Method and apparatus for providing permanent magnetic signatures in buried cables and pipes to facilitate long-range location, tracking and burial depth determination

摘要

Ferromagnetic material of an elongated object, such as a fiber optic cable, is permanently magnetized in a manner that results in a substantially cylindrically symmetric radial external magnetic field around the object. The produced magnetization is substantially greater than natural magnetization of the ferromagnetic material by the earth's magnetic field. The radial external magnetic field varies periodically along the length of the object. If the wavelength of the periodic variations is long compared to the width (diameter) of the object, the strength of the radial external magnetic field decreases approximately inversely with distance from the object for distances that are small compared to the wavelength. The periodic magnetic field variations may have a square wave or a sine wave pattern, for example. Magnetization of the ferromagnetic material is achieved by a magnetizer having pairs of counter-rotating mirror-image magnets adjacent to an object that is moved longitudinally relative to the magnets. By virtue of the magnetization, buried objects, such as pipes and cables can be detected magnetically at substantial distances from the objects.