Topographical effect of high embankments on resistivity investigation of the underlying permafrost table

Investigation of resistivity has been effectively used in assessing the risks of embankmentation and failure. A two-dimensional (2D) approximation of the surveyed object is commonly assumed for a survey line on the road surface. However, this approximation may not be met when resistivity investigations are conducted over a raised high embankment; under these conditions, regular inversions might yield erroneous results. This study explored the topographical effect of a high embankment on resistivity measurements by forward and inverse modeling of a 3D high embankment model. The results show that a 2D approximation of the survey lines on the road surface significantly increases the apparent resistivity within the depth of the raised embankment. Maximum relative errors reached 21% and 11% for the road shoulder and midline survey lines, respectively. The biased apparent resistivity resulted in an inverted resistivity that was higher than the true values, although resistivity contrasts can still identify interfaces between layers. A geometric factor was used to correct the biased apparent resistivity to eliminate the high embankment topographical effect. Inversion results of the corrected apparent resistivity agreed well with the forward model. The method was then verified by field application. The apparent resistivity of the field data collected on a high embankment in permafrost regions on the Qinghai-Tibet Plateau was corrected before inversion. The permafrost table derived from the inverted resistivity was verified based on borehole temperatures. These findings indicate that the topographical influence of high embankments on resistivity measurements needs to be considered. Correction of the apparent resistivity is indispensable for quantitative interpretation of the inverted resistivity.