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Thus, the normal magnetotelluric field varying linearly with the distance from
its source allows for determining the one-dimensional normal impedance Z N by
magnetotelluric and magnetovariational relationships (5.21) and (5.22). This is in
good agreement with the known definitions (1.2) and (1.3), suggested by Dmitriev
and Berdichevsky (1979) and Weidelt (1978) that are at the heart of one-dimensional
Tikhonov-Cagniard's magnetotellurics.
Now estimate a distance at which we can neglect the source effect and approx-
imate the normal field by a plane-wave. To this end we define the amplitude ratio
N between vertical and horizontal components of the normal magnetic field at the
centre M o of the observation domain. With a glance to (5.16),
H z o
H x o
|
o x o +
|
Z N (0)
1
r o
h eff
r o
N
N
o =
=
=
y o =
(5
.
23)
H y o
2
2
+
where
N is the normal apparent resistivity of the Earth:
2
N = |
Z N (0)
|
/ o ,
h eff is the effective penetration depth determined by the normal impedance or the
normal apparent resistivity:
N
o
h eff = |
Z N (0)
|
=
(5
.
24)
o
and r o is the distance between the centre M o of the observation domain and the
projection of the centre O of the source domain on the Earth's surface:
x o +
r o =
y o .
N does
not exceed 0.05. Assume that the lithosphere thickness is about 100 km. Then h eff
100 km and the condition
Let us disregard the vertical component of the normal magnetic field if
N
0
.
05 for polar magnetic perturbations is observed
at distances r o
2000 km, that is, at the middle and low latitudes.
5.1.2 MT and MV Response Functions in the Absence
of the Source Effect
E N
H N
We
have
divided
the
normal
field
{
,
}
into
two
plane-wave
modes
E N( m )
H N( m )
E N( 3)
{
,
} ,
=
,
{
,
m
1
2 defined by (5.17) and a source-effect mode
H N( 3)
}
defined by (5.18). If the vertical component of the normal magnetic field
 
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