Towards Multi-Channel Inversion of Electromagnetic Sea Ice Surveys
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Sea ice is a crucial parameter in climate research as it plays an important role in the interaction between oceans and the atmosphere in polar regions. It is considered a climate indicator and it is critical to observe its development in the context of climate change. While sea ice extent provides a picture of the surface conditions of the ice, ice thickness information is needed to fully understand the overall sea ice conditions. Frequency domain electromagnetic (EM) induction sounding is a non-invasive remote sensing method to measure sea ice thickness changes on the regional scale as well as provide a means to calibrate and validate satellite ice thickness data. This PhD thesis aims to advance the analysis of airborne and ground based sea ice thickness measurements from frequency domain EM sounding by improving ice thickness retrievals with concurrent use of Inphase and Quadrature instrument responses in a numerical inversion for multi- and single-frequency devices. The developed methods and algorithms include the forward modelling code and GUI ODFEM (One Dimensional Frequency domain Electromagnetic Model) to simulate the EM instrument responses for Inphase and Quadrature for different instrument and model settings. Furthermore, a brute force inversion method was established which can be used in combination with the 1D forward models created with ODFEM to invert single- and multi-frequency EM data into multi-layer ice thicknesses and ice conductivities. The performance of the developed brute force inversion algorithm is demonstrated on a variety of field data sets including an approach on how to resolve the thickness of a slush layer and its conductivity (wet saline snow) using a five-layer-model and the data of 3 measurement frequencies. Furthermore, how to measure the thickness and determine the conductivity of a sub-ice-platelet-layer commonly observed in Antarctica is demonstrated using an inversion of the two channel output of a single-frequency instrument (Inphase and Quadrature) in combination with in situ drill hole measurements. The methods and algorithms developed within this thesis provide new and more extended applications for a variety of EM devices for ground based and airborne ice thickness surveys.