P. Schmoldt, PhD - MTNet - DIAS

RMS−misfit
5
4
3
2
0.1 1 10 100
Roughness(1/τ)
10.2. Inversion for mantle structures
Solid lines:
halfspace
Dashed lines:
layered model
Iteration 1
Iteration 2
Iteration 3
Iteration 4
Iteration 5
Fig. 10.23.: RMS misfit for isotropic 3D inversion models at different iteration steps (colour-coded) using a 100 Ωm halfspace and a
layered arrangement as starting model (solid and dashed lines, respectively); see text for details about the layered starting model. This
initial inversion sequence only involves five iteration steps for each starting model. Results are plotted in terms of RMS misfit versus
roughness 6 for all models at each iteration step. The wsinv3d algorithm [Siripunvaraporn et al., 2005a] yields up to four models at
each iteration step and uses the lowest misfit model as starting model for the following inversion step (cf. Sec. 6.3).
and smoothness (i.e. inverse of model roughness) is observable for a higher number of
iterations, i.e. iteration 5 (models with higher roughness and misfit represent inferior
results). Inversion models of the fifth iteration step exhibit RMS misfits in the range
2.0 – 2.5 (halfspace starting model) and 2.3 – 2.7 (layered starting model); thus, model
responses exhibit an intermediate degree of similarity with the observed data. Despite the
increased misfit of these models (owing to the limited number of iteration steps), results
can be used to examine characteristics of the initial inversion sequence for both starting
models.
For sake of clarity and compactness, illustration is limited here to models from the fifth
iteration, which yield the respective lowest RMS misfit for both starting models (cf. Fig.
10.23). Herein, results are compared using a series of horizontal and vertical slices for
the two models (Fig. 10.24). The most striking difference between models using either a
halfspace or a layered starting model (labelled ‘a’ and ‘b’ in Figure 10.24, respectively)
is the electric resistivity of the region associated with the lithospheric-mantle beneath the
Tajo Basin (at depths between 30 km and 110 km). Inversion models exhibit a strong
correlation in the lithospheric-mantle with their respective starting models, i.e. 100 Ωm
and 1000 Ωm, respectively. Thus indicating that these deeper regions are not strongly
constraint by the data or not well fit by the model at the current inversion step.
Despite the small number of inversion steps, derived isotropic 3D inversion models can
be used to infer features of the Tajo Basin subsurface that are supported by data of the
PICASSO Phase I project. At crustal depths (≤ 30 km), models exhibit some degree of
similarity with each other as well as with the focussed isotropic 2D inversion for crustal
structures (cf. Sec. 10.1.5). However, due to increased cell size and small number of
iteration, some of the features are not as well established as in the focussed inversion. For
example, the extensive near-surface conductor that is modelled in the focussed inversion
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