Precise remote-monitoring technique of water volume and temperature of a crater lake in Aso volcano, Japan: implications for a sensitive window of a volcanic hydrothermal system

Abstract

A high-resolution Digital Surface Model and a commercial digital camera have enabled precise and continuous monitoring of the crater lake at Aso volcano. From July 2006 onwards, infrared (IR) thermometry has been used with this system, enabling more accurate measurements of lake volume and temperature based on simple and intensive observations than has been possible in any other previous studies. The heat discharge remained largely constant at approximately 220 MW, with the exception of an abrupt increase to 280 MW that coincided with a rapid decrease in the water level in August 2007. Simultaneously, an increase in temperature at a shallow depth was suggested by other observations. The crater lake was found to respond to even slight changes in volcanic fluid supply, which can be well quantified by our method. Thus, a crater lake can be monitored more precisely than subaerial fumaroles whose energy estimation is often accompanied by large uncertainties. Our monitoring technique of a crater lake provides information on the subsurface hydrothermal system beneath it, for which any in-situ measurements are practically impossible.

Thermal Activity Monitoring of an Active Volcano Using Landsat 8/OLI-TIRS Sensor Images: A Case Study at the Aso Volcanic Area in Southwest Japan

“…The temperature of the lake water is almost 60 °C in the calm period and is much higher (if the water even exists) in the active period. The observed heat discharge rate of this lake was consistently found to be about 220 MW in the calm period, except in 2007 when it increased to 280 MW as the water level rose [10]. The heat discharge rate in a new fumarole of the Yoshioka hot springs was estimated to be, since 2006, about 4.6 × 10 6 W, which is much higher than that found in a previous study of thermal activity in these fumarolic areas [8].…”

Volcanic smoke reduction in visible and thermal infrared imagery

“…Accordingly, it is likely that the apparent temperature of the lake surface in the right bottom side of the lake on the image (47-48 °C) is closest to the actual water temperature. Given that the IR thermometry gives systematically lower values than direct measurements from a buoy by 8.3 °C, which is empirically confirmed by Terada et al (2012), and assuming the skin effect (slight decrease in water temperature in a boundary layer at the very surface of the lake; Oppenheimer 1997) as 2 °C (Terada et al 2008), the real water temperature on that day is estimated to be 53-54 °C. Besides the zonal distribution, we recognize several hot spots in the central part of the lake.…”

“…8, in which we recognize a significant rightward shift and convergence of high-temperature component, while the background component of less than 20 °C stays almost unchanged. Conversion from surface temperature to heat discharge using the method of Terada et al (2008), based on Ryan et al (1974), results in an increase of approximately 20%, from ~ 18 to ~ 22 MW. As is seen from the visible imagery (Fig.…”

“…The first crater of Aso volcano hosts a hot acidic crater lake, which is locally called Yudamari in non-eruptive periods (Terada et al 2008). Temperature of the crater lake provides important information in evaluating the thermal discharge rate from the lake surface (Terada et al 2012;Terada and Hashimoto 2017).…”

“…Obviously, the greatest benefit of digital photographing is affinity to automatic recording and computer-based processing. For instance, Terada et al (2008) monitored the water level of the acidic crater lake of Aso volcano for 15 months by automatic interval recording of photographs from a fixed ground site. Rouwet (2011) collected and analyzed the photographs that were manually taken from the same spot at the crater rim of El Chichón volcano to discuss the morphological changes of the crater lake during the period 2001-2007.…”