COMPARISON OF SEA-ICE THICKNESS MEASUREMENTS UNDER SUMMER AND WINTER CONDITIONS IN THE ARCTIC USING A SMALL ELECTROMAGNETIC INDUCTION DEVICE

Drillhole-determined sea-ice thickness was compared with values derive d remotely using a portable small-offset loop-loop steady state electr omagnetic (EM) induction device during expeditions to Fram Strait and the Siberian Arctic, under typical winter and summer conditions. Simpl e empirical transformation equations are derived to convert measured a pparent conductivity into ice thickness. Despite the extreme seasonal differences in sea-ice properties as revealed by ice core analysis, th e transformation equations vary little for winter and summer. Thus, th e EM induction technique operated on the ice surface in the horizontal dipole mode yields accurate results within 5 to 10% of the drillhole determined thickness over level ice in both seasons. The robustness of the induction method with respect to seasonal extremes is attributed to the low salinity of brine or meltwater filling the extensive pore s pace in summer. Thus, the average bulk ice conductivity for summer mul tiyear sea ice derived according to Archie's law amounts to 23 mS/m co mpared to 3 mS/m for winter conditions. These mean conductivities caus e only minor differences in the EM response, as is shown by means of 1 -D modeling. However, under summer conditions the range of ice conduct ivities is wider. Along with the widespread occurrence of surface melt ponds and freshwater lenses underneath the ice, this causes greater s catter in the apparent conductivity/ice thickness relation. This can r esult in higher deviations between EM-derived and drillhole determined thicknesses in summer than in winter.