P. Schmoldt, PhD - MTNet - DIAS

4. Distortion of magnetotelluric data
Fig. 4.6.: Distortion of magnetotelluric (MT) responses due to a highly conductive feature at depth, causing resistivities of the TM
mode (solid squares) at periods greater than 0.2 s to decrease rapidly, followed by an adjustment to the values of the TE mode (open
squares) for longer periods. The drop is caused by a deviation of electric currents into direction of the conductor, decreasing the
electric field of the respective mode. This particular form of distortion is occasionally referred to as birdy, owing to the distorted
mode’s similarity in shape with an infantile drawing of a flying bird.
diagonal elements are increased accordingly. The degree of distortion on each mode,
caused by the current channelling effect, is therefore dependent on the orientation of the
distorter relative to the regional geoelectric strike direction. When the orientation of the
conducting distorter is close to the direction of one coordinate-system axis, i.e. either in
strike direction or orthogonal to it, the apparent resistivity of the respective mode will
exhibit a rapid drop in resistivity. For longer periods, the distorted mode will then, due
to mode mixing, approach the apparent resistivity level of the orthogonal mode. This behaviour
results in a very peculiar shape of the related mode, occasionally referred to as a
birdy owing to its similarity in shape with an infantile drawing of a flying bird (cf. solid
symbols in Figure 4.6).
In order to provide a quantitative description of the current channelling phenomenon
Edwards and Nabighian [1981] considered the ratio of current flowing through a conducting
dyke Jd to the total available current Jt, using the equation
Jd
Jt
= α
, (4.5)
1 + α
where α is the current channelling number. The authors examine the case of a dyke
with vertical extent h, infinitive lateral extent, and conductivity σd in a resistive host
medium with conductivity σh. For controlled source measurements α can be calculated
in dependence of the angular frequency ω and the distance down strike of the source L
when ωh