On the Resolution of Magnetotelluric Horizontal Magnetic Inter-station Transfer Function

Summary The horizontal magnetic inter-station transfer function (M) offers distinct advantages in magnetotelluric (MT) studies due to its reduced susceptibility to shallow galvanic distortions. While its utility in constraining subsurface conductivity through 2D inversions is established, its resolution characteristics in three-dimensional (3D) scenarios—particularly regarding depth sensitivity and lateral recovery—remain poorly quantified. Here, we conduct a systematic investigation of M through 3D forward modelling, sensitivity analysis, and inversion of synthetic models, followed by a validation using long-period MT array data from the Eastern Pamir. Key findings reveal: (1) M exhibits response patterns analogous to the impedance tensor (Z), with an inverse component correspondence where diagonal elements of Z align with off-diagonal elements of M; (2) Reference station placement critically controls the inversion performance of M—locations above conductive anomalies distort anomalous currents and degrade data fit, whereas resistive homogeneous backgrounds enhance lateral resolution; (3) M resolves shallow anomalies with lateral boundary definition and resistivity contrast recovery superior to those of Z and the tipper (T) in sparse arrays; (4) Sensitivity tests indicate that M provides limited resolution for deep structures beyond the effective inductive scale, where its recovery capability diminishes significantly compared to Z; (5) Application to field data demonstrates that incorporating M improves the resolution of complex shallow structures and enhances the fit of Z data, with the joint inversion of Z + M + T yielding the most robust electrical model. We recommend using multiple reference stations for field data inversion to mitigate the impact of variations in reference site locations. This work establishes a quantitative framework for deploying M in field surveys and 3D inversions.