Geophysical Methods for Detecting Permafrost in High Mountains

Five different geophysical techniques were tested for application for permafrost studies in high mountain regions within the PACE project (Permafrost and Climate in Europe). The aim was to determine suitable methods to detect, characterise, map and monitor permafrost in the context of possible slope instabilities resulting from climate induced thawing of permafrost bodies. Measurements were conducted on various field sites including pure bedrock and unconsolidated permafrost sites, such as rock glaciers and moraines. The applied geophysical techniques include DC resistivity tomography, refraction seismic tomography, frequency-domain and time-domain electromagnetic induction methods and passive microwave radiometry.

2-dimensional DC resistivity tomography turned out to be a suitable method for a number of permafrost related questions, such as detecting permafrost, mapping its horizontal extent, estimating the ice/unfrozen water content, determining the permafrost base for shallow permafrost occurrences and monitoring seasonal variations in the active layer. 2-dimensional refraction seismic tomography is equally well suited for these targets, except that interpretation of the results is more difficult, since the velocity contrasts of frozen and unfrozen ground is smaller than those of electrical resistivity. To remove interpretational ambiguities both methods should be combined.

Laboratory studies using a miniature DC resistivity tomography system were used to visualise freezing and thawing processes and to estimate the total water content at the field site. Permafrost monitoring on a monthly basis was conducted using DC resistivity tomography and a fixed electrode array. Resistivity changes were related to borehole temperature changes and the evolution of the unfrozen water content could be determined at different depths. As the unfrozen water content is a key parameter concerning slope instabilities induced by thawing permafrost, this approach is considered an important contribution to future monitoring studies.

Measurements with electromagnetic induction methods include conductivity mapping with frequency-domain electromagnetic instruments (EM31, GEM300) and transient electromagnetic soundings (PROTEM). Due to the small conductivity values encountered and the heterogeneity of most permafrost field sites in high mountain environments, conductivity mapping using the EM-31 and GEM-300 has to be combined with other geophysical techniques, for example DC resistivity. Once a permafrost occurrence is detected by a DC resistivity survey, its extent can easily be mapped with a conductivity meter, which is light-weight and therefore suitable to map large areas in short time by a single person. Electromagnetic soundings using the time-domain instrument PROTEM can be used to determine the depth of the permafrost base. The maximal investigation depth depends on the transmitter coil dimensions and the upper layer resistivity.

Passive microwave radiometry is used in winter to determine the bottom temperature of the snow cover (BTS) on a larger "footprint" than with the commonly used BTS-probes. As for conventional BTS surveys, an at least 0.8-1.0 m thick snow cover has to be present for some time in order to relate the temperature signal to permafrost occurrences. The microwave radiometer (11 GHz) can be mounted on a sledge or may be used from aboard a helicopter.

Even though the applicability of each method was tested separately in this study, it is recognised that in most cases an additional method is needed to unambiguously interpret the results in terms of permafrost prospection. A typical example in DC resistivity surveys is the differentiation between isolated rock, ice and air occurrences, each resulting in anomalously high resistivity values. In combining DC resistivity with refraction seismic surveys this ambiguity can be resolved as the seismic velocities of the three materials are markedly different.