The Characteristic States of the Magnetotelluric Impedance Tensor:   Construction, Analytic Properties and Utility in the Analysis of General   Earth Conductivity Distributions
  It is shown that the Magnetotelluric (MT) impedance tensor admits an anti-symmetric generalized eigenvalue - eigenstate decomposition consistent with the anti-symmetry of electric and magnetic fields referred to the same coordinate frame: this is achieved by anti-diagonalization through rotation by 2x2 complex operators of the SU(2) rotation group. The eigenstates comprise simple proportional relationships between linearly polarized eigenvalues of the input magnetic and output electric field along the locally resistive and conductive propagation path into the Earth, respectively mediated by the maximum and minimum characteristic values of the tensor (eigen-impedances). It is shown from first principles that the eigen-impedances are expected to be positive real (passive) functions, analytic in the entire lower-half complex frequency plane and with singularities confined on the positive imaginary frequency axis. Insofar as the impedance tensor is generated by isometric transformation of the eigen-impedances, it is also passive. The expected passivity is an effective means of appraising measured tensors for compliance with the basic tenets of the MT method: it can be violated only in the presence of sources in the Earth. In addition to extrinsic effects (e.g. noise), it is demonstrated with examples, that such sources may be secondary large or small scale inductive phenomena generated by realistic conductivity configurations. However, they may not be time-independent effects taking place in a passive induction context, such as steady-state current channelling, galvanic distortion and electric field reversals. In general, to assert whether violation of passivity has occurred, it is necessary to decompose the impedance tensor, refer it to its intrinsic coordinate frame and evaluate the compliance of the eigen-impedances with their expected analytic properties 
Andreas Tzanis
physics.geo-ph