A novel anisotropic inversion approach for magnetotelluric data from subsurfaces with orthogonal geoelectric strike directions

Abstract

The key result of this study is the development of a novel inversion approach for cases of orthogonal, or close to orthogonal, geoelectric strike directions at different depth ranges, for example, crustal and mantle depths. Oblique geoelectric strike directions are a well-known issue in commonly employed isotropic 2-D inversion of MT data. Whereas recovery of upper (crustal) structures can, in most cases, be achieved in a straightforward manner, deriving lower (mantle) structures is more challenging with isotropic 2-D inversion in the case of an overlying region (crust) with different geoelectric strike direction. Thus, investigators may resort to computationally expensive and more limited 3-D inversion in order to derive the electric resistivity distribution at mantle depths. In the novel approaches presented in this paper, electric anisotropy is used to image 2-D structures in one depth range, whereas the other region is modelled with an isotropic 1-D or 2-D approach, as a result significantly reducing computational costs of the inversion in comparison with 3-D inversion. The 1-and 2-D versions of the novel approach were tested using a synthetic 3-D subsurface model with orthogonal strike directions at crust and mantle depths and their performance was compared to results of isotropic 2-D inversion. Structures at crustal depths were reasonably well recovered by all inversion approaches, whereas recovery of mantle structures varied significantly between the different approaches. Isotropic 2-D inversion models, despite decomposition of the electric impedance tensor and using a wide range of inversion parameters, exhibited severe artefacts thereby confirming the requirement of either an enhanced or a higher dimensionality inversion approach. With the anisotropic 1-D inversion approach, mantle structures of the synthetic model were recovered reasonably well with anisotropy values parallel to the mantle strike direction (in this study anisotropy was assigned to the mantle region), indicating applicability of the novel approach for basic subsurface cases. For the more complex subsurface cases, however, the anisotropic 1-D inversion approach is likely to yield implausible models of the electric resistivity distribution due to inapplicability of the 1-D approximation. Owing to the higher number of degrees of freedom, the anisotropic 2-D inversion approach can cope with more complex subsurface cases and is the recommended tool for real data sets recorded in regions with orthogonal geoelectric strike directions.

Structures and geometries of the Tajo Basin crust, Spain: Results of a magnetotelluric investigation compared to seismic and thermal models

“…These modeling tests demonstrated that the data can be satisfactorily fit to models with an isotropic conductivity in each model cell, and more complicated anisotropic conductivity in some cells does not have to be appealed to. A numerical modeling study we undertook [ Schmoldt and Jones , ] demonstrated the validity of our approach.…”

“…Crustal structures can usually be determined by confining the modeled period range to crustal penetration depths; however, recovery of mantle structures is more challenging. No straightforward solution is currently known for the problem of deriving mantle structures for subsurfaces with significantly oblique strike directions, although Schmoldt [] and Schmoldt and Jones [] discuss approximate 1‐D and 2‐D modeling approaches using anisotropic techniques.…”

Reexamination of magnetotelluric responses and electrical anisotropy of the lithospheric mantle in the Grenville Province, Canada

“…Tikhonov regularization is implemented in terms of smoothness and anisotropy parameters. An isotropy weighting parameter ( τ iso ) is used to control the degree of anisotropy [e.g; Evans et al ., ; Evans et al ., ; Matsuno et al ., ; Miensopust and Jones , ; Le Pape et al ., ; Schmoldt and Jones , ]. Small values of this parameter will allow for variations between the results of the diagonal of the anisotropic resistivity tensor.…”

The Role of Electrical Anisotropy in Magnetotelluric Responses: From Modelling and Dimensionality Analysis to Inversion and Interpretation

“…Another study was performed by Schmoldt and Jones (2012), which presented an approach to invert 3D data with two oblique directions, using 1D anisotropic (Pek and Santos 2006) and 2D anisotropic (Mackie 2002) inversions. The tests were performed by inverting data from a 3D model with two orthogonal crust and mantle structures, along a profile oblique to both structures.…”