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Estimation des volumes d'icebergs qui se retournent: modélisation mécanique et analyse de signaux sismiques

Abstract : One main concern in climate science is to reduce uncertainties on sea level predictions. In particular, these uncertainties depend on the quantification of the mass losses of polar ice caps including Greenland ice sheet. Iceberg calving at Greenland glaciers accounts for up to half of ice losses at glacier termini. Some icebergs detaching from Greenland glaciers have the height of the glacier and are thin so unstable and capsize exerting a force on the glacier front which generates a seismic wave recorded at stations in Greenland and further away. These seismic signals have been recorded for the last twenty-five years by the permanent stations. The number of such events has increased and the spatial distribution has evolved. What is the evolution of the volume of capsizing icebergs ? Field data on capsizing icebergs are lacking, except for seismic data. Indeed, the database of seismic signals gives continuous information about iceberg capsize events: we aim to extract information from these data. The characteristics of seismic signals depend on the iceberg volume and the whole dynamic of the capsizing iceberg [2]. The global aim of this work is to calculate the volum of capsizing icebergs. To do this, we compare recorded seismic signals to synthetic seismic signals calculated using a model of iceberg capsize. Therefore, we solve an inverse problem to obtain information on the dynamic of the capsize and in particular an estimation of the volume of the iceberg. Iceberg capsize dynamics depends on complex phenomena: iceberg-water interactions, iceberg-glacier friction, glacier-sea floor friction, elasto-viscoplastic deformation of ice. Solving directly fluid flow, solid motion, and contact equations even in two dimensions is very costly and can hardly be used to generate catalogs and to solve inverse problem. Therefore, a simplified mechanical model of a capsizing iceberg in water has been developed based on few assumptions. The proposed model, named SAFIM (semi- analytical floating iceberg model) accounts for sea hydrodynamics only through hydrostatic pressure, pressure drag, and added mass, and it has been validated based on a separate state-of-the-art Compu- tational Fluid Dynamics code which can handle free surface and arbitrary iceberg configurations. The error on the horizontal force exerted by the fluid on the iceberg capsizing in open ocean (with no contact with the glacier) calculated with SAFIM goes from 4% to 20%, and the drag coefficient that minimizes this error goes from 1 to 3, depending on the aspect ratio of the iceberg.
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Pauline Bonnet, V Yastrebov, A. Mangeney, Olivier Castelnau, P. Queutey, et al.. Estimation des volumes d'icebergs qui se retournent: modélisation mécanique et analyse de signaux sismiques. 24ème Congrès Français de Mécanique, CFM 2019, Aug 2019, Brest, France. ⟨hal-02571877⟩

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