Variations of permafrost under freezing and thawing conditions in the coastal catchment Fuglebekken (Hornsund, Spitsbergen, Svalbard)
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Two seismic field surveys were organized in the Fuglebekken coastal catchment of Hornsund, Spitsbergen, Svalbard, to map frozen and unfrozen ground and assess the spatial and temporal state of the permafrost. Surveys were conducted during maximum thawing in September and maximum freezing in April of the following year. The obtained seismic wavefields were interpreted using three methods: the surface waves dispersion, seismic refraction, and traveltime tomography. The seismic experiments were supported by nearby boreholes with continuous thermal monitoring. In the frozen survey, a gradual increase in ice content of water-filled sediments was found, further from the coast. In September the shallow sensors in the boreholes validated positive ground temperatures down to 3.0 m depth, with below zero temperatures at greater depths. However, the seismic tomography indicated that the ground was unfrozen down to 30 meters. The ground probably remained unfrozen due to intrusion of high salinity seawater, even though it had been below 0℃. In April, in the area 300 m and farther form the coast, the ground below 3 m depth was frozen, except for a 19 m deep open talik identified in a borehole at the slope of Fugle Mountain. We attribute the complex spatial extent, form, and condition of permafrost in the Fuglebekken coastal catchment to multiple factors, including variable solar energy, snow and ground cover, thermal and humidity properties of the soil, subsurface water flow, and seawater intrusion. The presented combination of seismic methods provides a new robust and precise approach to access spatial variability of permafrost in a coastal environment. The proposed interpretation show deep percolation of subsurface flow into permafrost and it’s seasonal unfreezing at a depth of 30 m in both saltwater intruded zone and the slope area.