The present study revisits the theoretical foundation of the magnetotelluric (MT) method by addressing two fundamental questions that challenge key assumptions in current practice. The first concerns the validity of the conventional MT inversion framework, in which electromagnetic (EM) responses (impedances and tippers) observed at the Earth’s surface are matched to those computed in a Cartesian coordinate system. This raises the question: Is the geometrical inconsistency between spherical observations and Cartesian modeling physically justifiable? The second concerns the formulation of the MT impedance itself. While standard MT practice uses the impedance consisting of four elements (hereafter, 4-element impedance), we return to Maxwell’s equations and demonstrate that the impedance naturally consists of six elements (hereafter, 6-element impedance) at the Earth’s surface, regardless of coordinate system. This leads to a further question: What new framework for MT modeling and inversion emerges if we adopt the 6-element impedance? We explore these issues with attention to two key aspects of consistency: the internal consistency between observed and modeled responses, and the mutual consistency between Cartesian and spherical frameworks. Through theoretical analyses in both coordinate systems, we derive the following implications: These implications remain theoretical and require further verification through numerical and observational studies. Future work is essential to assess the practical applicability and potential benefits of the 6-element impedance framework. If confirmed, this formulation could offer a promising alternative to the conventional MT framework based on the 4-element impedance and tipper, potentially laying the foundation for a complementary MT approach with greater accuracy and physical fidelity.
Utada et al. (Thu,) studied this question.