Inductive detection of underground metallic pipes

A widely used instrument for locating underground metallic pipes consists of two coils: one of which (the transmitting coil) excites eddy currents in the pipe, and the other (the receiving coil) detects the magnetic field of. these eddy currents. Irregularities in the response of this type of instrument appear during certain search operations. These irregularities cause uncertainty about the position of the line of the pipe. They are explained quantitatively in this paper. The analysis shows how the reaction field of the pipe varies with the depth and diameter of the pipe, and with its electrical and magnetic properties. A theoretical estimate of the diameter of the smallest pipe that can be detected is given.