P. Schmoldt, PhD - MTNet - DIAS

10. Data inversion
the model into upper (0 – 10 km), intermediate (10 – 24 km), and lower crust (24 –
31.5 km) as well as the mantle (≥ 31 km).
• Features are manually removed or their resistivity values are modified in order to
test whether they become re-established in subsequent inversion steps and are therefore
likely to be data-supported structures.
• Focussed inversions are carried out for specific regions of the model by individually
inverting responses from selected stations 1 , namely
– stations pic013 – pic017, to refine the shape of the resistive body beneath the
Manchega plain (labelled ‘e’ in Figure 10.6); and
– stations pic004 and pic005, to investigate the lateral extent of the upper crustal
conductor in the north of the Tajo Basin (labelled ‘c’ in Figure 10.6).
• Anisotropic 2D inversion is carried out using a range of isotropy parameters (τiso)
in order to test for potential anisotropic structures with the results that even for
relatively low constraints (τiso = 10) the anisotropy magnitude is overall low and
mostly confined to the surficial conductive layer (cf. Fig. 10.7).
Misfit and sensitivity to regions of the Tajo Basin crustal model (Fig. 10.6) are examined
in the paragraphs below, followed by a discussion of model features and their
geological implication in the next subsection (‘Features of the crustal model’, Page 239);
deep-seated features at mantle depth are examined in the subsequent Section 10.2.
Model misfit and sensitivity
The concluding Tajo Basin crustal model fits observed MT data considerably well; the
model exhibits a RMS misfit of 1.47 using error floors of 5% for the phases and 10% and
20% for the apparent resistivities of TM and TE mode, respectively. During the inversion,
attention was also paid to the misfit distribution, assuring that the global misfit is not controlled
by the misfit of a small number of confined regions. As a result, a relatively even
distribution of the misfit is obtained using focussed inversions during the model identification
process, which reduce the misfit of respective regions (cf. Fig. 10.8). It should be
noted, however, that response data of stations in proximity of the DC train line had to be
truncated due to high noise levels; thus, structures in this region are less constrained (indicate
by white space in Figure 10.8). For the final crustal model (Fig. 10.6), all stations
exhibit a RMS misfit of 2.5 or lower with a relatively uniform distribution of the misfit
for all stations and periods. Detailed comparison of recorded data and model response for
each stations is given in the Appendix (Section A.4.1).
In order to determine reliability of regions within the inversion model, sensitivity analysis
is carried out following the approach by Schwalenberg et al. [2002]. Therein, a
1 alteration of other model regions during that process is restricted by fixing respective regions and setting
the parameter ‘solving for smoothest variation away from a priori model’
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