Acoustic vector sensor analysis of the Monterey Bay region soundscape and the impact of COVID-19.

From February 2019 through January 2021, data were collected by an acoustic vector sensor moored on the seafloor at a depth of approximately 900 m just outside of Monterey Bay, California, near a major shipping lane off the California coast. Analysis of the vector sensor data has shown the ability to accurately determine bearings to merchant vessels at ranges up to 60 km. This paper examines the features of the low-frequency soundscape using spectral probability densities and evaluates directional features through vector intensity processing as well as coherent linear and adaptive processing of the vector sensor channels. Merchant vessel acoustic data were analyzed using the 1/3 octave band centered at 63 Hz. Over the period analyzed, a reduction in merchant vessel noise was observed between February and June 2020 relative to the same period in 2019, consistent with a reduction in vessel traffic due to the worldwide response to COVID-19. The directional features of the data evaluated through adaptive processing methods also suggest this reduction can be most clearly distinguished towards the south, where the shipping lane is limited to transiting vessels, rather to the north-northwest, where merchant vessels tend to congregate on approach into the San Francisco Bay area.

Physics-based characterization of soft marine sediments using vector sensors.

In a 2007 experiment conducted in the northern North Sea, observations of a low-frequency seismo-acoustic wave field with a linear horizontal array of vector sensors located on the seafloor revealed a strong, narrow peak around 38 Hz in the power spectra and a presence of multi-mode horizontally and vertically polarized interface waves with phase speeds between 45 and 350 m/s. Dispersion curves of the interface waves exhibit piece-wise linear dependences between the logarithm of phase speed and logarithm of frequency with distinct slopes at large and small phase speeds, which suggests a seabed with a power-law shear speed dependence in two distinct sediment layers. The power spectrum peak is interpreted as a manifestation of a seismo-acoustic resonance. A simple geoacoustic model with a few free parameters is derived that quantitatively reproduces the key features of the observations. This article's approach to the inverse problem is guided by a theoretical analysis of interface wave dispersion and resonance reflection of compressional waves in soft marine sediments containing two or more layers of different composition. Combining data from various channels of the vector sensors is critical for separating waves of different polarizations and helps to identify various arrivals, check consistency of inversions, and evaluate sediment density.

Vector sensor cross-spectral density for surface noise in a stratified ocean-Formulas for arbitrary sensor geometries.

The cross-spectral density between pressure and particle velocity channels of two vector sensors with arbitrary orientation and separation is developed for a noise field consisting of directional sources distributed on a plane. Equations for arbitrary sensor orientations are necessary because cross-spectral density is an averaged product of two sensors' outputs that cannot be projected onto another direction after it is calculated. This prevents the use of existing methods, which calculate cross-spectral density for orthogonal particle velocity sensors, for sensors with different orientations. Analytic solutions to special cases for azimuthally symmetric environments and orthogonal velocity sensor orientations are developed and compared to previous results. The general equations are stated in a form suitable for use with any propagation model that produces Green's functions for pressure and orthogonal particle velocity components. Simulation examples demonstrate how sensor geometry and propagation conditions affect vector sensor array performance for wind-driven surface noise in deep and shallow water environments. Vector sensor beamforming is shown to provide additional gain in these directional noise fields as compared to isotropic noise.

Reciprocity relationships in vector acoustics and their application to vector field calculations.

The reciprocity equation commonly stated in underwater acoustics relates pressure fields and monopole sources. It is often used to predict the pressure measured by a hydrophone for multiple source locations by placing a source at the hydrophone location and calculating the field everywhere for that source. A similar equation that governs the orthogonal components of the particle velocity field is needed to enable this computational method to be used for acoustic vector sensors. This paper derives a general reciprocity equation that accounts for both monopole and dipole sources. This vector-scalar reciprocity equation can be used to calculate individual components of the received vector field by altering the source type used in the propagation calculation. This enables a propagation model to calculate the received vector field components for an arbitrary number of source locations with a single model run for each vector field component instead of requiring one model run for each source location. Application of the vector-scalar reciprocity principle is demonstrated with analytic solutions for a range-independent environment and with numerical solutions for a range-dependent environment using a parabolic equation model.