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those in the three-segment model (Fig. 7.25). Over the graben we observe an
abrupt drop in
E
y
, though without side minima associated with the current rear-
rangement effect. Here
E
y
<<
E
x
E
y
/
E
x
/
at
T
≥
100 s. It means that with
an isotropic normal field (
E
x
E
y
) the low frequency electric field is polarized
quasi-linearly along the graben (the channeling effect). Another characteristic fea-
ture of the model is a strong horizontal skin effect, which concentrates longitudinal
currents at the graben boundaries. It becomes distinctly apparent in side maxima of
H
y
(
T
=
10 s).
Finally we examine conditions favorable to the two-dimensional approxima-
tion of the three-dimensional graben shown in Fig. 7.49. Let us consider a set
of three-dimensional models with fixed parameters
=
1
,
1
=
10 Ohm
·
m
,
h
1
=
10
3
Ohm
0
.
3km
,
h
=
1
.
7km
,
2
=
·
m
,
h
2
=
99
.
7km
,
3
=
10 Ohm
·
m and variable
parameters
v
=
15 km
,
l
=
30
,
150
,
300 km;
v
=
30 km
,
l
=
60
,
300
,
600 km.
As in the case of the horst, we use the elongation
e
as an indicator
of quasi-two-dimensionality. Figure 7.50 demonstrates the three-dimensional and
two-dimensional (
l
=
l
/
2
v
=∞
) apparent-resistivity, impedance-phase and tipper curves
along a central
y
−
profile going across a graben 60 km wide and 60 km long
(
e
=
1). It is remarkable that the transverse curves for
yx
(3D)
,
yx
(3D), observed
over the graben edge (
y
=
29
,
31 km) and outside the graben (
y
=
40 km)
are close to the curves for
,
yx
(2D) and can be treated as quasi-two-
dimensional. At the same time the longitudinal curves for
yx
(2D)
xy
(3D)
,
xy
(3D) and
the tipper curves for Re
W
zy
(3D)
,
Im
W
zy
(3D) differ noticeably from the curves
for
Im
W
zy
(2D). One can arrive to conclu-
sion that the TM-mode is more robust to the current-gathering effect than the
TE-mode.
The current-gathering effect rather quickly attenuates as the graben elonga-
tion
e
increases. Figure 7.51 demonstrates the apparent-resistivity, impedance-
phase and tipper curves for a graben 60 km wide and 600 km long (
e
xy
(2D)
,
xy
(2D) and Re
W
zy
(2D)
,
=
10).
Now the three-dimensional curves for
xy
(3D)
,
yx
(3D)
,
xy
(3D)
,
yx
(3D) and
Fig. 7.45
Model of the
two-dimensional graben
