P. Schmoldt, PhD - MTNet - DIAS

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04-centre profile
The profile 04-centre runs parallel to the crustal strike direction and is located
on top of the more conductive crustal region (50 Ωm). The profile
contains stations synD04 – synM04 (see Fig. 8.4), which are decomposed
according to a geoelectric strike direction N45E. The inversion result for
the 04-centre profile features two regions of increased electric conductivity
located just below the crust mantle boundary (see Fig. A.5). Hence,
the erroneous features are likely to originate from the fixing of the crust at
100 Ωm in combination with the effect of different electric strike direction at
crust and mantle depth. The inversion model does not exhibit a continuous
sharp lateral change in resistivity that can be associated with the interface.
Instead, the model contains a highly resistive region in the northwestern to
central area at a depth between approximately 50 km and 200 km. Given
the relatively low RMS misfit of the model (2.42 with a 5% error floor for
phases and 10% error floor for apparent resistivities), an investigator might
erroneously infer a feature at one of these depth regions, e.g. an electric
asthenosphere at the bottom of the resistor. Like for the 3D-mantle profile
(Fig. A.4), the misfit of the impedance phase is mostly constrained to
the periods range related to the crust mantle boundary (10 – 100 s), and the
misfit for the TE mode, is higher than for the TM mode. The TM mode
apparent resistivity misfit exhibits a noticeable striping for periods related
to the crust, which is most likely associated with incorrect decomposition
of the modes at this depth range. For the case of the 3D model used in
this study, with the 90 degree difference between strike directions at crust
and mantle depths, the two modes are swapped. TE and TM modes are
sensitive to current flow parallel to and charge build up on the face of vertical
interfaces with a direction orthogonal to the profile, respectively. Thus,
swapping of the modes introduces additional vertical interfaces, which are
prohibited here due to the fixing of the crust; however the conductive features
at the crust mantle boundary could be indicators of effects due to the
mode swapping.
Depth (km)
Profile: 04-centre
TE
Log 10(periods)
Log 10(periods)
RMS
TM
NW SE
0
50
100
150
200
250
synD04
synE04
Apparent resistivity
synF04
synG04
synH04
300
0 16 32 48 64 80 96 112 128 144
Distance (km)
log10 (Wm)
1 2 3 4 5
-2
-1
0
1
2
3
4
-2
-1
0
1
2
3
4
synD04
synE04
synF04
synG04
synH04
synI04
synJ04
synK04
synL04
synM04
Misfit
(Total = 2.42)
Log 10(Wm)
Log 10(Wm)
synI04
synJ04
synK04
Phase
synD04
synE04
synF04
synG04
synH04
synI04
synJ04
synK04
synL04
synM04
Fig. A.5.: Isotropic 2D inversion results for the ‘04-centre’ profile on top of
the synthetic 3D model (see Figure 8.4 for station locations). An electric resistivity
interface at mantle depth is located between stations synI04 and synH04.
Periods between 10 s and 100 s are related to the crust–mantle boundary; see
Section 8.2.1 for a description of the model. During the inversion the crust is
kept fixed at an electric resistivity value of 100 Ωm. The misfit of the uppermost
region originates from the problematic of meeting the cell size requirements
for the highest frequencies.
synL04
synM04
degrees
degrees
A.3. Auxiliary inversion results for the synthetic 3D subsurface model