Improvements in Upward Looking Sonar-Based Sea-Ice Measurements: A Case Study for 2007 Ice Features in Northumberland Strait, Canada

Scientific and engineering studies of polar and marginal ice zones require detailed information on sea ice thickness and topography. Accurate information on sea ice thickness and topography data is required for basic ice-covered ocean studies and, increasingly, for addressing important navigation-, offshore structure design/safety- and climate change-issues. Since the early 1990's, upward-looking sonar (ULS) instrumentation have been developed and applied to provide under-ice topography data with high horizontal and vertical spatial resolution. Such internal recording ULS instruments, or ice profilers, are typically operated from the seafloor on taut line mooring systems. The ASL Model IPS4 Ice Profiler, which has been widely used in studies of the Arctic Ocean, as well as in numerous seasonal ice zones and in the Southern Ocean, is being upgraded to allow much expanded data storage capacity (from 69 Mbytes to 1-8 Gigabytes) and 16 bit A/D resolution for ice ranges and other parameters. With typical ping rates of 0.5 or 1 Hz, the enhanced capability of the Ice Profiler provides very high resolution measurements of ice keel drafts and the under-ice topography of sea-ice keel features. The Ice Profiler is often used in conjunction with an Acoustic Doppler Current Profiler which provides direct measurements of ice velocity. The combination of high resolution ice draft time series with ice velocities allows for computation of quasi-spatial ice drafts as a function of horizontal distance. The results from the first deployment of an upgraded Ice Profiler, operated just off the Confederation Bridge in Northumberland Strait, from November 2006 to April 2007, are presented and compared with the results of previous sea ice studies at the same location. The much larger onboard data capacity allows for realization of multiple targets for each ping and, on a subsampled basis, offers data on acoustic backscatter returns over the complete water column. This additional information is being analyzed to examine the nature and cause of occasional false target returns. In past measurement campaigns, there have been episodic occurrences of deep targets detected which are not consistent with sea ice features that can be reasonably expected to occur in Northumberland Strait. These features are often associated with occurrences of the largest (spring) tidal currents, leading to the hypothesis that these anomalous targets may be associated with velocity shears in the water column resulting from strong tidal flows past the bridge support structures. Based on these analyses, improvements to the target detection algorithm are being developed and tested.