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

Turu V.

Landform Analysis

2012

Vol. 21

57-74

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 magneticc resonance soundings were done following a 3 Kmprofile 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 (greater than 2 Mega Ohms meter for glacier ice) the surveyed depth is directly related to the loop area. For small loops (30msquare 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 less than 20 nV) but also the time decay (100 ms greater than =T*2 greater than 40 ms). Increasing loop sizes (90 and 120msquare loops), a slight increase in amplitude at the Larmor frequency, close to 30 nV, is observed with very high time decays (T*2 greater than 500 ms). Ground Penetrating Radar surveys were carried out in Hansbreen at the same location as theMRSsurveyed 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 conventionalMRSdevices. 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.

Remote sensing of eskers from Vormsi and Väinameri vicinity, northwestern Estonia

Aber J. S., Kalm V.

Geological Quarterly

2001

Vol. 45, No 4

365-372

We have utilised techniques of remote sensing in combination with ground observations in order to investigate eskers of the Vormsi and Väinameri region of northwestern Estonia. Landsat Thematic Mapper (TM) images were the basis for recognition and regional interpretation of esker systems, and kite aerial photography was employed for detailed, low-height views of selected eskers. A special Landsat TM composite was developed to enhance the display of shallow sea floor features. On this basis, we have extended known, land-based eskers across the sea floor, and we have identified additional probable eskers marked by shallow shoals and tiny islands. The known and suspected eskers of Vormsi and surroundings demonstrate a regular pattern in their distribution, which we suggest represents a subglacial drainage network that was anastomosing in character. The esker network is located along the central pathway of the Väinameri ice lobe, and the overall direction of drainage was toward the Palivere glacial limit. We interpret eskers of the Vormsi-Väinameri vicinity as evidence for substantial meltwater discharge beneath the Väinameri ice lobe, which terminated in a proglacial lake.