P. Schmoldt, PhD - MTNet - DIAS

4. Distortion of magnetotelluric data
10300 km
10300 km
28000 km
x y
Depth slice
3D body Dyke
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10000 km 10600 km
z y
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5 km
5 km
Map view
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x y
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Not to scale
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5 km
Log 10(Resistivity) (Wm)
Fig. 4.13.: Two subsurface models used to illustrate the effect of frequency-dependent dimensionality, each containing two halfspaces
of 100 and 1000 Ωm and a conductive distorter, i.e. a small-scale body (left-hand side) and a dyke structure (right-hand side). The two
models are identical in the y-z plane shown at the bottom of this figure. Responses for MT stations on top of the conductors (denoted
by inverted black triangle) are calculated to illustrate frequency-dependence of distortion; see text for details.
erated in the same manner, in order to compare the results of the 3D/2D model (‘Dyke’
in Fig. 4.13). The lateral extent of the local distorter is therein increased to the limits
of the model (>20000 km) in direction of the regional strike direction, making the body
virtually 2D. The models are certainly not an accurate representation of the true Earth,
in particularly their depth extends, but they are a useful tool to illustrate the connection
between observed dimensionality and frequency range.
The response for the MT station on top of the 3D/2D model (left-hand side in Fig. 4.13)
contains four different regimes, which can be related to different dimensional settings (cf.
Fig. 4.14). Going from short to long periods (left to right in Figure 4.14), the first regime,
comprising periods up to approximately 10 −2 s, is purely 1D. Responses in this period
range do not contain boundary effects and simply reflect the 10 Ωm conductivity of the
small-scale body. At longer periods, response curves exhibit a split, first in the phase data
followed by the apparent resistivity, indicating a 2D regime. The split in the period range
0.01−5 s is due to the lateral interface between the small-scale body and the host medium,
accordingly referred to as ‘local 2D’, as opposed to the ‘regional 2D’ for the interface
between the two quarter-spaces. For the period range 5 − 100 s, responses exhibit a static
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