External magnetic characterization of marine vehicles

A vessel can produce an external magnetic field by the deformation of the Earth´s field, and shipboard-type electrical equipment can in addition generate permanent or low frequency magnetic fields. Geophysical ships, as well as minehunters form a specific class of magnetically silent ships, for which it is desirable to significantly reduce the external magnetic field. The authors suggest using magnetic energy as a general measure of overall source intensity. The total magnetic energy outside a boundary surface can be expressed in terms of the harmonic expansion coefficients for the magnetic scalar potential. For specific characterization of the magnetic field distribution near the source, the authors suggest the use of isodynamic surfaces of constant magnetic field energy density (BH/2). The isodynamic surfaces are useful for identifying regions where the magnetic field extends furthest from the vessel or on-board equipment. They can also be used to determine magnetic properties of the source