Project HIAWATHA Long-Range Shallow-Water Sound Propagation in Lake Superior

Sound-propagation measurements were made along a 100-mile path in Lake Superior during September 1967 in an area in which water depth exceeded 600 ft. One-half-pound TNT charges were detonated at the axis of a sound channel and received on a hydrophone suspended at the same depth. Acoustic reception by totally refracted paths was analyzed in 10 frequency bands between 500 and 8900 Hz. The increase in propagation loss with range was found to be greater below 2 kHz than commonly observed in ocean sound channels and larger throughout the range of frequencies than theory would predict for an isotropic, freshwater medium. It is proposed that the largely unexpected results arise from local changes in the temperature structure within the first 150 ft of water depth which, because of the extreme asymmetry of the sound-velocity profile, scatter sound from the channel.