Darrer, BJ; (2017) Electromagnetic induction imaging through metallic shields. Doctoral thesis , UCL (University College London).

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Text B.J. Darrer - Electromagnetic induction imaging through metallic shields - University College London - 2017.pdf - Accepted Version Download (11MB) | Preview

Abstract

Electromagnetic induction imaging has wide potential application in the disciplines of medicine, security, industry, geophysics and scientific research in general. The present study focuses on the applications in the security industry and in particular on providing a new tool for cargo screening in the context of the detection of illicit trafficking of special nuclear materials. The thesis reports a proof-of-concept study of electromagnetic imaging of metallic objects concealed inside electromagnetic enclosures. The sample object is imaged via phase variation measurements between the driver and sensor coils due to inductive coupling between the coils and the object, these images being proportional conductivity maps. For effective imaging through conductive barriers, subtraction of images at different frequencies was carried out in order to isolate the contribution of the concealed object. The present study validates electromagnetic induction imaging for nuclear security applications. The resolution of the system was determined using an edge detection algorithm applied to the images and found to be ~30 mm. The instrumentation employs Helmholtz coils for the driving field and an array of 20 × 20 sensor coils mounted on a wooden apparatus, with fixtures being non-metallic to magnetically isolate the experiment. Further studies were made to determine the compatibility of the modality to image in 3D by imaging Copper and Aluminium disks raised above the sensor array. The experiment gave a positive result being able to detect up to 80 mm depth (lift-off height) for 150 mm diameter disks and up to 40 mm depth for the 20 mm diameter disks. A study was performed to determine the penetrating power of the system by imaging through Aluminium enclosures of varying thickness. It was found that a Copper disk of 40 mm diameter by 2 mm thickness could be imaged through an Aluminium box even when the wall thickness was 20 mm, at 10 to 200 Hz driving frequency.

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