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⊥
-anomalies caused by the
asthenosphere elevation come into play. At the same time the
⊥
- and
screening effect abates and the discernible
−
,
−
and
W
anomalies are hardly changed. The TM-mode is more sensitive to the litho-
sphere resistance than the TE-mode.
Another remarkable property of the TM-mode is that it may indicate the pres-
ence of conductive subvertical channels (fluid-saturated faults) crossing the resistive
litosphere. Let us revert to the model with the asthenosphere elevation shown in
Fig. 12.4 and introduce into it two two-dimensional vertical conductive channels
which intersect the lithosphere and connect the asthenosphere elevation with the
sediments. This model is shown in Fig. 12.6. Comparing Fig. 12.6 with Fig. 12.4,
we see, that the current channeling slightly affects the TE-mode, but it abates the
screening effect in the TM-mode so that the discernible
−
⊥
- and
⊥
-anomalies
occur reflecting the asthenosphere elevation.
12.4.2 Robustness of the TM- and TE-Modes to the 3D-Effects
The two-dimensional model is a convenient mathematical abstraction that separates
the magnetotelluric field into two modes of different physical nature: the TM-mode
associated with galvanic anomalies and the TE-mode associated with induction
anomalies. The question naturally arises: which of these modes is more robust to
the 3D effects caused by real geological bodies.?
Summing up the analysis carried out in Part II of our topic and taking into
account the estimates suggested in (Svetov, 1973; Kaufman, 1974; Berdichevsky
and Dmitriev, 1976; Veselovsky and Yudin, 1988; Berdichevsky et al., 1995), we can
say that the TM-mode is more robust to 3D effects caused by conductive bodies (that
is, by current gathering), while the TE-mode is more robust to 3D effects caused by
resistive bodies (that is, by current around-flow).
Let us exemplify this statement by two characteristic models that have given a
keen insight into the problem being a subject of rather long discussion.
Figure 12.7 presents a famous model of conductive sedimentary basin suggested
by Wannamaker et al. (1984). The model has a layered background and Fig. 12.7
contains an elongated rectangular prism of resistivity
=
2Ohm
·
m located in the
first layer of resistivity 400 Ohm
·
m. The prism's length and width are
l
=
35km and
w
=
3. The apparent-resistivity
curves have been computed for the central site O. In the left side of Fig. 12.8, we
show the three-dimensional longitudinal and transverse apparent-resistivty curves
of
15km, its elongation (aspect ratio) is
e
=
l
/w
=
2
.
3
D
and
3
D
oriented along and across the strike of the prism (in
x
and
y
direc-
tions). For comparison, the two-dimensional longitudinal and transverse apparent-
resistivty curves of
2
D
and
2
D
are shown too. They correspond to the TM- and
TE-modes generated in a 2D-model containing the prism with
l
→∞
. The locally
normal one-dimensional curve
n
is displayed as well. In the period range from
0.01 to 1 s, the three-dimensional curves of
3
D
and
3
D
virtually coincide with
n
−
−
the normal
curves admit the
1D-inversion determining a depth to the conductive prism. With lowering frequency,
curve. These high-frequency branches of the
A
