Semi-quantitative geophysical investigation of permafrost in northern Québec

Abstract Both continuous and discontinuous permafrost occur in northern Quebec. In contrast with permafrost in western Canada and Alaska, permafrost in eastern Canada contains no massive ice because of the regionally different climatic conditions and soil compositions. Site investigations related to infrastructures such as building foundations, airports, and roads frequently require a knowledge of the spatial distribution of ice content in permafrost since differential thawing can significantly affect the mechanical behavior of the soil. Delineation of permafrost ice content in northern Quebec is difficult because of its diffusion into the various unconsolidated deposits. Electrical and gravity surface surveys are appropriate methods for determining the horizontal and vertical extent of such permafrost conditions because the resistivity, IP chargeability, and density parameters are directly affected by the volume ice content and the amount of unfrozen water in the permafrost. This integrated surface geophysical approach was applied at three test sites in the Kangiqsualujjuaq (George River) area to yield a semi-quantitative volumetric estimate rather than a discrete spatial distribution of the excess ice content. The selected test sites at which this integrated method was demonstrated are representative of continental and supratidal cryogenic mounds of different dimensions in which negative residual gravity anomalies, large electrical resistivity contrasts, and near-zero IP chargeability were combined to delineate and characterize the excess ice zones. The relative magnitudes of these parameters are interpreted to yield a semi-quantitative estimate of the ice content in the anomalous regions. Test drill holes at the surveyed sites indicate that the integrated approach can provide useful geotechnical assessments of the integrity of foundations and other structures subject to permafrost environments.