Observation of channel pulse responses at long ranges in the Florida Straits.

Abstract

Acoustic propagation in shallow water is observed for transmission between a towed source (250 Hz m-sequence) and a bottom mounted horizontal array. A total signal processing gain of 45 dB results; 24 dB from m-sequence pulse compression and 21 dB from array gain. The tow is out to 60 km along a path parallel to shore. Individual and separable pulse arrivals associated with single normal modes of propagation are resolved in a parameter space of intensity, arrival time, out-of-plane angle of arrival, and Doppler. Two distinct groups of arrivals are observed, direct paths with near zero horizontal arrival angle and curved inshore, out-of-plane, paths with arrival angle of about 17 deg. A typical inshore path makes several bottom bounces both up and then down the sloping bathymetry. The inshore paths have more favorable transmission (less loss) and, at long ranges, exhibit a peculiar decrease in travel time with an increase in range. At short ranges “down” Doppler is observed, consistent with the opening range of the towed source, but at the longer ranges, “up” Doppler or closing range is observed even though the source is moving away from the receiver. A 3-D PE propagation model is applied to explain observations.