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Fig. 8.22 Two-dimensional
model of the asthenosphere
uplift
well-defined minima observed over the entire h -interval ( T
100-100000 s), while
in the TE-mode the minima of the longitudinal field E x disappear with lowering
frequency (low-frequency flattening). At the same time the H y -profile shows the
gentle central maximum, caused by excess current filling the uplift, and rather sharp
side maxima and minima, which arise due to the horizontal skin effect at the edges
of the uplift ( T
=
100 s). At lower frequencies the central maximum increases, while
the side extrema flatten out ( T
=
1000 s). But with further lowering frequency when
the most of current is induced in the homogeneous conductive mantle the magnetic
anomaly almost completely decays ( T
=
=
100000 s).
Now examine the apparent-resistivity and impedance-phase curves observed in
the two-dimensional model of the asthenosphere uplift. Figure 8.24 shows the trans-
verse and longitudinal curves
,
,
and
together with the locally normal
curves ˙
n over the uplift. They have been obtained
in the model from Fig. 8.22 at the different distances y from the epicentre of the
uplift.
Consider the apparent-resistivity and impedance-phase curves observed over the
central part of the uplift ( y
n ,
n outside the uplift and ¨
˙
n ,
¨
=
0
÷
125 km). Here the descending mantle branches
, are distorted by moderate screening effect. They
are shifted somewhat upward with respect to the locally normal curves ¨
of the transverse curves
n ,
n .At
¨
,
the same time the descending mantle branches of the longitudinal curves
are
virtually undistorted. They merge with the locally normal curves ¨
n .
Coming to the apparent-resistivity and impedance-phase curves observed outside
the uplift ( y
n ,
¨
300 km), we see that their descending mantle branches are
shifted somewhat downward with respect to the locally normal curves ˙
=
251
÷
n . These
distortions are accounted for by galvanic and induction effects of the conductive
uplift. At y
n ,
˙
, and
,
=
500 km both the effects attenuate. Here the curves
merge with the locally normal curves ˙
n .
It would be instructive to find out how the distortions of the apparent-resistivity
curves depend on the dimension of the uplift. Figure 8.25 shows the curves
n ,
˙
,
in the model from Fig. 8.22 with half-width
of the uplift varying from 5 km to
750 km. The observation site is located over the epicentre of the uplift ( y
v
=
0).
By analogy with (8.11) and (8.17), we estimate the intensity of the distortions
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