  We present a cosmic ray muon tomographic experiment for measuring the muon
flux and imaging the tunnel overburden structures in Changshu, China. The
device used in this study is a tracking detector based on the plastic
scintillator with SiPM technology, which can be conveniently operated in field
works. The compact system with sensitive area of $6400 cm^2$ can measure the
angular distribution of cosmic muons. It's able to image the overburden density
length from the surface of overburden to the detector along the muon tracks.
The open sky muon flux measurement outside the tunnel has a good agreement with
the modified Gassier Formula model. The distributions of muon flux at three
positions inside the tunnel are very similar to that of open sky. Assuming the
average density of overburden compact sandstone is $2.65 g/cm^3$, the
overburden thickness can be obtained from the density length derived from the
difference of muon flux inside and outside the tunnel. Moreover, for known
penetrated lengths (i.e., topography of overburden), the density anomalies of
the overburden can also been obtained. This study suggests a potential
application for imaging and detecting subsurface structures in civil
engineering, tunnels or caverns with the cosmic ray muon telescope.
