| abstract |
A method and apparatus for detecting ferrous changes passing axially through a cylindrical space. In one embodiment, the method comprises surrounding the cylindrical space with a nonmagnetic cylinder having an outer wall and a cylindrical axis; creating an alternating magnetic field in the cylindrical space, the magnetic field created by a rotatable permanent magnet; monitoring the magnetic field with magnetic flux sensors placed outside the outer walls; and detecting changes in the magnetic field as ferrous matter passes axially through the cylindrical space. In other embodiments, the apparatus identifies ferrous changes as the tool joints that connect a jointed tubing string as the tubing string is moved in or out of a well bore, or as the presence or absence of a coiled tubing string in the well bore. |