Measurement of sea-ice draft using upward-looking ADCP on an autonomous underwater vehicle

During March 2003, Autosub, an autonomous underwater vehicle (AUV) operated by the UK National Oceanography Centre, Southampton, was deployed under sea ice north of Thurston Island, Amundsen Sea, Antarctica (at similar to 71 degrees S, 100 degrees W). The vehicle was fitted with an upward-looking 300 kHz acoustic Doppler current profiler (ADCP) to provide current velocity above the AUV. The ADCP also recorded ranges to the ocean-ice interface. Such data can be used to derive sea-ice draft by using a number of novel processing steps such as correcting for the coordinate systems of the ADCP unit and the vehicle as well as corrections for changes in sound speed. This paper outlines the processing stages required to obtain a probability density function (PDF) of sea-ice draft and presents PDFs for the region north of Thurston Island. The distribution of ice draft was found to be unimodal, with modes between 2.2 and 2.4 m. Given the uncertainty in sound speed, the limit of accuracy was estimated as similar to 6 cm.