The use of Multispectral Satellite Data to retrieve temperatures from Volcanic Lakes

Introduction

With the exception of the pioneering work of Oppenheimer (1997), satellite remote sensing has been rarely used for the monitoring of volcanic lakes. During the last few years, significant developments in spaceborne sensor technology have been achieved offering new tools for the study of volcanic lakes by remote sensing methods.

ASTER satellite launched in 1999, is probably one of the most powerful satellite for monitoring volcanic activity (Ramsey, Flynn and Wright, 2004). ASTER has 14 different spectral bands in the visible (VNIR : 0.52-0.86µm), near-infrared (SWIR : 1.60-2.43µm) and thermal infrared (TIR : 8.12-11.65µm) wavelengths. The TIR bands with a spatial resolution of 90 meters give the ability to detect small thermal anomaly (a few degrees C), perform thermal mapping and monitor temporal variations in the lake surface temperatures.

Many volcanic lakes are located in remote areas with difficult accessibility, these lakes are rarely visited or monitored. For these lakes remote sensing by satellite sensors can provide very useful information at relatively low cost.

Ruapehu lake in New Zealand and ASTER TIR image of April 11, 2001 showing hot spots at the lake surface.

Volcanic lakes act as calorimeters trapping most of the heat released by the magmatic-hydrothermal system (see CVL fundamentals).

Their temperatures are reflecting the balance between heat input from hydrothermal fluids and heat output by radiation and evaporation of the lake surface to the atmosphere.

So, lake surface temperature is a key parameter to detect any changes occurring in the activity of the volcano.

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.

References:

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 to:

where t is the transmissivity of the atmosphere, e is emissivity, B(T) blackbody radiance, Ld: downwelling atmospheric irradiance and Lu: upwelling path radiance.

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 atmospheric effects.

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.

3. Validation of the method:

The method was developped on a low elevation volcano (Taal lake is at sea level) and for a tropical (humid) atmosphere. The validity of the AST_SW algorithm was thus checked on different lakes with different elevation and atmospheric conditions. Part of this validation was performed by Laura Trunk (University of washington) who applied independtly the algorithm developped here.

Here is an example of such validation made on Ruapehu volcanic lake in New Zealand (elevation: 2797 meters asl).