P. Schmoldt, PhD - MTNet - DIAS

11. Summary and conclusions
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. 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 later applied to the
PICASSO Phase I dataset from central Spain. Performance of the novel approaches were
therein compared to results of isotropic 2D and isotropic 3D inversion.
Structures at crustal depths were reasonably well recovered by all inversion approaches
in the synthetic model study, 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 artefacts in the synthetic model case and yielded implausible structures for the
real dataset, 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 perpendicular 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 Tajo Basin
subsurface the anisotropic 1D inversion approach did not yield a plausible model of the
electric resistivity distribution. Inadequacy of the derived model originates therein most
likely from inapplicability of the 1D approximation to the complex structures of the Tajo
Basin subsurface, exhibiting multiple indications of 2D and 3D features.
Owing to the higher number of degrees of freedom, the anisotropic 2D inversion approach
can cope with more complex subsurface cases and it yielded a reasonable reproduction
of the synthetic model as well as a plausible model for the Tajo Basin subsurface
using the PICASSO Phase I dataset. However, the anisotropic 2D inversion algorithm
used in this study requires coincident directions of structural strike and anisotropy. Thus,
the algorithm facilitates only a difference of 90 degrees between the strike directions of
crust and mantle, rather than the approximately 70 degrees determined for the Tajo Basin.
Hence, subsurface models obtained with the anisotropic 2D inversion approach for cases
with oblique strike directions that are significantly different from the orthogonal case must
currently be associated with a higher degree of uncertainty.
11.1.2. Suggestion for future work
Further development of the anisotropic inversion approaches are strongly linked to enhancements
of the inversion algorithms. Particularly useful enhancements of 2D algorithms
that would improve applicability of this novel inversion approach are:
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• Incorporation of anisotropy-axes directions that are independent of the inversion
mesh orientation. The 1D inversion algorithm ai1d by Pek and Santos [2006]
permits flexible anisotropy-axes directions, and the principle has been adopted for
a 2D algorithm with some success Pek et al. [2011]. However, the 2D algorithm