Experiments are described to demonstrate a new method of sonic signalling at extremely long ranges in the oceans, utilizing the natural sound channel. Signals were made by causing a 4-lb. charge of TNT to explode at about 4000-foot depth.
These signals have the following qualities:
(1) Extremely long-range transmission (probably 10,000 miles).
(2) Signal is positively identifiable.
(3) Abrupt termination of the signal allows the arrival time to be read with an accuracy better than 0.05 second. This permits location of the source to within a mile, if the signal is received at three suitably located stations.
(4) The relation of signal duration to distance is such that the distance may be estimated to 30 miles in 1000 from reception at a single station.
The limitations are:
(1) The great-circle path which the sound follows between source and receiver must lie entirely in deep water (probably at least 1000 fathoms).
(2) Sound travels in water at about 1 mile per second, so that the interval between the origin of the signal and its reception becomes sufficiently great to be a handicap for some uses, particularly with aircraft.
The signals were received to distances of 900 miles. In subsequent work, not included in this report, a J-lb. bomb was heard with abundant signal strength at over 800 miles, a 4-lb. bomb at 2300 miles, and a 6-lb. bomb at 3100 miles.
Two receiving arrangements have been used, a hydrophone hung 4000 feet deep over the side of a ship which was hove to, and a shore-connected hydrophone lying on bottom 4000 feet deep. Extrapolation of the results indicated a range of at least 10,000 miles from a 4-lb. charge. Recommendation is made to utilize a network of monitoring stations to locate planes, ships,and life rafts in distress on the open oceans. Three or more stations receiving a signal could locate the source within 1 mile. An experimental network of listening stations is being installed by the Navy Department in the western Pacific, and this system of signalling has been given the name SOFAR.
Three applications of SOFAR to geological problems have been proposed: (1) position fixing, (2) discovery of shoal areas, and (3) submarine volcano location.