 hr.