Design and operation of SQUID-based planar gradiometers for non-destructive testing of ferromagnetic plates

A planar second-order gradiometer coupled to a SQUID has been used with a persistent mode coil generating 0.02T perpendicular to a steel plate for the detection and characterisation of defects, such as cracks. We outline design criteria for the system, which is contact free, allows stand-off distances of up to 10cm, and can accommodate intervening media, both insulating (e.g. concrete) or conducting (e.g. sea-water or aluminium cladding). The sensitivity limit appears to be set by long-range permeability variations in the material of the plates, which may be due to unrelieved stress. We explain the use of digital filtering techniques to improve the resolution. Finally we discuss the use of higher-order gradiometers to improve the discrimination of localised defects from background variations.