Swath mapping the base of the arctic ice canopy

Satellite data have demonstrated that the areal extent of the Arctic ice canopy has decreased over the past few decades, however, whether sea ice thickness in the Arctic Ocean has substantially decreased remains controversial because of limited information available for the base of the ice canopy. Studies of sea ice thickness to date have involved analysis of ice draft data acquired by single-beam ice profiling sonars on US and British Navy submarines or single-beam moored sonars. During 1998 and 1999, a seafloor-mapping sonar mounted on the hull of the US Navy nuclear submarine USS Hawkbill serendipitously acquired sonar returns from the base of the arctic ice canopy despite design features intended to minimize such returns. Standard topographic processing of the data revealed coherent signals collected between the outgoing sonar pulse and the first returned bottom echoes. Existing processing software was modified to produce swath maps of the information collected from the beginning of ping transmission until the seafloor echoes were detected. These images demonstrate that the recorded water-column signals are different on the port and starboard sides of the system and that individual features can be traced from one side of the submarine to the other. Further software modification allowed us to produce the first wide-swath (2–6 km) sidescan images of the base of the Arctic ice canopy. Although the resultant maps cannot resolve the debate regarding changes in Arctic sea ice thickness, the approach will have far-reaching impact on the design of future systems, whether based on submarines or unmanned vehicles. Our analyses demonstrate that system performance is regulated by the ratio between sonar operating depth and height of the sonar above the seafloor. Where the ratio approaches 1, echoes from the seabed interfere with echoes from the ice canopy, preventing the creation of ice maps. When the sonar is operated below 135 m, signal attenuation results in ice maps with little useful information. However, where the sonar operational depth is less than 135 m and the total water depth ranges between 500 and 1200 m, our approach produces swaths up to 6 km wide exhibiting detailed lineations similar to those observed in higher frequency upward-looking sidescan sonar data. [Copyright &y& Elsevier]