The Interpretation Model - Models and Methods of Magnetotellurics - page 402

apparent-resistivity and principal phase curves related to the principal directions
of the regional impedance tensor.
Note that accuracy of dynamic corrections depends on the choice of the normal
conductance S N and the basement resistivity B . So, it would be reasonable to test

the distortion matrix [ e m ] obtained at different S N and B .

11.1.6 Saving the Static Shift Troubles

Can we dispose of static distortions? We manage to do this for the geoelectric noise
caused by the

effect. Making good use of the averaging and filtering techniques,
we smooth the local random distortions and pave the way for meaningful geophys-
ical interpretation. Certainly, on this way we lose some information, but it can be
possible to improve the MT-inversion by involving local TEMS- references.
In the case of static distortions caused by the S- effect we have a more complicated
situation. The scale of these distortions depends on dimensions of causing geoelec-
tric structures as well as on the galvanic transparency of resistive rocks underlying
the inhomogeneous upper layer. We can observe the local S -effects (stretching over
hundreds of meters) and the regional S -effects (extending for hundreds of kilome-
ters). It is evident that the averaging and filtering techniques are efficient only for
local S -effects. To eliminate the regional S -effect, we should use reference given by
the standard apparent-resistivity curve

−

st or apply the dynamic corrections (that is,
to solve the two-dimensional or three-dimensional problem for given distribution of
the upper layer conductance). Both approaches are associated with uncertainties that
are difficult to control and we not always manage to estimate the reliability of shift-
corrected apparent-resistivity curves. It is the S- effect that deserves the notorious
title of the main villain of the magnetotelluric piece.
Can we dispose of the S- effect? It would be possible, if we remove the inversion
uncertainties caused by near-surface galvanic anomalies of the electric field. We see
two different pragmatic approaches to this problem.
1. Development of a new interpretation strategy in which the magnetic field is
a primary source of information on the Earth's conductivity. The remarkable prop-
erty of the magnetic field is that with lowering frequency the anomalies caused by
near-surface inhomogeneities attenuate and the anomalies caused by deep inhomo-
geneities come to light. So the magnetic field can scan the Earth illuminating suc-
cessively deeper and deeper areas independently of near-surface inhomogeneities.
On this way we employ the MVS-MTS complex with MVS, which plays the role of
a basic method, and MTS, which controls and supplements the MVS data.
2. Interpretation of magnetotelluric and magnetovariational response functions in
the hypotheses test mode. Hypotheses are made on the basis of modern geological
and geophysical ideas (with all their controversies and viewpoints variety). On this
way we construct several starting models that correspond to the different hypotheses
and give unambiguous references for correcting the apparent-resitivity curves. The
most credible is a hypothesis that shows minimal model misfit.
In our topic we will give special prominence to both approaches.