Phase split investigation of different magnetotelluric data polarizations in electrically anisotropic media



Electrical anisotropy in the earth, the effect of current density dependency on the electric field orientation in a medium, has been considered significantly in recent Magnetotelluric (MT) observations. Several suggestions for electrical anisotropy are based upon MT observations of "phase splits", analogous to shear wave splits in seismology. The MT phase data is accentuated more than its amplitude responses since shallow small scale conductivity heterogeneities cause a significant distortion in MT amplitude responses (known as Galvanic Distortion), while MT phase responses remain immune.
To investigate the MT phase response we will use the tensor representation of the MT phase introduced by Caldwell et al. (2004). This representation has the advantage of considerably simplifying the analysis of the Galvanic distortion effect. Moreover no assumption about the dimensionality of the underlying regional conductivity structure is essential.
The properties of a generally asymmetric "phase tensor" are best understood in terms of the tensor's graphical representation as an ellipse (figure1). The tensor principal axes and principal values correspond to the major and minor axes and the lengths of the corresponding ellipse radii, respectively.
For a uniform conductivity half-space, a circle of unit radius represents the phase tensor at all periods. More generally, if the conductivity is both isotropic and 1-D, the radius of the circle will vary with the period according to the variation of the conductivity with depth. For example, the radius will increase if the conductivity increases with depth. In the 2-D case the strike of the regional conductivity distribution defines a natural orientation for the coordinate system with the x-axis parallel to the structural strike direction. The phase tensor principal axis is parallel and perpendicular to the strike direction of the regional conductivity structure.
The results of anisotropic models discussed in this paper were computed with the 2D anisotropic resistivity modeling code of Pek and Verner (1997) that uses a finite difference algorithm and makes it possible to consider 2D structures with an arbitrarily oriented anisotropy.
The numerical examples investigated in this paper show that MT phase response of an electrically uniform but anisotropic half-space is independent of the polarization direction and no phase split occurs. Using several 1D and 2D anisotropic models, it is demonstrated that MT phase splitting results from spatial differences or gradients in conductivity, not the inherent bulk properties of the anisotropic conductivity tensor. Hence in this respect MT phase splitting is fundamentally different from shear wave splitting in elastic anisotropy investigations.