P. Schmoldt, PhD - MTNet - DIAS

Scale Type Terminology Example
Atomic Intrinsic Lattice preferred
orientation (LPO)
Micro (crystal) Fabric Crystal preferred
orientation (CPO)
Macro Structural Shape preferred
orientation (SPO)
4.1.3. Anisotropy
4.1. Types of distortion
Point defects in the atomic lattice
Alignment of minerals with conductivity
dependent on crystal axis, e.g. olivine
Dyke swarms or fluid- or graphite-filled
micro-cracks system
Tab. 4.1.: Types of electric anisotropy divided according by size; see text for details.
Electric anisotropy denotes directional dependent conductivity of the subsurface due to
small-scale features, imbedded in a host medium of different resistivity. If one of the
feature’s dimensions is smaller than the sampling wave field it cannot be adequately resolved
by the MT method and is commonly attributed as distortion. Anisotropy can be
divided according to size, distinguishing between atomic, microscopic (or crystal), and
macroscopic cases. The first two refer to intrinsic and fabric anisotropy [e.g. Shankland
and Duba, 1990; Poe et al., 2010], whereas the latter is related to structural anisotropy
[e.g. Bahr, 1997; Jones et al., 1997; Wannamaker, 2005; Jones, 2006]; see Table 4.1 for
a summary of anisotropy types and examples.
In principle, many materials are anisotropic to a certain degree, i.e. exhibiting different
electric characteristics for different directions, in which adequate current density
description requires the general form of Ohm’s Law (eq. 3.5)
jx
jy =
jz
σxx σxy σxz Ex
σyx σyy σyz Ey . (4.10)
σzx σzy σzz Ez
When investigating the anisotropy of a material, the issue is usually reduced to considering
the material’s conductivity in three orthogonal spatial directions, with axis defined by
the direction of maximal and minimal resistivity (ρx, ρy, ρz); i.e. the inverse of conductivity.
These spatial directions are not necessarily aligned with regional geoelectric strike
(cf. Sec. 4.3).
Anisotropy can be caused by different geological features, associated with conductive
sheets or tubes (Fig. 4.9) dependent on the relation between the resistivity in the three
spatial directions (ρx, ρy, ρz). To distinguish between the two cases is often difficult as
ρz cannot be resolved by MT [e.g. Heise and Pous, 2001]; for the same reason a correct
determination of anisotropy dip and slant (Fig. 4.10) is usually not achieved. An examination
of anisotropic distortion in MT is therefore commonly reduced to the effect on the
horizontal EM-field components, i.e. on the elements of the impedance tensor Z. Following
the notation by Groom and Bailey [1989] and Garcia and Jones [2001], introduced in
Section 4.3, the impedance tensor in the case of a 2D subsurface with solely anisotropic
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