Models of Deep Faults - Models and Methods of Magnetotellurics - page 329

lithosphere had not been sufficiently dense, the regional palette of the magnetotel-
luric data would been consisted of the uniform bell-type apparent-resistivity curves
with verticall scattering of their mantle branches due to the near-surface S - effect.
It can be said without exaggeration that in many regions magnetotellurics owes its
information on crustal and mantle conductivity to the deep conductive faults cross-
ing the resistive lithosphere.

9.3 Current Channeling in Conductive Faults

The notion of the current channeling (conductive channeling) has been introduced
into magnetotellurics by Weidelt (1977). He used the thin sheet approximation and
examined a plane-wave model consisting of the upper inhomogeneous layer ( 1 ) and

the homogeneous two-layered substratum ( 2 = 1000 Ohm · m , h 2 = 250 km , 3 =

50 Ohm · m). The upper layer contains the homogeneous background of conductance

S n = 4000 S .

Figure 9.5 shows how the total and anomalous currents flow in the upper layer. It is
seen that the density of the anomalous current is significantly enhanced within the
conductive channel. This concentration effect received the name current channeling .
The current channeling is defined as concentration of induced current in highly con-
ductive elongated structures. The main idea in this definition is that currents induced
in a large remote area flow via conductive channels into domain under investigation
and generate here an anomalous magnetic field which has little in common with
the normal magnetic field representative for that domain (Weidelt, 1977). The early
works on the current channeling effect had been reviewed by Jones (1983). The
Simpson and Bahr monograph (2005) provides a present look on this effect.
It is generally taken that the current channeling effect is of a three-dimensional
nature and requires a three-dimensional treatment. But here we would like to note

500 S and a highly conductive cranked channel of conductance S =

Fig. 9.5 The Weidelt model of current channeling: systems of total and anomalous currents in the
layer with a cranked conductive inclusion, T = 1800 s