Decomposition of Ferromagnetic Signature Into Induced and Permanent Components

In this article, we present a new versatile algorithm for the decomposition of the ferromagnetic signature into permanent and induced magnetization components. It can be directly implemented at existing magnetic ranges for a more robust and less error-prone calibration of degaussing systems on naval vessels. A source model is formulated for describing the signature. The model consists of two collections of magnetic dipoles, representing the permanent and induced magnetization. The induced part of the source model depends on the heading of the ship relative to the geomagnetic field. A general linear relation between the induced moment and the external magnetic field is assumed. The parameters of the model are determined by formulating and solving a regularized inverse problem using magnetic-field data from multiple passages of the ship over the sensors. Data from at least three passages with different headings are co-processed to obtain an estimate of the model parameters. The analysis uses raw data from magnetic ranges without any assumptions on the induced field. We demonstrate how to apply the algorithm with two examples. The first is a finite-element study, with a single sensor line with three-axis magnetometers, where a surface ship is modeled by a generic geometry and magnetization state. The second is a study of a real ship engine, in an indoor magnetic ranging facility with full control of all parameters. For both, field maps of the permanent and induced fields are calculated and compared to data using the resulting source model.