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Table 8.1
Elongations of crustal conductor providing the tipper quasi-two-dimensionality (W-2D)
and the apparent-resistivity and impedance-phase quasi-two-dimensionality (
,
-2D
)
Lithosphere resistivity
Conductor width (km)
Conductor elongation
W-2D
,
-2D
1000 Ohm
·
m
15
20
25
30
15
20
60
10
15
10000 Ohm
·
m
15
30
50
30
25
35
60
20
25
of low-frequency magnetelluric anomalies. And finally we have to emphasize that
the quasi-two-dimensionality conditions inferred are valid not only over the crustal
conductor, but in its visinity as well,
|
y
| −
v
<<
0
.
5
l
.
8.1.5 Are Deep Crustal Conductors Isotropic or Anisotropic?
One of the significant problems of modern deep magnetotellurics is recognition
of anisotropy in the crustal and mantle conductive zones (Bahr and Duba, 2000;
Bahr and Simpson, 2002; Wannamaker, 2005). The difficulty is that isotropic and
anisotropic bounded deep conductors located in or under a highly resistive medium
may manifest themselves in the equivalent magnetotelluric and magnetovariational
response functions, which cannot distinguish between isotropy and anisotropy and
admit both the interpretations.
It is generally agreed that a stable difference between the principal values of the
impedance tensors observed over a large area with 2D indications counts in favor of
anisotropy. Is such an evidence reliable?
Let us consider two typical examples of crustal conductors and define conditions,
at which equivalency between isotropy and anisotropy is the case.
Figure 8.17 presents the two-dimensional layered models ICC (isotropic crustal
conductor) and ACC-I (anisotropic crustal conductor). They simulate the conductive
sediments and the resistive lithosphere underlaid with the conductive mantle. In the
Earth's crust, at a depth of 20-35 km, the model ICC contains the isotropic crustal
conductor in the form of the two-dimensional homogeneous prism of resistivity
of 10 Ohm
·
m and width of 44 km. In the model ACC-I we have the same prism
composed of alternating vertical layers of resistivities of 5 and 1000 Ohm
·
m. The
strike of vertical layers coincides with the prism strike. The prism can be considered
as an anisotropic (macroanisotropic) crustal conductor with the diagonal resistivity
tensor
⎡
⎤
xx
00
ACC - I
=
⎣
⎦
.
0
yy
0
00
zz
