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Fig. 7.51 Apparent-resistivity, impedance-phase and tipper curves in the model of a three-
dimensional graben shown in Fig. 7.49. Model parameters:
1
=
10 Ohm
·
m
,
h 1
=
0
.
3km
,
10 3
h
=
1
.
7km
,v =
30 km
,
l
=
600 km
, 2
=
Ohm
·
m
,
h 2
=
99
.
7km
, 3
=
10 Ohm
·
m
Re W zy (3D)
,
Im W zy (3D)
virtually
merge
with
the
two-dimensional
curves
for
(2D)
, (2D),
(2D)
, (2D) and Re W zy (2D)
,
Im W zy (2D). Thus, the condition
e
10 confidently provides quasi-two-dimensionality of the graben 60 km wide.
The same condition is found for the graben 30 km wide.
Let us compare the condition e
10 with conditions obtained in the elliptic-
cylinder model with equivalent contrast of conductances: m
S 1 /
S 1
=
=
( h 1
+
h )
/
h 1 =
6
.
7. Using estimates given by (7.132) for 10%-difference between
A (3D)
A (2D) , we get e xy
e yx
interval and e xy
e yx
and
13
.
1
,
8inthe S 1
107
.
3
,
16
interval. We see that the quasi-two-dimensionality in the graben
model may be provided by considerably smaller elongation than in the elliptic-
cylinder model. This can be accounted for by dominating role of the currents flowing
into the graben from above.
.
8inthe h
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