Optimized ERT Depth Electrode Placement for Permafrost Basal Thaw Imaging

Temporal degradation in extent of permafrost is a key indicator in monitoring the effects of global warming. The vast majority of permafrost studies focus on the active layer, i.e. the near surface permafrost layer which seasonally freezes and thaws. Yet, there is little research focused on the basal thaw processes of permafrost, i.e. thawing of the lower frozen boundary. While in many cases this boundary is controlled by the geothermal gradient, sub- and intra- permafrost groundwater infiltration have the potential to substantially alter subsurface permafrost extent. Unlike active layer thaw where much of the meltwater is retained by the underlying impermeable frost table, basal thaw will result in a significant net loss of water to the permafrost inventory.

Electrical resistivity tomography (ERT) has been used to effectively map lateral extent of permafrost with great success; however, this surface based imaging method is severely hindered when it comes to accurate detection of the lower permafrost boundary. Generally, ERT results over-estimate the depth of the permafrost layer considerably, contributing to an exaggerated permafrost volume estimate compared to actual conditions. Recent studies have introduced depth electrodes to address this issue. Here we present a series of forward electrical resistivity models examining optimal depth electrode placement and measurement geometry for enhanced permafrost basal thaw imaging. We interrogate multiple electrode arrays and consider discontinuous and continuous permafrost conditions in two-dimensions. Further, by varying permafrost and background starting resistivities, we can demonstrate the impact of large and small resistivity gradients on shadow zone extent.