Physical mechanisms of generation of the noise bursts associated with nonlinear internal tides in shallow water

Abstract

Recent paper of authors [J. Acoust. Soc. Am. 145(3, Pt. 2), 1670 (2019)] reported observations of repeated increase in acoustic noise intensity during passage of trains of nonlinear internal gravity waves. Periodic increases of pressure fluctuations by up to 35–40 dB in the frequency band from 10 to 5000 Hz were recorded on multiple near-bottom hydrophones during Shallow Water 2006 experiment. The present paper extends the earlier analysis to a more diverse set of nonlinear internal wave events by exploiting the acoustic data obtained at sites with a wider range of water depths and over a longer observation period. Three distinct physical mechanisms are identified, which are responsible for increased noise in different frequency bands. The observed noise bursts are attributed to turbulence of the water flow past hydrophones, sediment transport, and sediment saltation. Correlation between the velocity of the internal tide-induced near-bottom currents and noise intensity at low and mid-frequencies is investigated. [Work supported by ONR, NSF, and BSF.]