<title>Instrumentation and techniques for high-resolution magnetic imaging</title>

Detection of a Deep Flaw Inside a Conductor Using a Squid Magnetometer

“…Acoustic techniques are effective at detecting subsurface voids, but cracks immediately beneath the surface are difficult to discriminate from the surface signal. Superconducting QUantum Interference Device (SQUID) magnetometers, very sensitive instruments for measuring DC and low frequency fields, have been used for detection of flaws in conducting objects [1,2,3]. By injecting DC and low frequency AC currents into a brass bar, we have detected a subsurface flaw using a SQUID magnetometer, and shown these data to be consistent with our theoretical calculation.…”

“…J is the current density on the bounding surface, and Ti is an out ward unit vector normal to the surface S. We are interested in the field component in the direction normal to the surface only, which can be measured by our MicroSQUID magnetometer [2].…”

Imaging subsurface defects using squid magnetometers

“…By injecting a spatially uniform current into the conductors, such as a plate [2,3,4] or a thin-walled tube [5], and measuring the normal component of the magnetic field using a SQUID magnetometer, we can determine the dimensions and the location of the defects. Figure 1 is the calculated contour maps of the normal component of the magnetic field for a 12 mm long slot with angle of 90 0 , 30 0 , 100 and 0 0 with respect to the current injected in the y direction.…”

“…Over the last several years, SQUID magnetometers have been utilized for detection of defects in metal structures by measuring the magnetic field produced by injected current or permanent magnetization [1,2,3]. Because the amplitude of the output signal from the SQUID magnetometer is independent of the frequency of the magnetic field to be measured, SQUIDs are suitable for dc or low frequency measurements as required for detection of subsurface flaws inside a conductor.…”

A Comparison of Squid Imaging Techniques for Small Defects in Nonmagnetic Tubes

“…Our experiments were performed at the Vanderbilt University Electromagnetics Laboratory, which has been described in detail elsewhere [2]. Measurements were made with a first-order axial SQUID gradiometer (coil separation baseline = 3 cm); its small pickup coil (diameter = 3 mm) affords good spatial resolution for relatively small defects.…”