A novel anisotropic inversion approach for magnetotelluric data from subsurfaces with orthogonal geoelectric strike directions
Jan-Philipp Schmoldt and Alan G. Jones
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, e.g. crustal and mantle depths.
Oblique geoelectric strike directions are a well-known issue in commonly employed isotropic 2D 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 2D inversion in the case of an overlying region (crust) with different geoelectric strike direction.
Thus, investigators may resort to computationally expensive and more limited 3D 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 2D structures in one depth range, whereas the other region is modelled with an isotropic 1D or 2D approach; as a result significantly reducing computational costs of the inversion in comparison with 3D inversion.
The 1D and 2D versions of the novel approach were tested using a synthetic 3D subsurface model with orthogonal strike directions at crust and mantle depths and their performance was compared to results of isotropic 2D 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 2D inversion models, despite decomposition of the electric impedance tensor and using a wide range of inversion parameters, exhibited severe artifacts thereby confirming the requirement of either an enhanced or a higher dimensionality inversion approach.
With the anisotropic 1D 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 1D inversion approach is likely to yield implausible models of the electric resistivity distribution due to inapplicability of the 1D approximation.
Owing to the higher number of degrees of freedom, the anisotropic 2D inversion approach can cope with more complex subsurface cases and is the recommended tool for real datasets recorded in regions with orthogonal geoelectric strike directions.
Geophysical Journal International, 195, 1576-1593, doi:10.1093/gji/ggt355. [PDF]
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Alan G Jones / 14 November 2013 /