Three-dimensional magnetotelluric imaging including inter-station transfer function in Bayan Obo region, Inner Mongolia, China

The horizontal magnetic inter-station transfer function (hereinafter referred to as M) establishes the relationship between horizontal magnetic fields at the local site and the reference site (H^l = MH^r). 3D numerical simulation (Wen et al., in revision) indicates that M can effectively mitigate the "speckle-shaped artifacts" in shallow structures often observed in the inversion of long-period sparse arrays, and improve the recovery effect of the lateral boundaries of high-conductivity anomalies. Incorporating M data from a synchronized sub-array into the inversion, alongside the traditional impedance tensor (Z) and tipper (T), holds significant potential for enhancing the resolution of shallow electrical structures.

To verify the above conclusions and evaluate the practical application of M in complex geological environments, we conducted imaging of field data containing M data in the Bayan Obo area, Inner Mongolia, China. This region is located in the transition zone between the Yinshan Orogenic Belt and the North China Craton, characterized by significant lateral heterogeneity, making it an ideal site for testing resolution characteristics. We deployed an MT array with varying station spacings. While constructing a high-resolution reference model using the inversion of the dense station dataset, we also conducted inversion experiments on the sparse station data from multiple subsets of this dataset. By comparing the results of Z+T inversion with Z+M joint inversion, we verified and quantified the resolution capability of M for shallow electrical structures. Furthermore, by selecting sites with different electrical structures within the survey area as reference stations, we tested the impact of the subsurface electrical structure beneath the reference station on the resolution capability of M.

This research was supported by grants from National Major Science and Technology Projects of China: Deep Earth Probe and Mineral Resources Exploration (2024ZD1000200) and National Natural Science Foundation of China General Program (42474103).