Study of volcanic activity monitoring based on continuous measurement of vdcanic gas chemistry

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Monitoring of volcanic gas chemistry has been carried out only for ambient air of fumarolic fields. Until now there have been few trial to monitor volcanic (vent) gas directly, because common gas sensors do not work well in acid, wet and hot conditions such as in volcanic gas. Therefore, we have deveroped a new continuous monitoring system for volcanic gas chemistry. After high temperature steam of volcanic gas is cooled and removed completely using three types of dehydration apparatus, the chemical compositions of thus obtained dry gas are monitored continuously using an IR sensor (CO_2 and SO_2), a semiconductor sensor (H_2)and a zirconia sensor (O_2).This system was set up at an artificial fumarole inside the summit caldera of Ozu-Oshima volcano and has been in oparation for more than 2 years. This fumarolic gas is a mixture of CO_2 of magmatic origin and air component, and the mixing ratio changes with time both in long and short time ranges. The CO_2 shows an annual variation with high content in summer and low in winter. Generally decreasing trend of summer peak values in CO_2 may be coincide with the decreasing trend in magmatic activity of Izu-Oshima volcano after big eruptions in 1996. A short-term temporal variation of the gas chemistry is related with that in atmospheric pressure. The CO_2 content is inversely-correlated and O_2 is correlated with atmospheric pressure with 8 hour's delay. In constant, the H2 content increases intermittently up to 1000ppm 1 hour after sudden drop of atmospheric pressure. The differnt behaviors of CO_2 and H_2 suggest differnt origins of each species.