Three-dimensional (3-D) finite-difference modeling shows the sensitivity of the MT method to the electrical structure of volcanic zones. Modeling is done with the program FDM3D-MT, which allows relief topography, mixed conductivity structures (1-D, 2-D, and 3-D), different layered structures at infinity, and calculation of MT fields at arbitrary levels in the earth and atmosphere.
Numerical experiments with different field components and their transforms—including impedances, apparent resistivities, and magnetic transfer functions—indicate that conductivity gradients in volcanic zones can be identified on plots of isosurfaces of the complex electric field and on isosurfaces of the amplitudes and phases of tensor impedances. The actual conductivity values, however, are constrained poorly.
Interpretation is aided by reduction of the data to an artificial reference plane, positioned in the atmosphere above the top level of the relief surface, followed by the construction of 3-D pseudogeoelectrical structure. Examples show the possibility of 3-D imaging of volcanic environments with MT data measured not only at the Earth’s surface, but also at different levels in the atmosphere.