Observations of mid-frequency sound propagation on the Washington continental shelf with a subsurface duct

A joint oceanography and acoustics experiment was conducted on the Washington continental shelf in the summer of 2022. A towed system measured the in situ sound speed field along a 20 km track between acoustic sources and receivers. A weak but persistent subsurface duct was found with its sound speed minimum generally in the 50-100 m-depth range. The duct exhibited range and time dependence due to the internal tide, internal waves, and possibly other oceanographic processes. Mid-frequency (3500 and 6000 Hz) transmission loss (TL) was measured at 10 and 20 km ranges. The subsurface duct has a 10-13 dB effect on TL, depending on whether the sound source is inside or outside the duct. Measurements were also made using a bottom-mounted source, with transmissions every 3 min over several days. The sound intensity varies about 10 dB over a few minutes, while the scintillation index fluctuates between 0.5 and 1.5. Overall, it is found that mid-frequency sound propagation is variable at several temporal scales, ranging from minutes to hours, to days, or longer. Reducing the impact of these variabilities in acoustic applications would benefit from knowledge of the ocean processes at these different time scales.