3-D acoustic propagation through an estuarine salt wedge

Abstract

The estuarine salt wedge presents a dynamic and highly refractive waveguide, the acoustic propagation characteristics of which are controlled by the water column sound speed gradient and boundary interactions. Acoustically, the salt wedge consists of two isospeed layers separated by a thin, three-dimensional, high-gradient layer. The behavior of a broadband (500–2000 Hz) acoustic field under the influence of an estuarine salt wedge in the Columbia River estuary is explored using two 3-D acoustic propagation models: 3-D rays and 3-D parabolic equation (3DPE). These model results are compared to data collected during a field experiment in 2013. Results demonstrate that the dominant physical mechanism controlling acoustic propagation in this waveguide shifts from 3-D bottom scatter in a non-refractive waveguide (before the entrance of the salt wedge) to 3-D acoustic refraction with minimal bottom interaction in a refractive waveguide (when the salt wedge occupies the acoustic transect). Vertical and horizontal refraction in the water column and out-of-plane scattering by the bottom are clearly evident at specific narrowband frequencies; however, these mechanisms contribute to, but do not account for the total observed transmission loss.