1:45 PM - 3:15 PM
[SVC33-P06] Continuous observation system of CO2/ H2S ratios in volcanic gas at Hakone volcanoes
Keywords:Volcanic gas, Hakone volcano, Continuous monitoring system, CO2/H2S ratio
Hakone volcano, located in central Japan, has been active every few years since 2001, and a very small phreatic eruption occurred at Owakudani in 2015. The temporal variation of CO2/H2S ratio in fumarolic gas around Owakudani may be synchronized with crustal deformation and increase of the number of earthquakes caused by the volcanic activity (Mannen et al., 2021; Daita et al., 2021). Therefore, this geochemical observation is helpful for understanding the processes of volcanic eruptions and to evaluate the imminence of an eruption. However, conventional observation method of volcanic gas composition is low temporal resolution (at most, once a month) because field surveys are required.
The present study aims to establish and operate a continuous observation system for volcanic gas composition (CO2 and H2S).
The installation site of our observation system was the Kamiyu fumarolic field located about 300 m north of Owakudani in Hakone volcano. The fumarolic gas is suctioned through a tube with a liquid pump, and after gas-liquid separation in a plastic bottle, the gas phase is introduced into the measurement section. The CO2 and H2S concentrations in the gas were measured with a non-dispersive infrared absorption spectrometer (LI-840A, LI-COR Inc.) and a controlled potential electrolysis sensor (KHS-5P, Komyo Rikagaku Kogyo Co.), respectively. Volcanic gases were introduced for 10 minutes per hour. Data were taken every second. We used the average value for 1 minute after 8 minutes from the gas introduction for the observation data. The sensitivity of the CO2 spectrometer and H2S sensor was checked using the standard gases. Furthermore, we measured CO2 and H2S concentrations in the measurement section using the detector tube method about once a month.
The CO2 and H2S concentrations obtained by our observation system, for June to December 2022, range from 1500 ppm to 4000 ppm and from 20 ppm to 70 ppm, respectively. These concentration changes are synchronous. We corrected the obtained H2S values because the sensor sensitivity decreases over time. The CO2/H2S ratios ranged from 30 to 50. These values correspond to that of another device of the same specifications and by the detector tube method, indicating that the values of our observation system are reasonable. The temporal variation of the CO2/H2S ratio appear to be roughly linked to the frequency of shallow volcanic earthquakes at Owakudani reported by Kurihara et al. (this conference).
The present study aims to establish and operate a continuous observation system for volcanic gas composition (CO2 and H2S).
The installation site of our observation system was the Kamiyu fumarolic field located about 300 m north of Owakudani in Hakone volcano. The fumarolic gas is suctioned through a tube with a liquid pump, and after gas-liquid separation in a plastic bottle, the gas phase is introduced into the measurement section. The CO2 and H2S concentrations in the gas were measured with a non-dispersive infrared absorption spectrometer (LI-840A, LI-COR Inc.) and a controlled potential electrolysis sensor (KHS-5P, Komyo Rikagaku Kogyo Co.), respectively. Volcanic gases were introduced for 10 minutes per hour. Data were taken every second. We used the average value for 1 minute after 8 minutes from the gas introduction for the observation data. The sensitivity of the CO2 spectrometer and H2S sensor was checked using the standard gases. Furthermore, we measured CO2 and H2S concentrations in the measurement section using the detector tube method about once a month.
The CO2 and H2S concentrations obtained by our observation system, for June to December 2022, range from 1500 ppm to 4000 ppm and from 20 ppm to 70 ppm, respectively. These concentration changes are synchronous. We corrected the obtained H2S values because the sensor sensitivity decreases over time. The CO2/H2S ratios ranged from 30 to 50. These values correspond to that of another device of the same specifications and by the detector tube method, indicating that the values of our observation system are reasonable. The temporal variation of the CO2/H2S ratio appear to be roughly linked to the frequency of shallow volcanic earthquakes at Owakudani reported by Kurihara et al. (this conference).