A Gradient Scaling Scheme for the 3-D Magnetotelluric Inversion With Galvanic Distortion Correction

Galvanic distortion (GD) resulting from near-surface heterogeneity has long been considered a major impediment to the accurate interpretation of magnetotelluric (MT) data. As this GD effect is approximated by a real tensor, it can be solved numerically in the 3-D MT inversion. To achieve this goal in a practical way, we have implemented the inverse solution of both the GD and resistivity parameters in the widely used software package ModEM. To address the gradient domination problem of the distortion parameter over resistivity, a scaling scheme is proposed to balance their separate roles in the total gradient. Two synthetic datasets of the Block2-3D and the oblique conductor (OC) models provided in the ModEM package were distorted and then used to validate the new implementation. Synthetic inversions demonstrate that the artifacts introduced by fitting the GD effect can be removed and the regional structure is better resolved. A field data inversion test was conducted on the MT data from the Red River Fault zone and its adjacent areas on the southern margin of the Tibetan Plateau. Using the distortion-free phase tensor, we show that the new inversion significantly removes the GD artifacts from the shallower structure. It obtained a slightly different crustal structure and a clearer delineation of the upper mantle structure compared to the previous study.