Permafrost sounding (2003-2005) in the source area of the Yellow River, northeastern Tibet

Abstract

Present-day distribution and ongoing degradation of permafrost were evaluated by geophysical means in the source area of the Yellow River, located at the northeastern margin of the Tibetan Plateau. Seismic, electrical and/or thermal soundings were undertaken at 15 sites between 3260 m and 4790 m ASL in 2003-2005. High P-wave velocities (>2 km s-1) and relatively high DC resistivities (650-1100 Ωm) below a thin uppermost layer show that permafrost 10-30 m in thickness occurs above 4300 m ASL. In contrast, low P-wave velocities (<1 km s-1) throughout the uppermost ten to fifteen meters of sediments indicate that permafrost is absent below 4000 m ASL. On widespread alluvial plains between 4200 m and 4300 m ASL, some sites show subsurface intermediate P-wave velocities (1.5-1.7 km s-1) and low resistivities (30-140 Ωm) indicating the presence of unfrozen-saturated sediments, while others show high DC resistivities possibly indicating the presence of permafrost. Negative values of the mean annual ground surface temperature (MAST) also indicate widespread permafrost only above 4300 m ASL under the present climatic condition. Assuming that the inter-annual variation in MAST follows that in the mean annual air temperature, permafrost is estimated to have significantly thawed on the alluvial plains at 4200-4300 m ASL during the last half-century.