Advances in moored upward looking sonar systems for long term measurement of arctic ice and oceanography

A major impetus for scientific studies of climate change in the Arctic Ocean has been the reduction in the areal extent and thickness of its sea ice cover which has been experienced at accelerated rates in the past decade. These dramatic changes resulted in major climate science studies being conducted in the Arctic Ocean as well as opening the way for increased shipping and offshore oil and gas activities. An extended measurement record of the horizontal dimensions of this ice cover is available for the full Arctic Ocean Basin based upon a record compiled from nearly 40 years of relatively continuous satellite based measurements. Unfortunately, data accumulations for the ice cover's vertical dimension, i.e. sea ice thickness, as well as full temporal resolution ice velocity and under-ice ocean current profiles tend to be limited to a small number of year-long mooring data sets with durations that are only a few to several years, reflecting underlying greater measurement challenges. Moreover, the longest duration ice thickness data collection efforts, spanning more than 10 years, have been confined only to two specific portions of the Basin, namely, Fram Strait and the Canadian sector of the Beaufort Sea. However, in the past ten years, the available year-long ice and oceanographic mooring data sets have greatly increased in total number and in the number of sites. Advanced upward-looking sonar (ULS) instruments operated from subsurface moorings has been and continues to be the primary source of data with volumes and accuracy sufficient for meaningfully monitoring ice thickness, ice velocities, ocean current profiles and other in-situ water properties. The ice thickness, or more properly ice draft (underwater ice thickness) data is measured continuously with temporal resolution of 1 -2 seconds. Technological advances, since ULS instruments were first developed in the 1980's have led to new generations of iceprofiling sonar (IPS), incorporating much expanded on-board data storage capacities (up to 16 Gigabytes) and powerful realtime firmware which now allow unprecedented temporal (ping rates of up to 1 Hz). When combined with ULS Acoustic Doppler Current Profiler (ADCP) instrumentation using a special ice tracking mode (with a temporal resolution of a few minutes), details of the ice topography can be realized to resolutions of better than 0.1 m in the vertical and 1 m in the horizontal. These very high resolution ice draft measurements fully resolve individual ice features including undeformed level ice, brash ice, individual large ice keels including multi-year ice, hummocky ice rubble fields, glacial ice including icebergs and ice islands, and open water interruptions of the ice cover including leads between ice floes. Such continuous highly detailed ice measurements, along with concurrent measurements of ice velocities and ocean current profiles, are essential to understandings of mechanical and thermodynamical aspects of sea ice processes which govern ocean-atmosphere exchanges in polar waters, thereby determining ice extent and thickness parameters. The ice profiler ULS instrument can sample at higher sampling frequencies to measure non-directional ocean wave spectra and parameters (significant and individual maximum wave heights and peak periods) both during the period of mostly open water, often from mid-summer to mid-autumn, and also when ocean waves propagate into the periphery of the Arctic Ocean pack ice. Ocean wave interactions with pack ice are important in understanding the fracturing of sea ice floes and hastening the deterioration and melt of sea ice. The ULS data provide the first detailed measurements of such ocean wave - ice processes. A major challenge in moored ULS measurement systems is the inaccessibility of the measurement sites to ship logistics due to the very remote areas in the Arctic Ocean and its peripheral seas and the difficulty, resulting in very high logistic costs, of deployment and servicing the moorings due to the sea ice itself. This challenge is being addressed through the development of expanded capacity and more efficient internal power capability and increased onboard data storage, along with very high instrument reliability. With expanded alkaline battery packs, continuous operation for 2 to 3 years is now possible; lithium battery packs are being developed that will extend the in-situ ULS instrument operation to approximately five years. To provide access to the ULS data between mooring servicing intervals, two different approaches are being developed. In some areas cabled underwater observatory technology can be installed to provide real-time access to the ULS ice measurements in support of navigation and oil and gas exploration activities as previously described in Fissel et al. (2009) for sub-Arctic applications. The first such ocean observatory involving a ULS ice instrument was commissioned at Cambridge Bay in the Canadian Arctic in September 2012. At locations far from shorelines, the challenges become even greater. For offshore oil and gas drilling applications, an array of subsurface ULS moorings spanning distances of tens of kilometers, interconnected via bottom mounted fiber optics cable systems interfaced to the moored ULS instruments and to vessel platforms using acoustic modems, have been designed to provide tactical support for ice management operations in support of drilling activities. An alternative approach to provide yearly access to the multi-year moored ULS data sets is the development of small expendable buoyant “datapods” which store the ULS data on flash cards; during times of open water or very thin ice, the datapods are released from the mooring to float to the surface and the ULS data is then transmitted via satellite to provide remote access to the scientific users. There are variations on this approach involving aircraft landing on sea ice in the vicinity of the subsurface ULS moorings to access the data via on-command acoustic modem transmission of the data to acoustic receivers operated through ice holes.

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