Wave-wave interactions and deep ocean acoustics

TL;DR

This paper develops a theoretical framework for deep ocean acoustic signals generated by surface wave interactions, incorporating bottom effects, and validates it with observational data, enhancing understanding of ocean sound sources.

Contribution

It derives the spectral matrix of pressure and velocity near the ocean bottom, accounting for bottom effects and providing a weak standing wave approximation validated by data.

Findings

Spectral matrix ratios are universal constants without bottom effects.

Bottom effects alter the standing wave approximation, but a weaker form remains valid.

Data from Hawaii-2 Observatory agree with theory between 0.1 and 1 Hz.

Abstract

Deep ocean acoustics, in the absence of shipping and wildlife, is driven by surface processes. Best understood is the signal generated by non-linear surface wave interactions, the Longuet-Higgins mechanism, which dominates from 0.1 to 10 Hz, and may be significant for another octave. For this source, the spectral matrix of pressure and vector velocity is derived for points near the bottom of a deep ocean resting on an elastic half-space. In the absence of a bottom, the ratios of matrix elements are universal constants. Bottom effects vitiate the usual "standing wave approximation," but a weaker form of the approximation is shown to hold, and this is used for numerical calculations. In the weak standing wave approximation, the ratios of matrix elements are independent of the surface wave spectrum, but depend on frequency and the propagation environment. Data from the Hawaii-2 Observatory are in excellent accord with the theory for frequencies between 0.1 and 1 Hz, less so at higher frequencies. Insensitivity of the spectral ratios to wind, and presumably waves, is indeed observed in the data.