The Magnetotelluric Phase Tensor: A Critical Review

The magnetotelluric (MT) phase tensor is a property of the MT impedance that is resistant to a common form of distortion due to unresolvable local structure. Review of the theory leads to a new geometrical description that cleanly separates information about directionality and dimensionality of regional conductivity structure. This information is widely used to justify two-dimensional (2D) interpretation, but the case is seldom made convincingly. In particular, errors are largely ignored and it is not understood that full data covariance is essential for accurate error bars. It is also common to use 2D impedance tensor decompositions when the phase tensor shows this model to be inconsistent with the data. A phase tensor-consistent impedance tensor decomposition is introduced. Because the phase tensor is a distortion-free 3D response, it should be used as data for 3D inversions. Until codes for this become more developed, comparison of predicted and observed phase tensors can ascertain whether 3D aspects of the data have been adequately fit by other inversions or modeling.