Upward looking sonar-based measurements of sea ice and waves

With the recent reduction in summertime ice cover in the Arctic Ocean, year-long moored measurement programs require detailed information on sea ice thickness and topography data throughout most of the year, as well as ocean wave measurements during summer periods of major sea-ice retreat. This information 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 providing under-ice topography data with high horizontal and vertical spatial resolution. The internal recording ULS instruments, or ice profilers, are typically operated from the seafloor on taut line mooring systems. In the winter of 2007-2008, a new generation of ULS instrumentation was field tested, initially in Northumberland Strait near the Confederation Bridge separating the Canadian provinces of New Brunswick and Prince Edward Island. With typical ping rates of 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 upgrades intrinsic to the ULS instrument feature much expanded data storage capacity (from 69 Mbytes to 1-8 Gigabytes) and 16 bit A/D resolution for ice ranges and other parameters. The offered combination of much increased dynamic range (via the 16 bit A/D converter) combined with the greatly expanded data storage capacity enables the instrument to operate at much lower gain levels. This facility allows extraction of information on the strength of the backscattering associated with sea-ice in contrast to the larger amplitude acoustic returns from open water, as well as detection of multiple targets from each regular 1 Hz ping. The instrument's firmware also provides an ocean wave sampling mode in which a 2 Hz ping rate is used, typically over 20 minutes once each hour, from which non-directional wave spectra and wave parameters can be derived in post processing of the raw data. The new firmware allows the user to program the instrument to operate in up to 12 different sampling schemes over the course of the full deployment. For a typical Arctic Ocean deployment, this enables the instrument to be programmed to measure ocean waves in late summer and early autumn, then both waves and sea ice in autumn, sea ice in the winter and spring, sea ice and waves in the late spring and early summer. These features were utilized in the Northumberland Strait deployment, operated from Nov. 2007 to April 2008, to optimally detect the floating ice cover targets of interest, avoiding alternative false or null targets. Results are also presented on the measurement of ocean waves with wave heights of up to 3 m, and the early winter measurement of scattered ice keels in the presence of ocean waves.