Non-invasive monitoring survey in Beppu hot spring area using gravity, passive image interferometry, and InSAR

This paper presents the findings of a comprehensive non-invasive monitoring survey of the reservoir in the Beppu hot spring area, which has the largest discharge and the highest average temperature among hot springs in Japan. Interferometric synthetic aperture radar (InSAR) indicates a settlement of 15 mm over the past decade, even in areas with active geothermal use, demonstrating negligible subsidence compared to that in other geothermal areas reported globally. Gravimeters and seismometers were installed at the surface in two locations: one where geothermal manifestations and usage were particularly active and another where there were no discernible geothermal signs. Comparative observations over two and a half years revealed irregular variations in gravity and velocity at the high-geothermal activity site. Seasonal changes in gravity and velocity were observed at another site. The results of each non-invasive monitoring can be explained as follows: although gas saturation changes inferred from gravity and velocity occurred in the subsurface, reservoir volume reduction had not yet reached the threshold required to trigger significant subsidence. Monitoring gravity and velocity can reveal short-term variations in the subsurface and potentially contribute to the prevention of subsidence due to overproduction as well as InSAR deformation time-series.