Journal
SURVEYS IN GEOPHYSICS
Volume 26, Issue 6, Pages 733-765Publisher
SPRINGER
DOI: 10.1007/s10712-005-1832-1
Keywords
electromagnetics; anisotropy; magnetotellurics; solid-earth
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Electrical anisotropy, the effect of current density in a medium being a function of the orientation of the electric field, is being recognized increasingly as an important effect in explaining Earth electromagnetic observations. A consideration of anisotropy, however, in most cases is an admission of spatial aliasing in earth structure, wherein the averaging volume of diffusive EM fields may be greater than the characteristic dimensions of a family of oriented structures, thus leading to a response which is equivalent to a bulk anisotropic medium. Even for two-dimensional geometries, there can be strong non-parallelism of principal axes of vertical magnetic field relative to the impedance over broad areas, as well as impedance phase variations which leave normal quadrants, if there are multiple directions of anisotropy or anisotropy strike distinct from bulk geometric (2D) strike. This paper concentrates on experience with regional field studies in continental settings where bulk anisotropy is apparent. Upper crustal anisotropy may result from preferred orientations of fracture porosity, or lithologic layering, or oriented heterogeneity. Lower crustal anisotropy may result from preferred orientations of fluidized/melt-bearing or graphitized shear zones, but does not necessarily reflect current state of stress per se. In the upper mantle, the prior causes all may act in pertinent domains, but added to these is the possibility of strong electrical anisotropy due to hydrous defects within shear-aligned olivine crystals (solid-state conduction). Several field examples from continental MT investigations will be discussed, which roughly fall into active transpressional, active transtensional, and fossil transpressional regimes. A general challenge in interpreting data with apparent anisotropic effects is to establish the tradeoff between heterogeneity and anisotropy in the inversion of EM responses.
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