P. Schmoldt, PhD - MTNet - DIAS

10
Data inversion
Details about collection and processing of the PICASSO Phase I dataset were presented
in the previous Section 9, and this Chapter is concerned with inversion of the data and
evaluation of inversion model features. Due to the profoundly 3D nature of the Betic
Cordillera subsurface (cf. Secs. 7.2 and 9.6.1), and that fact that resistivity structures of
the Betics are relatively well studied by prior work, the focus of this investigation is on
the Tajo Basin region, which has hitherto not been studied using electromagnetic methods.
For depth ranges associated with the Tajo Basin crust and mantle, geoelectric strike
directions of N40.9W and N29.4E were determined, respectively (cf. Sec. 9.6.1). Derived
significant oblique strike directions between the crust and mantle are supported by
results of seismic tomography studies for the same region, determining changes of seismic
velocity along a NW-SE oriented interface at crustal depth and a NNE-SSW oriented
interface at mantle depths (cf. Sec. 7). Decomposing MT data with an incorrect geoelectric
strike direction introduces inversion artefacts (cf. Sec. 4); therefore, impedance
data were individually decomposed according to the strike direction of the crust and the
mantle followed by separate inversions for the two depth ranges. In addition, as shown by
Spratt et al. [2009] and Miensopust et al. [2011], focussed inversion for crustal regions
can enhance the quality of inversion models regarding local features. In inversions of data
related to a wide depth range, local features can be underfitted due to a global definition
of smoothing constraints and model misfit [e.g. Spratt et al., 2009].
Initially, isotropic 2D inversion, the common tool in modern MT investigation, is used
for data from the PICASSO Phase I stations located in the Tajo Basin owing to inferred
general suitability of the dataset with 2D inversion (cf. Sec. 9.6.1). In addition to isotropic
2D inversion, anisotropic 1D and 2D, as well as isotropic 3D inversions are conducted
with the PICASSO Phase I dataset in order to determine detailed information about the
subsurface. Application of anisotropic inversion approaches is motivated by their satisfactory
performance in a synthetic 3D model (cf. Sec. 8). Isotropic and anisotropic 2D
inversions are carried out using the program MT2Dinv [Baba et al., 2006], an enhanced
version of the algorithm developed by Rodi and Mackie [2001], as well as the commercial
225