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branches of the apparent-resistivity curves caused by bounded conductive zones in
deep layers of the Earth's crust (the deep
S
-effect) may severely spoil the static-shift
corrections.
Let us mention one more fitting technique which can be useful for qualitative
analysis of the apparent-resistivity curves. It is based on the vertical translation of
the
line, where
S
is a mean conductance of the upper conduc-
tive layer underlaid with a resistive substratum (Feldman et al., 1988; Berdichevsky
et al., 1988; Dmitriev-Berdichevsky, 1988). This approach is exemplified in
Fig. 12.54.
curves to the
S
A
−
−
11.1.5 Modeling the Distortions
Let some electromagnetic soundings (with direct or alternating current) give suf-
ficiently complete information on conductance
S
of the upper layer underlaid
with highly resistive rocks. Then we can construct a thin-sheet model reflect-
ing the horizontal variations in
S
and compute corrections for the distortions
caused by the
S
-effect. This idea has been realized by Fainberg et al. (1995)
in the
dynamic-correction method.
A distinguishing feature of this method is
that corrections can be applied in a wide frequency range (including transition
from undistorted branch of the apparent-resistivity curves to their statically shifted
branch).
We will illustrate the principle of the dynamic corrections by the example of mag-
netotelluric sounding aimed at studying deep conductive zones in the lithosphere or
asthenosphere (Berdichevsky, 1996). Assume that the inhomogeneous sediments
rest on the highly resistive layers of the crystalline Earth's crust. Following (1.75),
we represent the measured impedance tensor
Z
xx
Z
xy
[
Z
]
=
Z
yx
Z
yy
as
[
e
][
Z
R
]
[
Z
]
=
,
(11
.
13)
where
e
xx
e
xy
[
e
]
=
e
yx
e
yy
is the electric-distortion tensor, and
Z
xx
Z
xy
[
Z
R
]
=
Z
yx
Z
yy
