Field and Laboratory Investigation of Electrical Resistivity-Temperature Relationships, Southern Northwest Territories

摘要

DC electrical resistivity tomography (ERT) is increasingly being used to delimit permafrost bodies, but a threshold value to differentiate between frozen and unfrozen ground is needed for data interpretation. Here we compare resistivities measured in peat and silts across a range of temperatures in both the field and the laboratory. ERT surveys were conducted at four sites with instrumented boreholes in the southern NWT (60.9-62.5°N). In the laboratory, remoulded soil samples were wetted to field moisture content, frozen in closed containers, and resistivity was measured across a range of temperatures during thaw. Between 0℃ and -0.5℃, the resistivity of peat ranged from approximately 3,000-6,000 Ωm. For frozen silt, maximum resistivities were about 1,000 Ωm while the lowest was 85 Ωm, well below the value typically used to delimit permafrost. The relationship between resistivity and temperature allowed a two-dimensional temperature field to be generated from the tomograms, but predictions were not always consistent due to intra-site differences in moisture content and the presence or absence of segregated ice. This study illustrates the great range of resistivity values possible at temperatures close to 0℃, and demonstrates the importance of validating ERT results for warm permafrost.