Dependence on temperature

The dependence of resistivity on temperature differs for temperatures above and below the freezing point. At temperatures above the freezing point, a decrease in temperature changes the resistivity of the material only in so far as the resistivity of the pore water is changed. A decrease in temperature increases the viscosity of water, in turn decreasing the mobility of the ions in the water, which increases the resistivity. A relationship between p and temperatures T above the freezing point is given by where TfcC is the temperature of the freezing point. For a saturation exponent of n = 2 (commonly used for rock, Telford etal. 1990) and Tf = 0°C, Equation (6.5) describes simply an exponential decrease of the unfrozen water content with decreasing temperature (Riseborough 2002). The factor b controls the rate of decrease and can easily be determined from Equation (6.4) if resistivity data for different subzero temperatures are available. By choosing appropriate values for n and b, the temporal evolution of the unfrozen water content can be determined in a qualitative way.

6.3 FIELD SITE AND DATA ACQUISITION

The Schilthorn (2970m a.s.l., for details see Table 6.1) is located in the Bernese Oberland at the transition between the Prealps in the north and the principle chain of the Bernese Alps in the south (Figure 6.2). It is an east-west

Table 6.1 Characteristics of the Schilthorn field site

Catchment/study area

where p0 is the resistivity measured at a reference temperature T0 and aT is the temperature coefficient

Name of the basin/area Mountain range Elevation range of entire catchment Elevation range of individual sites Latitude and longitude Area in km2 Geology

% glacierized Vegetation type

(dominant) % forested

Mean Q at catchment outlet (mm)

Schilthorn

Bernese Alps

790-2970m

2900 m

ca40

Metamorphic sedimentary rocks, micaceous shales (Glockhaus formation)

Small to medium size debris, no vegetation

Not determined n

Figure 6.2 Location of the field site Schilthorn in the Bernese Alps, Switzerland

Figure 6.2 Location of the field site Schilthorn in the Bernese Alps, Switzerland striking crest with north and south facing slopes. The investigation site is located on a small plateau on the north facing slope at 2900 m a.s.l. The mean annual air temperature (MAAT) at the top of Schilthorn is —4°C (estimated from long-term data sets from neighbouring MeteoSwiss stations) and annual precipitation varies from about 1200 mm in the valley bottom (796 m a.s.l.) up to about 2700 mm on the top (Imhof et al. 2000). Because of the high amount of precipitation and additional snow input through wind transport, the snow cover persists usually from October to June (Imhof et al. 2000, Mittaz et al. 2002). Presence of permafrost has been found at the summit when the facilities for the cable car were built. During the construction of the buildings in 1965-67, several ice lenses with a thickness up to 0.5 m were encountered and special construction methods had to be used (Gurtner 1991). However, subsequent investigations showed highly heterogeneous subsurface conditions, both concerning the presumed permafrost distribution (Imhof etal. 2000) and geology (inferred from geophysical surveys, Hauck (2001)).

Starting in 1998, three boreholes (14m in 1998, 100 m and 100 m at an angle of 30° in 2000) were drilled within the PACE project (Vonder Muhll etal. 2000, Harris et al. 2003). The observed permafrost temperatures are comparatively warm, reaching —0.7°C at 14 m depth (Figure 6.3). Consequently, the ice content is low, and the unfrozen water content is high, leading to low resistivity values compared to typical mountain permafrost occurrences (Hauck & Vonder Muhll 1999, Vonder Muhll et al. 2000).

Repeated measurements throughout the year are required in order to correctly relate resistivity variations to changes in temperature and ice content. Monitoring of time-dependent processes (time-lapse) through repeated resistivity measurements has been used in hydrogeological tracer experiments in groundwater studies (e.g. Barker & Moore 1998). These repeated measurements are usually conducted on a timescale of hours or a few days to monitor the propagation of artificially induced tracers or natural rain and/or ground water. For permafrost, timescales of interest are of the order of weeks to months, and the focus is on monitoring the freezing and thawing processes.

A fixed-electrode array allowing repeatable resistivity tomography measurements along a 58m survey line throughout the year was permanently installed at Schilthorn in September 1999 (cf. Figure 6.1). The 30 stainless steel electrodes were buried 1 m into the ground. Each electrode was connected to a cable via shrinking

jp____©---

1. Oct 1999

1

—0— 1. Jan 2000

—a— 1. Apr 2000 -

—e— 1. Jul 2000

(a) .

Temperature (°C)

Temperature (°C)

at er 2

Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep 1999-2000

Figure 6.3 (a) Seasonal variation of the vertical temperature profile and (b) temporal evolution of temperature at 4 different depths in the 14 m deep borehole at Schilthorn (Bernese Alps, Switzerland). From Hauck, C. & Vonder MUhll, 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

tubes, which prohibit oxidation of the cable connectors. The cables were connected to a manual switchbox, which is accessible throughout the winter, and were buried for safety reasons in case of avalanches. Resistivity surveys were made by connecting the resistivity meter to the switchbox for each of the selected electrode configurations. This setup allows measurements to be taken throughout the year regardless of the snow cover thickness. An OYO McOhm resistivity meter was used for data acquisition. A whole survey takes approximately 90 minutes.

The measured apparent resistivities were inverted using RES2DINV. Between September 15, 1999, and August 28, 2000, eleven sets of DC resistivity tomography measurements were conducted with the fixed electrode array at Schilthorn. The time span between measurements was roughly one month except for the thawing season 2000, where measurements were conducted every two weeks (June/July). Additionally, an energy balance station including snow height, longwave and shortwave radiation balance, temperature, moisture and wind speed and direction measurements at 1.50 m above the surface was installed in 1999 (Mittaz 2002).

6.4 RESULTS

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