Earth Planets Space, Vol. 62 (No. 2), pp. 119-129, 2010
Hiroyuki K. M. Tanaka1, Tomohisa Uchida2, Manobu Tanaka3, Hiroshi Shinohara4, and Hideaki Taira1
1Earthquake Research Institute, the University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo 113-0032, Japan
2Department of Physics, the University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
3Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
4National Institute of Advanced industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
(Received November 28, 2008; Revised May 15, 2009; Accepted June 5, 2009; Online published February 22, 2010)
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.
Key words: Cosmic ray muon module, density path variation, time-dependent muon absorption, solid body, density structure.