Conventional 3D magnetotelluric (MT) inversion methods face substantial computational challenges when inverting large-scale data sets, demanding extensive processing time and computing resources. To address these limitations, we develop an advanced frequency-polarization joint parallel inversion framework for MT data that uses the nonlinear conjugate gradient (NLCG) method, integrating an adaptive algebraic multi-resolution sampling (AMRS) approach with a regularization technique. The AMRS approach adaptively discretizes modeling domains across different periods based on the normalized root-mean-square difference relative to the solution on the initial grid, which serves as a reference. The regularization technique ensures the conservation of current, which can significantly affect the efficiency of the forward-modeling problem and its adjoint when using iterative solvers, particularly at low frequencies. The reliability and efficiency of the regularization AMRS inversion are first examined by benchmarking it against the traditional NLCG inversion and the regularization inversion based on a two adjacent blocks model and a terrain model. Subsequently, the regularization AMRS inversion is applied to invert real MT data from the Cascadia region in the USA, demonstrating its capability to address real inverse problems.

The Pandian deposit is a newly discovered contact metasomatic skarn magnetite deposit found in the Cainozoic super-thick overburden on the northwest margin of Luxi Uplift (LXU). Presently, the horizontal scale of the deposit delineated by the potential field (gravity and magnetic method) has shown giant potential for ore deposits, and mapping the ore-controlling structures in the vertical scale becomes a primary task for metallogenic prediction. In our study, the wide-field electromagnetic method (WFEM), with a strong anti-noise ability in recording electromagnetic signals on the surface at multiple frequencies, is applied to characterize the deep conductivity distribution of the Pandian area. Based on the inversion results from two parallel WFEM profiles, which consist of 105 sites and previous geological and geophysical results, the 2D geoelectric models are established. The low-resistivity regions (with a typical range of 25~32 Ω·m) in the electrical models are proven to be ore bodies of Pandian deposit, which are developed along the contact zone between Yanshanian intrusive rocks and Paleozoic Ordovician strata. The scattered bodies (typically >32 Ω·m) in Ordovician limestone strata are probably caused by intrusive diorite pluton closely related to magnetite mineralization. Due to contact metasomatism, bedded limestone near magnetite was metamorphosed into marble and accompanied by low-resistivity skarn alteration, with resistivity much different from its high-resistivity protolith. The inverted geoelectrical models visually reflect the spatial distribution features of intrusive rocks and lithologic alteration/fracture zones.