P. Schmoldt, PhD - MTNet - DIAS

4.1. Types of distortion
the use of vertical magnetic field data. In the 1D anisotropic case no vertical magnetic field
should be prominent, whereas for the case of a 2D subsurface a vertical magnetic field,
related to the horizontal magnetic field parallel to the conductivity interface, is observable
[Kurtz et al., 1993; Kellett et al., 1992; Bahr et al., 2000; Heise and Pous, 2001]. However,
the presence of noise or deviation of the source from the plane wave assumption (cf.
Sec. 2.3) result in a vertical magnetic field, making impeding the separation of 2D and
anisotropic 1D cases. For the 2D case, the deviation of the induction arrows from the
expected 2D behaviour can be used as an indicator for potential presence of anisotropic
conductivity structures in the subsurface. The induction vectors in such cases are no
longer perpendicular to the strike direction, but they cannot be directly related to the
anisotropy direction either [Heise and Pous, 2001].
In many cases, determination of electric anisotropy can be aided by geological observations,
e.g through knowledge about dyke swarms in the study area or from the tectonic
history. Possible reasons for electric anisotropy in the upper mantle are directional variation
in connectivity of highly conducting mineral phases [Jones, 1992; Mareschal et al.,
1995] or hydrogen diffusion induced conductivity increase along olivine a-axes [Bahr and
Simpson, 2002]. One recently proposed cause of electric anisotropy in the transition zone
between lithosphere and asthenosphere is lattice preferred orientation of the present minerals
[e.g. Simpson, 2001; Bahr and Simpson, 2002; Hamilton et al., 2006]. The maximal
anisotropy for this latter case is controlled by the difference between highest and lowest
conducting direction of the present minerals, viz. 2.3 for dry olivine [Shankland and
Duba, 1990]. For wet (or hydrous) olivine the relation between conductivities of the three
axes is much more complex due to their H2O - dependent conductivity characteristics. Poe
et al. [2010] showed that the activation energy of intrinsic and extrinsic semiconduction
for olivine is H2O - dependent and that P-T - changes result in different σ - variation for the
three axes of wet olivine. As a result of this study, σ - ratios of the three wet olivine axes
are assumed to be P,T, and H2O - dependent, but further studies are needed to determine
reliable quantitative relations.
Anisotropy at the LAB is further proposed to originate from relative motion between
lithosphere and asthenosphere, “dragging along” and aligning material at the LAB. Anisotropy
direction in that case should coincide with relative plate motion direction. Identifying
features at such depth and relatively small thickness, however, is highly challenging
given the associated low resolution. Links between electric anisotropy at the LAB and
seismic anisotropy derived for the same depth [e.g. Vinnik et al., 1995; Silver et al., 2001;
Simpson, 2002b; Eaton et al., 2004; Debayle et al., 2005; Deschamps et al., 2008; Darbyshire
and Lebedev, 2009] are likely, since both parameters are presumably related to
the same tectonic mechanisms (cf. Chapter 5). Details about the relationship between
seismic and MT observations are presently still under debate, though [e.g. Ji et al., 1996;
Hamilton et al., 2006; Eaton et al., 2009; Roux et al., 2009]
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