The Interpretation Model - Models and Methods of Magnetotellurics - page 398

is evidently distorted and should be shifted downwards to match the xy - curve.

After correction we have two closely related xy - and yx -curves typical of a one-

dimensional model.
Lastly we consider a correction technique involving global magnetovariational
sounding, GMVS (Larsen, 1977; Rokityansky, 1982; Berdichevsky et al., 1989a, b;
Vanyan, 1997; Berdichevsky and Dmitriev, 2002). The technique is used to cor-
rect apparent-resistivity curves distorted by the

- and S- effects. Here the refer-
ence is given by the standard aparent-resistivity curve st reproduced from GMVS

and large magnetotelluric statistics collected in different geological provinces. The
st -curve defines an average planetary distribution of the Earth's conductivity that
can be adopted as a normal geoelectric model of our planet. Figure 11.28 shows
the standard

st -curve suggested by Fainberg in his pioneering works (1983a, b).
It is composed of GMVS data and magnetotelluric extension that can be formed
by slightly distorted MT-curves obtained in different geological provinces. We will
use the st -curve edited by Vanyan with regard to further generalizations of GMVS

and MT data (Vanyan, 1997; Berdichevsky et al., 1989a, b). Coordinates of this
st -curve are set out in Table 11.1. Figure 11.29 presents two examples of how
corrections are performed. Let a long-period apparent-resistivity curve

xy be dis-

torted by the -effect that persists throughout the entire MT recording range. Its

low-frequency descending branch lies much below the

st -curve. To remove the

xy -curve should be shifted upward so that its low-frequency descend-
ing branch fits the

-effect, the

st -curve. In the case of the S -effect, the correction procedure
becomes more intricate. We have to take into account that the high-frequency
ascending branch of the

xy -curve reflecting the upper layer conductance is slightly
distorted and does not need any correction. So only the low-frequency branch of

Fig. 11.28 Constructing the
standard apparent-resistivity
curve st