Depth estimation of fumarolic gas source deduced by fume pressure measurement

The origin of fumarolic gas is hydrothermal boiling in the active fumarolic area and magma degassing in the active volcano where high temperature gas is discharged. The determination of the fumarolic gas source is important for understanding the geothermal activity or the eruption process. However, it is difficult to determine the gas source by geophysical prospecting using seismic or geomagnetic techniques due to the resolution limitation of these techniques. New measurement tools are therefore necessary. The aim of this study was to determine the conduit length—i.e., the depth of the fumarolic gas source—by measuring the fume pressure, which in turn facilitates the determination of the pressure oscillation of a conduit generated by acoustic resonance. We initially assembled the measuring device and measured the fume pressure at an active fumarole in Kusatsu-Shirane volcano, Japan. Our measuring device succeeded in measuring the pressure oscillation generated by acoustic resonance, revealing that the fumarolic gas source at Kusatsu-Shirane volcano has a depth of approximately 40 m. We propose that the use of an acoustic resonance is an effective approach for deducing the fumarolic gas source.