P. Schmoldt, PhD - MTNet - DIAS

10.1. Inversion for crustal structures
southern edge of the profile at a depth of approximately 5 – 15 km (labelled ‘c’ in Figure
10.4). Existence of a conductor in this region is supported by inversions of each of
the modes as well as by the fact that a conductive structure is also apparent in the station
response data of the TE mode (for an impedance tensor decomposition according to
the crustal strike direction) at around 10 s beneath the southernmost stations; cf. Figure
9.12. However, the depth extent of this feature is less well-constrained due to the reduced
sensitivity of MT inversion below a conductive region (cf. Sec. 6.3).
The highly resistive region, modelled at the bottom of the southern half of the Tajo
Basin crust, (labelled ‘d’ in Figure 10.4) is present in all three inversion models; however,
its lateral extent, as well as its maximum resistivity, differs significantly between
the modes. In the TM-only inversion the anomaly is mostly confined to a region below
stations pic009 to pic019, whereas inverting data from the TE mode produces a more extensive
resistor, extending from beneath station pic007 to the southern edge of the profile.
The TE mode is commonly assumed to be more affected by 3D off-profile bodies (cf.
Sec. 4); given its location, it is possible that the feature is related to the Iberian Massif
(cf. Sec. 7). In that case, the anomaly could originate from charge accumulation along
the north-south oriented, thus parallel to the profile located, interface between the lower
crust of the Tajo Basin and the easternmost extent of the Iberian Massif. Alternatively, the
resistive feature may be related to distortion of MT data by the DC train line (cf. Sec. 4).
The two anomalies ‘e’ and ‘f’ are only supported by data of the TE and TM mode,
respectively (however, anomaly ‘e’ is also introduced into the combined mode inversion
model). For this initial inversion process, potential explanations of the two features are the
presence of a 3D body (anomaly ‘e’) and difference in induction depth of the two modes,
due to the different conductance modelled for the conductive layer ‘a’ above (anomaly
‘f’). Investigation, using refined and more detailed inversion as well as additional constraints,
will help to confine the different anomalies, thereby providing better information
about their possible causes.
10.1.4. Evaluating proposed layered crustal model
As illustrated in Section 6.3, MT inversion is non-unique, i.e. a range of models fit station
response data equally well within the given uncertainty levels. Conversely, a model that
can be rejected on the basis of its MT response misfit is definitely not representative of
the studied subsurface area. Therefore, MT is a formidable tool in rejecting proposed
subsurface structures.
Based on findings of seismic reflection and refraction studies a relatively levelled layer
structure has been presented for the Tajo Basin crustal region located slightly to the west
of the PICASSO Phase I profile (cf. Sec. 7.2.1). Seismic model were created by projecting
results from different studies in the proximity of the region; therefore, layering beneath
the PICASSO Phase I profile is potentially different from the seismic model. The hypothesis
of a levelled layer structure beneath the PICASSO Phase I profile is tested using
sharp-boundary inversion with so-called conductivity interfaces (in the following referred
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