Resonant and non-resonant mode propagation: Observation and modeling

Observations and modeling of underwater noise emission from a merchant ship are made as function range in waters of depth 77 m on the New England Mud Patch, an area distinguished by a low sound speed mud layer of thickness ∼10 m commencing at the water–sediment interface. The vessel emitted strong harmonics studied over 30 km in closing to 20 km in opening from the sensor, positioned 1.5 m above the seabed. A sequence in mode contribution is observed in both the data and modeling described: multi-model (>2), single mode, returning to multi-mode (2) propagation, for harmonics close to 5, 10, and 23 Hz, respectively. The data are forward-modeled using adiabatic normal modes based on a range-varying geoacoustic model for the upper sediments, whereas for the deep sediments a linear sound speed gradient governs an increasing compressional speed with depth to ∼1000 m. The modeling suggests dominant modes contributing to the observed field either reach a maximum amplitude within the water column (resonant mode) or within the sediment (non-resonant mode). We explore this interesting transition in mode propagation from the standpoint of a proposed, and alternative, geoacoustic model that include the influence of shear.