GEOSCAN, résultats de la recherche


TitreDelineation of permafrost and ice-rich terrain within fine-grained sediments using electrical resistivity, seismic and borehole data, Yellowknife, Northwest Territories
AuteurOldenborger, O A; Wolfe, S A; Morse, P D; Pugin, A
SourceXI. International Conference on Permafrost: Exploring Permafrost in a Future Earth, book of abstracts; par Günther, F (éd.); Morgenstern, A (éd.); 2016 p. 955-956, (Accès ouvert)
Séries alt.Secteur des sciences de la Terre, Contribution externe 20150457
ÉditeurInternational Permafrost Association
RéunionXIth International Conference on Permafrost: Exploring Permafrost in a Future Earth; Potsdam; DE; juin 20-24, 2016
Mediaen ligne; numérique
SujetsNature et environnement
ProgrammeGéosciences de changements climatiques, Infrastructures terrestres
Diffusé2016 06 01
Résumé(disponible en anglais seulement)
The properties of frozen ground make electrical geophysics a potential tool for permafrost characterization. Permafrost can have a strong electrical signature where low temperatures reduce the mobility of charge-carrying ions and the freezing of water greatly reduces the availability and connectivity of pore fluid for electrolytic conduction. Electrical resistivity is typically observed to increase significantly for cryotic ground, and even more so for ice-rich permafrost. Consequently, in permafrost terrain, measurements of electrical resistivity provide valuable information for interpretation of some combination of the material type and the amount of ground ice (e.g., Oldenborger and LeBlanc, 2015). However, the relations between material type, thermo-physical state and electrical resistivity are non-unique and the imaging of the ground as typically achieved through inversion of geophysical data has resolution-dependant limitations. Qualitative interpretation or quantitative utilization of electrical resistivity images (ERI) should be done in the context of ERI uncertainty. We utilize ERI results over a lithalsa (ice-rich permafrost mound) associated with a retrogressive thaw slump on an island in the Yellowknife River, Northwest Territories (Figure 1A). The lithalsa has formed by permafrost aggradation within silty-clay of alluvial and lacustrine origins (Wolfe and Morse, 2015). Observations along the headwall of the scarp reveal up to 70% visible ice content in the near surface (Figure 1A inset). ERI surveys were conducted to confirm the presence of ground ice at depth and to map the distribution of permafrost and ice-rich ground in the area. The geophysical results show distinct high resistivity anomalies consistent with occurrence of ice-rich ground (Figure 1B). These anomalies occur in the context of moderately resistive ground interpreted to represent ice-bonded permafrost, and also conductive ground interpreted to represent unfrozen silt and clay. The ERI results support the hypothesis of ice-cored terrain (lithalsa) and provide further insight into permafrost conditions such as the near-shore extent of permafrost, potential locations of thin permafrost aggradation, and the presence of pond-related thaw bulbs and through-taliks. Additional data are used to further inform the interpretation of the ERI results. Several water jet-drilled boreholes provide observations on thaw depth, base of permafrost, and bedrock depth. Continuous water-borne single-channel seismic reflection data were acquired on the Yellowknife River adjacent to the island to provide additional estimates of bedrock topography and also information on underlying sediments. These data corroborate the ERI results in places, but also serve to identify limitations and regions of uncertainty or inconsistent data. In particular, the highly resistive ground associated with massive ice, combined with the highly conductive silty active layer result in limited depth of investigation despite very high quality data. Practically speaking, for ice-rich regions ERI may be limited to detection of resistive bodies as opposed to estimation of their actual resistivity. Furthermore, ERI may be unable to discriminate bedrock from massive ice, making estimates of potential heave or subsidence difficult based on these results alone. The base of permafrost is observed to be multi-valued in terms of electrical resistivity, demonstrating that the approach of using iso-resistivity surface for mapping thermos-physical transitions is flawed. Oldenborger, G.A., LeBlanc, A.-M., 2015. Geophysical characterization of permafrost terrain at Iqaluit International Airport, Nunavut. Journal of Applied Geophysics 123, 36¿49. Wolfe, S.A., Morse, P.D., 2015. Holocene lake-level recession, permafrost aggradation and lithalsa formation in the Yellowknife area, Great Slave Lowland, Proceedings of the Canadian Permafrost Conference, 236.
Sommaire(Résumé en langage clair et simple, non publié)
Les propriétés du gélisol font de la géophysique un outil qui peut potentiellement nous permetre de caractériser le pergélisol. Nous utilisons les résultats d'imagerie de résistivité électrique (IRE) sur une lithalse (une butte cryogène riche en glace) associée à un glissement régressif dû au dégel, situé sur une île de la rivière Yellowknife, Territoires du Nord-Ouest. À des fins pratiques, dans les régions riches en glace l'IRE peut se limiter à la détection de corps résistifs plutôt que d'estimer leur résistivité réelle. En outre, il peut être difficile pour l'IRE de différencier entre le substratum rocheux et la glace massive, rendant ainsi difficile d'estimer des soulèvements ou affaissements potentiels en se basant seulement sur ces résultats.