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On the suitability of capacitive resistivity imaging (CRI) for permafrost monitoring

Uhlemann, Sebastian. 2012 On the suitability of capacitive resistivity imaging (CRI) for permafrost monitoring. ETH Zürich, Department of Earth Sciences, Institute of Geophysics, Masters Thesis, 86pp.

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Abstract/Summary

Permafrost covers about 24% of the exposed land in the northern hemisphere and is abundant in various regions in Europe, mainly as mountain permafrost. This type is very sensitive to climate warming, because its temperature is only marginally below the freezing point. In recent years permafrost degradation frequently caused large rockfall events, which represent a fatal danger for the population in the alpine regions. Therefore, modelling and monitoring of permafrost is becoming a focus of several research groups. This thesis investigates the suitability of capacitive resistivity imaging (CRI) for obtaining supporting information for the different areas of permafrost research, in terms of subsurface characterisation and estimation of temperatures; there is a strong relationship between resistivity and temperature, especially for values below the freezing point. For this purpose numerical simulations of a quasi-static quadrupole over a homogeneous halfspace and a finite rock sample are carried out and intensive laboratory studies on a Tuffeau chalk sample are made. The numerical simulations indicate that dipole combinations employing small dipole separations are prone to error, because the electrodes cannot be considered as point poles and geometric effects become important for small separations. Moreover, the results indicate a higher sensitivity of the imaginary part of the complex transfer impedance to the geometric configuration of the quadrupole than the real part shows. Therefore, the experimental use of the imaginary part is limited to be only qualitative. It is shown, that these results are in accord with previous theoretical estimations. By comparison, the real parts of the complex transfer impedance obtained from numerical simulations and experimental work are found to be consistent, supporting the validity of both estimates. This highlights the benefits of numerical simulations in the evaluation of the properties of the experimental setup. To study the effects of changing sample conditions on the performance of the CRI system, the laboratory sample was exposed to two freezing cycles. The results indicate the suitability of CRI for laboratory studies on permafrost samples, but also reveal problems in the experimental setup. A comparison of apparent resistivities derived from CRI and electrical resistivity tomography (ERT) showed a good accordance, but a resistivity-dependent offset was observed. Nevertheless, large reciprocal errors of CRI and ERT data sets throughout the experiments were found. Possible explanations were given for these errors. The obtained resistivity-temperature relationship showed a common observation: at positive temperatures, resistivity changed linearly with changing temperatures, whereas below the freezing point, strongly increasing resistivities were found. The reciprocal datasets were inverted and resistivity models of the sample obtained. These models showed a high resolution of the resistivity structure of the Tuffeau chalk. By using a derived calibration curve, the resistivity models were transformed into temperature models, which showed a high accuracy below the freezing point; other factors dominate the resistivity above the freezing point. The presented results proved the suitability of CRI for permafrost monitoring, since they showed the possibility to obtain high resolution images of the resistivity distribution of the chalk sample in a steady and transient manner.

Item Type: Publication - Thesis (Masters)
NORA Subject Terms: Earth Sciences
Glaciology
Physics
Date made live: 19 May 2014 15:21 +0 (UTC)
URI: http://nora.nerc.ac.uk/id/eprint/500982

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