Ice Thickness: Instruments

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The Ice Profiling Sonar Mooring

(Click here for information on the Mooring program and Mooring data.)

      The central component of the ice thickness program is an ice profiling sonar, operated at a safe depth of about 45 m. This purpose-built upward looking sonar provides an Eulerian look at the submergence depth (i.e. draft) and bottom topography of pack ice drifting overhead. The data from the ice-profiling sonar are used to calculate ice draft with a vertical accuracy of 5 cm. The ice draft is an excellent proxy for the ice thickness except on very small (10 m) scales in rough ice.

      An acoustic Doppler current profiler is used to measure the motion of the ice. Measurements of ice drift are needed to convert time-series observations into topographic profiles, from which the ice-draft distribution can be derived. Drift measurements also provide insights into the place of origin and fate of ice observed at the site.

      The Doppler sonar also provides measurements of ocean current and acoustic backscatter at 8-m increments of depth up to the surface. We position this sonar as deep as practical to provide these measurements all the way to the surface. Because the echoes received by the sonar come from zooplankton, the scattering time series is a complementary depth-time proxy for zooplankton abundance at CH01.

      These instruments are secured and supported at their operating depths through use of the sub-sea mooring anchored to the seabed, illustrated on the right. One drawback with the mooring is that data are recorded within the sub-sea instruments and are not available until the mooring is recovered. The objective is to recover the data once a year, but the instruments are designed to operate without servicing for 2-3 years in case ice prevents ship access in any particular year.

Sketch of Installed Mooring

(Click sketch below for larger image of Mooring)

Sketch of Mooring installed at CH01

 


      

The sketch (above, right) shows the mooring that has been at CH01 since October 2008. At this time, three new instruments were added to supplement the two sonar operating since 2003: an AURAL ambient sound recorder and temperature-salinity recorder at the depth of each sonar. The AURAL provides almost continuous recordings of sounds from marine mammals (e.g. bowhead, beluga, bearded seal, walrus), human activity (e.g. aircraft, shipping, distant seismic survey) and natural processes (ice fracture, ridge building, wind waves, blowing snow). More information on the Mooring program or data can be found at:

http://imb-crrel-dartmouth.org/imb.crrel/thickdat.htm.