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Table 12.4
Misfits of transverse apparent resistivities and phases
Station
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
⊥
,
% 2 2 1 0 8 6 3 1 87 6 1 2 2 9
⊥
,
deg
2.5
2.9
2.6
1.9
2.2
3.6
1.0
2.4
4.5
1.8
1.5
2.2
1.9
1.9
1.4
⊥
,
g 0 9 4 8 1 9 2 8 7 1 3 4 2 8 9
12.8 From Two-Dimensional Inversion to Three-Dimensional
Inversion
Let us consider two peculiarities that distinguish the three-dimensional inversion
of magnetovariational and magnetotelluric data from the two-dimensional inversion
discussed above.
The electromagnetic field studied in two-dimensional models is of simple struc-
ture. The transverse current flows in the vertical plane perpendicular to the model
strike (gathers in the conductive zones and flows over and under the resistive
zones), while the longitudinal current flows horizontally (along the model strike).
In three-dimensional models the electromagnetic field is dramatically complicated.
Considering the three-dimensional models, we observe strong lateral effects (lateral
flow-around and current-gathering) and reveal rather complex trajectories, along
which the currrent transfers the information on the conductivity distribution. It is
evident that adequate description of real asymmetric locally inhomogeneous media
requires a great number of the geoelectric and geometric parameters (resistivities
and distances in
x
z
). Complication of the interpretation model and an increase
in the number of its parameters lead to the extension of the set of equivalent solutions
and impair the inversion stability. Thus, we arrive to the conclusion of necessity of
strong constraints imposed on the three-dimensional interpretation model.
The properties of the two-dimensional magnetotelluric field depend on its ori-
entation in reference frame formed by the model strike (to separate the induction
and galvanic anomalies, it is enough to orient the magnetotelluric field along and
across the model strike). In the three-dimensional asymmetric model such a refer-
ence frame is absent. So, we lose that simple physical basis, on which the strategy
of two-dimensional inversion separating inductive TE mode and galvanic TM mode
has been constructed. It would be helpful to use the experience of two-dimensional
interpretation and to create the strategy of multicriterion three-dimensional inver-
sion, which is an analog of multicriterion two-dimensional inversion, but takes into
account the properties of the three-dimensional magnetotelluric field.
We see the following approaches to the three-dimensional interpretation of mag-
netovariational and magnetotelluric date.
1. Let a single observation profile cross an elongated three-dimensional struc-
ture at acute angle. Here the quasi-two-dimensionality conditions are not observed,
but we can accomplish the formal two-dimensional inversion and introduce
three-dimensional corrections determined by the hypotheses testing (hypothetical
three-dimensional structures of different strike and different elongation are tested).
,
y
,
