To study the acoustic signal precisely in a crater lake we carried out an acoustic measuring remote station and installed it in November 1994 in the crater lake of the TaaL volcano (Philippines). This station acquires every fifteen minutes temporal samples of the acoustic signal (at a 40 meters depth) in three different frequency bands: low frequencies( <500hz), intermediate frequency (<12,5 kHz) and high frequencies (<500kHz). Data processing aIIow the representation of a sonogramme. An important variation of the acoustic signal was induced by the earthquake of Mindoro (magnitude 7.1) of 15 November 1994 located at 46 km of Taal. The thermal power brought to the lake falls rapidly of 50%, the gas flow is slowed down a tenth. Liquid and gas flows return in their initial state only 5 days afterwards. A transient phenomenon is observed in all the measured parameters: tiltmeters, acoustics, thermal power. From the temperature measurement of the Taal crater lake and weather values, we made a calculation of the energy contribution of the volcanic system to the crater lake from 1990 to 1995, this shows a renewal of the activity of the Taal volcano since 1990, the calculated thermal power was increasing from 50MW to 250MW. Gas flow measured are too low to bring such a power, that it is in Taal or Ruapehu, we conclude that heat is brought mainly by Iiquid exchanges. We in addition show that the variations of the slope of the volcano are strongly correlated with the variations of the thermal power. The measurement of acoustic signal in a volcanic medium is thus a promising method which deserves to be implemented in other volcanoes.
