Overcoming data scarcity of active layer thicknesses using geophysics
Résumé
The thermal imprint of a typical river in the continuous permafrost of Central Yakutia (Siberia, Russia) is studied by active layer thickness measurements along six cross sections, either directly or with geophysical methods. Ground-Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT) measurements provide permafrost depths comparable to direct measurements acquired in boreholes and pits when available. Geophysical data interpretations complement non-instrumented cross-sections and show that the unfrozen/frozen interface is deeper below the riverbed for all studied cross sections and depends on the local river width. GPR data provide depths of 1 to 2 m with a sampling step as small as 5 cm when the riverbed width is less than 8 m. Where riverbeds are wider, ERT data are more adapted than GPR in imaging the unfrozen layer. ERT data indicates a permafrost boundary as deep as 6 m for the wide river cases, compared to less than 2 m for the narrower ones. The geophysical data enabled us to reveal an exceptional spatial variability in active layer depths that could not be attributed with in situ measurements before.
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