Estimates of compressional and shear speed attenuation in the 10–50 Hz frequency range based on underwater noise from a merchant ship

Observations of underwater noise emission from a merchant ship are made as a function range in waters of depth 77 m on the New England Mud Patch (NEMP), as part of the 2022 Seabed Characterization Experiment with primary objective being study of sound propagation involving seafloors consisting of fine-grained sediments. In this region the seafloor is characterized by a mud-like layer of order 10 m thickness, commencing at the water–sediment interface. The ship traversed the NEMP producing a 30 km observation transect for which the water and mud-layer depth slowly varied with range. Data are forward-modeled using adiabatic normal modes based on a range-varying geoacoustic model for the upper sediments that incorporates stratigraphic information from the experimental area. For the deep sediment structure, the compressional speed is a constant, sand-like layer of speed 1810 m/s for O(100) m, after which speed increases before ultimately terminating at a basalt basement of speed 5000 m/s. For simplicity, compressional attenuation is estimated upon assumption of realistic values for the Poisson ratio and shear speed quality factor Q. For frequencies near 10 Hz, estimated compressional wave attenuation is reduced by approximately 30% relative to corresponding estimates that exclude elastic effects.