An approximate analysis of the behavior of electromagnetic waves in a conducting medium is given. The approximations consist of replacing pairs of electrodes by dipoles and of using only the first order images in the case of a layered earth. It is shown that under typical conditions one can measure the depth of an electrical interface 6,000 feet deep by means of electromagnetic waves. It is further shown that even a thin layer of salt water or oil bearing sand at a depth of 6,000 feet gives an effect that is easily measurable.The response of the earth to a d.c. step function is analysed for the case in which the displacement currents are negligible. This approximation is valid for all geological formations. The shape of a commutated direct current is shown as a function of the distance of travel.