Backscattering of sound from salinity fluctuations: Measurements off a coastal river estuary

In this work a set of near co-located acoustic intensity observations and environmental measurements is presented which allowed quantification of the source of the acoustic scattering and confirmation of backscattering contributions from salinity microstructure. Observations were made by the T-REMUS Autonomous Underwater Vehicle (AUV) in a fresh water plume off a coastal river estuary characterized by strong salinity stratification and intense turbulence. Velocity and density microstructure measurements were used to calculate the expected acoustic backscattering cross-section from salinity fluctuations, which was then compared to near coincident acoustic intensity measurements. Surrogate measures of discrete particle scattering sources (i.e., biologic and mineral) for comparison with measured backscattering were also obtained. A series of regression analyses (single and multi-variable) were performed in an attempt to account for the observed acoustic variability. It was found that most of the variability in the acoustic return signal was explained by a linear combination of the independent variables selected. The results suggest that most of these regressions were dominated by biological and microstructure sources, with a fair amount of cases that were either completely biologically or microstructure dominated. The results led to the conclusion that measurements of salinity backscattering agree with theoretical predictions within experimental error of +/-2 dB (on average) for a wide range of turbulence levels, with an observed underestimate of the salinity backscattering of at least 1.5 dB under high turbulence conditions. These results strongly support the idea of performing an inversion of the acoustic signal to obtain the salinity variance dissipation rate.