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Can we predict (at least roughly) the effect of finite strike of elongated structures?
It wouldn't be dramatically wrong if we assume that the conductive structure (say,
graben) in the TM-field and the resistive structure (say, horst or ridge) in the TE-field
allow for the two-dimensional approximation provided their elongation exceeds
5-10. But conditions of the two-dimensional approximation of elongated structures
dramatically change when we consider the conductive structures in the TE-field and
the resistive structures in the TM-field. The question is about elongations that range
up to 15-25 (and even more) depending on geoelectric situation. Such elongated
structures, if they exist, are a rarity in nature. We believe that any two-dimensional
inversion may need a posteriori estimation if not correction of model errors caused
by 2D approximation.
12.4.3 Susceptibility of the TM- and TE-Modes to Near-surface
Galvanic Distortions
In the abstract two-dimensional model, only the TM-mode suffers from the near-
surface galvanic distortions which manifest themselves in static shift of apparent-
resistivity curves. But in actual practice, we may deal with superposition of
large-scale elongated structures and small-scale three-dimensional near-surface
inhomogeneities of more or less complicated form. In that events, both the field
modes, TM and TE, are distorted and hence not only the transverse but also the lon-
gitudinal apparent-resistivity curves suffer from static shift. Fortunately, the experi-
ence suggests that in many cases (even in mountains) the longitudinal
−
curves
⊥
−
are less disturbed than the transverse
curves (Kovtun, 1989; Moroz, 1991;
Dyakonova et al., 1986; Alperovich et al., 1980; Berdichevsky and Dmitriev, 2002).
This is typical for regions with a predominance of elongated inhomogeneities with
a common strike in the absence of pronounced local 3D-inhomogeneities.
Let us show three indicative examples.
In Fig. 12.12 we see the longitudinal and transverse apparent-resistivity curves,
⊥
, obtained in the vicinity of the Urals (Dyakonova et al., 1986). Here
the longitudinal
and
-curves experience far lesser static shift than the transverse
⊥
-curves. The descending mantle branches of the longitudinal curves are close
to each other. They gravitate to the standard curve of
st
. At the same time, the
corresponding transverse curves cross the
st
-curve and their mantle branches are
shifted upward by decade and a half.
The longitudinal and transverse apparent-resistivity curves,
⊥
, obtained
on the Kola Peninsula (Djakonova et al., 1986) are presented in Fig. 12.13. Here
almost the same pattern is observed. The longitudinal curves seem to be slightly
distorted. Their left descending branches merge together, reflecting a conductive
layer in the lower part of the Earth's crust. The mantle branches of these curves
are arranged about the standard curve of
and
st
, though with a slightly different slope.
Compare the longitudinal curves with transverse ones. The transverse curves cross
the
st
-curve. They are drastically shifted upward (up to decade).
