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Fig. 8.7
Transverse and longitudinal magnetotelluric curves obtained at the epicentre of the
conductive zones of different half-width
v
. The mode is shown in Fig. 8.3. Model parameters:
,
2
=
2
=
h
2
=
h
2
=
1
=
10 Ohm
·
m
,
h
1
=
1km
1000 Ohm
·
m
,
19 km
,
15 km
,
c
=
10 Ohm
·
m
,
2
h
2
=
500 Ohm
·
m
,
=
65 km
,
3
=
10 Ohm
·
m
distorted by the deep
S
-effect. With further increasing
v
, the deep
S
-effect attenuates,
the
⊥
-curve is normalized, and at
v
=
850 km both the curves,
⊥
and
, coincide
with the locally normal curve ¨
n
. Note that in the model under consideration the
-curve allows for the one-dimensional inversion if the half-width
v
of the con-
h
2
ductive prism is 5 times larger than its depth
h
1
+
(see Fig. 8.3), whereas the
⊥
-curve is justified if the half-width
v
is 42.5
one-dimensional inversion of the
h
2
. This is pay for the screening effect and the deep
times larger than the depth
h
1
+
S
-effect.
Let us discuss these effects at greater length. It would be instructive to answer
two questions: (1) how does the galvanic-screening effect depend on resistivity
2
of the layer overlying the crustal conductive zone? (2) how does the deep
S
-effect
depend on resistivity
2
of the layer underlying the crustal conductive zone?
Figure 8.8 shows the transverse apparent-resistivity
⊥
-curves in the model from
2
Fig. 8.3 with half-width of the conductive prism
v
=
500 km and resistivity
of
the overlying layer varying from 1000 Ohm
m. The observation
site is located at the epicentre of the crustal conductive zone (
y
·
m to 100000 Ohm
·
=
0). The screen-
2
=
ing effect of the overlying layer with resistivity
1000 Ohm
·
m is rather slight.
⊥
-curve merges with
Here, in a wide range of high and medium frequencies, the
the locally normal ¨
n
-curve. It has a distinct minimum reflecting the conductive
prism. With increasing
2
this minimum is smoothed and at
2
=
100000 Ohm
·
m
⊥
-curve with no evidence of crustal conductor (complete
screening). Intuition suggests that the screening effect can be roughly estimated by
means of adjustment distance
d
we get the bell-shaped
S
1
R
2
, where
S
1
=
=
h
1
/
1
and
R
2
=
h
2
2
. When
