Geophysical measurements DC resistivity tomography

The direct current (DC) resistivity technique is based on electrical resistivity differences between different subsurface materials. For typical permafrost material, a marked increase in resistivity at the freezing point was shown in several field and laboratory studies (Hoekstra et al. 1975, King et al. 1988). Consequently, the application of electric and electromagnetic techniques has a long tradition in the study of permafrost (for a review, see Scott et al. 1990, Vonder Muhll et al. 2001). With the development of fast, commercially available two-dimensional tomographic inversion schemes, the DC resistivity method has been increasingly applied, especially in mountainous terrain (Hauck & Vonder Muhll 1999, 2003a, Vonder Muhll etal. 2000, Kneisel et al. 2000, Ishikawa et al. 2001, Isaksen et al. 2002, Marescot et al. 2003, Hauck et al. 2003, Ishikawa 2003, Delaloye et al. 2003, Reynard et al. 2003). As the heterogeneous surface and subsurface characteristics of mountain permafrost terrain often prohibit the application of plane-layer approximations used in standard data processing for 1-dimensional soundings, the 2-dimensional tomographic method greatly improves the quality of data interpretation in resistivity studies on permafrost.

In DC resistivity surveys, electrical current is injected into the ground via two current electrodes. The resistance of the ground is then determined by measuring the electric potential between two other electrodes and dividing by the current. By multiplying the resistance with a geometrical factor depending on the distance between the electrodes and choosing different electrode spacings and locations, the so-called apparent electrical resistivity is determined on a 2-dimensional grid. By using a tomographic inversion scheme (RES2DINV, Loke & Barker 1996), these apparent resistivities can be inverted to yield a 2-dimensional specific resistivity model of the ground. For monitoring purposes, these measurements are repeated at certain time intervals using a permanently installed electrode array, which allows for measurements independent of the snow cover thickness (Figure 6.1). Furthermore, the fixed-electrode array effectively filters resistivity variations because of variable electrode contacts or geological background variations, as mainly temporal resistivity changes are determined (Hauck 2001, 2002).

Resistivity meter

Figure 6.1 Installation setup of the fixed-electrode array at Schilthorn, Switzerland ooooooo oooooo ooo

Resistivity meter

Figure 6.1 Installation setup of the fixed-electrode array at Schilthorn, Switzerland

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