Inversion of the Magnetic Polarisability Tensor at a Single Frequency from Walk-through Metal Detector Measurements

A tomographic metal detection and characterisation system has been designed and built with the intention of recovering detailed information about magnetic and/or conductive objects within the detector space. This information is gathered as a result of a “walk-through” scan of a candidate in the same manner as would be expected at a typical security metal detector archway. Following the passage of the candidate, the system uses measurements from an array of coils to calculate the polarisability tensor, which is considered to describe the low frequency electromagnetic characteristic of a small metallic object when it interacts with a magnetic field. The tensor is known to vary according to the excitation frequency of the applied magnetic field, however in this system only a single frequency measurement is considered. In addition to the magnetic polarisability dyadic tensor the position of the perturbation is also determined as a product of the inversion algorithm.The inversion algorithm used in this paper is based on the implementation of a model which is calculated by the multiplication of the transmit and the receive magnetic field strength vectors with the magnetic polarisability dyadic. The inversion algorithm seeks to minimise the error between the actual measurements and the simulated measurements through the modification of an estimated object tensor and the variation in estimated object location.The system has been tested and is capable of inverting object tensors with