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| Titel |
Quantitative Pseudo-3D electrical resistivity tomography in steep instable permafrost rocks |
| VerfasserIn |
Michael Krautblatter, Adrián Flores-Orozco, Sarah Verleysdonk, Andreas Kemna |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250035079
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| Zusammenfassung |
| Temperature is a key control of rock- and ice mechanical properties of instable permafrost
rocks. As boreholes cannot be installed in instable rocks and thermal modelling is difficult to
perform in highly dissected bedrock, temperature-referenced geophysics could (in future)
become a key method for the assessment and monitoring of hazardous permafrost rocks. Here
we show the first approach to measure quantitative pseudo-3D electrical resistivity
tomography (ERT) in permafrost rocks.
In 2006, a 3D ERT array was installed across a NE-SW exposed crestline at 3150 m a.s.l.
in the Steintaelli, Valley of Zermatt, Switzerland. The 3D array consists of five parallel
41-electrode arrays with midpoints on the crestline. To cope with the heterogeneity of
dissected rock, an in-line electrode spacing of 2 m was applied and the offset between the five
arrays was 4 m. Electrode positions were aligned with a laser tachymeter and the x, y and z
information was used to create a decimetre-resolution digital elevation model. Topographic
information was accommodated in the finite-element grids underlying the ERT modelling
with adjusted boundary conditions. The 3D array was measured repeatedly in 2006, 2007 and
2008. Three temperature loggers recorded rock temperatures in 10 cm depth in the field.
The temperature-resistivity behaviour of two rock samples was measured in the
laboratory.
To obtain quantitatively reliable ERT values, we used an appropriate ERT data error
model, derived from the analysis of normal-reciprocal measurement discrepancies, in the
smoothness-constrained inversion code CRTomo. The error analysis yielded a relative
resistance error of 8-9 % for high resistances in all transects. Repeated laboratory
measurements of water-saturated paragneiss samples from the study site indicate an
equilibrium freezing point at –0.1 Ë C (spontaneous freezing point –1.1 Ë C) with resistivity
values ranging from 11-14 kΩm. Resistivity at –2 Ë C approaches values between 24-32
kΩm.
ERT images show consistent results for all transects. Decametre large frozen rock
bodies (>104.2 Ωm = 16 kΩm) dominate NE-exposed slopes. Permafrost at depth
ranges between 104.2 and 104.5Ωm (= 32 kΩm), which refers to laboratory values
between –0.1Â Ë C and –2Â Ë C, and is surrounded by a large zone in the range
of 104.0-104.2Ωm indicating freezing or melting around –0.1Â Ë C. The presence
of ice-filled crevices on the crestline and the NE face appears to have a crucial
influence on spatial and temporal permafrost development. Next to 3-4 m deep
melting from the surface, elongated recesses of unfrozen rock indicate melting by
cleft water up to 10 m depth. ERT images in August 2006, after the cool winter
2005/2006, indicate the presence of massive ice-intercalations in two transects,
while those of 2007 and 2008 indicate gradual widespread permafrost degradation. |
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