Electromagnetic interrogation of the anisotropic Earth: Looking into the Earth with polarized spectacles

Electrical conductivity of common Earth materials ranges over many orders of magnitude, from >106 S/m (sulphides, graphite) to <10−6 S/m (competent shield rocks). Such a large range of the physical parameter being sensed facilitates high resolution of conducting structures, but an important consequence is that over a large region electromagnetic fields can have vastly different penetration depths from site to site. Thus, adopting interpretation methods used in seismology without appropriate adaptation is inappropriate. In particular, geoelectrically determined strike arrows at a given period should not be portrayed as equivalent to maps of SKS arrows with the same inherent depth information. In addition, there are structures for which depth penetration is radically different for the two modes of propagation (TE and TM) in 2D structures. An extreme example, but encountered in the real Earth, is a class of structures with significant responses in the MT mode with electric field parallel to structures (TE mode) to the presence of anomalous conductivity, but for which the responses in the mode with the electric field perpendicular to structures (TM mode) are negligible. In this case, the upper anisotropic layer acts as a polarizing filter, and only TM mode fields penetrate through it. Thus no tensor information is available for the lower layers, and variation in structural strike directions with increasing depth is irresolvable.