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Monitoring of tidal variations of apparent resistivity

Universitat de Barcelona
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Possibilities of using tidal phenomena for the development of a monitoring technique of apparent resistivity Rho(a) variation registration, and the analysis of previous studies, are considered in this paper. The properties of tidal deformation of the Earth’s crust are analyzed. The high accuracy of Rho(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 Rho(a) variations with amplitudes of 3-8 % and variations of the vertical tidal deformations, dH. Tidal Rho(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 Rho(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 Rho(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 Epolarization 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.

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