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High-Frequency Acoustic Propagation in Shallow, Energetic, Highly-Salt-Stratified Environments

Abstract : The long term goal of this research is to measure and understand high-frequency, line-of-sight acoustic propagation in an estuarine environment characterized by strong tidal flow, often large salinity stratification, high shear, high dissipation rates of turbulent kinetic energy, and increased water property variability. Acoustic propagation techniques provide a means for remote-sensing of the path-averaged statistical structure and motion of the intervening flow, providing information on the 2-dimensional characteristics of turbulence, microstructure, and advection. Estuaries provide an excellent environment to quantify stratified turbulence and its influence on acoustic propagation, as these environments provide a broad range of stratification and turbulence intensities within a single tidal cycle. The primary objective is to conduct high-frequency, line-of-sight acoustic propagation measurements in Fall 2012 in the Connecticut River estuary, at the same location and time of year as previous direct measurements of turbulence parameters and broadband acoustic backscattering have been performed in 2008 and 2009 (funded through ONR Physical Oceanography). In addition, shipboard measurements of high-frequency broadband acoustic backscattering, currents (using a 1.2 MHz ADCP), and continuous CTD measurements will be performed in order to support the interpretation of the scintillation measurements. Secondary objectives include testing the validity of the existing theoretical framework for propagation of high-frequency sound through a turbulent medium, determining the range of conditions under which it is accurate, and quantifying the importance of anisotropy.