Direct helicopter EM — Sea-ice thickness inversion assessed with synthetic and field data

Abstract

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.

Developments in frequency domain AEM: Tackling drift and noise with a ferrite-core, receiver triplet.

“…The results showed that the underestimation of the thickness of so-called pressure ridges is significant and variable over a range of 20% to 50% (Hendricks 2009). Modelling results are consistent with findings in the field based on drilling profiles (Pfaffling et al 2007) that traditional HEM systems are not capable of resolving the maximum thickness of pressure ridges and their porosity. Thus, an enhanced HEM ice thickness retrieval is needed to enable the investigation of the role of pressure ridges in the open sea and shallow coastal waters.…”

“…Whenever sea ice thickness is mentioned in an EM context, it actually refers to the total thickness (ice thickness + snow thickness). We provided a detailed description and discussion of AEM ice thickness retrieval in Pfaffling et al (2007). In contrast to fairly one-dimensional level ice, pressure ridges are porous, blocky structures formed in deformation events, when two convergent ice floes break due to lateral stress.…”

“…A 1D interpretation by direct inversion of the Inphasecomponent (Pfaffling et al, 2007 andHaas et al, 2009) for three frequencies was used to calculate ice thickness and to illustrate the accuracy of the system (Figure 7). The 500 Hz frequency was omitted due to the insufficient signal to noise level as the frequency is not suitable for ice thickness retrieval but rather bathymetry surveying.…”

SMOS-derived thin sea ice thickness: algorithm baseline, product specifications and initial verification

“…Hence, the obtained ice thickness is the ice plus snow thickness from the difference between the laser range and the EM-derived distance. The accuracy over level sea ice is on the order of 10 cm (Pfaffling et al, 2007). Uncertainties in the ice thickness measurements may arise from the assumption that sea ice is a non-conductive medium.…”

Sea ice and snow thickness and physical properties of an ice floe in the western Weddell Sea and their changes during spring warming

“…The accuracy of helicopter-borne and ground-based EM thickness measurements is better than 70.1 m over level ice (Haas et al, 1997;Pfaffling et al, 2007). However, over ridges and deformed ice, due to the footprint of the measurements of up to 3.7 times the flying altitude, and due to the porosity of ridge keels, the maximum thickness of pressure ridges and deformed ice can be underestimated by as much as 50% (Haas et 2003; Reid et al, 2006a).…”