High resolution mapping of sound speed and density with broadband echo sounders

Abstract

Advances in acoustic propagation and physical oceanographic modeling necessitate improved spatiotemporal resolution of water column properties (temperature, salinity, sound speed, and density) to inform and validate models. Shipboard profiling provides adequate vertical resolution, but high-resolution horizontal sampling is expensive and time consuming. Stationary arrays generate critical long-term data sets and high vertical resolution, however, there is a trade-off between dense horizontal sampling and total spatial coverage. Towed systems provide dense coverage horizontally but lack vertical resolution in a single location. During the ONR funded New England Shelf Break Acoustics experiments in 2021, sparse shipboard CTD and expendable bathythermograph (XBT) casts were combined with backscatter measurements by shipboard broadband echo sounders to map water column physical properties of the New England Shelf Break front with high resolution vertically and horizontally. The shipboard echo sounders detected backscatter from the strong gradients of the shelf break front, physical microstructure, and internal waves present at the frontal boundary. Acoustically tracking the front between CTD casts resulted in high resolution profiles of water column properties. Acoustic propagation modeling at 500 Hz and 4 kHz was compared when using the acoustically determined water column profile and when using profiles from CTD and XBT casts alone.