Seasonal Variations in Specific Resistivity in the Upper Layers of the Earth Crust

Abstract

To predict the effects of strong earthquakes, it is important to know the soil characteristics. The accepted methods for such assessment ignore the fact that soils properties can vary greatly throughout the year. This is apparently due to the lack of data required for such consideration. In this paper, based on an analysis of long-term monitoring data by vertical electric sounding (VES) in a stationary array with a large number of spacing, we analyze seasonal variations in specific resistivity at different near-surface depths of a section in the area of the Peter the Great Range in Tajikistan. Based on long-term precision daily measurement data, a horizontally four-layered model of the geoelectric section was constructed. The amplitude of seasonal variations in the specific resistivity in each layer was estimated as the ratio of the standard deviation of the seasonal variation to the mean interannual specific resistivity. In the upper part of the section (depth 0–1.5 m), the amplitude of the seasonal variation reaches 20%, and its shape agrees well with the seasonal variation of the apparent resistivity at small spacings. In the second (depth 1.5–10 m) and third (depth 10–66 m) layers, the amplitude of the seasonal wave decreases rapidly, being less than 1% in the third layer. In the fourth layer (depth from 66 m or more), this amplitude again increases, reaching 2%. The difference between the maximum and minimum values of the seasonal wave (i.e., its range) reaches 7%. One possible explanation for such a high amplitude of seasonal variations in specific resistivity at depths of hundreds of meters is the presence of a deep aeration zone with annual regulation of the level and salinity of groundwater. The results should be taken into account in exploration geophysics, in engineering surveys, and in accounting for soil properties when predicting the possible consequences of strong seismic impacts.