Modelling and interpretation of polarimetric scattering from sub-arctic lakes at L-Band

Kurzfassung

The presented work contributes to the interpretation of fully polarimetric SAR (Synthetic Aperture Radar) data over (shallow) sub-arctic lakes, on the purpose of investigating SAR capabilities in revealing facts about the subsurface and the inhomogeneities within the ice layer which are dominated mainly by methane bubbles. A model for the polarimetric backscattering from a two layer structure observed by a fully polarimetric side looking synthetic aperture radar at low frequencies (L-Band) is developed in [1]. The upper layer thickness is few metres thick and contains inhomogeneities. The developed model describes the backscattering as the incoherent sum of the three main contributions: subsurface (X-Bragg), volume and dihedral backscattering. The volume is modelled as a cloud of partially vertically oriented ellipsoids. The dihedral backscattering component is modelled as the component reflected on the subsurface after being scattered according to Rayleigh theorem by the same cloud of particles that is responsible for the volume backscattering. This dihedral component predicts a larger horizontally polarized backscattering than vertically polarized, a zero copolarization phase shift for small local incident angles. The mechanism could describe a great part of the backscattering from the sub-arctic lakes. The model simulations are compared to experimental Quad-pol data obtained by ALOS-PALSAR over frozen shallow sub-arctic lakes acquired over several regions in the northern wetlands. Over the Churchill site located on the shore of Hudson Bay in Manitoba (province of Canada), the lake samples are divided into two groups, one with high backscattered power that is assumed to be floating ice and the other with low backscattered power that is assumed to be grounded ice. The forward simulation with a water subsurface and a certain range of volume characteristics matched the observed polarimetric parameters for floating ice lake samples observed by ALOS. The exact same simulation with the same assumption is performed for grounded ice and it matched the observed polarimetric parameters for grounded ice lake samples observed by ALOS. Hence the model shows the same trend as the observed data for grounded and floating ice. Over the Inuvik site located in the Northwest Territories of Canada, a similar investigation is performed, while assuming the first acquisition to be grounded ice and the second to be floating ice, as ice melting before the second acquisition is verified by temperature measurements. Hence the model could explain this temporal change. Based one those simulations an Entropy-Alpha colour scheme is generated and Entropy-Alpha colour maps (power normalized) for ALOS images are presented. The images show the temporal changes of the ice and the difference between grounded and floating ice. [1] Al-Kahachi, Noora ; Papathanassiou, Konstantinos P.: Polarimetric scattering model for methane bubbles trapped in the ice of sub-arctic lakes: Proceedings of (IGARSS), 2012, Munich, Germany.