Gravity Anomaly Over the Washout Zone of the St. Petersburg Metro

The washout zone in the St. Petersburg metro is an accident between the stations "Lesnaya" and "Ploshad Muzhestva." This accident occurred on April 8, 1974. The accident was caused by the impact of quicksand in the tunnels. The washout area was closed twice. The train service was restored on June 26, 2004, when new roundabout tunnels were commissioned. In the summer of 2018, at night, the authors performed the ground gravimetric survey along two profiles (above the old tunnel and above the new tunnel). High accuracy of observations was achieved (3 μGal for gravity survey and 5 mm for referencing absolute values of height). High accuracy allows to use the pickets as the points for gravitation field monitoring. As a result of processing, local anomaly of the gravity field was revealed. The position of the anomaly coincides with the position of the quaternary paleochannel. The width and the morphology of the anomaly made it possible to estimate the parameters of the incision-surface of the paleo-canyon.