Imaging the conduit size of the dome with cosmic‐ray muons: The structure beneath Showa‐Shinzan Lava Dome, Japan

Abstract

We developed a radiographic technique to image a subsurface conduit shape using cosmic‐ray muons. The test measurement was performed in Showa‐Shinzan lava dome located in Hokkaido, Japan as an example. A muon detector with an area of 6000 cm2 was set up at the foot of the lava dome. Muon tracks recorded in nuclear emulsion films in the detector were analyzed to determine the level of energy absorption along different ray paths through subsurface beneath the lava dome. A typical angular resolution of the muon detector of 10 mrad corresponds to a spatial resolution of 10 m at a distance of 1 km, which is difficult to be addressed with seismological technique. We mapped differentially absorbed cosmic‐ray muons, which depend upon the varying thickness and density beneath the dome. We successfully imaged the conduit shape and determined a conduit diameter of 102 ± 15 m, assuming the observed high absorption region beneath the dome is localized in the vent area.

Possible application of compact electronics for multilayer muon high-speed radiography to volcanic cones

“…Since we did not have three-dimensional tomographic information in 1910 lava dome, we cannot assume the shape of the conduit. However, by assuming the density of dacitic lava in 1910 lava dome and the surrounding soil are both close to the average density measured in 1944 lava dome (Showa shinzan; 2.7 g cm −3 ) and surrounding soil (1.8 g cm −3 ) with muon radiography (Tanaka et al, 2007b), it is concluded that the circular conduit cannot explain the data. As can be seen in Fig.…”

“…4), muon radiography was already applied by Tanaka et al (2007b) and the result was interpreted by Tanaka and Yokoyama (2008). The temporal sequence of the dome formation was visualized by the results of repeated precise levels (Minakami, 1947 andYokoyama, 2002): the top of magma branch was below the aquifer that ranged from 200 m b.s.l.…”

“…Based on this test measurement it was clarified that the BG rate with four PSDs are comparable to the muon penetration flux for 1000 m rock. Tanaka et al (2007b) reported that the observation time to resolve the density structure of the similar sized target (Showa-shinzan lava dome in Fig. 4) was four months.…”

“…In 2007, density distribution of 1944 Usu lava dome (Tanaka et al, 2007b) was measured with muon radiography. Considering the typical thickness of ∼ 500 m of the target volume, the muon intensity of 3000 sr day −1 is expected.…”

The MU-RAY project: Summary of the round-table discussions

“…The muon telescope will be available for the radiography of other volcanoes, in particular Stromboli. The methodology, established and successfully applied to several volcanoes in Japan (Tanaka et al, 2005(Tanaka et al, , 2007a(Tanaka et al, -c, 2008Tanaka and Yokoyama, 2008) relies on the observation of the absorption by the rock of quasi-horizontal muons produced by the decay of secondary particles in the cascades originated by the interaction of the primary cosmic radiation with the earth atmosphere. So far, angular resolutions down to 30 mrad were obtained with this method, corresponding to spatial resolutions of 30 meters for a detector positioned at 1 km from the center of the volcano.…”