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30 km they virtually vanish. Let us take a look at their frequency
dependence. On a period
T
y
= |
y
| −
v
=
1s we observe a bell-like maximum of
E
x
and a
bowl-like minimum of
H
y
fringed with sharp side maxima. The minimum of
H
y
is caused by current deficiency within the central resistive segment, while the side
maxima reflect the concentration of longitudinal currents in the neighborhood of the
central segment (horizontal skin effect). Such events are characterized by the condi-
tion
h
eff
<<
2
=
(the effective depth penetration is much less than the width of central
segment). At low frequencies, when
h
eff
>>
2
v
v
, these anomalies flatten (
T
=
100s)
and vanish (
T
10000 s). This effect is referred to as
inductive flattening
. Quite
different picture is exhibited by the galvanic TM-anomaly. Over the central segment
we have a box-like maximum of
E
y
whose amplitude slightly depends on frequency.
Outside the central segment two types of galvanic anomalies are observed. At high
frequency (
T
=
1s) we have sharp side minima of
E
y
associated with rearrange-
ment of transverse currents due to different skin depth in the central segment and in
the bordering side segments. With lowering frequency these minima vanish and we
see a slow decrease of
E
y
caused by current leakage (transverse currents penetrate
into the underlying medium and flow under the resistive segment). In the
h
-interval
(
T
=
10000 s) this effect extends to distances about 1000 km.
The electric and magnetic profiles in a model with the conductive central segment
are presented in Fig. 7.25. First consider the inductive TE-anomalies. The central
segment manifests itself in a bowl-like minimum of
E
x
(
T
=
1
s
,
h
eff
<<
2
=
v
). At
the low frequencies this minimum flattens and vanishes (
T
=
100
−
10000
s
,
h
eff
>>
2
). A decrease in
E
x
is accompanied with an increase in
H
y
. Note that
at high frequencies the horizontal skin effect makes itself evident within the central
segment: the excess longitudinal currents are concentrated at its boundaries caus-
ing the sharp side maxima of
H
y
(
T
v
1s). At the low frequencies the excess
currents are distributed uniformly and the anomaly of
H
y
assumes the form of a
bell-like maximum (
T
=
10000
s
this maximum decays. The
anomalies of
E
x
and
H
y
attenuate rather quickly with distance. At the distances
=
100 s). At
T
=
>>
100 km they virtually vanish. Coming to the galvanic TM-
anomalies, we observe an abrupt drop in
E
y
over the central segment (current rear-
rangement effect, which shows up in sharp side minima). Note also that within the
y
= |
y
| −
v
central segment
E
y
<<
E
x
E
y
/
E
x
/
for
T
≥
100 s. If the normal field is isomet-
y
), then
E
y
<<
ric (
E
N
x
E
N
. The low-frequency electric field is quasilinearly
polarized along the conductive central segment, which serves as a current channel.
This effect looks like a
channeling effect
.
Figure 7.26 displays the
W
zy
-profiles. We see here the same zigzag anomalies of
Re
W
zy
and Im
W
zy
as in the dike model (compare Fig. 7.26 with Fig. 6.13). Note
that the real and imaginary tippers are intensive in the
S
1
-interval and they quickly
decay in going to the
h
-interval.
=
|
E
x
|
7.2.4 The Screening Effect
In closing we consider a case that highly resistive intermediate layer inhibits the
current rearrangement and impairs or even blocks the access to information on the
