Quantifying the contribution of ship noise to the underwater sound field

A combined noise source model based on vertical coherence to quantify the proportions of two types of noise power

With the vigorous development of maritime trade, the frequency band from 100 to 1500 Hz of shallow-sea ambient noise is not only affected by surface wind-induced noise but also the contribution of ship noise. Shallow-sea ambient noise can be described by a linear combination of surface wind-induced noise sources and ship noise sources. By using the correspondence between the real part of the vertical coherence and vertical energy flux, this work establishes a combined noise source model based on vertical coherence. A 64-element vertical array is used to measure the ocean ambient noise 103 m deep in an area of the South China Sea. Two scenarios, one dominated by surface wind-induced noise and one dominated by wind-induced noise and ship noise, are selected for investigation. By comparing the theoretical and measured values, the accuracy of the model proposed in this paper and its ability to remove the ship noise reliably are verified. This approach can be used to quantify the proportion of ship noise power in shallow-sea low-frequency environments and evaluate the contributions of wind-induced noise and ship noise at different times.

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.

Impact of the COVID-19 pandemic on levels of deep-ocean acoustic noise

The extraordinary circumstances of the COVID-19 pandemic led to measures to mitigate the spread of the disease, with lockdowns and mobility restrictions at national and international levels. These measures led to sudden and sometimes dramatic reductions in human activity, including significant reductions in ship traffic in the maritime sector. We report on a reduction of deep-ocean acoustic noise in three ocean basins in 2020, based on data acquired by hydroacoustic stations in the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty. The noise levels measured in 2020 are compared with predicted levels obtained from modelling data from previous years using Gaussian Process regression. Comparison of the predictions with measured data for 2020 shows reductions of between 1 and 3 dB in the frequency range from 10 to 100 Hz for all but one of the stations.

Maximum likelihood separation of anthropogenic and wind-generated underwater noise

A method is presented for simultaneous estimation of the probability distributions of both anthropogenic and wind-generated underwater noise power spectral density using only acoustic data recorded with a single hydrophone. Probability density models for both noise sources are suggested, and the model parameters are estimated using the method of maximum likelihood. A generic mixture model is utilized to model a time invariant anthropogenic noise distribution. Wind-generated noise is assumed normally distributed with a wind speed-dependent mean. The mean is then modeled as an affine linear function of the wind-generated noise level at a reference frequency, selected in a frequency range where the anthropogenic noise is less dominant. The method was used to successfully estimate the wind-generated noise spectra from ambient noise recordings collected at two locations in the southern Baltic Sea. At the North location, 3 km from the nearest shipping lane, the ship noise surpasses the wind-generated noise almost 100% of the time in the frequency band 63-400 Hz during summer for wind speed 7 m/s. At the South location, 14 km to the nearest shipping lane, the ship noise dominance is lower but still 40%-90% in the same frequencies and wind speed.