P. Schmoldt, PhD - MTNet - DIAS

J
s 0
s 0
4.1. Types of distortion
Fig. 4.5.: The principle of current channelling, denoting a deflection of the Electric current J by a highly conductive anomaly σ1 in a
more resistive surrounding medium σ0; cf. Figure 4.3.
• information about the resistivity structure of the subsurface is known from other
methods like transient or time domain EM (TEM) [e.g. Andrieux and Wightman,
1984; Sternberg et al., 1988; Pellerin and Hohmann, 1990] or Geomagnetic Deep
Sounding (GDS) [e.g. Wolfgram and Scharberth, 1986],
• statistical approaches are used [e.g. Jones, 1988, and references within], or
• distorting bodies are incorporated in the inversion process.
Magnetic galvanic distortion
The magnetic galvanic distortion effect is frequency dependent, and the decay of its magnitude
is approximately proportional to √ T [Chave and Smith, 1994]. The magnetic
galvanic distortion effect of a surficial body can therefore be considered negligible if the
period is long compared to the depth extend of the distorting body [e.g. Groom and Bailey,
1989]. However, further magnetic galvanic effects can be caused by additional distorting
bodies in the subsurface, affecting the frequency range that is related to the body’s location.
Naturally, the magnetic galvanic effect of such bodies deteriorates in a similar way
and becomes negligible again for periods that are comparatively long relatively to the dimension
of these bodies [e.g. Garcia and Jones, 2001]. Multiple subsurface distorters at
different depth ranges will therefore result in several magnetic galvanic distortion effects
that are mostly confined to data of the period range related to the location of each distorter.
Current channelling
A special case of galvanic distortion is the channelling of electric currents into a 3D body
of higher conductivity, following the orientation of the body, which provides the path of
least resistance (Fig. 4.5). Even though this situation can be adequately described by
electric and magnetic galvanic distortion, it is addressed in a separate paragraph owing
to its distinct emergence in MT recordings. A thorough overview about the problem of
current channelling was given by Jones [1983a].
Current channelling due to an elongated 3D distorter deflects parts of the electric current
into a direction parallel to the orientation of the conductor, thus causing a deviation
from the orthogonality of magnetic and electric field. Therefore, off-diagonal elements
of the impedance tensor (Eq. 3.34), related to TE and TM mode, are decreased, whereas
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