Monitoring of tidal variations of apparent resistivity

Abstract

Possibilities of using tidal phenomena for the development of a monitoring technique of apparent resistivity (ρ a ) variation registration, and the analysis of previous studies, are considered in this paper. The properties of tidal de- formation of the Earth's crust are analyzed. The high accuracy of ρ a measurements (not more than 1% error) using modern audiomagnetotelluric (AMT) instruments allows measurement of small changes in the apparent resistivity (2-3 %). The results of AMT monitoring in different geological settings show a good correlation between ρ a varia- tions with amplitudes of 3-8 % and variations of the vertical tidal deformations, dH. Tidal ρ a variations are easily observed in heterogeneous geological environments, but they are not observed at sites with a thick homogeneous sedimentary cover. Both direct and inverse correlations between ρ a and dH plots were observed, which can be explained by the dependence of the degree of water saturation in the rocks: direct correlations between ρ a and dH appear in dry rocks with high resistivity and inverse correlations are observed in water saturated rocks with low resistivity. As a result of the monitoring performed at a site with anisotropic rocks, an inverse correlation for E- polarization and a direct correlation for B-polarization were observed. These results show that the AMT sounding method can be applied to the monitoring of the stress-strain state of rocks prior to earthquakes. Audiomagnetotelluric monitoring. Apparent resistivity. Tidal variation.