Acoustic backscatter from salinity microstructure

The contribution of salinity changes to sound speed fluctuations is often n eglected in estimating the scattering cross section at high frequencies (>1 0 kHz). To examine when salinity might be important, an expression is formu lated for the scattering cross section a that includes salinity and an esti mate of the cospectrum of temperature and salinity. Profiles from the south ern New England shelf, the Bosphorus, and Puget Sound are used to estimate levels of a as a function of depth and acoustic frequency. Salinity can inc rease a by more than an order of magnitude. particularly at frequencies gre ater than 100 kHz, when salinity controls the density field. The cospectrum is expected to be large under the same conditions and can potentially nega te strong scattering at lower frequencies. An f(+1) dependence of a is expe cted over two decades in frequency when salinity controls density. Multifre quency acoustic systems may be able to distinguish biology and microstructu re based on this spectral dependence.