Ice thickness is a key parameter describing the state of the ice cover. During HOTRAX, we measured thickness in three ways: the ice watch, drilling holes, and EM-31surface surveys.

Surface ice surveys followed horizontal transects, which ranged from lengths of hundreds of meters to several kilometers, along which ice thickness and snow depth were measured every five meters. Ice thickness was determined using an EM-31 electromagnetic induction sensor, and snow depth was measured using a calibrated ski pole.

The Geonics EM-31is an electromagnetic induction device that determines the apparent conductivity of the underlying medium based on measurements of the secondary electromagnetic field induced in the underlying ice and the seawater by a transmitter coil . Sea ice tends to have low sea ice conductivities (<50 mS m^-1) compared to high seawater conductivities (>2500 mS m^-1). The signal received by the instrument is controlled by eddy currents generated at the ice-seawater interface and its strength is related to the ice thickness. The two coplanar transmitter and receiver antenna coils are mounted at a spacing of 3.66 m and operate at 9.8 kHz. The instrument package was carried as shown on the right.