We have developed a portable assembly type cosmic-ray muon telescope system with power-effective real-time
readings to monitor the internal structure of a volcano. Using this system, we have performed measurements at
the Satsuma-Iojima volcano and studied the feasibility of using a continuous flux of cosmic-ray muons over the
observation period. The system is based on the measurement of time-dependent muon absorption along different,
nearly horizontal paths through a solid body. The rationale is that one can deduce the time-dependent changes
in the density distribution of muon absorption in the interior of the object where an absorption variation, i.e., a
density path variation, becomes an intensity variation since the muon energy spectrum is exponential or, expressed
otherwise, it drops rapidly when the energy threshold increases. The muon telescope, which has a surface area
of 1 m2, was installed at the observation point located 1.2 km from the summit crater of Satsuma-Iojima. Muon
tracks within scintillator layers in the telescope were analyzed continuously by real-time three-dimensional image
processing to measure the level of absorption along different ray paths through the summit crater region. A typical
angular resolution of the muon detector of ±16 mrad corresponds to a spatial resolution of ±20 m at a distance
of 1.2 km. Our results show the density structure determined in Satsuma-Iojima volcano, Japan, which is located
above sea level. A density structure situated above sea level can be analyzed at a resolution that is significantly
higher than is possible with conventional geophysical measurements.