Sea ice is an important climate variable and is also an obstacle for marine operations in polar regions. We have developed a small and lightweight, digital frequency-domain electromagnetic-induction (EM) system, a so-called EM bird, dedicated for measurements of sea ice thickness. 3.5 m long and weighing only 105 kg, it can easily be shipped to remote places and can be operated from icebreakers and small helicopters. Here, we describe the technical design of the bird operating at two frequencies of f1 = 3.68 kHz and f2 = 112 kHz, and study its technical performance. On average, noise amounts to ±8.5 ppm and ±17.5 ppm for f1 and f2, respectively. Electrical drift amounts to 200 ppm/h and 2000 ppm/h for f1 and f2, during the first 0.5 h of operation. It is reduced by 75% after two hours. Calibration of the Inphase and Quadrature ppm signals varies by 2 to 3%. A sensitivity study shows that all these signal variations do affect the accuracy of the ice thickness retrieval, but that it remains better than ±0.1 m over level ice in most cases.This accuracy is also confirmed by means of comparisons of the helicopter EM data with other thickness measurements. The paper also presents the ice thickness retrieval from single component Sea ice is an important climate variable and is also an obstacle for marine operations 3 in polar regions. We have developed a small and lightweight, digital frequency-4 domain electromagnetic-induction (EM) system, a so-called EM bird, dedicated for 5 measurements of sea ice thickness. 3.5 m long and weighing only 105 kg, it can 6 easily be shipped to remote places and can be operated from icebreakers and small 7 helicopters. Here, we describe the technical design of the bird operating at two 8 frequencies of f1 = 3.68 kHz and f2 = 112 kHz, and study its technical performance. 9On average, noise amounts to ±8.5 ppm and ±17.5 ppm for f1 and f2, respectively. 10Electrical drift amounts to 200 ppm/h and 2000 ppm/h for f1 and f2, during the first 11 0.5 h of operation. It is reduced by 75% after two hours. Calibration of the Inphase 12and Quadrature ppm signals varies by 2 to 3%. A sensitivity study shows that all 13 these signal variations do affect the accuracy of the ice thickness retrieval, but that it 14 remains better than ±0.1 m over level ice in most cases. This accuracy is also 15 confirmed by means of comparisons of the helicopter EM data with other thickness 16 measurements. The paper also presents the ice thickness retrieval from single

[1] Helicopter-borne electromagnetic sea ice thickness measurements were performed over the Transpolar Drift in late summers of 2001, 2004, and 2007, continuing ground-based measurements since 1991. These show an ongoing reduction of modal and mean ice thicknesses in the region of the North Pole of up to 53 and 44%, respectively, since 2001. A buoy derived ice age model showed that the thinning was mainly due to a regime shift from predominantly multi-and second-year ice in earlier years to first-year ice in 2007, which had modal and mean summer thicknesses of 0.9 and 1.27 m. Measurements of second-year ice which still persisted at the North Pole in April 2007 indicate a reduction of late-summer second-year modal and mean ice thicknesses since 2001 of 20 and 25% to 1.65 and 1.81 m, respectively. The regime shift to younger and thinner ice could soon result in an ice free North Pole during summer.

Accuracy and precision of helicopter electromagnetic ͑HEM͒ sounding are the essential parameters for HEM seaice thickness profiling. For sea-ice thickness research, the quality of HEM ice thickness estimates must be better than 10 cm to detect potential climatologic thickness changes. We introduce and assess a direct, 1D HEM data inversion algorithm for estimating sea-ice thickness. For synthetic quality assessment, an analytically determined HEM sea-ice thickness sensitivity is used to derive precision and accuracy. Precision is related directly to random, instrumental noise, although accuracy is defined by systematic bias arising from the data processing algorithm. For the in-phase component of the HEM response, sensitivity increases with frequency and coil spacing, but decreases with flying height. For small-scale HEM instruments used in sea-ice thickness surveys, instrumental noise must not exceed 5 ppm to reach ice thickness precision of 10 cm at 15-m nominal flying height. Comparable precision is yielded at 30-m height for conventional exploration HEM systems with bigger coil spacings. Accuracy losses caused by approximations made for the direct inversion are negligible for brackish water and remain better than 10 cm for saline water. Synthetic precision and accuracy estimates are verified with drill-hole validated field data from East Antarctica, where HEM-derived level-ice thickness agrees with drilling results to within 4%, or 2 cm.

Existing estimates of footprint size for airborne electromagnetic (AEM) systems have been based largely on the inductive limit of the response. We present calculations of frequency-domain, AEM-footprint sizes in infinitehorizontal, thin-sheet, and half-space models for the case of finite frequency and conductivity. In a half-space the original definition of the footprint is extended to be the side length of the cube with its top centered below the transmitter that contains the induced currents responsible for 90% of the secondary field measured at the receiver. For a horizontal, coplanar helicopter frequency-domain system, the in-phase footprint for induction numbers less than 0.4 (thin sheet) or less than 0.6 (half-space) increases from around 3.7 times the flight height at the inductive limit to more than 10 times the flight height. For a vertical-coaxial system the half-space footprint exceeds nine times the flight height for induction numbers less than 0.09. For all models, geometries, and frequencies, the quadrature footprint is approximately half to two-thirds that of the in-phase footprint. These footprint estimates are supported by 3D model calculations that suggest resistive targets must be separated by the footprint dimension for their individual anomalies to be resolved completely.Analysis of frequency-domain AEM field data acquired for antarctic sea-ice thickness measurements supports the existence of a smaller footprint for the quadrature component in comparison with the in-phase, but the effect is relatively weak. In-phase and quadrature footprints estimated by comparing AEM to drillhole data are considerably smaller than footprints from 1D and 3D calculations. However, we consider the footprints estimated directly from field data unreliable since they are based on a drillhole data set that did not adequately define the true, 3D, sea-ice thickness distribution around the AEM flight line.