P. Schmoldt, PhD - MTNet - DIAS

4. Distortion of magnetotelluric data
Fig. 4.9.: The two types of electric anisotropy ρx = ρz ρy and ρx ρz = ρy refer to two different geological settings, i.e. anisotropic
sheets (e.g. dyke swarms) and anisotropic tubes (e.g. ore veins).
Fig. 4.10.: Illustration of basic anisotropy parameters; from Pek and Santos [2002]
distortion (Z ani
2D ) can be written as product of the undisturbed 2D impedance tensor (Z2D)
and a tensor containing the anisotropic contribution (A)
Z ani
2D = A · Z2D = (1 + S 2 D) −1/2 1+S D
Thus, the off-diagonal elements of (Z ani
2D )
Z ani 1 + S D
xy = Zxy √
1 + S D
0
0 1−S D
0 Zxy
Zyx 0 . (4.11)
> Zxy, (4.12)
and
Z ani 1 − S D
yx = Zyx √ < Zyx, (4.13)
1 + S D
are always greater for Zxy and smaller for Zyx since S D is a positive, real number (cf. Sec.
4.3).
Given a sufficiently large array of MT recording station it is possible to investigate
whether the subsurface comprises either regional 2D structures or a 1D subsurface with
anisotropic structures. Whereas for the 2D case TE and TM modes exhibit significant
variation for stations at different distances to the location of the conductivity interface
(cf. Fig. 3.4), the horizontal transfer function and the phase split are constant over a
large horizontal region in case of a anisotropic 1D subsurface. Moreover, it is in principle
possible to distinguish between a 2D subsurface and a 1D subsurface with anisotropy by
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