P. Schmoldt, PhD - MTNet - DIAS

8. Recovering a synthetic 3D subsurface model using lower-dimensional inversion schemes
inversion is reasonable fast; for most profiles, inversions of one sequence is carried out
in under four hours using one processor of an Intel Xeon CPU X5680 dual core machine
with 3.33 GHz for a mesh with 108×146 cells and 1600 data points (product of number
of stations, number of period estimates, and impedance tensor elements).
The ai2D inversion algorithm [Baba et al., 2006] yields electrical resistivity models for
the direction parallel and orthogonal to the profile, i.e. ρxx and ρyy. Thus, no rearrangement
of data vectors for the different cells of the model (like in the case of anisotropic 1D
inversion) is required. The ρxx model can be used to recover the resistivity distribution of
the region that is not in agreement with the assumption of isotropic strike direction; i.e.
the mantle in approach 1 and the crust in approach 2 (cf. Fig. 8.15). Results of the ρyy
model are therein shown for comparison.
The second approach currently suffers unfortunately from a systematic problem. Longperiod
data, sensing the mantle region, are affected by the resistivity distribution of regions
above. Hence, results obtained in step one are biased and, even though crustal
structures can be recovered to some degree using anisotropic inversion during step 2,
mantle structures remain erroneous (cf. Fig. A.11). Subsequent isotropic inversion of
the mantle (in a third inversion sequence) destroys the anisotropic crustal structures due
to the inherent isotropy constraints. An anisotropic inversion in the second sequence, on
the other hand, contradicts the anisotropic inversion approach by introducing anisotropic
features to the mantle region. For a successful application of the second anisotropic inversion
approach ‘anisotropy zone’ assignment would be required, but this is not yet
implemented as already noted in the previous section. As a result, realisation of approach
2 has to be postponed for the time being. This is unfortunate, since approach 2 is likely to
yield superior inversion results for the mantle given its isotropic (instead of anisotropic)
inversion of mantle range using the true mantle strike direction. It is therefore strongly
recommended that performance of the second approach is thoroughly investigated, once
anisotropy-zones are implemented in the inversion code
Approach 1 does not suffer from the lack of anisotropy-zones, because the isotropic
inversion of shorter periods is conducted prior to the anisotropic inversion of long-period
data. Fixing the crustal structures at their isotropic values does not impede anisotropic
inversion in the secondary sequence and approach 1 yields ρxx inversion models that exhibit
resistivity distributions similar to the true model (cf. Fig. 8.17). Crustal structures
are recovered reasonable well for both anisotropy directions (ρxx and ρyy) and in the ρxx
model the resistivity interface at mantle depth is considerably well resolved. The ρxx
model exhibits a distinct lateral change from intermediate resistivity values in the south
of the profile to high resistivity values in the north, whereas the ρyy model contains a less
distinct lateral change. The change of electric resistivity is facilitated through a changing
degree of anisotropy magnitude (ρAA plot in Figure 8.17).
Exceedingly high values of the northern mantle region as well as smooth variation in
anisotropy magnitude, hence the less distinct lateral interface in the ρxx model, are due to
smoothness constraints of the inversion process (cf. Sec. 6.3). The agreement of ρxx inversion
models with the synthetic 3D model can be enhanced by choosing a lower smoothing
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