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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
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