Heating episodes, sometimes cyclic, are relatively frequent in some volcanic lakes (like Ruapehu) and usually reflect changes in the flow rate or in the enthalpy of hot fluids entering the lake. These heating episodes always represent an alarming situation because an increasing lake temperature could be a precursory signal for the renewal of magmatic activity as was observed 3 months before the 1990 eruption of Kelud volcano (Vandemeulebrouck et al. 2000).
A sensor like ASTER with multispectral TIR is crucial for obtaining accurate surface temperatures. ASTER is clearly an improvment compared to Landsat TM or ETM+ which have only one TIR band and for which the correction of atmospheric effects is much more difficult or impossible.
Delmelle, P. and Bernard, A. 1999. Volcanic lakes. In: Encyclopedia of volcanoes. Ed. H. Sigurdsson. Academic Press, pp: 877-895.
Oppenheimer, C. 1993. Infrared surveillance of crater lakes using satellite data. Journal of Volcanology and Geothermal Research, 55:117-128.
Ramsey, M., Flynn, L. and Wright, R. 2004. Volcanic Observations from Space: New Results from the EOS Satellite Instruments. Journal of Volcanology and Geothermal Research, 135 1/2.
Vandemeulebrouck, J., Sabroux, J. C., Halbwachs, M., Surono, Poussielgue, N., Grangeon, J. & Tabbagh, J. 2000. Hydroacoustic noise precursors of the 1990 eruption of Kelut volcano, Indonesia. Journal of Volcanology and Geothermal Research, 97: 443-456.
1. Development of a new method:
The retrieval of temperature by remote sensing methods is based
on the measurement of the spectral radiance emitted by the target (Planck’s
law). Radiance is a function not only of temperature but also emissivity and
the radiance (Ll) measured
by a satellite sensor need to be corrected for atmospheric effects according
Ll = t [e B(T) +((1-e)
where t is the transmissivity of the atmosphere, e is emissivity, B(T) blackbody radiance, Ld:
downwelling atmospheric irradiance and Lu: upwelling
A new technique for the retrieval of surface temperatures of volcanic
lakes with ASTER data has been developped. This technique is based on the classical
Split-Window method which has been successfully used with other satellites to
obtain accurate sea surface temperatures (SST). The Split-Window method is based on establishing
an empirical relationship between water temperatures measured at the surface
and the brightness temperatures measured in at least two TIR channels. The use
of two TIR channels enables a differential absorption measurement in order to
remove the effects of atmospheric vapor and other absorbing constituents. This
approach has the principal advantage to correct the atmospheric effects on a
pixel by pixel basis i.e. for the local atmosphere and at the exact time the
satellite is collecting TIR data. With ASTER data, the difference in brightness
temperatures between bands 13 and 14 (BT13-BT14) was calculated to remove the
ASTER On-Demand L2 Brightness Temperature at the Sensor or the AST_04 product is used for brightness temperature data. This product is available from the NASA LP DAAC web site at a small cost (80 US$). More details on these data can be found here.
The crater lake of Taal volcano (Philippines) was used as a test
site to develop the method. Taal volcano is a 15 x 22km prehistoric caldera
occupied by the large freshwater Lake Taal (LT) and an active vent complex
of the small Volcano Island where an acidic lake the Main Crater Lake (MCL) is present. The 1.2 km2 acidic lake in the Main Crater is only 2 m above the
level of LT and its volume is estimated at 40 millions m3 (Delmelle et al.,
1998). Since both lakes have almost the same elevation, the caldera lake can
be used as a reference to remove the influence of atmospheric temperatures.
Delmelle, P., Kusakabe, M., Bernard, A., Fisher, T., DeBrouwer, S. and Del Mundo, E. 1998. Geochemical and isotopic evidence for sea water contamination of the hydrothermal system of Taal volcano, Luzon, the Philippines. Bulletin of Volcanology, 59:562-576.