Guilizzoni, Roberta; (2018) A resonant system for electromagnetic induction imaging of concealed conductive targets. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The possibility of revealing the presence and identifying the nature of electrically conductive targets is of central interest in many fields, including security, medicine, industry, archaeology and geophysics. In many applications, these targets are electromagnetically shielded by other external materials and thus cannot be directly accessed and detected. Hence, material interrogation techniques are required that allow penetration through the shielding materials, in order for the targets to be identified. Electromagnetic interrogation techniques represent a powerful solution to this challenge, as they enable penetration through conductive shields. Two resonant electromagnetic induction imaging (EII) methods, based on the use of LCR circuits, were developed in this research work. These proof-of-principle EII methods were based, respectively, on position-resolved-measurements of resonant frequency and Q-factor shifts, which occurred as a consequence of eddy current induction inside the conductive targets to be detected. The proposed techniques were applied to 2D imaging of conductive targets (having conductivities σ ranging from 0.54 to 59.77 MSm-1), both unshielded and shielded by an aluminium shield (1.5-mm-thick). The experimental results achieved in the first part of this work highlighted a limitation in the LCR resonant circuits used for EII investigations, linked to the Q-factor low absolute 5 values (between 7 and 23). Therefore, investigations were conducted, leading to the implementation of an improved version of the EII system, based on active bandpass filters. The sensitivity of the novel EII system was found to be larger by a factor of 3.5 compared to the previously used LCR-based system, when both systems were used for imaging copper. Additionally, the new system allowed achieving images with higher contrast. The research work reported in this thesis led to establishing a proof-of-principle method for EII of conductive samples, also in the interesting scenario where the samples were shielded by conductive shields preventing them from being directly accessed.

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