Canonical correlation analysis as a statistical method to relate underwater acoustic propagation and ocean fluctuations.

Numerical models are currently used to understand how environmental fluctuations impact acoustic propagation. Such a process can be tedious in complex fluctuating environments. This letter proposes a complementary approach based upon canonical correlation analysis (CCA) to determine statistical relationships between two sets of observed acoustic and oceanographic variables. It is shown, as an example, how CCA puts forward the impact of external and internal tide on shallow water propagation. Results are consistent with the physical understanding of tide impact on acoustic propagation. They encourage the use of CCA for complex studies, in particular, for environments fluctuating under several environmental phenomena.

Scattering of low-frequency acoustic waves from a moving source by the sea surface.

This paper examines the scattering of a monochromatic acoustic wave by sea-surface gravity waves in the 1-200 Hz frequency range. The source is moving in a straight line at a constant speed, and the acoustic waves are traveling upward in a refractive channel. Considering the scales of the problem, the small perturbation method coupled with the normal-mode theory and an asymptotic analysis are used to derive the first-order scattered pressure field p1. This method, established by Labianca and Harper [J. Acoust. Soc. Am. 61(2), 378-389 (1977)], allows p1 to be expressed with normal-mode functions, which are computed numerically using the in-house modal propagation code MOCTESUMA for any sound-speed profile. The pressure field is calculated in a deep-water configuration with a moving source inside a summer thermocline. First, the spatial distribution of p1 is found to follow the diffraction grating formula. Particular attention is drawn to the border between the propagative and evanescent regimes in which singularities in the theory lead to computational difficulties. Subsequently, the power spectral density of the pressure field is computed and the Doppler sidebands, asymmetrically shifted from the carrier frequency, are examined.

Understanding deep-water striation patterns and predicting the waveguide invariant as a distribution depending on range and depth.

The Waveguide Invariant (WI) theory has been introduced to quantify the orientation of the intensity interference patterns in a range-frequency domain. When the sound speed is constant over the water column, the WI is a scalar with the canonical value of 1. But, when considering shallow waters with a stratified sound speed profile, the WI ceases to be constant and is more appropriately described by a distribution, which is mainly sensitive to source/receiver depths. Such configurations have been widely investigated, with practical applications including passive source localization. However, in deep waters, the interference pattern is much more complex and variable. In fact the observed WI varies with source/receiver depth but it also varies very quickly with source-array range. In this paper, the authors investigate two phenomena responsible for this variability, namely the dominance of the acoustic field by groups of modes and the frequency dependence of the eigenmodes. Using a ray-mode approach, these two features are integrated in a WI distribution derivation. Their importance in deep-water is validated by testing the calculated WI distribution against a reference distribution directly measured on synthetic data. The proposed WI derivation provides a thorough way to predict and understand the striation patterns in deep-water context.