Iceberg Thickness from BIOMASS Polarimetry

Kurzfassung

Height information of semi-transparent media is usually derived with interferometric or tomographic SAR techniques, such as forest height or the penetration depth into glaciers. In contrast, BIOMASS data present the opportunity to derive the thickness of icebergs solely from SAR polarimetry. The large penetration of the long-wavelength signals into ice and the quad-pol polarimetry allow to observe signals originating from the bottom of an iceberg, the ice-water interface. This bottom signal appears like a range-delayed replica of the direct signal from the top of the iceberg with distinct polarimetric characteristics. The targets of interest are tabular icebergs with sizes of often several kilometers. They are calving from ice shelves, as opposed to their non-tabular, irregularly shaped, smaller siblings calving from marine terminating glaciers. Further, tabular icebergs have a more or less rectangular profile, with a flat top, steep sides, and a relatively flat bottom. Even though this rectangular profile degrades over time, it allows to formulate a first-order model of the range delay between the backscatter from the top and the bottom of the iceberg. More specifically, the range distance between the top and bottom signals, as well as the length of the delayed bottom signal, depends on the thickness and the ground-range length of the iceberg. The permittivity of ice is considered for calculating refraction angles and range delays. Using Archimedes principle, the thickness can be separated into freeboard and submerged draft in order to consider the signals penetrating not only through the top of the iceberg, but also through the frontal freeboard wall. In single-pol backscatter images, it can be difficult to discriminate the iceberg bottom signal from a top signal. Further, the bottom signal can be also hidden in the surrounding sea ice backscatter. The quad-pol data of BIOMASS allows a clear distinction, in most cases, between the top and bottom signals of an iceberg. An interesting polarimetric pattern can be observed: Icebergs that show a surface-scattering mechanism in the top backscatter, with low polarimetric entropy and alpha parameters, have a strong dihedral contribution in the range-delayed bottom signal, with high alpha values and large phase differences between HH and VV. In contrast, icebergs that show a medium entropy and medium alpha scattering mechanism in the top backscatter, have similar scattering properties also in their bottom signal. A first theory is a stronger volume scattering contribution from inside the iceberg causing both the top and the bottom signal to appear volume-like. So far, the investigation concentrated on one BIOMASS acquisition from the commissioning phase, where channel imbalance phase and Faraday rotation were corrected, so that the phase differences between different channels have been widely calibrated, making the data ready for polarimetric analysis. A first estimation, with the first-order geometry model and by discriminating top and bottom signals according to their polarimetric characteristics, resulted in a thickness of 130 m. This iceberg is located in front of the Jelbart ice shelf, which has reported ice thicknesses of about 200 m at the calving front. Further investigations will refine the model formulation and the understanding of the polarimetric characteristics, as well as increase the number of estimated icebergs and include validation.