Three-Dimensional Inversion of Magnetotelluric Data for a Resistivity Model With Arbitrary Anisotropy

Electrical anisotropy is increasingly recognized as an important aspect of the resistivity models required to explain magnetotelluric (MT) observations. However, a limited number of practical MT inversion algorithms that can consider anisotropy have been published to date. To address this problem, we have developed a three-dimensional (3-D) MT inversion algorithm that recovers a 3-D resistivity model that considers arbitrary electrical anisotropy. The inversion uses the same inversion algorithm as the widely used ModEM inversion algorithm, and a novel forward modeling algorithm to consider the anisotropic Earth. The algorithm was tested on both synthetic and field MT data. Inversions considered both a completely general anisotropy tensor with six components and approximations with less parameters. Synthetic inversions show that the two horizontal components of resistivity and the anisotropy strike can be well recovered, while the vertical component of resistivity is poorly resolved, primarily because current flow in MTs is dominantly horizontal. The synthetic examples confirm the limitation of the axial anisotropic inversion technique when applied to MT data produced by a resistivity model with arbitrary anisotropy. The synthetic inversions also showed that inversion of data from an isotropic model will not result in an artificially anisotropic model. Compared to the isotropic inversion model of the real MT data, the anisotropic model clearly shows some features that are consistent with the mapped geology. As expected, the results showed that a given data set can be fit by a range of models, with an inherent trade-off from 3-D heterogeneity to 3-D anisotropy. This uncertainty can be reduced with the use of prior information in the inversion.

Automated summary

Electrical anisotropy is crucial in resistivity models for interpreting magnetotelluric observations. A new 3-D MT inversion algorithm has been developed to recover a 3-D resistivity model considering arbitrary electrical anisotropy, tested on synthetic and field data. The algorithm demonstrates limitations in resolving vertical resistivity components due to dominant horizontal current flow in MTs and highlights the importance of prior information in reducing uncertainty.