Acoustic Backscatter from Salinity Microstructure

Abstract

The contribution of salinity changes to sound speed fluctuations is often neglected in estimating the scattering cross section at high frequencies (>10 kHz). To examine when salinity might be important, an expression is formulated for the scattering cross section σ that includes salinity and an estimate of the cospectrum of temperature and salinity. Profiles from the southern New England shelf, the Bosphorus, and Puget Sound are used to estimate levels of σ as a function of depth and acoustic frequency. Salinity can increase σ by more than an order of magnitude, particularly at frequencies greater than 100 kHz, when salinity controls the density field. The cospectrum is expected to be large under the same conditions and can potentially negate strong scattering at lower frequencies. An f+1 dependence of σ is expected over two decades in frequency when salinity controls density. Multifrequency acoustic systems may be able to distinguish biology and microstructure based on this spectral dependence.