Sound Transmission from Deep to Shallow Water

The propagation of sound from deep to shallow water was studied by a direct field experiment. Two aircraft were used: one to drop explosive sound signals detonating at 800 and 2000 ft out to 100 miles in two directions off the coast of Delaware, the other to monitor a pair of sonobuoys located in 100 f of water at the break of the continental shelf and at a point 20 miles toward shore in shallow water. The field-recorded data were converted to transmission loss by standard techniques. Surprisingly good transmission was observed. The 100-f receiver showed lower transmission losses for the deep axial shots than it would have if it had been located on the channel axis in deep water. This good transmission is possibly due to two causes: a kind of focusing effect, wherein the continental slope acts to increase the density of rays at its top, plus a near-optimum slope condition enabling axially transmitted sound to take but a single bounce up into shallow water. The 800-ft shots were also well-received in spite of the absence of all-water ray paths, probably as a result of conversion to seismic waves along the slope.