Acoustic propagation characteristics of the estuarine salt wedge

Abstract

The estuarine environment often hosts a salt wedge—a layer of denser seawater advected by the rising tide under fresh water discharged by the river. The nature of the stratification is a function of the tide's range and speed of advance, river discharge volumetric flow rate, and river mouth morphology. The competing effects of temperature and salinity on sound speed present the question: Is the salt wedge acoustically observable? Using temperature and salinity profiles collected in situ, numerical results show that the salt wedge can impact acoustic propagation. Acoustically, this environment consists of two isospeed layers separated by a thin gradient. While this three-layer very shallow water acoustic waveguide is typically dominated by high angle multipath propagation, refraction occurring in the gradient layer allows some low-angle energy from near-surface sources to be trapped in the upper layer. Acoustic fluctuations observed at an upstream or downstream receiver depend upon the interaction between the advancing and receding tide and the river discharge, which can include the presence of internal waves propagating along the top of the salt wedge.