An inverted echo sounder for monitoring layer thickness in stratified oceans

Inverted echo sounders (IESs) measure the two-way travel time of an acoustic pulse travelling between the sea bed and the surface. This time depends upon the barotropic movements of the surface and upon variations in the internal properties of the water column. Sound velocity is principally a function of temperature; salinity and pressure have a second order effect. Traditionally, IESs have been used to anomalies particularly related to the surface thermocline. In this application an IES was modified to collect and store the acoustic return signals from each point in the water column above the instrument, with the intention of extracting signals scattered from the bottom layer comprising the Arctic overflow water. If the technique proved viable then it would be possible to monitor the seasonal, interannual and long term variation in thickness of the Arctic overflow layer. The instrument was deployed in 2000 m of water, together with a bottom pressure recorder for calibrating the barotropic signal, in the Denmark Strait at position 63°22´N 36°04´W from the FS Poseidon during August 1996 and recovered one year later from the FS Meteor. Its operation consisted of launching an acoustic pulse at intervals of two hours, then digitising and storing the analogue return signals. Preliminary analysis of the data is shown