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note that (1) the phase
brd
is much less subjected to near-surface distortions than the
apparent resistivity
brd
, (2) in the model under consideration, the phase immunity
to near-surface distortions is quite comparable with the immunity of the magneto-
variational invariants
W
and
M
, and (3) the resolution of the phase
brd
leads
in depth the resolution of the apparent resistivity
brd
and the magnetovariational
invariants
W
and
M
: whereas
brd
,
W
and
M
reliably detect the mantle
prism in the range of periods 250-10000 s, the phase
brd
reveals the mantle prism
in the interval 40-250 s.
The pseudo-topography of the apparent phase
A
determined from the phase ten-
sor [
] is displayed in Fig. 11.54. Comparing the pseudo-topographies of
A
and
brd
, we see that in the model under consideration the phase
A
has approximately
the same immunity to near-surface distortions as the phase
brd
. In both pseudo-
topographies, the effect of the near-surface prism virtually vanishes at the period
T
=
3 s, and the effect of the crustal prism decays at a period of about 10000 s.
Finally, we consider pseudo-topographies of the apparent resistivity
6
.
brd
and
phase
brd
determined in the model with the first sedimentary layer containing
chaotically distributed small-scale heterogeneities. The pseudo-topography of
brd
is shown in Fig. 11.55. We see that the geoelectric noise produced by local hetero-
geneities fills all levels of the pseudo-topography, from
T
10000 s. In
this “forest” the near-surface, crustal and mantle prisms are lost. The pseudotopogra-
phy of
=
1s to
T
=
brd
shown in Fig. 11.56 is much more informative. Here, the levels
T
=1,6.3,
and 40 s are filled with smoothed geoelectric noise which does not cover the crustal
prism. And the levels
T
= 250 and 10000 s are almost free of the noise. Against
this background, the effects of the crustal and mantle prisms can be identified quite
reliably.
It seems that the pseudo-topography technique can prove helpful for constructing
the interpretation model. The main advantage of this technique is that the topog-
raphy of magnetotelluric and magnetovariational invariants is constructed without
any restrictions and can give a fairly comprehensive idea of the resolution of dif-
ferent response functions, mutual position and shape of geoelectric structures, the
superposition of their effects, the thickness and height of the “forest” produced by
geoelectric noise.
11.5 Mapping the Sediments Conductance
The sediments conductance
S
is a traditional geoelectric parameter widely used in
all methods of the electric and electromagnetic soundings with direct and alternating
current. This parameter readily determined from the apparent-resistivity curves can
give a qualitative information on the topography of the crystalline basement and
variations in the thickness and resistivity of the sedimentary layers. Maps of
S
may
provide the geolectric zoning of vast areas. They are helpful in recognizing local and
regional structures as well as in analyzing the informativeness of the magnetotelluric
and magnetovariational response functions.
Determining the sediments conductance by magnetotelluric sounding, we usually
take the
eff
−
curves and apply a simplified technique which is valid for the 1D
