Three-dimensional magnetotelluric inversion using frequency-domain survey decomposition and adaptive mesh decoupling

SUMMARY 3-D magnetotelluric (MT) inversion is a powerful tool for imaging the Earth’s electrical structure, yet its computational demands remain a major challenge, particularly when simulating responses across broad frequency bands. Conventional inversion schemes rely on a unified mesh for forward simulation at all frequencies, which inflates the number of model parameters and greatly extends runtime. To overcome these limitations, we present a finite-volume-based frequency-domain survey decomposition (FVFSD) method that adaptively constructs forward simulation meshes according to the skin depth of each frequency while maintaining a fixed horizontal discretization. This design decouples forward simulation meshes from the inversion mesh, striking a balance between accuracy and efficiency. To further improve the treatment of resistivity contrasts, an equivalent circuit scheme is employed to compute effective conductivities within control volumes, outperforming conventional volume-weighted averaging. We validate the proposed method through comprehensive numerical experiments, including synthetic benchmarks and real-world case studies from the Akebasitao region (China) and the Southern African MT experiment. Results demonstrate that FVFSD achieves accuracy comparable to standard finite-difference forward simulations, while significantly reducing computational time. In large-scale MT inversions, this acceleration directly translates into faster convergence and more efficient recovery of lithospheric-scale resistivity structures. The method is fully compatible with existing inversion algorithms and solvers, making it straightforward to integrate into standard workflows. Overall, FVFSD provides a scalable and accurate strategy for advancing 3-D MT inversion, with clear implications for lithospheric studies, resource exploration and tectonic investigations.