P. Schmoldt, PhD - MTNet - DIAS

8. Recovering a synthetic 3D subsurface model using lower-dimensional inversion schemes
8.4. Summary and conclusions
Oblique geoelectric strike directions in the crust and mantle are a common issue in MT
investigation, particularly causing problems during 2D inversion due to the requirement
of defining a common strike direction. Different approaches can be conceived, of which
constrained isotropic 2D inversion, anisotropic 1D inversion, and anisotropic 2D inversion
attempts were studied in this Chapter. After illustrating general applicability of anisotropic
inversion approaches using basic mathematical relations for anisotropy effects
on MT data, the different approaches were applied to data from a synthetic 3D model with
orthogonal strike direction in crust and mantle (Fig. 8.3) and results for different profiles
over the 3D model were analysed. In particular, inversion models for a profile containing
a set of stations that represent MT recording sites of the PICASSO Phase I investigation
were evaluated and results of the different approaches were contrasted.
For isotropic 2D inversion, a set of inversion parameters was identified which yields
a subsurface model closest to the original model (for this inversion approach). Whereas
crustal structures were reconstructed reasonable well, the electric resistivity distribution
of the mantle was not particularly well recovered (Fig. 8.8). Even the optimal isotropic
2D model contained significant inversion artefacts, in particular a resistive body at mantle
depth. Using only TM mode data for the isotropic 2D inversion process did not result in
a more adequate reproduction of mantle structures. Results of isotropic 2D inversion for
subsurface cases similar to the 3D model used in this study are therefore to be used with
caution and its applicability to the Tajo Basin subsurface is questionable.
Anisotropic 1D inversion yielded models that are close to the 3D subsurface model,
thereby indicating the potential of anisotropic inversion for the case of complex subsurface
structures. In the anisotropic 1D inversion approach the crust was approximated
by a quasi - isotropic 1D layer and the mantle was imaged by an anisotropic 1D structure.
Crustal structures of the synthetic 3D model were in general adequately reproduces
by the anisotropic 1D inversion; the vertical electric resistivity interface at crustal depth
were imaged by a step-like change of resistivity between stations at the respective location
(Fig. 8.11). Mantle structures were recovered reasonably well using resistivity values for
the anisotropy direction perpendicular to the mantle strike direction. This finding demonstrates
practicality of anisotropic inversions in resolving certain types of 3D subsurface
models. However, due to inherent limitations of 1D inversion (e.g. less likely to adequately
recover a model containing more complex structures) its results may not be used
as a final model, but rather to aid subsequent 2D inversion.
In anisotropic 2D inversion, anisotropic structures can be introduced for certain regions
of the model in order to account for effects of oblique geoelectric strike directions in different
depth regions, e.g. crust and mantle. Those effects originate from the inevitable erroneous
assignment of TE and TM mode in either crustal or mantle region during the 2D
inversion. For the case of two oblique strike directions at different depth regions the coordinate
system used for the inversion was aligned with one of the strike directions, thereby
facilitating isotropic 2D inversion of the respective region. The electric resistivity distri-
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