An experiment of muon radiography at Mt Etna (Italy)

Abstract

Interactions of conduit geometry with gas-liquid flows control volcanic activity, implying that the evaluation of volcanic hazards requires quantitative understanding of the inner structure of the volcano. The more established geophysical imaging techniques suffer from inherent ambiguity, may require spatially dense measurements in active areas and may not provide sufficient spatial resolution in the uppermost part of the conduit system. It is thus desirable to develop new imaging techniques allowing a better spatial resolution of a volcano's upper feeding system, with reduced ambiguity and a low level of risk for operators. Muon particles can be utilized to image the internal density distribution of volcanic structures. The principle of muon radiography is essentially the same as X-ray radiography, except for substituting penetrating particles in place of photons. Muons are more attenuated by higher density parts inside the target and thus information about its inner structure are obtained from the differential muon absorption. We report on a muon-imaging experiment that was conducted at Mt Etna in 2010. The target structure was one of the summit craters of the volcano. This experiment was performed using a muon telescope suitably designed to withstand the harsh conditions in the summit zone of a high volcano. We found a marked difference between synthetic and observed attenuation of muons through the target. This discrepancy is likely due to the bias on the observed flux, arising from false muon tracks. They are caused by low-energy particles that, by chance, hit simultaneously the two matrixes of the telescope, leading to detection of a false positive. We separated the useful from the unwanted signal through a first-order model of the background noise. The resulting signal is compared with the corresponding synthetic flux. Eventually, we found regions of higher-and lower-than-expected muon flux, that are possibly related to inner features of the target crater.

Development of the very long-range cosmic-ray muon radiographic imaging technique to explore the internal structure of an erupting volcano, Shinmoe-dake, Japan

“…However, the telescopes have needed to be placed in the vicinity (0.5-1.6 km) of the volcano craters (Tanaka et al, , 2010b(Tanaka et al, , 2009a(Tanaka et al, , b, 2007aDe Lellis et al, 2014;Tanaka and Yokoyama, 2013;Carbone et al, 2013;Kusagaya et al, 2013;Lesparre et al, 2012;Hernández et al, 2013;Miyamoto et al, 2012) (Fig. 1).…”

Instant snapshot of the internal structure of Unzen lava dome, Japan with airborne muography

“…This technique has been applied to explore the internal structures of volcanoes9101112131415, faults1617, caves181920, carbon capture and storage (CCS)2122, industrial plants23, and historic architecture24. The flux of open-sky vertical muons is ~70 m −2 s −1 sr −1 (ref.…”

“…The resultant image quality depends on the number of surviving muon events that are acquired within the given observation period. Conventional land-based muographic measurements have required a measurement time of at least several days26 and sometimes longer91011121314 to image the internal structure of a volcano. This is mainly due to strong attenuation of the muon flux after passing through a long distance of rock.…”

Muon tomography applied to activevolcanoes

“…The global DAQ system is a reduced version of the OPERA one [17] and built as a network of "smart sensors", synchronized by a common GPS-disciplined clock unit. Seven Diaphane telescopes have been recording data on active volcanoes (Mount Etna in Sicily [5], the Mayon in the Philippines and the Soufrière in Guadeloupe) or in undergound laboratories (the Mont-Terri in Switzerland [2] and the Tournemire laboratory in France). The underground experiments allowed to validate the detectors design and to measure their performance.…”