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Fig. 11.24
North-West
filtering technique:
a - filtering a graph obtained
at
T=
14 s, b - multiplying
the graphs, obtained at
T
>
14 s, by correction
factors
The most beneficial are techniques relying on extraneous references. We will
consider two techniques of that kind: (1) using transient electromagnetic sounding
(TEM-Sounding, TEMS), and (2) using global magnetovariational sounding (GMV-
Soundig, GMVS).
It seems that Andrieux and Wightman (1984) were the first who tried to eliminate
the
effect by applying TEMS. The idea was widely discussed in the literature.
This approach can be efficient if near-surface local inhomogeneities are embedded
in a host one-dimensional medium.
Sternberg et al.(1988) use a TEMS-system consisting of a square transmitter loop
and recording central loop. There is good reason to believe that the observed mag-
netic field is hardly distorted by small surficial bodies. In order to compare TEM-
and MT-data, the skin-depths in the frequency- and time-domains are equated.
This yields the relation
T
−
=
.
5
15t between magnetotelluric period
T
and transient
