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Listening to Crater Lake

Scientists with hydrophones in Mt Ruapheu's Crater Lake aren't keeping an ear out for Russian submarines. Instead, they're listening for sounds of volcanic activity.

Dr Tony Hurst of the Institute for Geological and Nuclear Science is working with French scientists in the study. He believes that the acoustic system appears to be more sensitive and have a wider range than normal seismometers.

The researchers have installed an automatic monitoring station by the crater lake. Powered by solar panels and batteries, the station acquires the lake temperature and an acoustic signal, which it processes and transmits to the researchers.

One of the problems in the project was how to transmit the data from the depths of the crater to a receiving station. The answer was to use satellites as a relay. The ARGOS system uses US satellites to provide a cheap way of receiving small amounts of data from isolated sites. Ten times a day, an ARGOS satellite passes within reach of the crater lake station and picks up data. This is relayed to scientists at the Universite de Savoie who do preliminary processing and return the results to IGNS.

A similar project by the Universite de Savoie in Indonesia found that a major eruption sequence of Kelut volcano was preceded by a rapid increase in acoustic noise in the crater lake there.

And what does it all sound like? Hurst says the dominant noises at Crater Lake are very low frequencies, at and below the bottom of the human hearing range, giving a low rumble. At some sites there are harmonically related frequencies around 100-300 Hz that would sound reasonably musical, and then a peak caused by bubbles at 500 Hz, which gives a succession of tinkling sounds.