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) p roject. In this paper the applicability of the different methods for permaf rost studies in mountainous regions is evaluated and the results are compar ed with borehole data. The EM-31 proved highly effective for mapping shallo w permafrost and for active layer studies in various permafrost environment s. The localities around each of the PACE boreholes were investigated for n ear-surface heterogeneities. Results are presented as relative conductivity variations along the survey lines. Sensitivity studies concerning snow cov er 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, frequ ency and conductivity). However, care must be taken in choosing the appropr iate frequency in GEM-300 surveys, as the instrument response depends on th e frequency as well as the ground resistivity. For high-resistivity materia ls (e.g. rock glaciers with a high ice content), high frequencies (19,975 H z with the GEM-300 or the single-frequency 9800 Hz for the EM-31) are appro priate. In relatively low-resistive material, resistivity variations with d epth may lead to different responses for different frequencies. The time-do main system PROTEM was used to determine permafrost depths. The exploration depth ranged between 100 and 300 m, depending on the upper layer resistivi ty. The results were especially encouraging for sites with a large contrast between the resistivity of the frozen and unfrozen layers (e.g. rock glaci ers). Estimated permafrost thicknesses are in good agreement with borehole data. Copyright (C) 2001 John Wiley & Sons, Ltd.