  Muon radiography is an innovative and non-destructive technique for internal
density structure imaging, based on measuring the attenuation of cosmic-ray
muons after they penetrate the target. Due to the strong penetration ability of
muons, the detection range of muon radiography can reach the order of hundreds
of meters or even kilometers. Using a portable muon detector composed of
plastic scintillators and silicon photomultipliers, we performed a
short-duration(1h) flux scanning experiment of the overburden above the
platform and tunnel of the Xiaoying West Road subway station under
construction. With the observation direction facing up, the detector is placed
on the north side of the track and moved eastward from the platform section
inside the station to the tunnel section. The scanning length is 264m and a
total of 21 locations are observed. By comparing the observed and predicted
values of the muon survival ratio at different locations, the experiment
accurately detects the jump in thickness at the interface of the platform
section and tunnel section. Furthermore, unknown anomalies caused by random
placed light brick piles and side passage mouth above the observation locations
are detected and confirmed later. This experiment verifies the feasibility of
using natural muons to quickly detect abnormal structures of the overburden of
tunnel, and shows that muon radiography has broad application prospects in
tunnel safety and other similar aspects.
