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.
phases,
This relation between the tipper and the longitudinal impedance is in
agreement with decomposition (4.79) suggested in (Zhang et al., 1993).
Next we will consider the electric and magnetic profiles related to the TE- and
TM-modes.
Figure 7.13 shows E x - and H y -profiles crossing the two-segment model. The
fields E x ,
H y are components of the TE-mode. They are normalized to the right
normal fields E N
x
H N
y
H N
y . At high frequencies the longitudinal electric field E x
rises steeply up from 0.1 E x ( T = 1 s) and from 0.22 E x ( T =15s) to E x . With low-
ering frequency this sharp transition monotonously flattens ( T =40
,
=
100 s). Areas
of observable changes in E x spread up to 400 km from the boundary between
the segments of different resistivity. The electric anomaly is accompanied by the
well-defined maxima and minima of the transverse magnetic field H y . These
extrema reflect the concentration and deconcentration of the electric currents at
the segment boundary. The higher frequency, the narrower the concentration and
deconcentration zones and the sharper the magnetic extrema. Here we again see
the horizontal skin effect. It clearly manifests itself in the range covered T =
1
÷
40 s and embraces the areas of about 400 km wide. At T > 250 s the skin effect
attenuates.
Figure 7.14 shows the E y -profile (the TM-mode). The transverse electric field
E y is normalized to the normal field E y . It has a jump on the boundary between
the left (conductive) and right (resistive) segments. Over the left segment we see
afallof E y , which is accounted for by rearrangement of the transverse current
due to different skin depths in the left and right segments and by current leakage
(the current meets the resistive segment and flows under it - let us recall an old
geophysical joke “the current is not a fool”). The effect of skin-depth difference is
characteristic of high frequencies ( T
÷
Its distortion zone does not exceed
1 km. At low frequencies the under-flow effect dominates ( T
=
0
.
15 s)
.
1500 s). We
can estimate its remote action by the adjustment distance d . Over the left segment
it is observed at distances numbered in the hundreds and even thousands of km
( d
=
15 s
,
3 d ). The distortion of the high-frequency field E y over the right segment is
rather weak ( T
1500 s), we
notice a pronounced anomaly of E y , which attenuates due to leakage of excess cur-
rent into the underlying medium. Here the distortion zone numbers in the hundreds
of km (1
=
0
.
15 s). But with lowering frequency ( T
=
15 s
,
5 d ).
Finally, we consider the W zy -profiles (Fig. 7.15). Here Re W zy is pos-
itive everywhere, while Im W zy changes the sign with frequency. The
Re W zy -profiles have a nearly symmetric maximum, whose form slightly depends
on frequency. The highest maximum of Re W zy is observed in the S 1 -interval
( T
.
10 s). The Im W zy -profiles show maxima or minima which alternate with
frequency. In the h -interval ( T
=
1000 s) tippers vanish. The real inductive arrows
are shown at the bottom of Fig. 7.15. They radiate from left to right, that is, from
higher to lower conductivities.
=
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