ASA 125th Meeting Ottawa 1993 May

1pAO7. Fluctuations and predictability of sound propagation at moderate frequencies in bottom-limited sound channels.

Harry A. DeFerrari

Michael G. Brown

Frederick D. Tappert

Charles Monjo

Lan Nghiem-Phu

Div. of Appl. Marine Phys., RSMAS/U.M., 4600 Rickenbacker Cswy., Miami, FL 33149-1098

Sound propagation in the moderate frequency range (depth to wavelength ratio, h/(lambda), between 20 and 100) in shallow water environments with bottom roughness is extremely complicated, variable, and poorly understood. Multipaths are generally neither resolvable (assuming Q>1) nor stable. Fluctuations are dominated by a combination of continual formation and destruction of multipaths as well as by perturbations to otherwise stable paths---as is the case in deep water. Propagation characteristics observed in the Straits of Florida for several ranges have consistent features. Sound fields are slowly varying and phase coherent. Large fluctuations with periodicities of a few minutes to a few hours can be qualitatively accounted for as a slowly modulated complicated caustic structure. The quantitative description of these phenomena is beyond the domain of validity of existing theories because variables do not separate and rays behave chaotically. Advances in both theory and experiments are required to improve understanding of these propagation issues.