Analysis of mid-frequency sound intensity fluctuations on the Washington shelf

A joint Oceanography/Acoustics experiment was conducted 15 July–13 August 2022 in Washington’s shallow coastal shelf waters to investigate mid-frequency sound intensity fluctuations and the oceanographic mechanisms driving them. Acoustic pulses centered at 3.5 kHz and 6 kHz were transmitted up-shelf from a stationary source and recorded by receivers on two moorings at 10 km and 20 km ranges. A ship-towed profiler, the Shallow Water Integrated Mapping System (SWIMS), repeatedly sampled the ocean between the source and receiver moorings, providing sound speed measurements as a function of range, depth, and time. In addition, five oceanographic moorings distributed along the acoustic path took additional ocean data over time. A roughly 15- dB ping-to-ping TL variability and scintillation index of near unity are found at the 20 km range. The intensity data show strong correlation between pings separated by a few minutes, beyond which it is uncorrelated. After smoothing, they also show trends at roughly 12-hour intervals. Assisted by environmental data and numerical modeling, the oceanographic mechanisms for these observed fluctuations are investigated. Implications for signal processing in the presence of such intensity fluctuations are also discussed. (Work supported by Office of Naval Research.)