P. Schmoldt, PhD - MTNet - DIAS

10.1. Inversion for crustal structures
Depth (km) Description Resistivity (Ωm)
0 - 3 Sediments 20
3 - 5 Sediments with fluid intrusion 10
5 - 10 Upper crust 15
10 - 24 Intermediate crust 70
24 - 31 Lower crust 100
≥ 31 Lithospheric-mantle 100
Tab. 10.2.: Layers of the Tajo Basin lithosphere with layer boundaries based on seismic reflection studies and estimates of electric
resistivity values inferred from horizontal averaging of inversion models constructed using different sets of smoothing parameters;
see text for details. Therein, average resistivity values of the layers contain contributions of the (more resistive) crustal rocks and
conductive anomalies such as fluid phases and ore bodies. The lithospheric-mantle resistivity is underestimated, presumably a result
of low sensitivity to the mantle region for the used period range (10 −3 – 10 2 s), which was chosen to suit investigation of the crust.
Note that thicknesses of sedimentary and upper crustal layer are increased to facilitate a minimum thickness of 2 km.
resulting decreased sensitivity at greater depths. The average resistivity determined for
the whole length of the PICASSO Phase I profile is certainly affected by the increased
resistivities inverted for the subsurface region in proximity of the train line. Therefore,
average resistivity values are potentially too high. Even though they only represent part of
the profile, average resistivities for the northernmost stations are more reliable given their
lower disturbance and untruncated response curves. The starting model for subsequent
inversions is therefore created on the base of the average resistivity–depth profile for the
longer period range and northernmost stations (green lines in plots at the bottom of Figure
10.3); note that the difference between average resistivity–depth profiles from inversion
with different smoothing parameters is negligible. Resulting electric resistivity values for
crust and lithospheric-mantle are summarised in Table 10.2.
10.1.3. Investigating characteristics of TE and TM mode response
data
The two modes in 2D MT investigation, TE and TM, relate to the off-diagonal elements
of the 2D MT impedance tensor (Eq. 3.39) and are affected to different degree by the
characteristics of the subsurface; see Section 4 for a detailed discussion of subsurface
characteristics and their effect on the two modes. It is therefore usually useful to invert
each of the modes separately to identify similarities and differences of the models in order
to infer contribution of the modes to the combined-mode inversion model.
The PICASSO Phase I dataset for the Tajo Basin crust is inverted for each of the modes
individually (‘TE-only’ and ‘TM-only’), as well as for both modes together. The inversion
follows the Jones Catechism (Sec. A.2.3) and uses the optimal smoothing parameter combination
determined in the previous Sections 10.1.1 and 10.1.2 with a 100 Ωm halfspace
as starting model. Resulting inversion models are displayed in Figure 10.4. The RMS
misfit of the three models is above acceptable: 3.01 (TE and TM mode), 3.13 (TE-only),
and 2.63 (TM-only); however, the aim of this inversion process step is not to determine
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