Detailed ground-penetrating radar (GPR) surveys at 50 and 200 MHz on Hansbreen, a polythermal glacier in southern Svalbard, are presented and interpreted. Comparison of the variations in character of the radar reflections with borehole thermometry and water levels in moulins suggests that GPR can be used to study the hydrothermal properties of the glacier. The high resolution of the GPR data shows that the hydrothermal structure of the glacier is highly variable both along the centre line and on transverse profiles. Water contents for many places and depths within the glacier were calculated by estimating radar-wave velocities to point reflectors. We find typical water contents of 1-2% for the temperate ice, but wetter ice associated with surface crevassing and moulins (typically 4% water content). There is evidence that wet ice sometimes overlays drier ice. The hydrothermal structure is thus shown to be very complex. Temperature gradients in the cold ice indicate freezing rates of temperate ice below cold ice of 0.1-0.5 ma-1, while isolated point reflectors within the cold ice indicate large water-filled bodies that are probably related to the regular drainage structure of the glacier.

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ABSTRACT. D etailed ground-penetrating rada r (GPR ) surveys a t 50 a nd 200 MHzon H ansbreen, a p oly therm al glac ier in southern Svalbard , are presented a nd interpreted . C omparison of the variati ons in cha racter of th e r a d a r refl ecti ons with borehole therm ometry and wa ter levels in m oulins suggests tha t GPR can be used to study the hydroth ermal properti es of the glacier. The hi gh reso luti o n of the GPR d a ta shows tha t the hyd ro therm a l structure of th e glacier is hi ghl y va riabl e both along th e centre line a nd o n transverse profil es. Water contents for many pl aces a nd depths within the glacier were calcul ated by es timating radar-wave vel oc iti es to point refl ectors. \Ve find typica l wa ter contents of 1-2% for the temper a te ice, but wetter ice associated with surface crevassin g a nd moulins (typically 4% water content). There is evidence that wet ice sometimes overlays drier ice. Th e hydrothermal st ructure i thus shown to be ve ry compl ex. Temperature g r adi ents in the co ld ice indicate freez ing rates of tem perate ice below co ld ice of 0.1 -0. 5 m a I, whil e isola ted point re fl ec tors within th e co ld ice indi cate la rge water-fill ed bodi es that a re probably related to the reg ular dra i nage st ructure of the g lacier. INTRODUCTIO N

Ice temperature measurements were taken from three shallow and five deep (to bedrock) boreholes on Hansbreen, Svalbard, in selected years between 1988 and 1994. In general, results show a subpolar, polythermal structure. The glacier accumulation zone is of warm ice within the entire vertical profile except in the uppermost layer of seasonal temperature fluctuations where there is an upper cold ice layer in the ablation zone which vanes in thickness and may even be absent in the western lateral part. The upper layer of cold ice thins along the glacier centre-line from the equilibrium line altitude down to the glacier front. The depth of the pressure melting, indicating the base of the cold ice layer, was defined at the borehole measurement sites but was not manifested as an internal reflection horizon using multi-frequency radar methods. The isotherm lies about 20 m above a radar internal reflecting horizon near the equilibrium line altitude and about 40 m above it in the frontal part of the glacier. The internal reflection horizon almost certainly reflects the high water content within temperate ice and not the cold/temperate ice interface. At 1 0 m depth, the temperatures are 2-3°C higher than the calculated mean annual air temperatures, demonstrating the importance of meltwater refreezing on the release of latent heat.