A study of the physics of telluric current flow at very low frequencies in the Earth's crust

Summary. Interpretation of the effects of natural electromagnetic induction is often in terms of models in which changes in resistivity at great distances from the point of measurement are not taken into account. In this paper we will try to show that this is not a valid approximation for very low-frequency telluric currents, flowing near the surface in the Earth's crust, when the material separating the crust from the mantle has a sufficiently high electrical resistance. In this case the shape and dimensions of the circuit, as well as the resistivity distribution along the whole length of the path followed by the currents, play a part in determining the induced electromagnetic field. A number of experimentally verifiable consequences follow from this.