Comparison between energy balance ground temperature and resistivity evolution

Figure 6.9 shows a comparison between the temperature change in the borehole, the radiation balance and the total resistivity variation at the borehole location. Thereby, the dominant role of the snow cover evolution becomes visible. A permanent snow cover was established at the end of October (Figure 6.9c) and persisted until mid-June. During that time, the temperature within the uppermost 10 m of the borehole remained almost constant (Figure 6.9a), as the ground temperature regime was effectively decoupled from the atmosphere and temperatures stayed at the freezing temperature of the ground. The

Figure 6.8 Resistivity-temperature relationship for the monitoring data from Schilthorn. The data are in good agreement with Equation (6.3) (dashed line) for temperatures above the freezing point, and split into three branches for temperatures below the freezing point. Each branch shows an exponential increase of resistivity with decreasing temperature, but at different rates (factor b in Equation (6.4), solid lines). The exponential increase rates of the laboratory measurements with the sample material from Schilthorn (saturated and dry state) are shown for comparison (dotted and dashed-dotted lines). Reproduced by permission of American Geophysical Union

Temperature (°C)

Figure 6.8 Resistivity-temperature relationship for the monitoring data from Schilthorn. The data are in good agreement with Equation (6.3) (dashed line) for temperatures above the freezing point, and split into three branches for temperatures below the freezing point. Each branch shows an exponential increase of resistivity with decreasing temperature, but at different rates (factor b in Equation (6.4), solid lines). The exponential increase rates of the laboratory measurements with the sample material from Schilthorn (saturated and dry state) are shown for comparison (dotted and dashed-dotted lines). Reproduced by permission of American Geophysical Union net radiation (being the dominant energy flux, see Mittaz et al. (2000)) during that time is negative, meaning that cooling takes place at the snow surface (Figure 6.9b). But as the energy flux through the snow cover is negligible during winter (less than 1 W/m2, Figure 6.9e), the freezing processes in the subsurface can only be induced by the cold October temperatures, which penetrated into the ground before the snow cover arrived and propagated to larger depths through heat conduction. After the melting of the snow cover in June, temperature variability in the borehole is high, coinciding well with the observed variability of the radiation balance (Figure 6.9b). This agreement confirms again the dominant role of the radiation balance for ground temperatures in mountain permafrost terrain.

Figure 6.9(d) shows the evolution of the unfrozen water content, which was calculated using Equation (6.5). The parameter b was chosen from the respective resistivity-temperature relation shown in Figure 6.8. The results for four different depths are shown. In the uppermost layer

(0.5 m), the unfrozen water content starts to decrease at the end of October, corresponding to the onset of the negative radiation balance seen in Figure 6.9(b). The minimum is reached in February and subsequently later at greater depth (beginning of June at 8.7 m depth). At larger depths, the evolution of S is nearly sinusoidal, corresponding to the seasonal variation of ground temperature. The minimal value of S is smallest at larger depths (0.2-0.3 below 6m for n = 2) and largest at intermediate depths (0.6-0.8 at 2-4m for n = 2), but depends on the choice of parameters b and n. The larger the n, the smaller the variations of S. King et al. (1988) examined a large number of permafrost samples from the North American Arctic. At -2°C, they found unfrozen water contents as high as 0.9 (clay) and as low as 0.2 (sands) depending on the material type.

Finally, Figure 6.9(f) shows the total resistivity variation at the borehole location, calculated as weighted vertical mean. Total resistivities increase steadily until a maximum is reached for the April measurement. From

Unfrozen water content, S

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