P. Schmoldt, PhD - MTNet - DIAS

a (horizontal smoothing)
10. Data inversion
Period range: 10
5.0
4.5
a5b1t3
-3 (a) – 10 s
(b)
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
a3b1t6
a2b1t310
0 1 2 3 4 5 6 7 8 9 10
t (global smoothness)
a (horizontal smoothing)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
Period range: 10 -3 – 100 s
a5b1t3
a3b1t6
a2b1t310
0 1 2 3 4 5 6 7 8 9 10
t (global smoothness)
RMS
misfit
6.0
Fig. 10.1.: RMS misfit for models of the Tajo Basin crustal structures with different combinations of global (τ) and horizontal smoothing
parameters (α), using (a) a period range of 10 −3 −− 10 s and (b) of 10 −3 −− 100 s. Values are obtained through linear interpolation
of 42 α,τ-combinations for each frequency range with applied parameters summarised in Table 10.1. Red-yellow markers, together
with the respective model name, indicate combinations chosen for further investigations.
These six models (three for each period range) are kept for further investigation in subsequent
inversion steps.
10.1.2. Starting model construction
Results of non-stochastic MT inversion are, in general, dependent on the starting model.
For an unsuitable starting model the inversion may yield a model related to a local, rather
than the global, misfit minimum (cf. Sec. 6.3). A uniform halfspace, often 100 Ωm,
is commonly used as a starting model for MT inversion as it is assumed firstly to be
less likely to introduce inversion artefacts, and secondly to be relatively close to average
resistivity values of the Earth at crustal and upper mantle depths (cf. Sec. 5.2). However,
using a priori information about the subsurface, e.g. from 1D inversion of MT data or
results of other geophysical or geological investigations, an enhanced starting model can
(and should be) created that is superior to the halfspace model approach. For the Tajo
Basin, supplementary information about the subsurface is available, in particular, from
seismic reflection and refraction studies, inferring four layers within the crust and a Moho
depth of approximately 31 km (see Table 7.2 for thickness of crustal layers). Presently,
there is no reliable relation between seismic velocity and electric resistivity at crustal
depth that allows for a straightforward construction of a MT starting model from seismic
velocity results. Instead, electric resistivity values for the four crustal layers are inferred
from horizontally averaged resistivities of the six models chosen during the smoothing
parameter analysis (Sec. 6.3.3) using an in-house script developed by Mark Muller. The
script yields mean and standard deviation resistivity values for each row of the model
within the specified horizontal region of the profile.
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5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0