Invariant features of deep ocean ambient noise.

Abstract

Very low‐frequency, deep ocean ambient noise is generated by non‐linear interactions (collisions) between surface waves with nearly equal frequency, propagating in nearly opposite direction. It is shown that the usual “standing wave” approximation used to compute this noise predicts that the ratios of entries in the power spectral density matrix, obtained from auto and cross‐correlations of acoustic velocity and pressure fluctuations, are universal constants. This prediction is too strong and not observed. A careful consideration of bottom effects invalidates the standing wave approximation. Nevertheless a weak version of the standing wave approximation still holds and has interesting implications. While the ratios of entries in the power spectral density matrix due to surface wave generated noise are not universal constants, they are insensitive to the details of the surface wave spectrum. This theoretical prediction is borne out by data from the Hawaii‐2 Observatory. [Work supported by ONR.]