  Cosmic Ray Muon Radiography utilizes highly-penetrating cosmic ray muons to
image the density profile of an object of interest. Here we report on the first
trial to use a portable field-deployable cosmic ray tracking system in order to
image the whole overburden of a UK railway tunnel with short duration scans (c.
30 minutes). A unknown overburden void was identified and, post-trial,
confirmed by railway authorities. These experiments demonstrate the
identification of hidden construction shafts with high levels of statistical
significance as density anomalies within the data.
