Sea ice thickness mapping by airborne and ground EM methods

Abstract

An experimental campaign was carried out in 1991-1997 in the Baltic Sea, a body of brackish water with resistivities of 3 to 4 ohm-m, to test the ability of airborne and ground electromagnetic methods to map sea ice thickness. The measurements were made as part of the European research projects Baltic Experiment for ERS-1 and Local Ice Cover Deformation and Mesoscale Ice Dynamics. Measurements on sea ice were carried out using a multifrequency, horizontal and vertical coplanar loop-loop system with a coil separation of 10 m. The purpose of these measurements was to obtain detailed information about conductivity properties of the ice, including keel ice rubble. The first interpretation was made by using a layered-earth model. To correct for small variations in coil separation which primarily influences the in-phase component, coil separation was treated as an unknown parameter in the inversion. A three-dimensional model was used to interpret more complicated ice geometry in ridge areas. The results obtained were validated by drilling. Airborne measurements were made with a vertical coplanar coil configuration where the coils were mounted on the wing tips of a Twin Otter aircraft, at a frequency of 3.1 kHz. A laser profilometer was used to map surface topography. Test measurements were made along 1 to 2 km long profiles. In good conditions the thickness accuracy achieved was ±0.2 m for undeformed ice, but worse for deformed ice with complicated geometry. EM measurements on sea ice were found to be a practical tool if the detailed structure of ice and ridges is needed whereas AEM is superior if large coverage is essential.