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, phase-mixing, and zero-offset errors can be derived using least-squares 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 frequency-domain EM data for Antarctic sea-ice thickness measurements. Comparison of coincident EM measurements made using three EM31 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 thickness measurements over the footprint of the EM instrument, and seawater and sea-ice conductivities were determined using independent measurements. Best-fit scaling, phase-mixing, and zero-offset errors were determined via inversion of the geometrical sounding data, with the sea-ice thickness and seawater conductivity fixed at their known values. After application of the calibration corrections, sea-ice thicknesses derived from the three instruments agreed closely with each other and with drilling results. Calibration corrections derived in this manner have been shown to be valid over a period of at least several weeks.