P. Schmoldt, PhD - MTNet - DIAS
P. Schmoldt, PhD - MTNet - DIAS P. Schmoldt, PhD - MTNet - DIAS
10. Data inversion Depth (km) Depth (km) 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Anisotropy direction Anisotropy magnitude pic020 pic019 pic017 pic015 pic013 pic011 pic009 pic007 pic005 pic004 pic003 pic002 pic001 Degrees +80 r ||/r 8 +60 7 +40 +20 0 -20 -40 -60 -80 r model r || model pic020 pic019 pic017 pic015 pic013 pic011 pic009 pic007 pic005 pic004 pic003 pic002 pic001 Log10(Wm) Log10(Wm) 4.0 4.0 3.5 3.0 2.5 2.0 1.5 1.0 pic020 pic019 pic017 pic015 pic013 pic011 pic009 pic007 pic005 pic004 pic003 pic002 pic001 pic020 pic019 pic017 pic015 pic013 pic011 pic009 pic007 pic005 pic004 pic003 pic002 pic001 Fig. 10.17.: Results of anisotropic 1D inversion for Tajo Basin subsurface structures using the ai1D algorithm by Pek and Santos [2002]. Top-left: anisotropic strike direction, i.e. direction of maximum electric resistivity; top-right: magnitude of electric resistivity, i.e. ratio of maximum and minimum electric resistivity; bottom-left and bottom-right: electric resistivities perpendicular ρ⊥ and parallel ρ to the mantle strike direction, respectively; see text for details. impedance data aims to yield a dataset free from distortion effects that are suitable for 2D inversion; thus, this may provide a dataset more adequate for the anisotropic 1D inversion approach. Decomposition is carried out using the strike program by McNeice and Jones [2001] and a strike direction of N29.4E; related inversion results are shown in Figure 10.18. Evidently, the anisotropic 1D inversion of the decomposed dataset does not result in a more plausible subsurface model than for the undecomposed dataset. It is therefore concluded that anisotropic 1D inversion is not suitable for recovering complex structures like those observed for the heterogeneous Tajo Basin subsurface, thereby confirming prognoses made during the synthetic 3D model study (Sec. 8.3.2). Anisotropic 2D inversion may yield superior results for these more complex subsurface structures, since therein no inherent 1D assumption for the subsurface are made. 10.2.3. Anisotropic 2D inversion Due to implausible results of isotropic 2D and anisotropic 1D inversion in recovering the Tajo Basin subsurface, the anisotropic 2D inversion approach is applied, which showed satisfactory results for the synthetic model study (cf. Chap. 8). Due to current limitations of the MT2Dinv algorithm [Baba et al., 2006], an enhanced anisotropic version of the algorithm by Rodi and Mackie [2001], only the first approach is use. The first ap- 256 6 5 4 3 2 1 3.5 3.0 2.5 2.0 1.5 1.0
Depth (km) Depth (km) 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Anisotropy direction Anisotropy magnitude pic020 pic019 pic017 pic015 pic013 pic011 pic009 pic007 pic005 pic004 pic003 pic002 pic001 +40 +20 0 -20 -40 -60 -80 10.2. Inversion for mantle structures Degrees +80 r ||/r 8 +60 7 r model r || model pic020 pic019 pic017 pic015 pic013 pic011 pic009 pic007 pic005 pic004 pic003 pic002 pic001 Log10(Wm) Log10(Wm) 4.0 4.0 3.5 3.0 2.5 2.0 1.5 1.0 pic020 pic019 pic017 pic015 pic013 pic011 pic009 pic007 pic005 pic004 pic003 pic002 pic001 pic020 pic019 pic017 pic015 pic013 pic011 pic009 pic007 pic005 pic004 pic003 pic002 pic001 Fig. 10.18.: Results of anisotropic 1D inversion for Tajo Basin subsurface structures using the ai1D algorithm by Pek and Santos [2002] and a dataset decomposed according to the strike direction of the mantle (N29.4E) using the strike program by McNeice and Jones [2001]. Top-left: anisotropic strike direction, i.e. direction of maximum electric resistivity; top-right: magnitude of electric resistivity, i.e. ratio of maximum and minimum electric resistivity; bottom-left and bottom-right: electric resistivities perpendicular ρ⊥ and parallel ρ to the mantle strike direction, respectively; see text for details. proach comprises initial isotropic 2D inversion for the crustal structures and subsequent anisotropic 2D representation of the mantle; a thorough description of the approaches is given in Section 8.3.3. Since the first anisotropic 2D inversion approach uses data that are rotated according to the strike direction of the crust (N40.9W), the previously obtained isotropic 2D inversion results for crustal structures (Sec. 10.1.5) can be used for the first sequence. The starting model for the second sequence contains the crustal inversion model shown in Figure 10.6 and a mantle which consists of either a 100 Ωm halfspace or a 100 Ωm lithospheric-mantle and a 10 Ωm asthenosphere (cf. Fig. 10.19). In consideration of isotropic 2D inversion results (Sec. 10.2.1) as well as findings of collocated studies presented in Section 7.3.2, the lower boundary of the resistive lithospheric-mantle region in the second starting model is set to 110 km. Crustal structures are kept fixed and only long-period data are used for the anisotropic 2D inversion in order to focus the inversion onto the mantle region. Different combinations of smoothing parameters (α, β, and τ) as well as electric resistivity gradient and Laplacian regularisation (cf. Sec. 6.3) are used for the inversion process with an inversion sequence according to the Jones Catechism (Sec. A.2.3). Inversion models exhibit different subsurface structures dependent on the combination of constraints, because improper regularisation can dominate the objective function (cf. Sec. 6 5 4 3 2 1 3.5 3.0 2.5 2.0 1.5 1.0 257
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10. Data inversion<br />
Depth (km)<br />
Depth (km)<br />
0<br />
50<br />
100<br />
150<br />
200<br />
250<br />
300<br />
0<br />
50<br />
100<br />
150<br />
200<br />
250<br />
300<br />
Anisotropy direction Anisotropy magnitude<br />
pic020<br />
pic019<br />
pic017<br />
pic015<br />
pic013<br />
pic011<br />
pic009<br />
pic007<br />
pic005<br />
pic004<br />
pic003<br />
pic002<br />
pic001<br />
Degrees<br />
+80<br />
r ||/r 8<br />
+60<br />
7<br />
+40<br />
+20<br />
0<br />
-20<br />
-40<br />
-60<br />
-80<br />
r model r || model<br />
pic020<br />
pic019<br />
pic017<br />
pic015<br />
pic013<br />
pic011<br />
pic009<br />
pic007<br />
pic005<br />
pic004<br />
pic003<br />
pic002<br />
pic001<br />
Log10(Wm) Log10(Wm) 4.0<br />
4.0<br />
3.5<br />
3.0<br />
2.5<br />
2.0<br />
1.5<br />
1.0<br />
pic020<br />
pic019<br />
pic017<br />
pic015<br />
pic013<br />
pic011<br />
pic009<br />
pic007<br />
pic005<br />
pic004<br />
pic003<br />
pic002<br />
pic001<br />
pic020<br />
pic019<br />
pic017<br />
pic015<br />
pic013<br />
pic011<br />
pic009<br />
pic007<br />
pic005<br />
pic004<br />
pic003<br />
pic002<br />
pic001<br />
Fig. 10.17.: Results of anisotropic 1D inversion for Tajo Basin subsurface structures using the ai1D algorithm by Pek and Santos<br />
[2002]. Top-left: anisotropic strike direction, i.e. direction of maximum electric resistivity; top-right: magnitude of electric resistivity,<br />
i.e. ratio of maximum and minimum electric resistivity; bottom-left and bottom-right: electric resistivities perpendicular ρ⊥ and<br />
parallel ρ to the mantle strike direction, respectively; see text for details.<br />
impedance data aims to yield a dataset free from distortion effects that are suitable for<br />
2D inversion; thus, this may provide a dataset more adequate for the anisotropic 1D inversion<br />
approach. Decomposition is carried out using the strike program by McNeice<br />
and Jones [2001] and a strike direction of N29.4E; related inversion results are shown<br />
in Figure 10.18. Evidently, the anisotropic 1D inversion of the decomposed dataset does<br />
not result in a more plausible subsurface model than for the undecomposed dataset. It is<br />
therefore concluded that anisotropic 1D inversion is not suitable for recovering complex<br />
structures like those observed for the heterogeneous Tajo Basin subsurface, thereby confirming<br />
prognoses made during the synthetic 3D model study (Sec. 8.3.2). Anisotropic<br />
2D inversion may yield superior results for these more complex subsurface structures,<br />
since therein no inherent 1D assumption for the subsurface are made.<br />
10.2.3. Anisotropic 2D inversion<br />
Due to implausible results of isotropic 2D and anisotropic 1D inversion in recovering the<br />
Tajo Basin subsurface, the anisotropic 2D inversion approach is applied, which showed<br />
satisfactory results for the synthetic model study (cf. Chap. 8). Due to current limitations<br />
of the MT2Dinv algorithm [Baba et al., 2006], an enhanced anisotropic version of<br />
the algorithm by Rodi and Mackie [2001], only the first approach is use. The first ap-<br />
256<br />
6<br />
5<br />
4<br />
3<br />
2<br />
1<br />
3.5<br />
3.0<br />
2.5<br />
2.0<br />
1.5<br />
1.0
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