Conductivity tomography for non-destructive evaluation using pulsed eddy current with HTS SQUID magnetometer

In standard eddy current nondestructive evaluation, the depth of investigation is restricted to a limited range around the skin depth of the chosen excitation frequency. We developed a pulsed eddy current NDE technique which allows simultaneous analysis of the sample at all depths. The advantage of SQUID magnetometers over induction coil sensors is that the field does not decay as rapidly as its time derivative, allowing for a broader range of investigated depths. Using rectangular excitation pulse trains, eddy currents are induced in the samples. The generated magnetic response field is recorded in offtime between the transmitter pulses by an HTS SQUID magnetometer. The early time data of the transient signal corresponds to the upper layers of the sample, while the late time data deliver information from deep layers. The transient responses are averaged over a number of alternating excitation pulses in order to enhance the signal-to-noise ratio and to eliminate drift. Scan measurements of different aluminum samples are presented. From the measured data, a 3D apparent conductivity image of the sample is calculated by applying a technique known from geophysical data interpretation. From this tomographic conductivity image of the sample, the position and depth of flaws in the material is readily determined.