Detection method of identifying metallic structures below a surface with a scanning detector having 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), the method comprising the steps: determining a movement of the scanning detector along a path on the surface with a movement sensor; injecting a sinusoidal excitation current into the excitation coil arrangement (31, 32); repeatedly switching a frequency of the sinusoidal excitation current among at least a first frequency in a coupling regime between 15 kHz and 30 kHz and a second frequency outside the coupling regime; measuring signal amplitude and signal phase (p) of a signal induced in the pair of detection coils (8, 9) for several positions along the path; determining a material composition of the metallic structure based on the sign of a derivative of the phase (p) along the path; determining an amplitude ratio for signals of the first frequency to signals of the second frequency; selecting a first parameterization among a set of stored parameterizations of amplitude ratio and diameter relations based on the determined material composition; determining a diameter of the metallic structure based on the first parameterization and the amplitude ratio; selecting a second parameterization among a set of stored parameterizations of amplitude and depth relations based on the determined material composition and the determined diameter; determining a depth by which the metallic structure is buried below the surface based on the second parameterization and the amplitude; communicating the material composition, the diameter, and the depth to a display or to a transceiver. determining a movement of the scanning detector along a path on the surface with a movement sensor; injecting a sinusoidal excitation current into the excitation coil arrangement (31, 32); repeatedly switching a frequency of the sinusoidal excitation current among at least a first frequency in a coupling regime between 15 kHz and 30 kHz and a second frequency outside the coupling regime; measuring signal amplitude and signal phase (p) of a signal induced in the pair of detection coils (8, 9) for several positions along the path; determining a material composition of the metallic structure based on the sign of a derivative of the phase (p) along the path; determining an amplitude ratio for signals of the first frequency to signals of the second frequency; selecting a first parameterization among a set of stored parameterizations of amplitude ratio and diameter relations based on the determined material composition; determining a diameter of the metallic structure based on the first parameterization and the amplitude ratio; selecting a second parameterization among a set of stored parameterizations of amplitude and depth relations based on the determined material composition and the determined diameter; determining a depth by which the metallic structure is buried below the surface based on the second parameterization and the amplitude; communicating the material composition, the diameter, and the depth to a display or to a transceiver.