Detection method of identifying metallic structures below a surface with a scanning detector having a pair of laterally offset detection coils and an excitation coil arrangement symmetrically arranged with respect to the detection coils , 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 ; 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 (A) and signal phase (p) of a signal induced in the pair of detection coils 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 (R) 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 (R) 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 (R); selecting a second parameterization among a set of stored parameterizations of amplitude (A) 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 (A); 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 ; 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 (A) and signal phase (p) of a signal induced in the pair of detection coils 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 (R) 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 (R) 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 (R); selecting a second parameterization among a set of stored parameterizations of amplitude (A) 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 (A); communicating the material composition, the diameter , and the depth to a display or to a transceiver . <EOS>