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Fig. 11.43
Electric and
magnetic effective intensities
D
eff
and
M
eff
along the
profile Urals-Tobol'sk;
1-
D
eff
(conventional units),
2-
M
eff
(conventional units),
3 - Mezozoic bottom from
MTS-data, 4 - Mezozoic
bottom from seismic data
Consider an example of such relations taken from the MT-surveys on the Siberian
platform. The terrigenous sediments in the region under consideration contain
trappean intrusions (Upper Cambrian) and salt-bearing strata (Lower Cambrian).
Figure 11.44 presents a map (
a
) for the depth of reflecting horizon in the Lower
Cambrian and for the sediments conductance
S
. The contours on the seismic horizon
outline two deep structures: the Sokolov depression (SD) and the Romanov uplift
(RU). At the same time the conductance contours show a submeridional uplift at
the place where the reflection seismics gives the Sokolov depression. Explanation
to this contradiction is that the
S-
map reflects topograthy of near-surface trappean
intrusions. Let us compare the map A with a map (
b
) for the telluric and magnetic
effective intensities,
D
eff
and
M
eff
.The
S
- and
D
eff
−
maps look similar. A submerid-
ional maximum of
D
eff
outlined in the vicinity of the village Sokolov corresponds
to the minimum of
S
elongated approximately in the same direction. By and large
the seismic map and the
M
eff
-map are also similar. The minimum of
M
eff
outlined
in the vicinity of the village Romanov corresponds (with some displacement) to
the Romanov uplift. Summing up, we can say that
D
eff
-map characterizes (at least
qualitatively) the trappean intrusions, while the
M
eff
-map provides a rough idea of
the basement topography.
Next we will show a synthetic example. Let us turn again to the superimposition
model from Fig. 2.3 containing a near-surface local resistive
-shaped inclusion and
a deep regional conductive two-dimensional prism. Figure 11.45 shows the contour
maps of the apparent resistivities
Z
xx
Z
yy
−
Z
xy
Z
yx
o
Z
xy
−
Z
yx
2
2
2
eff
=
|
Z
eff
|
brd
=
|
Z
brd
|
=
,
=
,
o
o
4
o
Z
xy
Z
yx
Z
yy
2
2
2
2
Z
rms
o
=
|
Z
xx
|
+
+
+
rms
=
2
o
