Experimental Confirmation of Horizontal Refraction of Sound Propagation in a Wedge-Like Ocean

Abstract

Analysis of experimental data obtained during the SET 1 exercise in the region of the East Australian Contentinental Slope has confirmed earlier theoretical predictions of energetic horizontal refraction in underwater acoustic propagation due to sound-slope interaction. The observed general modal behavior of the sound, the azimuthal characteristics of the field and the effects of bottom absorption were in accordance with the theory of sound transmission in wedge-like ocean acoustic environments. A experiment was conducted in which two ships, one towing a sound projector and the other a towed array, started at a 400 meter depth contour with an initial separation of 33 kilometers and proceeded out to deep water on diverging, constant line of bearing courses chosen so as to maintain the relative experimental geometry. Beamform data demonstrated the existence of horizontally refracted arrivals which showed significant bearing shifts due to repeated reflections from the ocean surface and bottom. The obliqueness of the azimuthal launch angle with respect to the shore line prevented the local grazing angle from becoming large enough to produce much bottom loss which is in accord with predictions. A combination of the particular source-receiver position and the critical angle effects serve to limit both the azimuthal extent of the sound reception and the total number of modes that arrive at any one position.