Recent development and application of inverted echo sounders in observational physical oceanography

Abstract

Since the first experiment carried out by Rossby in Bermuda, inverted echo sounders have been used in physical oceanography observations for half a century. The inverted echo sounder measures the round-trip acoustic travel time from the sea floor to the sea surface, thus acquiring vertically integrated information on the thermal structure, from which the first baroclinic mode of thermocline motion can be inferred. Arrays of inverted echo sounders have been deployed almost all over the global ocean to observe internal waves, mesoscale eddies, western boundary currents, etc., providing valuable targeted data for physical oceanographers. Acoustic aspects of the inverted echo sounder performance have been recently examined. Sources of error affecting instrument performance include tidal effects, barotropic adjustment, ambient acoustic noise and sea surface roughness. The latter two effects are explored with a simulation that includes surface wave reconstruction, acoustic scattering based on the Kirchhoff approximation, wind generated noise, sound propagation, and the instrument's signal processing circuitry. Not only does the analysis enhance our understanding of the acoustic travel time data but also suggests new approaches to extend the application of inverted echo sounders, for example, for sensing wind over the sea. New deployments of inverted echo sounders and recent developments in hardware system and auxiliary accessories are also introduced.