P. Schmoldt, PhD - MTNet - DIAS

A.2.2. Oblique strike intricacy
A.2. Auxiliary information regarding inversion processes
The problematic in using locally true resistivity values stems from the oblique geoelectric
strike direction at crust and mantle depths. For any point on the model, a projection of
structures along the mantle strike direction result in a homogeneous halfspace, due to
orthogonality of the strike direction in the two depth regions. To illustrate this point a
gedankenexperiment is conducted here, using Figure A.3 to illustrate the steps within:
consider a projection of recording stations (grey) onto a profile orthogonal to the crustal
strike direction (black) as shown in plot ‘a’ of Figure A.3. For that profile an inversion for
crustal structures can be carried out using short-period data of the projected stations. The
inversion result shown in plot ‘b’ of Figure A.3 comprises a very simple 2D subsurface
model for the crust (more complex models are certainly possible, but this model is chosen
to illustrate the issues of oblique strike direction even for a very simple subsurface case).
Plot ‘c’ in Figure A.3 displays the same model together with the locations of recording
sites and projected stations in 3D view for clearness. Plot ‘d’ in Figure A.3 illustrates
the projection of the recording stations onto four profiles that are orthogonal to the strike
direction at mantle depth (any other profile parallel to the the profiles 1 to 4 is valid as well,
but for the sake of simplicity the display is limited to those four profiles). The location
of the four profiles orthogonal to the mantle strike are shown on top of the crustal model
in plot ‘d’ in Figure A.3 with dashed-dotted lines indicating the strike direction at mantle
depths. Each of the four projected profiles is located on top of a crustal region that is only
valid for one of the stations (grey-coloured station in plot ‘e’ in Figure A.3). Certainly,
the true subsurface between each station is defined by the crustal model, but the effects
of vertical interfaces on the TE and TM mode are interchanged due to the swapping of
transverse and parallel components at crustal and mantle depth.
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