The technology of investigation of the condition of tunnel lining by probing is intended for monitoring the lining condition and searching for voids under concrete slabs.

The essence of the method is point mechanical impulse excitement of the surface of the tunnel lining, and defining its condition by the time, magnitude, frequency and phase properties of the acoustic echo-signal. The technology was applied in 2003 in the investigation of the lining of tunnels 1 and 2 (the Outwash section) between Lesnaya and Ploshchad Muzhestva stations of the St Petersburg underground, with the purpose of determining the depth of the protective reinforcing matrix layer and the existence of voids in the tubed space.