Effect of Meteorological and Hydrological Factors on the Electrical Resistivity of the Upper Layers of the Earth’s Crust: Correlation Analysis of Seasonal and Residual Components of the Time Series

Abstract

This article continues a series of publications by the authors with the results of studying the variations in electrical resistivity in four layers of the geoelectric section. Precision soundings using the VES method have been carried out on a stationary multielectrode array with a maximum spacing of the supply electrodes of 3 km in the central part of the Garm geophysical test site in Tajikistan daily for 12 years. As a result of observations, a profile containing about 4500 pickets was obtained. Unlike conventional soundings, each picket of this profile corresponds not to a geographic point, but to a certain point in time. For the inversion of the time profile, specially developed precision algorithms with additional regularization of the inverse problem are used. The solution error is controlled by numerical simulation methods, during which the direct and then the inverse VES problem is solved first for profiles simulating the experimental profile, and the actual solution errors are estimated. Analysis has shown that the error in calculating the seasonal component of resistivity variations in layers 1–4 is 1–2%, and the error for flicker noise is from 1.3 to 3%. In this case, the total amplitude of seasonal variations in resistivity in the upper layer is more than 50%, and in the second layer it is 5.5%; the amplitude of variations in flicker-noise components for the same layers is estimated as 54 and 24%, respectively. In this paper we analyze the effect of external (exogenous) factors on resistivity variations at different depths. The fact of a significant effect of the groundwater level and soil temperature on the resistivity of the upper layer of the section with a thickness of 1.5 m has been established. For temperature, the coefficient of proportionality averaged over the entire layer is –0.58 ± 0.12%/deg, for the groundwater level, it is –0.8%/cm. For the second layer of the section (depth 1.5–10.2 m), an exact coincidence of the form of the seasonal variation of resistivity and atmospheric pressure is found. This coincidence is not accidental, since both atmospheric pressure and resistivity in layer 2 are characterized by an anomalous form of seasonal variation with two maxima and two minima during the year, which is completely atypical for seasonal changes in resistivity. At the same time, for relatively higher frequency (HF) variations (periods from several days to several weeks), there is no correlation effect. The authors attribute the possible reason for the observed effect to pressure regulation of competitive sources of groundwater inflow into the aquifer confined to the second layer of the section. It is assumed that the salinity and conductivity of water in competing sources differ sharply, which is quite plausible from geological considerations. Changes in atmospheric pressure change the inflow of water from these two sources, which leads to a change in the conductivity of the layer. The inertia of water exchange processes determines the absence of dependence of resistivity on atmospheric pressure at relatively high frequencies and the presence of such a relationship for variations with a characteristic duration of more than a month.