Acoustic resonances within the surficial layer of a muddy seabed.

This is an investigation of sound propagation over a muddy seabed at low grazing angles. Data were collected during the 2017 Seabed and Bottom Characterization Experiment, conducted on the New England Mud Patch, a 500 km area of the U.S. Eastern Continental Shelf characterized by a thick layer of muddy sediments. Sound Underwater Signals (SUS), model Mk64, were deployed at ranges of 1-15 km from a hydrophone positioned 1 m above the seafloor. SUS at the closest ranges provide measurements of the bottom reflection at low grazing angles (<3 deg). Broadband analysis from 10 Hz to 10 kHz reveals resonances in the bottom reflected signals. Comparison of the measurements to simulated signals suggest a surficial layer of mud with a sound speed lower than the underlying mud and overlying water. The low sound speed property at the water-mud interface, which persists for less than 1 m, establishes a sound duct that impacts mid-frequency sound propagation at low grazing angles. The presence of a low-speed surficial layer of mud could be universal to muddy seabeds and, hence, has strong implications for mid-frequency sound propagation wherever mud is present.