P. Schmoldt, PhD - MTNet - DIAS

RMS−misfit
3.0
2.5
2.0
1.5
1.0
0.1 1 10 100
Roughness(1/τ)
10.2. Inversion for mantle structures
Iteration 0
Iteration 1
Iteration 2
Iteration 3
Iteration 4
Iteration 5
Iteration 6
Iteration 7
Iteration 8
Iteration 9
Iteration 10
Fig. 10.25.: RMS misfit for isotropic 3D inversion models at different iteration steps of the second inversion sequence using the lowest
misfit model of the initial inversion sequence as starting model; i.e. the model obtained from the fifth iteration with a halfspace starting
model and a model roughness of 10 (cf. Fig. 10.23). The RMS misfits for the fifth iteration of the initial sequence are indicated by
markers connected with the dashed black line. The wsinv3d algorithm [Siripunvaraporn et al., 2005a] determined up to four models
at each iteration step and uses the lowest misfit model as starting model for the following inversions (cf. Sec. 6.3). The overall lowest
RMS misfit is obtained during the second inversion sequence after eight iterations (purple markers). Results are plotted in terms of
RMS misfit versus roughness 6 for all models at each iteration step.
displaying individual misfits for each station are moved to the Appendix (Sec. A.4.2).
It is possible that the model of the eighth iteration step represents a local, rather than the
global, minimum (cf. Sec. 6.3), but owing to the considerably low misfit of this model
the second inversion sequence is terminated after ten iterations (resulting in a total of 15
iteration steps).
Results of the second 3D inversion sequence are presented in terms of the lowest misfit
model with the highest model roughness (RMS = 1.50, 1/τ = 10), the model with the
lowest misfit for a slightly lower roughness (RMS = 1.59, 1/τ = 3.16), and the lowest
roughness model (RMS = 1.75, 1/τ = 0.32); labelled ‘a’, ‘b’, and ‘c’ in Figure 10.27,
respectively. The three models exhibit a strong mutual similarity, i.e. all models comprise
a relatively conductive region ≤100 Ωm in the centre of the PICASSO Phase I profile at
a depth range associated with the lithospheric-mantle beneath the Tajo Basin (i.e. 30 –
110 km). For the Earth’s lithospheric-mantle usually electric resistivities in the range of
1000 Ωm or greater are determined (cf. Sec. 5.2.2). A reduction in the order of one magnitude
in electric resistivity implies an unusual geological setting of the lithospheric-mantle,
e.g. significantly increased temperatures or well-connected networks of conducting constituents
(cf. Sec. 5.1).
Ten additional iterations are carried out to ensure that the relatively low electric resistivity
of the lithospheric-mantle in the centre of the profile is not the result of smoothing
constraints used during the inversion process in combination with low sensitivity of the
PICASSO Phase I dataset to the lithospheric-mantle. Therein, a layered a priori model
containing a 100 Ωm crust (≤31 km), a 1000 Ωm lithospheric-mantle (31 – 110 km), and
100 Ωm sublithospheric mantle is used, which enforces a more resistive lithosphericmantle.
Inversion models obtained using the layered a priori model exhibit a more re-
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