Detection of deep flaws by using a HTS-SQUID in unshielded environment

We reported the nondestructive evaluation of flaws in conductive samples with the high-T/sub c/ SQUID operated at 77 K. By nulling the SQUID with an additional compensation, we can sensitively detect the excited magnetic field due to the eddy current around a deep flaw. To investigate the effects of the size and the depth of flaws on the induced magnetic field, the excited magnetic fields as a function of the depth were measured for buried slots with various widths and thickness. Besides, the amplitude and the phase with different excitation frequencies for a fixed slot size are also studied. It was found that for a fixed excitation frequency, the phase of the measured signal linearly responds to the depth of the slot, while the amplitude correlates with the height of the slot. The results are valuable for quantitative evaluation of flaws.