Observations and modeling of range-dependent propagation in low-frequency tones emitted by a container ship

Observations of tonal emissions over frequencies ∼ 9.5-36 Hz from a merchant vessel along a 31.5-km track are used to study seabed properties 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 data are forward-modeled using adiabatic normal modes based on a range-varying geoacoustic model for the upper sediments, reproducing key effects such as changing mode interference patterns. For the deep sediments, a proposed linear sound speed gradient governs an increasing compressional speed with depth. Bayesian inversion yields frequency-dependent estimates of the compressional wave attenuation within the deep sediments using data from the lower frequencies for which it is possible to disambiguate the effect of attenuation from the upper mud layer. At ∼16 Hz, the highest frequency meeting this condition, this estimate is 0.17 ±0.03 dB/λ, and these inversion results are used to infer corresponding values for the higher frequencies. A compressional wave attenuation of 0.05 dB/λ in the mud layer is applied across all frequencies. Elastic effects are included in the geoacoustic candidate models used in the inversion; removing them increases compressional wave attenuation 10%-40% depending on frequency.