MAGNETOTELLURIC ANALYSIS USING MOHR CIRCLES

The Mohr circle, most commonly met in the analysis of mechanical stress, is used to depict magnetotelluric impedance information, taking the real and quadrature parts of magnetotelluric tensors separately. The magnetotelluric concepts of two-dimensionality, three-dimensionality, skew and anisotropy are then all given quantitative expression on a diagram, as are various magnetotelluric invariants. In particular, a new invariant, the ''central impedance,'' becomes evident in a discussion of effective impedances. Some insight is gained into impedance rotations, and an anisotropy angle is defined, analogous to skew angle. Mohr circles are also tested to depict the effects of the shear and twist operations on a regionally two-dimensional structure. Generally, the application of shear or twist results in an impedance tensor with a Mohr circle of typical three-dimensional form.