  We present a comprehensive mathematical study of the Magneto-Telluric (MT)
method, on bounded domain in $\mathbb{R}^3$. We show that electrical
conductivity and magnetic permeability, assumed to be $C^2$, can be uniquely
recovered from MT data measured on the boundary of the domain. The proof is
based on the construction of complex geometric optics solutions. Furthermore,
we obtain a unique determination result in the case when the MT data are
measured only on an open subset of the boundary. Here, we assume that the part
of the boundary inaccessible for measurements is a subset of a sphere.
