Post-processing calibration of frequency-domain electromagnetic data for sea ice thickness measurements

Abstract

Sea ice thickness measurements using electromagnetic (EM) instruments require accurate data. Calibration of sea ice thickness data acquired using a low induction number (LIN) EM sensor can be performed by conducting a geometric sounding at a range of heights over level sea ice of known thickness, and by comparing the observed data with the expected layered-Earth response. Calibration corrections for scaling, phasemixing and zero-offset errors can be derived using leastsquares inversion to minimise the misfit between the observed data and the theoretical response, and can be incorporated in modelling algorithms used to determine sea ice thickness. This paper presents a case history illustrating identification and correction of calibration errors in low induction number EM data for Antarctic sea ice thickness measurements. Comparison of coincident EM measurements made using three identical LIN instruments showed that measured apparent conductivities disagreed by up to around 100 mS/m, resulting in errors in the estimated sea ice thickness of up to 60%. Separate calibration corrections were determined for each instrument by analysis of geometrical sounding data acquired over level sea ice. Sea ice thickness at the calibration site was determined by making a large number of drilled thicknesses over the footprint of the EM instrument, and seawater and sea ice conductivities were determined using independent measurements. After application of the calibration corrections, sea ice thicknesses derived from the three instruments agreed closely with each other and with drilling results.