P

Ap per day 06.07.

Ap per day 28.08.

20 40 Distance (m)

20 40 Distance (m)

Figure 6.7 (Plate 8) (a) Resistivity model for the measurement on 15.9.1999 as determined by the inversion. (b)-(k) Resistivity difference per day based on the September measurement (a). White and dark shading denote resistivity increase and decrease, respectively. From Hauck, C. & Vonder Miihll, D. 2003b. Permafrost monitoring using time-lapse resistivity tomography. In: Phillips, M., Springman, S.M. & Arenson, L.U. (eds): Permafrost. Proc. 8th International Conference on Permafrost, 21-25 July, Zurich, Switzerland, Vol. 1, 361-366, published by Balkema, Lisse. Reproduced by permission of Taylor and Francis

be visualised by plotting ratios of successive resistivity measurements instead of cumulative differences (Hauck 2002, not shown here). During the phase transition, the temperatures remain close to 0°C (the so-called zero-curtain effect), while the resistivities increase, as the unfrozen water content is diminished. From the borehole temperature data shown in Figure 6.3b, it is seen that the zero-curtain effect started at the end of October and lasted until end of December at 1 m depth and until beginning of February at 4 m depth (right hand panel).

In the beginning of May, the temperatures near the surface approached 0° C, and melting of the uppermost layer started. Again, temperatures remained almost constant at 0°C during the phase transition. At the time of the first ''summer'' resistivity measurement (June 2000), most of the frozen water in the uppermost 2-3 m had already melted, which led, together with additional water input by rain, to a wet soaked surface layer, decreasing the resistivity strongly near the surface (grey shading in Figure 6.7g). Between June and July 2000, temperatures increased at all depths down to 10 m with a corresponding resistivity decrease throughout the major part of the survey area. This decrease continued until end of August 2000, thereby almost totally equalising the resistivity increase of the winter months. This can be seen by the predominantly grey shading in Figure 6.7(k) indicating only small resistivity differences between the measurement at the end of August 2000 and the reference measurement of September 15th 1999.

6.5 DISCUSSION

The results shown in the previous section indicate that geophysical methods are not only applicable for the detection and mapping of permafrost occurrences in mountainous areas but can also be used for monitoring purposes of freeze and thaw processes in the shallow subsurface. By conducting 2-dimensional tomographic surveys, the spatial variability of time-dependent processes can be detected. This is in contrast to the commonly used (and much more expensive!) point measurements in boreholes, which may not be representative for the whole study area. The horizontal variability of the temporal resistivity changes shown in Figure 6.7 indicate that freezing and thawing do not occur homogeneously along the 60 m survey length in the presented field case.

Comparing the obtained resistivity data with the temperature data from the borehole and incorporating the theoretical considerations and laboratory results, the vertical and temporal variability of the unfrozen water content (and therefore the relative ice content) can be assessed. In addition, monitoring results from the energy balance station for the same time span are used to relate the resistivity and temperature evolution to the dominant forcing variables, for example, the radiation and the snow cover duration.

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