Three-dimensional magnetotelluric inversion with structurally guided regularization constraint

Abstract

We propose a novel technique for the structurally regularized three-dimensional magnetotelluric inversion scheme. In the proposed method, the structural resemblance between the inverted electrical resistivity model and the independently derived guiding model is enforced by controlling the weights of the roughness operator for regularization. Using a simple mathematical function, we forced the weights to be small across presumed discontinuities in the guiding model, allowing for sharp changes in resistivity values across the discontinuity. We forced the weights to be large along blocks with similar values in guiding physical parameters, enforcing smooth changes of resistivity along the blocks. Numerical inversion tests indicate that the new guided inversion produces a resistivity model that closely approximates the true model, surpassing the results of both conventional smooth inversion and cross-gradient inversion, particularly when a perfectly correlated guiding model is used. We applied our guided inversion method to magnetotelluric data in Southern Tohoku, Northeast Japan, with structural constraints from a seismic velocity structure. Given the non-uniqueness problem of MT inversion, guided inversion enables the exploration of alternative resistivity structures that are more consistent with geological or geophysical models than those produced by conventional smooth inversion, therefore expanding the possibility of interpretation. We suggest that the proposed inversion method can serve as an alternative to the commonly used cross-gradient method, especially in the absence of objective schemes for the simultaneous selection of multiple trade-off parameters in the objective function.