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quasi-two-dimensionality. This clearly suggests that in the models under considera-
tion the local induction is weak and magnetelluric anomalies are of galvanic nature.
8.2.5 Are Asthenospheric Structures Isotropic or Anisotropic?
In studying the upper mantle, we may face the anisotropy-isotropy equivalence.
Asthenospheric conductive zones located under the highly resistive lithosphere can
manifest themselves in the equivalent response functions, which do not distinguish
between isotropy and anisotropy and admit both the interpretations.
Let us consider two examples concerning the asthenosphere uplift.
Figure 8.32 shows the two-dimensional models IAU-I (isotropic asthenosphere
uplift) and AAU-I (anisotropic asthenosphere uplift). The model IAU-I contains the
isotropic asthenosphere uplift of resistivity of 9.1 Ohm
m, width of 200 km and
amplitude of 50 km. In the model AAU-I we have the same uplift composed of
alternating vertical dykes of resistivities of 5 and 50 Ohm
·
m (the strike of vertical
dykes coincides with the uplift strike). The uplift can be considered as an anisotropic
(macroanisitropic) conductor with the diagonal resistivity tensor
·
⎡
⎤
xx
00
AAU - I
=
⎣
⎦
.
0
yy
0
00
zz
where
m.
The calculations performed by the finite-element method (Wannamaker et al.,
1986) show that the apparent-resistivity and tipper curves obtained in the models
IAU-I and AAU-I are close to each other. The models demonstrate the anisotropy-
isotropy equivalence.
Next consider Fig. 8.33, which presents the two-dimensional models IAU-II
(isotropic asthenosphere uplift) and AAU-II (anisotropic asthenosphere uplift).
The
xx
≈
9
.
1Ohm
·
m
,
yy
=
27
.
5Ohm
·
m
,
and
zz
≈
9
.
1Ohm
·
model
IAU-II
contains
the
isotropic
asthenosphere
uplift
of
resistiv-
ity of 10 Ohm
·
m, width of 200 km and amplitude of 50 km. In the model
AAU-II
we
have
the
same
uplift
composed
of
alternating
horizontal
lay-
ers
of
resistivities
of
5
and
1000
Ohm
·
m.
The
uplift
can
be
considered
as
an
anisotropic
(macroanisotropic)
conductor
with
the
diagonal
resistivity
tensor
⎡
⎤
xx
00
AAU - II
=
⎣
⎦
.
0
yy
0
00
zz
where
m.
The apparent-resistivity and tipper curves obtained in the models IAU-II and
AAU-II are also close to each other (anisotropy-isotropy equivalence).
xx
≈
10 Ohm
·
m
,
yy
≈
10 Ohm
·
m
,
and
zz
≈
500 Ohm
·
