Imaging of Small Defects in Nonmagnetic Tubing Using a SQUID Magnetometer

Abstract

Abstract Using a recently developed superconducting quantum interference device (SQUID) magnetometer system which offers higher resolution and greater sensitivity than previous designs, we have created high-resolution spatial maps of the magnetic anomalies created by small defects in thin-walled, nonmagnetic tubes. Images obtained using three different sensing methods (injected current, induced current, and ferromagnetic decoration) are presented, and compared in terms of sensitivity and signal-to-noise ratio (SNR). All three methods possess the sensitivity to detect holes on a tube's outer diameter as small as 0.37 mm diameter × 0.54 mm deep, with varying SNR, as well as holes on the tube's inner diameter as small as 1.0 mm diameter × 0.3 mm deep. The ferromagnetic decoration method produces the strongest signals and excellent SNRs. Using this approach, we have detected inner- and outer-diameter slots with dimensions 0.76 × 0.15 × 0.08 mm3. In addition, we examine results of numerical inversion calculations applied to the experimental images. With inversion techniques it is possible to determine the original current density distribution given the measured magnetic field, hence revealing the approximate shape and location of defects.