Detection of metal deformation using a high-T-c rf SQUID planar gradiometer

Using a high-T-c radio frequency superconducting quantum interference devic e planar gradiometer and eddy-current method, artificial deformation and ho le defects on an aluminum plate were successfully detected in both the ampl itude and phase signals, even though they were covered by 2 or 4 mm aluminu m plate. To reduce the influence of boundary effect of metal structure, a d ifferential method was used. All the experiments were done in our laborator y of unshielded environment. (C) 2001 Elsevier Science B.V. All rights rese rved.