Retrieval of voluminous defects in a non-magnetic metal plate from maps of time-harmonic anomalous magnetic fields in air due to a source nearby is performed by a contrast-source, gradient-type technique which uses the binary aspect of the sought conductivity distributions. The functional analysis within a vector inte-gral formulation of the diffusive fields is sketched and the efficiency of the approach is discussed in the light of preliminary inversion results obtained with synthetic data.