Experience in VES investigation of resistivity structure of the Tuymaada Valley, middle Lena River

Abstract

This paper discusses experience from the application of direct current vertical resistivity sounding in engineering surveys conducted in 1973–1995 in the Tuymaada Valley of the middle Lena River. Using 349 borehole measurements, the probabilistic variability of three main parameters of resistivity sections is examined for the northern and southern parts of the valley outside of Yakutsk. The first two parameters are electrical resistivities of the frozen Quaternary deposits and the underlying bedrock consisting of sedimentary rocks. The third parameter is bedrock depth. The resistivities of the Quaternary deposits and the bedrock depths show normal probability distributions. The sample estimate of the arithmetic mean resistivity values for the Quaternary deposits in the 95 % confidence interval is 3519 ohm-m, with individual values varying from 2052 to 5238 ohm-m. The probability distribution of the bedrock resistivities is not normal and consists of three groups with modes of 350, 780 and 2500 ohm-m. The multimodal distribution suggests that the bedrock is lithologically inhomogeneous containing a range of rock types with different strengths. The resistivities in each mode group are normally distributed. The overall sample estimate of the most probable mean modal resistivity of the rocks in the southern and northern parts of the valley is 779 ohm-m, while the measured values range from 30 to 3347 ohm-m. The higher mean resistivities compared to Yakutsk are due to the lower anthropogenic salinity of the Quaternary deposits and the higher density of the rocks. The bedrock depths determined by VES are generally consistent with drilling data, with errors mostly less than 20 %. In order of magnitude, the error of the VES method in the Tuymaada Valley is similar to that observed in other permafrost regions of northeastern Russia. The similar levels of error in the available local- and regional-scale VES measurements indicates that VES provides a reliable tool for accurately describing the resistivity structure of river valleys in permafrost regions of Yakutia and elsewhere in Eastern Siberia and the Far East.