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12.4 Two-Dimensional Bimodal MV-MT Inversion
This approach is widely used in investigating locally inhomogeneous layered
media that have well-defined extended strike and allow for two-dimensional
approximation.
The condition of
quasi-two-dimensionality
of the structures under study can be
established by means of magnetovariational and magnetotelluric tests (Sect. 11.3.1,
11.3.2) and with a priori 3D modelling. A convincing a posteriori evidence of
quasi-two-dimensionality is that the two-dimensional inversions of different mag-
netovariational and magnetotelluric response functions (tippers, magnetic tensors,
impedance tensors) agree with each other.
The two-dimensional magnetotelluric field consists of the TM- and TE-modes.
The TM-mode is related to the H-polarized wave generating the transverse
MT-curves (telluric current flows across the structures, its magnetic field is directed
along the structures and does not depend on the Earth's conductivity). The TE-mode
is related to the E-polarized wave generating the longitudinal MT-curves together
with the tippers and horizontal magnetic tensor (telluric current flows along the
structures, its magnetic field is directed across the structures). The main difference
between these modes is that the TM-mode charges the structures, and its anoma-
lies are of galvanic nature, but the TE-mode does not charge the structures, and its
anomalies are of inductive nature.
The TM- and TE-modes offer different sensitivity to near-surface and deep con-
ductivity and provide different accuracy of 2D-approximation of real 3D structures.
These properties of the TM- and TE-modes dictate the philosophy and practical
strategy of the two-dimensional interpretation of MT-data collected in the regions
with elongated structures.
The problems of two-dimensional interpretation of magnetotelluric and mag-
netotevariational soundings have been discussed in many papers and monographs
(Svetov, 1973; Kaufman, 1974; Berdichevsky and Dmitriev, 1976, 2002; Jupp and
Vozoff, 1977; Kaufman and Keller 1981; Veselovsky and Yudin, 1988; Park et al.,
1983, 1991; Wannamaker et al., 1984; Berdichevsky and Zhdanov, 1984; Park,
1985; Mackie et al., 1988; Wannamaker et al., 1989a, 1991; Berdichevsky et al.
1992b, 1995, 1998, 2003; Weaver, 1994; Zhdanov and Keller, 1994; Gupta and
Jones, 1995; Banks et al., 1996; Zhdanov, 2002; Mehanee and Zhdanov, 2002;
Ledo et al., 2002; Ledo, 2006). The discussion exhibits a wide range of vari-
ous and sometimes conflicting viewpoints, from “the TM-impedance functions are
emphasized because theory and experiment show that they are more robust to three-
dimensional effects
” (Wannamaker et al., 1989) and “Two-dimensional modeling
should emphasize the TM-mode impedance
...
...
because it is more immune to finite
strike effects.” (Wannamaker, 1999) to “
inverting both the TE- and TM-modes
results in models that fit the TM locally without resolving large-scale structure
evinced only by the TE-mode.” (Banks et al., 1996) and “
...
the most comprehen-
sive and reliable information on the Earth's conductivity can be derived by means of
bimodal inversion, that is using both the TM- and TE-modes.” (Berdichevsky et al.,
1998).
...
