Abstract: According to the method, a primary electromagnetic field, defined by a waveform of known shape and containing a plurality of frequency components, is directed towards a target zone. Signals are received in the vicinity of the target zone which are attributable to eddy currents in conductors present in the target zone and direct coupling with the primary field. A reference signal having a complex waveform similar to that of the primary field is generated artificially and the reference signal is compared with the received signals in a digital computer in such a manner as to minimize the amount of signal present in the received signal which is attributable to direct coupling with the primary field. The resulting received signal may then be correlated with a plurality of stored reference waveforms in order to obtain the best fit.