Geophysica (2009), 45(1–2), 77–92 Frequency Domain Electromagnetic Soundings of Canadian Deep Permafrost Kimmo Korhonen, Timo Ruskeeniemi, Markku Paananen and Jukka Lehtimäki Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland (Received: April 2009; Accepted: September 2009) Abstract Frequency domain electromagnetic soundings were carried out at the Lupin gold mine and Ulu gold prospect in Northern Canada by the Geological Survey of Finland to investigate the lateral and vertical conditions of deep permafrost. The results indicate a deep seated electrical conductor in approximately half of the soundings. The detected conductor is interpreted as unfrozen and porous rock underneath the permafrost base saturated with saline groundwater. The mean depth of the permafrost base is 589 m and 476 m at the Lupin mine and Ulu prospect, respectively. The permafrost base, as deep as few hundreds of meters, can be detected using frequency domain electromagnetic soundings. Key words: frequency domain electromagnetic soundings, permafrost, Canada 1. Introduction Perennially frozen ground or permafrost is widespread at high latitudes and altitudes (e.g., Yershov, 1991; Heginbottom et al., 1995; Brown et al., 1998). It has a strong impact on landscapes, ecosystems and man-made infrastructures. Thus, the climate warming in the permafrost areas has created a concern about the degradation of permafrost. The impacts of this degradation include the thickening of the active layer and the consequent instability of the land surface and in longer term the changes in flora and fauna (Harris, 2005). Failing ground under buildings and oil and gas pipelines are widely reported from Russia and North America. Even more concerning is the potential release of methane from the melting soil (Walter et al., 2006; Mastepanov et al., 2008). A large number of monitoring stations have been established all over the world to measure the changes in surface ground temperatures (CALM; http://www.udel.edu/ Geography/calm/). Temperature profiling has also been conducted in numerous deep boreholes to assess the rate and impacts of the warming. Furthermore, there is a strong demand to obtain information covering larger areas and deeper regimes. Thus, various geophysical methods have been tested for this purpose. A wide range of geophysical methods have been used in mapping the distribution of permafrost laterally and vertically (e.g., see the review by Scott et al., 1990). The Published by the Geophysical Society of Finland, Helsinki