Applicability of frequency-domain and time-domain electromagnetic methods for mountain permafrost studies

The electromagnetic instruments EM-31, GEM-300 and PROTEM have been tested at various field sites during the PACE (Permafrost and Climate in Europe) project. In this paper the applicability of the different methods for permafrost studies in mountainous regions is evaluated and the results are compared with borehole data. The EM-31 proved highly effective for mapping shallow permafrost and for active layer studies in various permafrost environments. The localities around each of the PACE boreholes were investigated for near-surface heterogeneities. Results are presented as relative conductivity variations along the survey lines. Sensitivity studies concerning snow cover and instrument drift were also performed. The multi-frequency instrument GEM-300 was compared with the EM-31 and showed good agreement for similar induction numbers (the relation between transmitter-receiver spacing, frequency and conductivity). However, care must be taken in choosing the appropriate frequency in GEM-300 surveys, as the instrument response depends on the frequency as well as the ground resistivity. For high-resistivity materials (e.g. rock glaciers with a high ice content), high frequencies (19,975 Hz with the GEM-300 or the single-frequency 9800 Hz for the EM-31) are appropriate. In relatively low-resistive material, resistivity variations with depth may lead to different responses for different frequencies. The time-domain system PROTEM was used to determine permafrost depths. The exploration depth ranged between 100 and 300 m, depending on the upper layer resistivity. The results were especially encouraging for sites with a large contrast between the resistivity of the frozen and unfrozen layers (e.g. rock glaciers). Estimated permafrost thicknesses are in good agreement with borehole data. Copyright (C) 2001 John Wiley & Sons, Ltd.