The monitoring of volcanic lakes in Indonesia: from ground to spacemeasurements.
A. Bernard1, A. Mazot1, B. Barbier1 and K. Huda2.
1: Université Libre de Bruxelles, Belgium.
2: DVGHM, Indonesia.

Indonesia has a chain of 129 active volcanoes extending nearly 7000 km from the Northernend of Sumatra through Java, Sulawesi and Halmahera. Several eruptions of these volcanoes
have been catastrophic in the past both in terms of the loss of human life and the partial or
total destruction of infrastructure and economy. Millions of Indonesians are currently living
on volcanoes and civil infrastructures are progressively developing on their slopes. Due to theeconomic and demographic pressures that exist in Indonesia today, it is clearly not possible toprohibit settlement of population around these active volcanoes. One of the challenges forIndonesia during the 21st century will be to use and manage this land resource as well as tominimize the risk to humans and long-term economic effects of future eruptions.
15 active Indonesian volcanoes have their crater filled with an aqueous lake. These lakespresent a hazardous situation because they contain large volume of waters like Dempo inSumatra with 8.5 millions m3 or Ijen in Java with 35 millions m3. None of these lakes isequipped with a monitoring instrumentation and their geochemistry or internal physicalstructure is generally poorly known.
Many of these lakes are located in remote areas with difficult accessibility and it is obviousthat the use of spaceborne platforms is an advantageous and cost effective solution for themonitoring of these remote lakes. The recent developments of new sensors like ASTER withan enhanced resolution and multispectral capabilities offer a new way to monitor changes inthe thermal activity of these lakes. We have developed a new algorithm based on the classical
Split-Window method for the retrieval of accurate surface temperatures from volcanic lakes(Figure 1).

Figure 1: temperature data for Ijen lake recorded by ASTER during 2000-2005. Lake
temperatures peaked at 48.1°C on July 13, 2004. During the same period, signs of increasing
activity were reported by local observers. See Bulletin of the Global Volcanism Network,
11/04.

A large majority of volcanic lakes have acid-sulfate-chloride (ASC) waters which reflect thedirect absorption of magmatic gases into a sub-surface hydrothermal system.
In Indonesia, two lakes have a contrasting composition where neutral Na/K chloride waters
dominate the chemistry: Kelud in Java and Segara Anak (Rinjani) in Lombok. Neutral Na/K
chloride fluids are usually typical of aged hydrothermal systems where the emission of acidmagmatic gases has stopped and the acidity has been completely neutralized by fluid-rockhydrolysis reactions. The discharge of neutral-chloride fluids at the top of volcanic edifices
which have experienced significant magmatic eruptions in the recent past is a relatively raresituation.
For Kelud lake, we have data collected during the past 12 years (1993-2005). These data show
a continuous evolution of both temperature and composition of the lake. The initial lakechemistry (1993-1997) was dominated by Na-K chloride waters. Today, Ca-Mg sulfate waters
are the main component in the lake. This evolution suggests that two independenthydrothermal systems with distinct compositions are present within the volcanic edifice andcontribute to the chemistry of the lake waters.
Two notable events were observed in 1996 and 2001 when a sudden increase in laketemperatures occurred. In both events, the lake temperature peaked at 50°C and a strong
increase in the intensity of the degassing from subaqueous fumaroles was observed. The twoheating episodes lasted about 18 months and were not followed by any eruptive activity.
Heating episodes, sometimes cyclic, are relatively frequent in some crater lakes and reflectchanges in the flow rate or in the enthalpy of hot fluids entering the lake. However, theseheating episodes always represent an alarming situation because an increasing laketemperature can be a precursory signal for the renewal of magmatic activity as was observed 3
months before the 1990 eruption (Badrudin, 1994; Vandemeulebrouck et al. 2000).

Figure 2: temporal evolution of the chemical and isotopic compositions of Kelud volcanic
lake during 1993-2005.