An Experiment on Transmission Loss and Fluctuations of 3-kHz Sound in Shallow Water.
NAVAL RESEARCH LAB WASHINGTON D C
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The experiment investigated certain aspects of the 3-kHz sound field produced by a source at middepth in shallow water off Panama City, Florida. The soundspeed profile consists of a weak surface channel in the upper half of the water column and a strong negative gradient in the lower half. A two-layer normal-mode model predicts that due to the source placement and high directivity, modes of lower order than the fourth or higher order than the twelfth should not be significantly excited. The prediction concerning the absence of the second and third modes tends to be supported by the amplitude distribution over depth observed at ranges of 15 to 27 km during a towed-hydrophone run the data are inconclusive concerning the first mode. In the second part of the experiment the anchored receiving ship recorded about 48 hours of CW signals at each of two ranges 7.0 km and 27.6 km. Over the total time span of the data the amplitude fluctuations of signals received on three hydrophones at different depths were observed to be weakly correlated, and there was some indication of variation at the tidal frequency for the 7.0-km data. On a shorter time scale, semiperiodic fluctuations at periods from 30 sec to 30 min occurred at times during these runs. Author