19322 IEEE SENSORS JOURNAL, VOL. 21, NO. 17, SEPTEMBER 1, 2021 Measuring the Magnetic Polarizability Tensor Using an Axial Multi-Coil Geometry Toykan Özde˘ ger , John L. Davidson , Wouter van Verre , Liam A. Marsh , Member, IEEE, William R. B. Lionheart , and Anthony J. Peyton Abstract —The Magnetic Polarizability Tensor (MPT) is a representative property of an electrically conducting or mag- netic object that includes information about the object’s char- acteristics such as, shape, size and material. The MPT is especially relevant to metal detection (MD) and can be used to improve MD performance by helping to distinguish between objects. This paper describes an instrument intended to mea- sure the MPT of objects such as anti-personnel landmines and metallic clutter, up to 130 mm in diameter. The instrument uses a novel multi-coil geometry to generate a uniform elec- tromagnetic field over the volume containing the test object to accurately determine the MPT. Performance tests of the system shows peak variance in the MPT is approximately 15 mm 3 . Typical experimental repeatability is better than one percent for tests involving copper disks. Additionally, simulated data as well as previously published simulated and experimental data are used as a validation method of the experimental results. Good agreement between these and the measured MPTs of example targets are seen, proving the system’s capability of characterizing metallic objects. Index Terms— Electromagnetic Induction Spectroscopy, Magnetic Polarizability Tensor, metal detection, metal classi- fication. I. I NTRODUCTION A NTI-PERSONNEL (AP) landmines continue to be a significant problem in post-conflict areas as they can remain armed long after the conflict has ended. Clearing an area of landmines is a slow and expensive process and so landmines may remain undisturbed for many years. This renders large areas of land unusable, preventing people from using this land for agriculture, social and economic activity, or housing. However, the biggest problem is that a landmine is victim activated and cannot distinguish between military personnel or civilians. Therefore, landmines in post-conflict areas mostly cause damage to non-military people [1], [2]. Manuscript received May 10, 2021; accepted May 28, 2021. Date of publication June 14, 2021; date of current version August 31, 2021. This work was supported in part by the Engineering and Physical Sciences Research Council under Grant EP/R002177 and in part by the Sir Bobby Charlton Foundation. The associate editor coordinating the review of this article and approving it for publication was Dr. Jürgen Kosel. (Corresponding author: Toykan Özde˘ ger.) Toykan Özde˘ ger, John L. Davidson, Wouter van Verre, Liam A. Marsh, and Anthony J. Peyton are with the Department of Elec- trical and Electronics Engineering, The University of Manchester, Manchester M13 9PL, U.K. (e-mail: toykan.ozdeger@manchester.ac.uk; j.davidson-2@manchester.ac.uk; wouter.vanverre@manchester.ac.uk; liam.marsh@manchester.ac.uk; a.peyton@manchester.ac.uk). William R. B. Lionheart is with the Department of Mathematics, The University of Manchester, Manchester M13 9PL, U.K. (e-mail: bill.lionheart@manchester.ac.uk). Digital Object Identifier 10.1109/JSEN.2021.3088809 It is estimated that there are tens of millions of buried AP landmines worldwide, which means with the current rate of demining, it will take many years to clear all post-conflict areas of landmines [3], [4]. Metal detection (MD) has been the common way to detect landmines in humanitarian demining, which has more recently been augmented by ground penetrating radar (GPR). However, land in post-conflict areas usually have a high density of metallic clutter in the soil, which results in a high false alarm rate (FAR) from MD, which can hamper the demining process. In some fields, deminers can find 100 clutter objects for each landmine [5]. Clearly better differentiation would help. The Magnetic Polarizability Tensor (MPT) is a representa- tive electromagnetic object property, which depends on the size, material and shape of an object. Recent progress in mathematical theory has demonstrated that the MPT is the coefficient array in the first term in an asymptotic expansion of the perturbed field [6]–[9]. Although the motivation of this paper is toward humanitarian demining, the MPT and Electromagnetic Induction Spectroscopy have also been suc- cessfully applied widely in MD, for example to the detection of unexploded ordnance [10]–[12], walk-through metal detec- tors [13], metal recycling [14], workpiece recognition [15], non-destructive testing [16]–[18] and buried object detec- tion [19]. MPT and broadband inductive sensing have also been previously proposed for landmine detection [20]–[25]. This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/