1. A method for analysing a magnetic signal and/or field registered by a multi-channel measuring device in a geometry in which an interesting source is in measurement volume V 1 , the sensors measuring the field or the components thereof outside volume V 1 inside volume V 2 , and the sources of magnetic interferences outside volume V 1 and V 2 in volume V 3 , which can be infinite, characterised in that parametrising in volume V 2 the magnetic field produced by the interesting sources disposed in volume V 1 as a sum of elementary fields, each of which is irrotational, sourceless and infinite outside volume V 1 in such a manner that for the parametrised magnetic field, a representation of the desired accuracy in volume V 2 is obtained; and parametrising in volume V 2 the magnetic field produced by the interference sources disposed in volume V 3 as a sum of elementary fields, each of which is irrotational, sourceless and infinite outside volume V 1 in such a manner that for the parametrised field, a representation of the desired accuracy in volume V 2 is obtained; and calculating the signal vector of the measuring device corresponding to each elementary field; and measuring the magnetic signal by sensors; and separating the fields that were produced from sources disposed in different volumes by calculating the components of the measured signal vector in the basis formed by the signal vectors associated with the elementary fields.