Combined eddy current and leakage field detector for well bore piping using a unique magnetizer core structure

Pipe-inspection apparatus adapted for locating magnetically-anomalous defects in or on either the interior or exterior walls of a string of well-bore piping having a known minimum internal diameter, a known relative magnetic permeability and a known transverse cross-sectional metal area, and comprising:

• an elongated body adapted for suspension in such a piping string from a well-logging cable and including upper and lower longitudinally-spaced pole pieces respectively having a maximum outer diameter somewhat less than said known internal piping diameter and magnetically coupled to the opposite ends of an elongated reduced-diameter core member of a magnetic metal having a relative magnetic permeability no greater than said known piping permeability and a transverse cross-sectional metal area less than said known piping metal area;
• flux-inducing means adapted for establishing a longitudinally-oriented unidirectional magnetic flux along an incremental portion of such well-bore piping located between said upper and lower pole pieces and including a magnetizing coil coaxially wound around said core member;
• eddy-current inducing means adapted for establishing eddy currents along and adjacent to the interior wall of said incremental piping portion and including an electrical oscillator, and a plurality of output coils respectively coupled to said oscillator and cooperatively arranged at circumferentially-spaced intervals around said core member for being operatively positioned immediately adjacent to said interior wall to induce eddy currents therein;
• defect-sensing means including a plurality of differentially-wound pairs of detecting coils cooperatively arranged at circumferentially-spaced intervals around said core member for being operatively positioned immediately adjacent to said interior wall to simultaneously detect flux-leakage variations as well as eddy-current variations appearing thereon and caused by magnetically-anomalous defects in said incremental piping portion during the movement of said detecting coils therethrough;
• and
• defect-responsive circuit means operatively coupled to said detecting coils and cooperatively arranged for providing first and second output signals in response to said flux-leakage variations and said eddy-current variations respectively.