Acoustic fluctuations due to shallow-water internal waves

Abstract

Oscillations of up to 20 dB in level with total duration of about 1 1/2 hours have been extensively seen in acoustic propagation experiments, and are due to internal waves. The transmission distances varied from 2 to 23 km in water 35 m deep, with frequency typically about 2 kHz. The internal waves and their acoustic effects were virtually switched on and off through the tidal cycle. This is due to the delicate and varying balance between the vertical density gradient and the shear gradient which together determine the Richardson number. The oceanographic structure was complicated, and included strong horizontal temperature gradients. For 1964 and 1965 experiments in the eastern part of the working area the effects tended to occur near slack water (+2 h relative to local high water) with periods of about 5 min and wavelengths apparently about 50 m. For 1967–1969 experiments in the western part the effects tended to occur near times of maximum flow (−1 and +5h) with periods of about 20 min and wavelengths apparently about 500 m. Measurements from five other indicators of internal waves are included.