Detecting geyser activity with infrasound

Johnson, J.; Anderson, J.; Anthony, R.; Sciotto, M.

We monitored geyser activity in the Lower Geyser Basin (LGB) of Yellowstone National Park with dual four-element microphone arrays separated by ~600 m. The arrays were independently used to identify incident coherent planewave energy, then conjoint cross beamback-azimuths fromthe two arrayswere used to precisely locate signal sources. During a week in August 2011 we located repeating infrasound events, peaked in energy between 1 and 10 Hz, originating fromat least ve independent geothermal features, including the episodically erupting Great Fountain, Fountain and Kaleidoscope Geysers, aswell as periodic infrasound fromnearby Botryoidal and persistent sound from Firehole Spring. Although activity from nearby cone-type geysers was not detected in the infrasound band up through 50 Hz, the major fountain-type geysers (i.e., with columns greater than 10 m) could be detected at several kilometers, and two minor geysers (i.e., a few meters in eruption height) could be tracked at distances up to a few hundred meters. Detection of geyser activity was especially comprehensive at night when ambient noise was low. We conclude that infrasound monitoring of fountain-type geysers permits convenient tracking of geyser activity, episodicity, signal duration, energy content, and spectral content. These parameters enable objective statistical quantication of geyser behavior and changes over time that may be due to external forcing. Infrasonic study of geyser activity in an individual basin has great monitoring utility and can be reasonably accomplished with two or more distributed sensor arrays.