Geophysics

Kneisel et al. (2008), Maurer and Hauck (2007), Musil et al. (2002) and Hauck (2001) present thorough overviews on geophysical methods for detecting permafrost in high mountains. The most important methods used for 2D or even 3D determination of the depth and lateral variability of permafrost are:

o Electrical resistivity tomography (ERT) o Capacitively coupled ERT

o Frequency-domain electromagnetic induction (FEM) mapping o Time-domain electromagnetic induction (TEM) sounding o Seismic refraction tomography o Ground penetrating radar (GPR)

It is recognised that in most cases one method is not sufficient and at least one additional method is needed in order to interpret the results unambiguously in terms of the existence of permafrost or not. A typical example in DC resistivity surveys is the difficulty in differentiating 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.

In addition to surface geophysics, geophysical methods can also be used in boreholes for permafrost detection (Arenson, 2002; Vonder Muhll and Holub, 1992). By using cross-hole geophysical methods, 3D soil models can be created (Maurer and Hauck, 2007; Musil et al., 2003) that provide important information on the heterogeneity of the permafrost distribution. Borehole stability is often an issue in coarse, temperate frozen soils, and a casing, that influences the geophysical readings, may be required. Further methods that require the borehole to be filled with water might not be possible.

Note that geophysical investigations detect change in the physical properties of the ground, i.e. ice, water, mineralogy, or density. However, permafrost is thermally defined and if there is no change in the ground's physical properties, such as frozen pore water, no geophysical investigation method is capable of detecting permafrost. Temperature measurements with depth are therefore indispensable. Calibration of geophysically determined stratigraphies using borehole information is strongly suggested.

It is recommended to extend geophysical survey lines into non-permafrost if these zones are known, e.g. over the edge of a rock glacier. Such an extension can be used for reference when looking for permafrost features in the geophysical profiles.

Was this article helpful?

0 0

Post a comment