Mapping discontinuous permafrost in the Canadian Sub‐Arctic using a combination of airborne and surface geophysical surveys

Abstract

Shallow geological, geotechnical, and environmental investigations in sub-arctic regions require a unique approach because of complex near-surface conditions. The surficial geology in these regions includes a thin layer of muskeg, silt, or sand. Bedrock can be sedimentary, metamorphic, or igneous, and is usually shallow. Permafrost is present as either a continuous or discontinuous layer with a thin, seasonally active layer. A combination of surface and airborne geophysical surveys, supported by drilling, sampling, and borehole geophysical logging, has proven to be an effective way of characterizing the subsurface extent of the permafrost and cover. Terrain conductivity surveys showed the seasonal development of an active (unfrozen) zone of highly variable depth. Electrical resistivity surveys produced cross-sections of the subsurface down to depths of 30 m, that showed the presence of permafrost lenses and taliks (wedges of thawed overburden). Borehole geophysical logs in shallow drill holes (<10 m) were used to confirm the depth extent of a conductive active layer, while logs from deeper holes mapped the top of bedrock and the base of the permafrost. Airborne magnetic and electromagnetic (EM) surveys were used to provide a complete coverage of the region at a reconnaissance level. Inversion of the airborne EM data produced a regional model of a resistive permafrost layer (0 – 60 m thick) sandwiched between a conductive surface layer and a conductive bedrock. On a regional scale the permafrost thickness map, derived from the airborne and ground investigations, is useful in planning linear infrastructure routes (e.g. pipelines and roadways) that will have engineering requirements dependant on permafrost conditions.