P. Schmoldt, PhD - MTNet - DIAS

3.5. The influence of electric permittivity
Fig. 3.4.: Behaviour of TE and TM mode for the same period in the presence of a conductivity contact zone, displaying a smoothly
varying continuous TE mode and a jump in the TM mode at the interface with the adjustment distance depends on the local resistivity
values.
3.4.2. Lateral interfaces
The behaviour of electric currents, and hence of electric and magnetic fields, in the presence
of lateral conductivity interfaces can be derived from Maxwell’s Equations (Sec.
3.1.1) and is summarised in Table 3.2; a comprehensive overview about lateral boundary
effects is given in Chapter 4 and here only the basic principles are illustrated. For MT it
is useful to consider EM fields in terms of their contribution to the TM and TE modes,
i.e. the combination of normal electric field EN and transverse magnetic field HT forming
the TM mode, and transverse electric ET and normal magnetic field HN forming the TE
mode.
For the sake of demonstration, two homogeneous quarter-spaces are considered here,
exhibiting different values of resistivity and are connected along an interface parallel to
the x-axis. In order to investigate the behaviour of the two modes, it is assumed that
MT data have been continuously collected along a profile parallel to the y-axis, crossing
the conductivity interface from the relatively conductive to the resistive side (Fig. 3.4).
Far away from the contact zone, in an inductive distance sense, both modes will simply
represent the conductivity of each quarter-space. For the TE mode the transition between
the two regions will be smooth since only EN is discontinuous on conductivity interfaces.
The TM mode, on the other hand, exhibits a jump at the interface caused by the deviation
of electric currents towards the interface on the resistive side and parallel to the interface
on the conductive side, making TM the favourable mode to detect lateral conductivity
changes. The adjustment distance, i.e. the distance from the interface where the effect of
the conductivity contact is comparably small, is dependent on period range and resistivity
for each quarter-space analogue to the vertical skin depth (Sec. 3.3).
3.5. The influence of electric permittivity
In MT, it is commonly assumed that the influence of electric permittivity is small in comparison
with the effect of electric conductivity, which is dominating the relationship be-
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