The inventive detection method identifies metallic structures 2 below a surface 3 with a scanning detector 1. The scanning detector has a pair of laterally offset detection coils 8, 9 and an excitation coil arrangement 31, 32 symmetrically arranged with respect to the detection coils 8, 9. A movement of the scanning detector 1 along a path on the surface 3 is determined with a movement sensor 26. A sinusoidal excitation current 33 is injected into the excitation coil arrangement 31, 32. A frequency of the sinusoidal excitation current 33 is switched among a set of frequencies which contains a first frequency in a coupling regime between 15 kHz and 30 kHz and a second frequency outside the coupling regime. Signal amplitude A and signal phase p of a signal 7 induced in the pair of detection coils 8, 9 is measured for several positions along the path by an electric sensor. A material composition of the metallic structure is determined based on the sign of a derivative of the phase p along the path. An amplitude ratio R for signals of the first frequency to signals of the second frequency is determined. A first parameterization is selected among a set of stored parameterizations of amplitude ratios and diameter 12 relations based on the determined material composition. A diameter 12 of the metallic structure 2 is determined based on the first parameterization and the determined amplitude ratio R. A second parameterization is selected among a set of stored parameterizations of amplitude and depth 11 relations based on the determined material composition and determined diameter 12. A depth 11 by which the metallic structure 2 is buried below the surface 3 is determined based on the second parameterization and the signal amplitude A. The material composition, diameter 12, and depth 11 are transmitted to a display 21 or to a transceiver 51.