Gpr And Dc Resistivity Application To Discontinuous Permafrost Delineation, Eielson Air Force Base, Alaska

Abstract

Discontinuous permafrost is present in interior Alaska to depths as great as 45 m. The surface active layer seasonally thaws to depths > 2m and in many locations groundwater can exist above, within, and below permafrost. The study area soils consist of organic material and loess draped over well sorted alluvial gravels. Vegetation is predominately black spruce forest with sedges while grasses predominate in inactive stream channels. In this setting we conducted a geophysical site investigation using ground penetrating radar (GPR) and two-dimensional DC resistivity. Measurements were made in late winter and late summer to delineate permafrost distribution beneath four parallel 500 m transects. 400-MHz GPR detected active layer depth and thaw zones while 100-MHz GPR detected thaw zones and the permafrost bottom. We compared results of Wenner and dipole-dipole resistivity with GPR and found that, in most cases, the permafrost interpretations from the dipole-dipole data closely match the GPR data permafrost interpretations; however, no boreholes are yet available for ground truth. Permafrost was detected on average between 0.7 and 20 m below the surface except along a north-south trending power line clearing where permafrost was mostly absent. Within some inactive fluvial channels permafrost was sporadic or absent. The GPR and resistivity methods worked well in concert to delineate permafrost and stratigraphy, providing high quality data to depths of 30 m.