1. Phenomenological approach

1.1 Basics of conduction in the earth

Earth is a conductive medium whose equivalent resistivity ranges from 10 Ωm to over 1,000 Ωm for very rocky terrain. This physical property makes it possible to use this medium as a natural conduction element linking several distant sources. Despite its resistivity, which remains high, the volume involved can easily transform this conductor of mediocre resistivity into a "good" conductor.

As with any physical system, this problem is governed by Maxwell's equations [equations (1)...