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

Anisotropy versus heterogeneity in continental solid earth electromagnetic studies: fundamental response characteristics and implications for physicochemical state

Practical Magnetotellurics: References

The study of electrical anisotropy in the Earth, defined as the electrical conductivity varying with orientation, has experienced important advances in the last years regarding the investigation of its origins, how to identify and model it, and how it can be related to other parameters, such as seismic and mechanical anisotropy This paper provides a theoretical background and a review of the current state of the art of electrical anisotropy using electromagnetic methods in the frequency domain, focusing mainly on magnetotellurics The aspects that will be considered are the modelling of the electromagnetic fields with anisotropic structures, the analysis of their responses to identify these structures, and how to properly use these responses in inversion and interpretation Also, an update on the most recent case studies involving anisotropy is provided

http://diposit.ub.edu/dspace/bitstream/2445/161860/1/617102.pdf

The Role of Electrical Anisotropy in Magnetotelluric Responses: From Modelling and Dimensionality Analysis to Inversion and Interpretation

In this paper, some recent topics on the modeling of magnetotelluric data are introduced. The focus is on the handling of real field data for two-dimensional resistivity modeling. First, the removal of the effects of near surface heterogeneity is reviewed. It covers telluric distortions (phase mixing and static shift) and magnetic distortions using conventional Groom-Bailey type 3D/2D model (three-dimensional local anomaly underlain by regional two-dimensional structure). The extension of a 3D/2D distortion model for multi-site, multi-frequency is a new development. Magnetic distortion seems to be less significant for land observations, but significant for sea floor data, where the regional magnetic field is weak due to seawater. In special cases involving for example, distortion due to topography and bathymetry, explicit removal is possible. There are some schemes proposed for a 3D/3D model (three-dimensional local anomaly underlain by regional three-dimensional structure). Along with the removal of the distortion, it is important to recognize the dimensionality of the dataset prior to modeling. A property using strike estimates for each site is an indicator for dimensionality which is unaffected by local distortion. Mapping the local strike or a rose diagram is an effective visualization of the dimensionality. Two-dimensional inversion is becoming routine. For the fast calculation of derivatives, approximate calculation, reciprocity or conjugate gradient methods are used. In order to incorporate a priori information and to overcome the intrinsic ill-posed nature of the inversion problem, imposing constraints on the model structure is important. A proper tradeoff between the data fit and constraints should be optimized to obtain minimum structures that are required by the field data. However, the choice of constraints is rather subjective and depends on the geological situations. For field data, two-dimensional inversion has limits on modes, area, and period range. Special care must be taken for the structure outside the profile. Two-dimensional inversion incorporating anisotropy is interesting and becoming popular, but the structure may not be unique. Future development in three-dimensional inversion for real datasets should take the above points into consideration.

On Two-Dimensional Modeling Of Magnetotelluric Field Data

Magnetotelluric impedances and parametric sensitivities for 1-D anisotropic layered media

SUMMARY An algorithm for the numerical modelling of magnetotelluric fields in 2-D generally anisotropic block structures is presented. Electrical properties of the individual homogeneous blocks are described by an arbitrary symmetric and positive-definite conductivity tensor. The problem leads to a coupled system of partial differential equations for the strike-parallel components of the electromagnetic field, Ex and H,. These equations are numerically approximated by the finite-difference (FD) method, making use of the integro-interpolation approach. As the magnetic component H, is constant in the non-conductive air, only equations for the electric mode are approximated within the air layer. The system of linear difference equations, resulting from the FD approximation, can be arranged in such a way that its matrix is symmetric and band-limited, and can be solved, for not too large models, by Gaussian elimination. The algorithm is applied to model situations which demonstrate some non-trivial phenomena caused by electrical anisotropy. In particular, the effect of 2-D anisotropy on the relation between magnetotelluric impedances and induction arrows is studied in detail.

/pdf/finite-difference-modelling-of-magnetotelluric-fields-in-two-4tvdxb6ktg.pdf

Finite-difference modelling of magnetotelluric fields in two-dimensional anisotropic media

Government Support of Science and the Impact of the Crisis: The Case of the EU Countries

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Finite-difference modelling of magnetotelluric fields in two-dimensional anisotropic media

Government Support of Science and the Impact of the Crisis: The Case of the EU Countries