The performance of the superconducting quantum interference device (SQUID) nondestructive evaluation (NDE) system with a shielded magnetometer and unshielded gradiometers were investigated. In the magnetometer NDE system, the environmental noise was reduced with the SQUID seating inside a μ-metal cylinder, whereas the environmental noise was eliminated with electronic subtraction in the gradiometer NDE system. For the shielded magnetometer NDE system, the signal-to-noise ratio was improved with a higher excitation field to enhance the defect signal. For the gradiometer NDE system, the signal-to-noise ratio was improved as a result of the reduction of environmental noise. The performance of two SQUID NDE systems is compared and the results are discussed.
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