Advancement of the capacitive resistivity method to estimate ice content in permafrost.

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Permafrost ist permanently frozen ground that exists predominantly in polar regions and high-altitude mountains. The global abundance of permafrost is essential for ecology, infrastructure and the carbon inventory of the earth. In the context of global warming, the mapping and observation of permafrost occurence becomes increasingly important; a task where geophysics can make substantial contributions. Refraction seismics and DC resistivity methods are most frequently used, however, they can be logistically difficult to apply in rough terrain. In this project, we investigate the feasibility of capacitive resistivity for permafrost research. Instead of metal sticks, electrode sheets are being used, which may inject a high-frequency alternating current without direct contact to the ground. The high-frequency alternating current offers the opportunity to determine electrical permittivity, which constitutes important complementary information in addition to the electrical conductivity. In the frequency range between 10 Hz and 100 kHz, the permittivity of water ice displays a characteristic frequency dependence, which shall be exploited to estimate the ice content in frozen rocks. So far, we developed and tested suitable equipment in cooperation with the manufacturer. With an appropriate theoretical description of the measured impedance, we demonstrated at several locations that the typical frequency spectrum of permittivity may be measured under field conditions. Here, we propose to continue the project by applying the method to several specific problems. At three locations in Switzerland, which are frequently investigated by other working groups and where extensive a priori information is available, we will characterize the inner structure of the permafrost bodies and estimate the ice content. In another region located in Siberia, we will, in addition to the estimation of the ice content, delineate the boundary between the frozen and unfrozen ground and map its lateral variation. The application requires further methodological requirements. In our previous experiments, the penetration depth was limited to a few meters; we intend to push the limits as far as possible. In particular, we will investigatie and avoid distortion by electromagnetic coupling effects, which become increasingly important for larger electrode spacing. We also will adapt inversion and interpretation methods to the large bandwidth and the large phase phase shift of the impedance compared to more conventional techniques.