The transient electromagnetic method (TEM) is a geophysical method for detecting underground geological bodies by following the principle of electromagnetic induction, which has been widely used in permafrost exploration. In the practical applications of the TEM to investigate permafrost, it is found that in certain areas with shallow buried bedrock, the electrical resistivity near the surface cannot be obtained, and both frozen soil and underground bedrock exhibit a high electrical resistivity, so it is difficult to determine the distribution characteristics of the permafrost thickness. Based on this background, by analyzing measured data, it is considered that the reason for this situation is that the noise superposition effect generated by the receiving coil under the action of the primary field forms a shallow detection blind area. This study uses equivalent anti-flux opposing coils to eliminate the abovementioned blind area and realize measurement in the permafrost area of Mahan Mountain in Lanzhou. The results showed that the opposing coils transient electromagnetic method (OCTEM) can clearly detect low-resistivity anomalies near the boundary and permafrost base in the Mahan Mountain area, solve the problem of the shallow detection blind area of the conventional TEM, effectively eliminate the interference caused by the primary field, and greatly improve the horizontal and vertical resolutions.

Advective heat transported by water percolating into discontinuities in frozen ground can rapidly increase temperatures at depth because it provides a thermal shortcut between the atmosphere and the subsurface. Here, we develop a conceptual model that incorporates the main heat-exchange processes in a rock cleft. Laboratory experiments and numerical simulations based on the model indicate that latent heat release due to initial ice aggradation can rapidly warm cold bedrock and precondition it for later thermal erosion of cleft ice by advected sensible heat. The timing and duration of water percolation both affect the ice-level change if initial aggradation and subsequent erosion are of the same order of magnitude. The surplus advected heat is absorbed by cleft ice loss and runoff from the cleft so that this energy is not directly detectable in ground temperature records. Our findings suggest that thawing-related rockfall is possible even in cold permafrost if meltwater production and flow characteristics change significantly. Advective warming could rapidly affect failure planes beneath large rock masses and failure events could therefore differ greatly from common magnitude reaction-time relations. Copyright (C) 2011 John Wiley & Sons, Ltd.