Spectral broadening by ocean microstructure

Abstract

In the past decade there has been a great deal of interest in the use of a sonar to infer physical properties of the ocean. Acoustic Tomography and FLIP high‐frequency active sonar system are examples of successful efforts. Recently there has been interest in whether high‐frequency sonar can be used to measure the smallest scale of ocean fluctuations, microstructure, typically of order meters and smaller in the open ocean. A critical problem which arises in attempting such measurements is that the scattering volume is typically of order of the scale of the variability resulting in Doppler spread as well as Doppler shift. In this work the nature of this spread is examined using a realistic form of microstructure field—one which takes into account both space and time variability. A Brownian motion type of model for the scatterers, which is often employed, will be shown in general to be inappropriate. It will be shown that the temporal correlation of the returned signal is not ergodic. This result is related to the nature of the fourth‐order moment, which unlike the second‐order moment contains information on the spatial correlation of the scatterer motion.