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Of course, the parallel inversion is the simplest approach to a multicriterion
problem, and apparently this is the reason why it is popular among geophysicists
fascinated by the possibility of automatic inversions eliminating the necessity of
comprehensive analysis. The technique of successive partial inversions undoubt-
edly complicates the work, and this is a possible reason for the objections raised in
discussions. However, our experiments on the integrated interpretation of MV and
MT data indicate that the game, albeit more difficult, is worth the candle.
In the last sections of our topic we would like to look upon two case histories
illustrating the interpretation of magnetotelluric and magnetovariational data by
means of hypotheses testing and succession of partial inversions.
12.6 Geoelctric Model of the Baikal Rift
Magnetotelluric studies in the Baikal rift zone have been conducted for many years,
beginning in 1960s (the Eastern Geophysical Trust, Institute of the Earth's Crust,
and the University of Moscow). These pioneering works, initiated by V. Pospeev
(Pospeev, 1976), deeply influenced the development of geoelectrics in our coun-
try. During three decades, nearly 1000 MT-soundings were carried out within the
rift zone and adjacent areas. Unfortunately, the data obtained are nonuniform in
their quality. A considerable part of this vast territory was studied at the dawn
of magnetotellurics, in the 1960s and 1970s, when observations were made with
analog medium-frequency instrumentation and were processed manually, by rough
(visual or approximate) methods. Interpretation of these data is usually reduced to
one-dimensional inversion of effective apparent-resistivity curves, thereby ignoring
the distortions caused by near-surface inhomogeneities. Authentic magnetotelluric
measurements using digital broadband instrumentation as well as efficient methods
of spectral analysis and geoelectric noise suppression were started in the late 1970s,
but by that time the research activities had been shifted from the Baikal rift into
Trans-Baikal areas and further to the east. It was not until 20 years later that an
attempt was made to analyze and generalize these obsolete results obtained in the
Baikal rift zone, to select the most reliable data and construct a meaningful geoelec-
tric model of this province, typical of continental rifting (Berdichevsky et al., 1999;
Berdichevsky and Dmitriev, 2002). It has been shown that magnetotellurics may
play a decisive role in choosing between the competing conceptions for the deep
structure of the Baikal rift. Let us consider this instructive two-dimensional inter-
pretation accomplished as a sequence of partial inversions of apparent-resistivity
curves in the hypothesis test mode.
12.6.1 Two Concepts of the Baikal Rift Zone
Two different concepts of the Baikal rift zone are discussed in the literature:
1. The mantle-diapir concept. Figure 12.29a presents a schematic cross-section
of the Baikal rift zone along a profile transecting the middle part of Lake Baikal.
