P. Schmoldt, PhD - MTNet - DIAS

Method Applicability
Swift angle 2D
Groom and Bailey decomposition 3D/2D
Bahr parameters 3D ⋆
Weaver et al. invariants 3D ⋆
Caldwell et al. phase tensor 3D ⋆
4.4. Removal of distortion effects
Tab. 4.6.: Comparison of commonly used analysing tools for MT distortion and their applicability depending on the dimensionality
of the subsurface. The Swift angle determination [Swift, 1967] can only be used in a 2D environment and fails for settings of higher
dimensionality while the Groom and Bailey decomposition [Groom and Bailey, 1989] is designed to retract the 2D regional structure
in the presence of small-scale 3D structures but cannot practical in a fully 3D situation. The methods by Bahr [1988], Weaver et al.
[2000], and Caldwell et al. [2004] are able to identify the dimensionality of the MT tensor for 1D, 2D, 3D, and mixed mode settings,
but have the drawback that no direct conclusion about the values of resistivity and phase for the regional structures can be drawn.
4.4.6. Conclusion
Today, various attempts have been published that aim to cope with the effects of distortion
onto MT data, among which the methods by Groom and Bailey [1989], Weaver et al.
[2000], and Caldwell et al. [2004] proofed most applicable (Tab. 4.6). It is not recommended
to use the approaches by Swift [1967] and Bahr [1988] anymore as they fail for
certain subsurface characteristics and have been succeeded by the WAL method [Weaver
et al., 2000] and the phase tensor method [Caldwell et al., 2004]. The latter two are limited
to an analysis of the subsurface dimensionality and solely give resistivity and phase
values for an approximated 1D scenario (WAL); hence, these approaches can only be used
as an indicator for subsequent methods that provide quantitative results for the different
regimes. The technique by Groom and Bailey [1989], on the other hand, evaluates the fit
for a proposed strike direction and provides quantitative results but fails for cases where
the subsurface is highly 3D and no simplification like a 3D/2D scenario (Sec. 4.2) can be
identified.
For a typical MT fieldwork, where data are collected along a profile, a combination of
the methods by Weaver et al. [2000], Caldwell et al. [2004], and Groom and Bailey [1989]
can be most effective. An analysis of the subsurface dimensionality with the WAL and
Phase tensor techniques can specify the region suitable for Groom-Bailey decomposition
or help to weigh the contribution from stations and frequency ranges during the estimation
of Groom-Bailey decomposition.
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