A combined GPR and SNMR monitoring of a drained intraglacial water pocket located into the polythermal glacier of Tête Rousse

Formation of intraglacial lakes in polythermal glaciers results from specific climatic, topographic and aspect conditions. Their occurrence constitutes a significant threat to the valleys located below in case of sudden discharge occurring without any warning prior signs. Such case occurred in 1892 at Saint-Gervais (French Alps), where a deadly water wave reached the town a few tens of minutes after a water-filled cavity ruptured within the Tête Rousse glacier located upstream. Further multi-method investigations of this glacier began from 2007, based initially from surface Ground Penetrating Radar (GPR) imaging and then supplemented with Surface Nuclear Magnetic Resonance (SNMR) surveys and borehole measurements. These combined study allowed first to detect and characterize a water-filled water pocket located 40 m below the surface within the glacier, and second to monitor its changes after water was drained in fall 2010 to reduce risk. Here, we will present the main GPR results obtained in this unique and complex 3D case, which will be quantitatively compared with other geophysical and borehole measurements. This case study shows the potential of combining GPR and SNMR methods for the study of polythermal glaciers.