ASA 127th Meeting M.I.T. 1994 June 6-10

3aAO10. Temporal modulations of pulse response amplitude in shallow water resulting from a combination of sound-speed variability and bottom loss.

Charles L. Monjo

Harry A. Deferrari

Dept. of Appl. Marine Phys., Univ. of Miami RSMAS, 4600 Rickenbacker Cswy., Miami, FL 33149

Reciprocal transmission tomography experiments in the Florida Straits have yielded a month long time history of reciprocal pulse responses over three ranges approximately 22 km in length and 250 m in depth. A transmitted pulse of 8.75 ms in duration is spread out approximately 100 ms. In a previous paper a strong correlation was reported between the sound-speed gradient at middepth and the total pulse width. A plausible explanation, that the modulation of the sound-speed profile was causing a temporal modulation of the ray arrivals was set forth as a hypothesis for this study to be examined with range-dependent ray and PE models. The models predict a different and unexpected relation. Variations in the sound-speed profile shape from linear to convex increase the bottom grazing angle to the early arrivals, while decreasing the bottom grazing angles of the slower arrivals. This results in an attenuation of the early arriving portion of the pulse to below ambient noise and therefore an apparent narrowing of the pulse. Likewise, a change in profile shape from linear to concave, produces smaller bottom grazing angles for the fastest arrivals, less loss, and an overall wider appearing pulse. [Work supported by ONR.]