On the potential for a bottom active layer below coastal permafrost: the impact of seawater on permafrost degradation imaged by electrical resistivity tomography (Hornsund, SW Spitsbergen)

This paper presents the results of two-dimensional electrical resistivity tomography (ERT) of permafrost developed in coastal zone of Hornsund, SW Spitsbergen. The measurements were made using the Wenner-Schlumberger electrode array with an electrode spacing 5 m for overview and 1.5 or 1 m spacing for detailed imaging. Using the ERT inversion results, we studied the ‘sea influence’ on deeper parts of the frozen ground. During the investigations we tested hypotheses that the operation of seawater on shoreface may cause changes in the shape of the coastal permafrost base, and that the impact of seawater on more inland permafrost depends on the shape of the shoreline (differently in the embayment, and differently in a headland exposed to the open sea). Our study was inspired by previous ground temperature measurements conducted in several boreholes located in study area which captured the propagation of ground heat waves from the base of permafrost. Our resistivity models indicate a major differentiation in terms of resistivity of permafrost in the coastal zone. The resistivity measures obtained in deeper layers of ground were so low (<100 Ω·m) that in the ‘warm permafrost’ conditions they exclude a possibility of freezing the coastal sediments and bedrock from the side of the sea. Low values continue further inland, going down under the surface layer of permafrost with higher resistivity. We interpret this situation as an influence of seawater's temperature and salinity on deeper parts of permafrost. Based on the measurements conducted within two years, we stated a change in the distribution of resistivity, both in the active layer, and in coastal front of permafrost in deeper parts of the ground. As observed in the inverse models, the geometric arrangement between the fields of extreme resistivity indicates the existence of a bottom active layer by the permafrost base, depending on thermal and chemical characteristics of seawater. The measurements conducted in the embayment, as well as on the headland exposed to the operation of storm waves, proved strong differences in the scale of the impact of seawater on permafrost.