P. Schmoldt, PhD - MTNet - DIAS

10.1. Inversion for crustal structures
During the initial inversion sequence interfaces are kept fixed at their location along
the layer boundaries, allowing only lateral variation of electric resistivity values within
the layers. The RMS misfit is reduced to a value of 4.56 through the lateral variation
of electric resistivity within layers; most layers exhibit relatively higher resistivity in the
southern region of the model (middle plot in Figure 10.5). Owing to the prevailing increased
misfit, additional CI-inversions are conducted in which conductivity interfaces
are no longer kept fixed along layer boundaries. Instead, deviation of conductivity interfaces
from their initial location along the layer boundaries is only constrained; i.e.
changes of their locations are added to the objective function (cf. Sec. 6.3). CI-inversions
with constrained interface locations (bottom plot in Figure 10.5) yield inversion models
with interfaces that deviate significantly from a levelled layer case. In the northern region
of the model, layer boundaries of the CI-inversion model exhibit some agreement with
proposed seismic layer locations. However, in the south and centre of the model, location
and thickness, particularly, of the intermediate crustal layer are significantly different.
The RMS misfit for the constrained CI-inversion model remains unacceptable high (4.67)
and reduction of the misfit can only be achieved through further deviation from the levelled
layer case. Hence, a perfectly levelled layer structure of the Tajo Basin crust is not
in agreement with MT response data of the PICASSO Phase I project. Results of seismic
reflection studies are usually accurate and reliable; it is therefore concluded that the
layered subsurface model, derived by Díaz and Gallart [2009] through their compilation
of results from seismic studies in Central Spain, cannot be projected to the region of the
Tajo Basin beneath the PICASSO Phase I profile. Alternatively, the seismically derived
layers may not exhibit a significant difference in terms of their electric conductivity properties
in comparison to lateral changes of electric conductivity beneath the Tajo Basin.
Respective prevailing lateral changes in crustal layers may originate, for example, from
compositional differences or the presence of a highly conducting phase in subareas of the
crustal layers (cf. Sec. 5.2.1). Lateral changes of electric conductivity are indicated by
results of initial inversions (Fig. 10.4) and subsequent inversion steps will provide more
detailed information about the nature as well as possible interpretations of this lateral
discontinuity.
10.1.5. Final model of the Tajo Basin crust
The advanced starting model determined in Section 10.1.2, containing layers inferred
from seismic reflection studies (Sec. 7.3.2) and electric resistivity values derived through
averaging of inversion results with a range of smoothing parameters, is used to obtain an
enhanced model of the crustal structures beneath the Tajo Basin; see Table 10.2 for depth
extent and electric resistivity values of the starting model. Inversion for Tajo Basin crustal
structures is carried out according to the Jones Catechism (Sec. A.2.3) using a range of
adaptive processes during the inversion, which are described below.
• During initial inversion steps four tear zones are applied to maintain separation of
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