Measurement of GMPT Coefficients for Improved Object Characterisation in Metal Detection

Magnetic polarizability tensors (MPTs) have become popular for characterising conducting permeable objects and assisting with the identification of hidden objects in metal detection for applications in security screening, humanitarian demining and scrap sorting. A rigorous mathematical justification of the complex symmetric rank 2 MPT object characterisation has been established based on the leading order term in an asymptotic expansion of the perturbed field for small objects. However, the accuracy of an MPT object characterisation is limited by the tensor's small number of independent coefficients. By considering higher order terms in the asymptotic expansion, generalised magnetic polarizabilty tensors (GMPTs) have been introduced and the purpose of this work is to show that GMPT coefficients can, for the first time, be measured in practice. GMPTs offer the possibility to better discriminate between objects and, hence, the potential for better classification and identification, overcoming the limitations of a rank 2 MPT object characterisation. In a metal detector, the low-frequency background fields generated by a set of coils is almost always non-uniform and using GMPTs allow us to make a virtue of this. In this work we include both measurements and simulations to demonstrate the advantages that using GMPTs offer over using an MPT characterisation alone.