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Fig. 12.11
Top: longitudinal
(
) and transverse (
⊥
)
apparent-resistivity curves
along a profile crossing the
Precaucasian foredeep.
Bottom: geophysical
cross-section; C -the
Caucasian ridge, PCF- the
Precaucasian foredeep;
surface of the Paleozoic
basement from
-curves (1),
from seismics (2), from
drilling (3)
-curves are bowl-type throughout the entire profile, 150 km
long. Their 1D-interpretation yields the Paleozoic basement topography that is
in close agreement with seismics and drillings. At the same time the transverse
⊥
-curves change their shape, from the bowl-type at the distance of 120-150 km
from mountains to a falling type in the vicinity of mountains. Small wonder that 1D-
and even 2D-interpretation of the transverse
The longitudinal
⊥
-curves would give incongruous (say,
crazy) results that have nothing in common with seismic data and general geological
ideas of the region's structure. Suffice it to say that with TM-inversion, the resistivity
of sediments falls to 0.1 Ohm
m, while the solid high-ohmic lithosphere wedges out
and the conductive “asthenosphere” appears at the depth of about 8-10 km. It seems
that this remarkable situation is the same as in model with a three-dimensional
horst shown in Figs.12.9 and 12.10. The resemblance in behavior of experimental
and model MT-curves is striking. It is evident that within the Precaucasian fore-
deep, we meet an intensive 3D-effect connected with currents flowing around the
high-ohmic Caucasian Ridge. The flow-around effect dramatically distorts the trans-
verse
·
-curves (the
TE-mode). So, one cannot agree with the statements like “2D-interpretation of the
TM-mode is more accurate than the TE-mode in the presence of 3D-bodies” (Park,
1996) or “2D-modelling should concentrate on the impedance in the TM-mode
because of its low sensitivity to the effect of finite strike” (Wannamaker, 1999).
In fact, the TM-impedance is more robust to 3D-effects caused by conductive struc-
tures (that is, by current gathering), but the TE-impedance may be more robust (even
considerably more robust!) to 3D-effects caused by resistive structures (that is, by
current around-flow).
⊥
-curves (the TM-mode) and scarcely affects the longitudinal
