Three-dimensional modeling of magnetotelluric sounding of volcanoes

Recent methodological results obtained by the author in the field of magnetotelluric (MT) tomography and monitoring of volcanoes are reviewed. The MT field transforms considered in the paper allow one to obtain the most focused image of the inner structure of a volcano and to monitor the electric conductance in a magma chamber as a function of physical characteristics of magma. Model constraints from data for shield and central-type volcanoes showed that the best image of the geoelectric structure of volcanoes can be obtained from impedance phases. On the other hand, the apparent resistivity calculated from the impedance determinant was shown to be most suitable for reliable MT monitoring of the resistivity in a magma chamber, because variations in these two quantities have the same sign. The magma feeder effect on the magnetotelluric sounding resolution is estimated for various heights of a volcanic edifice. A method proposed for the interpretation of MT data measured at the Earth's surface eliminates the topography effect and weakens the influence of the geological noise produced by shallow inhomogeneities. Methods of 3-D tomography of volcanoes and monitoring of magma resistivity variations from MT data measured on the surface are proposed.