Airborne electromagnetic measurements of sea ice thickness: methods and applications

Alfred Wegener Institute operates two helicopter-borne electromagnetic (EM) sounding devices dedicated to the measurement of sea ice thickness. With the method, level total (ice plus snow) thickness can be determined with an accuracy of ±0.1 m. However, due to the footprint of the method and due to the porosity of unconsolidated ridge keels, deformed ice thickness can be strongly underestimated. The paper summarizes the retrieval of ice thickness from the EM data, and shows some validation results. Applications of the methods show that level ice thickness has decreased by 20% in the Transpolar drift between 1991 and 2001, but remained constant at 2.0 m between 2001 and 2004. Surveys in 2004 and 2005 in the Lincoln Sea revealed very thick ice with modal thicknesses between 3.9 and 4.2 m, and an increase of mean ice thickness from 4.67 m in 2004 to 5.18 m in 2005. As the EM instruments also comprise a laser altimeter and a differential GPS receiver (DGPS), independent measurements of ridge distributions and surface roughness can be obtained. In addition, coincident measurements of total thickness and surface elevation allow the retrieval of snow thickness.