Geomagnetic induction in a concentrically stratified Earth

The ratios of the internal to external source terms of the earth's surface potential for certain geomagnetic fluctuations are indicative of the electrical conductivity of the earth's interior. A nonlinear first-order differential equation is derived for these ratios as functions of depth and conductivity in a spherically symmetric earth. This equation may be solved numerically to match an observable surface ratio with a conductivity-depth profile, or it may be solved analytically for a few of the infinite number of possible profiles. This approach provides a good qualitative insight into the problem of geomagnetic induction in a concentrically stratified earth.