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In-Depth Information
Fig. 8.13
The
p
-dependence
of
|
D
|
p
=
a
/
.
(8
.
4)
a
3
D
H
o
of an equivalent magnetic
dipole situated at the centre of the sphere. It characterizes the intensity of local
induction. The
p
-dependence of
The parameter
D
defines the moment
M
=
2
π
|
D
|
is shown in Fig. 8.13. We distinguish here two
ranges: (1)
p
<
1
5, strong local induction.
The point is which of these ranges the fluidized and graphitized crustal conductors
fall into.
Conductive zones of the fluid nature usually occur at a depth of about 15-25 km.
Their thickness amounts to 15-20 km, while their resistivity may reach 10-15
Ohm
.
5, weak local induction, and (2)
p
1
.
·
m. To test intensity of local induction, we consider a cubic conductor 20
×
20
m and the upper face at a depth 15 km. Assume
that observations cover a period range from 25 to 3000 s. For rough estimation, take
an equivalent sphere with
a
×
20 km with resistivity 10 Ohm
·
=
10 km and
h
=
15 km. Now calculate the anomalous
magnetic field
H
A
H
o
appearing due to local induction within the conductor.
According to (8.2), (8.3) and (8.4), we get for
T
≥
25 s:
≥
7
.
96 km
,
p
≤
H
A
H
o
≤
.
,
|
| ≤
.
,
.
1
006. This is a case of weak local induction: the
induced magnetic field is negligibly small against the inducing magnetic field.
Next test the intensity of local induction in conductive zones of the graphite
nature. Their resistivity reaches 1 Ohm
25
D
0
2
0
m (and even less). They usually occur at
a depth of about 3-15 km and their thickness does not exceed 3-4 km. Consider
a cubic conductor 4
·
m and the upper face at a
depth 3 km. Assume that it manifests itself at periods from 10 to 1000 s. Taking the
equivalent conductive sphere with
×
4
×
4 km with resistivity 1 Ohm
·
=
1Ohm
·
m and
a
=
2km
,
h
=
3 km, we get
,
H
A
H
o
≤
for
T
0064. Once again
we meet a case of weak local induction within a crustal conductor.
Clearly these rough estimations are preliminary. Satisfactory analysis can be
performed by modeling specific three-dimensional structures. The distinguish-
ing feature of the weak local induction is that magnetotelluric anomalies decay
when we increase the resistivity of the host medium which contain conductive
bodies.
≥
10 s :
≥
1
.
59
,
p
≤
1
.
26
,
|
D
| ≤
0
.
2
0
.
