The main purpose of this paper is to study the effect on a natural electromagnetic field of a lateral variation in the physical properties of the ground. An exact mathematical solution is given for two media of different resistivities in contact along a vertical plane (fault) overlying a horizontal basement that is taken as being either infinitely resistive or infinitely conductive, or at infinite depth. Results are given in the form of curves along profiles perpendicular to the fault. Some practical inferences are drawn from the shape of the curves and from their comparison.