Anisotropy Versus Heterogeneity in Continental Solid Earth Electromagnetic Studies: Fundamental Response Characteristics and Implications
for Physicochemical State |
| |
| Authors: | Email author" target="_blank">Philip?E?WannamakerEmail author |
| |
| Institution: | (1) Energy & Geoscience Institute, University of Utah, 423 Wakara Way, Suite 300, Salt Lake City, UT, 84108, U.S.A |
| |
| Abstract: | 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. |
| |
| Keywords: | Electromagnetics anisotropy magnetotellurics solid-earth |
| 本文献已被 SpringerLink 等数据库收录！ |
|
