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From Kirchhoff's laws, we have:
xx
≈
10 Ohm
·
m
,
yy
≈
10 Ohm
·
m
,
and
zz
≈
500 Ohm
m. The apparent-resistivity and tipper curves computed for different dis-
tances from the model centre (
y
·
80 km) are also closely related to each
other and cannot distinguish between isotropy and anisotropy.
The isotropy-anisotropy equivalence has simple physical interpretation. The TE-
mode is associated with longitudinal currents. It provides the anisotropic-isotropic
equivalence since these currents penetrate into the anisotropic and isotropic con-
ductors in a similar way and their integral effect is almost the same. The TM-
mode reflects the behavior of the transverse currents, which penetrate into the
anisotropic and isotropic conductors in a different way and hence may detect the
difference between anisotropy and isotropy. But the TM-mode is subjected to gal-
vanic screening and its informativeness depends on the screening-effect intensity.
We suggest a straightforward rough criterion for the anisotropy-isotropy equiva-
lence: the deep isotropic and anisotropic conductors are equivalent provided that
w
=
0
,
−
22
,
−
=
√
S
sed
R
cr ust
is the adjust-
ment distance determined by the sedimentary conductance
S
sed
and resistance
R
cr ust
of crustal strata separating the conductor from the sediments. In the equivalent mod-
els ICC and ACC-I as well as ICC and ACC-II (see Figs. 8.17 and 8.18), we have
d
<
2
d
,
where
w
is the width of the conductor and
d
Widening the crustal conduc-
tor or decreasing the resistance of the overlying strata, we arrive at models with
transverse apparent-resistivity curves which distinguish between the isotropy and
anisotropy. In models with a moderate resistance of the upper crust, say
R
cr ust
≈
10
7
=
43
.
6
−
97
.
5 km and
w
=
44 km
.
Here
w
<<
2
d
.
m
2
, typical for active regions, the difference between the isotropic and
anisotropic crustal conductors, 100-150 km wide, may be seen. Under these con-
ditions, the studies of anisotropic crustal conductors make undoubted practical
sense.
Ohm
·
8.2 Models of Asthenosphere Conductive Zones
Figure 8.19 presents the apparent-resistivity curves calculated for a one-dimensional
model with the sediments of conductance 150 S and the asthenosphere of conduc-
tance from 0 to 20000 S. The horizontal asthenospheric layer occurs at a depth of
90 km. It shows up rather vividly when its conductance exceeds 2000-3000 S. In
that event the apparent-resistivity curves have a steep descending branch close to
the line
h
100 km.
Seen below are several models illustrating the magnetotelluric anomalies caused
by asthenosphere conductive zones.
=
8.2.1 The Dmitriev-Mershchikova Cosine-Relief Model
Giving credit to simple analytical solutions, we begin with a two-dimensional model
suggested by Dmitriev and Mershchikova (1974). This three-layered model is shown
