Using high-T-c SQUIDs, we have designed a nondestructive evaluation system
to probe buried flaws. The frequency of the excitation magnetic fields was
varied from 10 to 800 Hz and the magnetic fields generated from the eddy cu
rrent was lock-in detected. The SQUIDs output data revealed the most effect
ive excitation frequencies for the flaws with different depths and widths.
The signal-to-noise ratio of the deep flaws can be further enhanced either
by using higher excitation fields or using the SQUIDs with higher low-frequ