Improvements inUpwardLooking Sonar-Based Sea- IceMeasurements: A CaseStudy for2007Ice Features inNorthumberland Strait, Canada

Scientific andengineering studies ofpolar andmarginal icezonesrequire detailed information onseaicethickness and topography. Accurate information onseaicethickness andtopography dataisrequired forbasic ice-covered oceanstudies and, increasingly, foraddressing important navigation-, offshore structure design/safety- andclimate change-issues. Since theearly 1990's, upward-looking sonar(ULS)instrumentation havebeendeveloped andapplied toprovide under-ice topography datawithhigh horizontal andvertical spatial resolution. Suchinternal recording ULSinstruments, oriceprofilers, aretypically operated fromthe seafloor ontautline mooring systems. TheASLModelIPS4IceProfiler, whichhasbeenwidely usedinstudies oftheArctic Ocean, aswellasinnumerous seasonal ice zonesandintheSouthern Ocean, isbeing upgraded toallow muchexpanded datastorage capacity (from 69Mbytes to1-8Gigabytes) and16bitA/Dresolution foriceranges andother parameters. Withtypical pingrates of0.5or1Hz,theenhanced capability oftheIce Profiler provides veryhighresolution measurements oficekeeldrafts andtheunder-ice topography ofsea-ice keelfeatures. TheIce Profiler isoften usedinconjunction withanAcoustic Doppler Current Profiler whichprovides direct measurements oficevelocity. Thecombination ofhighresolution icedraft timeseries withicevelocities allows forcomputation ofquasi-spatial icedrafts asa function ofhorizontal distance. Theresults fromthefirst deployment ofanupgraded IceProfiler, operated just offtheConfederation Bridge inNorthumberland Strait, fromNovember2006toApril 2007, arepresented andcompared withtheresults ofprevious seaicestudies atthesamelocation. Themuchlarger onboard datacapacity allows forrealization ofmultiple targets foreachpingand,onasubsampled basis, offers data onacoustic backscatter returns overthecomplete watercolumn.Thisadditional information isbeing analyzed toexamine thenature andcauseofoccasional false target returns. Inpastmeasurement campaigns, there havebeenepisodic occurrences ofdeeptargets detected whicharenotconsistent withseaicefeatures thatcanbereasonably expected tooccurinNorthumberland Strait. These features areoften associated withoccurrences ofthelargest (spring) tidal currents, leading tothehypothesis thatthese anomalous targets maybeassociated withvelocity shears inthewatercolumnresulting fromstrong tidal flows pastthebridge support structures. Basedonthese analyses, improvements tothetarget detection algorithm arebeing developed andtested.