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is the resistance of the crustal layers between the sediments and the conductive
zone. Reasoning from model estimates, we believe that a three-dimensional conduc-
tive zone with maximum horizontal diameter
D
max
manifests itself rather notably
if
D
max
>
10
7
Ohm
m
2
3
d
.Let
S
sed
=
100
S
and
R
crust
=
·
(conductive zone is over-
laid with a crustal layer of resistivity 1000 Ohm
·
m and thickness 10 km). Then
d
100 km is
expected to be quite detectable. But it is of no use to hope that such a conductor can
be excited galvanically if the resistance of upper layer exceeds 10
8
=
31
.
6 km. Here the crustal conductor with horizontal diameter
D
max
=
m
2
.
The inductive mechanism is associated with local induction in the crustal con-
ductors. This mode seems to have the advantage that the buried three-dimensional
conductors may be excited and detected, no matter how resistive the surroundings
are (Berdichevsky et al., 1984, 1992; Vanyan et al., 1986, 1988, 1991; Egorov,
1987). Let us estimate the intensity of local induction in the real crustal conductors
(fluidized, graphitized). Following (Kaufman, 1994), consider a conductive sphere
with a radius
a
and resistivity
10
8
Ohm
÷
5
·
·
placed in the homogeneous non-conductive space
with a uniform magnetic field
H
o
directed along the horizontal
x
-axis (Fig. 8.12).
Examine the anomalous magnetic field
H
A
arising due to local induction in the
,
|
|
=
sphere. Over the sphere, at its epicentre (
x
=
0
,
y
=
0
z
a
+
h
), we have
a
3
1
2
D
H
o
H
A
=
h
)
3
,
(8
.
1)
(
a
+
from which
=
H
A
H
o
a
3
1
2
|
D
|
h
)
3
,
(8
.
2)
(
a
+
where
−
3 coth
p
(1
i
)
3
2
p
2
i
−
D
=
+
1
.
(8
.
3)
−
p
(1
i
)
Here
h
is a distance between the observation site and the surface of the sphere, while
p
is the
induction-fa
ctor
defi
ned as the ratio between the radius
a
of the sphere and
the skin-depth
=
√
2
/
o
:
Fig. 8.12
Coducting sphere
of resistivity
in a uniform
magnetic field
H
o
;
H
A
- magnetic field of the
currents induced within the
sphere