A comparison is made using a 3-D model between the response of the magnetic prospection (M.) and that of electromagnetic prospection (E.M.) for the same feature, and the investigation depth of both methods is discussed. Also studied are: the influence of the shape of the feature and of the primary field, the significance of the measurements, and the influence of parasites. Contrary to some preceding studies, the conclusion points out that the E.M. Method is more interesting to develop than the M. Method. See also AATA 21-264.