Inversion Strategy - Models and Methods of Magnetotellurics - page 470

longitudinal curves exhibit distinctive minima and clearly outlined descending man-
tle branches, which are close to the standard st -curve (small static shift !). It seems

that these curves are weakly distorted and may be used for rough 1D-estimates. One
can presume that minima of the -curves are caused by a crustal conductive layer.

It lies at a depth of about 20-30 km, and its conductance increases from 200-300
S in the north (MTS-1) to 1000-1500 S in the south (MTS-4). These estimates are
in accordance with a model constructed from the tipper inversion (Fig. 5). Quite
different pattern is given by the transverse curves. The crustal conductor is clearly
evidenced only by the ⊥ − curve from MTS-4, carried out near a deep fault. But

with distance from the fault the crustal conductor is screened and the apparen-
resistivity minimum degenerates into gentle bendings (MTS-1, MTS-2). What is
more, the mantle branches of the ⊥ −

curves are significantly displaced upward

and downward from the standard st -curve (large static shift).

Beyond question, in the above examples the longitudinal − curves are less

⊥ − susceptible to near-surface galvanic distortions than the transverse curves.

12.4.4 Informational Complementarity of the TM- and TE-Modes

We have examined the main properties of the TM- and TE-modes in the presence
of elongated target structures. The results of this consideration are summarized in
Table 12.1, which shows the susceptibility of the TM- and TE-modes to near-surface
static distortions and their accuracy in the 2D-approximation of elongated structures
as well as their sensitivity to near-surface and deep structures, the lithosphere resis-
tance and conductive faults.
The TM-mode ( ⊥ , ⊥ ) provides the better accuracy in the 2D-approximation of

conductive structures and the better sensitivity to near-surface structures as well as to
the lithosphere resistance and deep faults, but it suffers from the screening effect and
may miss the deep structures (for instance, conductive zones in the high-resistive
lithosphere).
The , , ) ensures the better accuracy in the

2D-approximation of resistive structures and the better sensitivity to deep structures,
but it is open to large errors in the 2D-approximation of conductive structures. Also,
if the transverse apparent-resistivity curves in the TM-mode suffer dramatically
from the static shift, the TE-mode may change the situation for the better (the tipper
and longitudinal impedance-phase curves do not experience static distortions, while
the longitudinal apparent-resistivity curves under favorable conditions are slightly
distorted).
The TM- and TE-modes nicely complement each other: gaps left by one mode
are filled by another mode. In this sense we say that the TM- and TE-modes satisfy
the principle of informational complementarity .
The complementarity principle forms a sound basis for the 2D-interpretation
strategy.
The sensitivity to the target structures is of critical importance. Consider, for
instance, a magnetotelluric survey designed to studying conductive zones in deep

TE-mode (Re W , Im W