Exploiting environmental asymmetry for vessel localization from the vertical coherence of radiated noise.

A method to determine the range and bearing of a moving broadband acoustic source, such as a surface vessel, using the coherence measured on two omni-directional, vertically separated hydrophones is demonstrated using acoustic data recorded near Alvin Canyon on the New England shelf break. To estimate the vessel's range, two theoretical approaches, a half-space model and a Pekeris waveguide model based on normal modes, establish simple relationships between the broadband signal coherence and frequency, source range, and the vertical separation of the receiver hydrophones. A brute force inversion produces a passive acoustic estimate of vessel range. Rapidly changing bathymetry with large features, such as that near Alvin Canyon, produces azimuthal asymmetry in the plan-view coherence pattern about the receivers due to horizontal refraction, focussing, and the up- (down-) slope compression (extension) of modal interference patterns. For vessels with a constant speed and heading, this generates an asymmetry in the received power and vertical coherence fringing pattern. This effect is first demonstrated using reciprocal three-dimensional parabolic equation and raytracing models in an idealized Gaussian canyon, then observed in Alvin Canyon measurements. By comparing the experimental observations to the modeled coherence, the vessel's bearing and range relative to the receivers are obtained.

Opportunistic ship source level measurements in the Western Canadian Arctica).

Increased ship traffic due to climate change increases underwater noise in the Arctic. Therefore, accurate measurements of underwater radiated noise are necessary to map marine sound and quantify shipping's impact on the Arctic ecosystem. This paper presents a method to calculate opportunistic source levels (SLs) using passive acoustic data collected at six locations in the Western Canadian Arctic from 2018 to 2022. Based on Automatic Identification System data, acoustic data, and a hybrid sound propagation model, the SLs of individual ships were calculated within a 5 km radius of each measurement site. A total of 66 measurements were obtained from 11 unique vessels, with multiple measurements from the same vessel type contributing more SLs. For vessels with propeller cavitation, measured SLs correlated positively with vessel parameters, such as speed and length. SL and speed did not correlate well for vessels without propeller cavitation. The JOMOPANS-ECHO SL model produced good agreement with measured SL for certain ship types (container ships, a tanker, and a passenger vessel). However, significant differences between measurement and model are evident for certain polar-class ships that travel in the Arctic, indicating that more controlled SL measurements are needed.

Ambient noise levels with depth from an underwater glider survey across shipping lanes in the Gulf of St. Lawrence, Canada.

A two-month-long glider deployment in the Gulf of St. Lawrence, Canada, measured the ambient sound level variability with depth and lateral position across a narrow channel that serves as an active commercial shipping corridor. The Honguedo Strait between the Gaspé Peninsula and Anticosti Island has a characteristic sound channel during the Summer and Fall due to temperature variation with depth. The experiment comprised continuous acoustic measurements in the band 1-1000 Hz and oceanographic (temperature and salinity) measurements from a profiling electric glider down to 210 m water depth. The mean observed ambient sound depth-profile was modeled by placing a uniform distribution of sources near the surface to represent a homogeneous wind-generated ocean wave field and computing the acoustic field using normal modes. The measurements and predictions match within the observed error bars and indicate a minimum in the sound channel at 70 m depth and a relative increase by ∼1 dB down to 180 m depth for frequencies >100 Hz. The impact of detector depth, the distance to a busy shipping corridor, wind noise, flow noise, and self-noise are discussed in the context of passive acoustic monitoring and marine mammal detection.

Quantifying the contribution of ship noise to the underwater sound field.

The ambient sound field in the ocean can be decomposed into a linear combination of two independent fields attributable to wind-generated wave action at the surface and noise radiated by ships. The vertical coherence (the cross-spectrum normalized by the power spectra) and normalized directionality of wind-generated noise in the ocean are stationary in time, do not vary with source strength and spectral characteristics, and depend primarily on the local sound speed and the geoacoustic properties which define the propagation environment. The contribution to the noise coherence due to passing vessels depends on the range between the source and receiver, the propagation environment, and the effective bandwidth of the characteristic source spectrum. Using noise coherence models for both types of the sources, an inversion scheme is developed for the relative and absolute contribution of frequency dependent ship noise to the total sound field. A month-long continuous ambient sound recording collected on a pair of vertically aligned hydrophones near Alvin Canyon at the New England shelf break is decomposed into time-dependent ship noise and wind-driven noise power spectra. The processing technique can be used to quantify the impact of human activity on the sound field above the natural dynamic background noise, or to eliminate ship noise from a passive acoustic monitoring data set.