P. Schmoldt, PhD - MTNet - DIAS

4. Distortion of magnetotelluric data
Fig. 4.2.: The effects of a small-scale 3D body onto the modes of the magnetotelluric (MT) measurement depend on the orientation
of the 3D body with respect to the 2D regional structure. (a) 3D structure normal to regional structure: TE mode is affected mainly
by galvanic effect, TM mode is affected by galvanic and inductive effects. (b) 3D structure parallel to regional structure: TE mode is
affected by galvanic and inductive effects, TM mode is affected mainly by galvanic effect; after [Ledo, 2005].
4.1.1. Galvanic distortion
The principle of galvanic distortion can be illustrated using a subsurface model with lateral
change of electric conductivity, e.g. two quarter-spaces with different conductivity
along a vertical interface (cf. Fig. 3.4). Applying an electric field with a component
orthogonal to the conductivity interface yields a charge build-up on either side of the interface
(with different signs). The charge build-up affects electric current and electric field
in the related region, whereas the magnetic field remains comparatively undistorted (cf.
Eqs. 4.2, 4.3) [Price, 1973; Kaufman, 1985, e.g.].
Distortion of the EM field by a small-scale body in an otherwise homogeneous region
is dependent on the resistivity difference between the body and the surrounding region.
A body which is more conductive than the background acts as an attractor for electric
currents, causing the electric field lines to curve towards the inclusion, whereas a more
resistive body forces the field lines to bend away from its position (cf. Figure 4.3). This
behaviour can be adequately described by the concept of vectorial addition of primary
fields with secondary fields generated by the charges at the boundaries, providing traditional
terminology like current channelling, flow around effect, etc. with mathematical
framework [Jiracek, 1990]. From Figure 4.3 and Equation 4.2, it is apparent that the total
electric field is enhanced above a resistive body and reduced over a conductive one. Therefore,
the respective TM mode of MT curves is shifted upwards when the electric field is
measured right on top of a surficial resistive body and downwards over a conductive body
[Jiracek, 1990], presuming that the points used for the voltage difference measurement
are not crossing the boundaries of the distorter. A quantitative analysis about the relationship
between the amplitude of static shift and the location of the current electrodes with
respect to the distorter is given by Pellerin and Hohmann [1990] and Spitzer [2006].
Berdichevsky and Dmitriev [1976a] and Ranganayaki and Madden [1990] introduce the
concept of adjustment distance Λ (also referred to as horizontal skin depth) in order to
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