P. Schmoldt, PhD - MTNet - DIAS

8.2. Generating synthetic 3D model data
Fig. 8.3.: Subsurface model with orthogonal geoelectric strike directions at crust and mantle depths. Constructed to derive an optimised
1D or 2D inversion approach that provides an optimal recovery of the resistivity distribution at mantle depth for such a case of oblique
strike directions in crust and mantle. The approximate location of the projected PICASSO Phase I profile is indicated by the dashed
line.
impedance values of each station, i.e. a variance of 10 −21 . This step is required to permit
subsequent calculations, which otherwise would fail due to attempted divisions by zero.
Performance of the different approaches for datasets with higher noise levels is studied at
a later stage of this investigation. From those files with minuscule uncertainty levels two
different types of datasets are created, i.e ‘rotated’ and ‘decomposed’, which are used for
anisotropic and isotropic inversion processes, respectively (see Sec. 8.3).
Creation of the first dataset type simply involves rotating of data from all stations to
N45W and N45E using a script by Xavier Garcia (personal communication, 2008). Note
that N45W and N45E represent the respective strike directions of crust and mantle, and
that TE and TM modes are swapped for datasets with a difference of rotation by 90 degrees.
For these rotated datasets, diagonal elements of the impedance tensor are in general
non-zero and are used for the novel anisotropic inversion approach.
The second type of data (decomposed) is created using the program strike by Mc-
Neice and Jones [2001], based on the theory by Groom and Bailey [1989] (Sec. 4.4.4),
commonly used to provide datasets suitable for isotropic 2D inversion. Therein, two
datasets are generated that are adequate for the strike directions at either crustal or mantle
depth, i.e. N45E and N45W, respectively. For these decomposed datasets, the diagonal
elements of the impedance tensor (Zxx and Zyy) are considered insignificant and are not
used during isotropic 2D inversion.
Prior to inversion of forward responses from the synthetic 3D model, data are analysed
to identify characteristics of the responses which help to understand applicability of the
different inversion approaches. First the response data are visualised using maps of four
different periods (periods are used as a depths proxy, cf. Section 6.3.1), in which North
is located towards the top left (Fig. 8.6); figures are rotated anticlockwise by 45 degrees
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