Magnetotelluric Response of Laterally Inhomogeneous and Anisotropic Media

Reddy, I. K.; Rankin, David

doi:10.1190/1.1440579

Cited by 51 publications

(31 citation statements)

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“…Finite source fields in principle have some sensitivity to anisotropy dip, but this was judged to be weak for realistic anisotropy contrasts (Pek, 2002). Perhaps the best-known algorithm for MT responses over 2D structures with generally oriented anisotropy is that of Pek and Verner (1997), building on the work of Reddy and Rankin (1975). One motivation for this development was the observation in field data (to which we will return later) that sometimes there is a systematic discrepancy between the MT impedance principal directions and those implied by the vertical magnetic field (H z ) induction arrows.…”

Section: State Of Numerical Simulation Of Anisotropic Mt Responsesmentioning

confidence: 98%

Anisotropy Versus Heterogeneity in Continental Solid Earth Electromagnetic Studies: Fundamental Response Characteristics and Implications for Physicochemical State

Wannamaker

2005

Surv Geophys

134

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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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Section: State Of Numerical Simulation Of Anisotropic Mt Responsesmentioning

confidence: 98%

Anisotropy Versus Heterogeneity in Continental Solid Earth Electromagnetic Studies: Fundamental Response Characteristics and Implications for Physicochemical State

Wannamaker

2005

Surv Geophys

134

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“…In the first place, the FEM requires to derive integral equations (Galerkin equations) from these differential equations (for example, CONNOR andBREBBIA, 1976 or REDDY andRANKIN, 1975 noted that the internal boundary conditions are automatically satisfied and we need not treat them explicitly. This equation can be derived from the energy minimization criteria using variational approach (COGGON, 1971).…”

Section: Electromagnetic Induction For Two-dimensional Model By a Finmentioning

confidence: 99%

Two-dimensional modelling of resistivity structure beneath the Tohoku district, northern Honshu of Japan, by a finite element method.

Ogawa

Yukutake

Utada

1986

J. geomagn. geoelec

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“…medium can be represented by an arbitrary symmetric, positive-definite matrix, thus allowing the anisotropy strike, has not been too much progress whenReddy & Rankin (1975) is taken as a reference basis. Their analysis of a slightly less dip, and slant to be introduced; and (ii) under the quasisteady state conditions, the H-mode equations need not be Surface plots of the apparent resistivity pf;, and phase q$yx for the model fromFig.…”

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confidence: 99%

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

Pek¹,

Verner²

1997

Geophysical Journal International

146

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S U M M A R YAn 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.

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Magnetotelluric Response of Laterally Inhomogeneous and Anisotropic Media

Cited by 51 publications

References 0 publications

Anisotropy Versus Heterogeneity in Continental Solid Earth Electromagnetic Studies: Fundamental Response Characteristics and Implications for Physicochemical State

Anisotropy Versus Heterogeneity in Continental Solid Earth Electromagnetic Studies: Fundamental Response Characteristics and Implications for Physicochemical State

Two-dimensional modelling of resistivity structure beneath the Tohoku district, northern Honshu of Japan, by a finite element method.

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

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