Surface NMR survey on Hansbreen Glacier, Hornsund, SW Spitsbergen (Norway)

Abstract

Glaciers are widely spread on polar and sub-polar regions but also on middle latitude mountains, where cold-dry type glaciers, polythermal glaciers and temperate-wet glaciers are respectively present. Polythermal glaciers have a cold-ice layer (temperature below the pressure melting point) overriding a temperate-ice layer. Nineteen magnetic resonance soundings were done following a 3 Km profile on Hansbreen front. Resistivity on the glacier surface, magnetic susceptibility of rocks, electromagnetic noise and total earth's magnetic field measurements confirm that the MRS survey took place in the best conditions. MRS data show different signals amplitudes at the Larmor frequency according to the loop dimension. In a very high electrical resistive context (>2 Mega Ohms meter for glacier ice) the surveyed depth is directly related to the loop area. For small loops (30 m square loop) amplitudes around 50 nV are common as well as some decay time (T*2) above 300 ms. Enlarging the loop size (60 m square loop) it is possible to observe a decrease of the signal amplitude at the Larmor frequency (E0 < 20 nV) but also the time decay (100 ms >= T*2 >40 ms). Increasing loop sizes (90 and 120 m square loops), a slight increase in amplitude at the Larmor frequency, close to 30 nV, is observed with very high time decays (T*2 >500 ms). Ground Penetrating Radar surveys were carried out in Hansbreen at the same location as the MRS surveyed zone. Available GPR data show a water content of 2,5% on the cold-ice layer (the first 35 m depth) and 2% of water content on the temperate-ice layer but a 4% of water content can also be detected. Both geophysical methods are not convergent because some water content on ice has too short relaxation times being undetectable with conventional MRS devices. In that sense the low T*2 time decays data from large MRS loops elucidates that at the temperate-ice layer water flows by seepage through veins and microfractures at a very low rate toward the glacier bottom and a large amount of free water is close to the cold/temperate transition surface. In the cold-ice layer large T*2 time decays are common because water flows through fissures or karstic like conduits. In summary, combining the MRS and GPR techniques gives glaciologists a powerful toolkit to elucidate water flow-paths on glaciers, supercooled meltwater content and subglacial water or aquifers.