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Volcanic activity frequently generates sound waves in the atmosphere. Much of the energy is below the limit of human hearing and hence is termed infrasound. Low frequency sound waves are minimally attenuated in the atmosphere allowing observations to be made from great distance and infrasound to be used for remote monitoring of eruptions. Volcano infrasound observations can be used to infer eruption properties such as total volume ejected or plume height. Harmonic infrasound, characterized by an impulsive onset and a gradually decaying coda, has been recorded at open-vent volcanoes, including Villarrica (Chile), Cotopaxi (Ecuador), and Kilauea (Hawaii). The harmonic infrasound signals are due to resonance of the air mass within the crater, similar to a brass instrument such as a trombone. The observed infrasound signal is sensitive to the crater geometry and depth. We are working on several projects examining how properties of the volcano can be inferred from the observed infrasound data.At Villarrica (Chile) the infrasound signal changed in the lead up to a paroxysmal eruption on 3 March 2015. We inverted the observed infrasound signals to estimate the position of the lava lake and showed that the lava lake rose up in the crater immediately prior to the eruption. This study demonstrated the utility of infrasound monitoring at open-vent volcanoes.At Cotopaxi (Ecuador) repeating harmonic infrasound signals of exceptionally low frequency were recorded following the 2015 eruptive period. We used the infrasound observations to estimate the crater geometry. The stable nature of the infrasound indicates that the crater geometry was unchanged in the six months after the cessation of the eruptive activity.
The physical properties of the Earth are investigated by imaging the subsurface with waves, similar to how x-rays are used to image the human body during medical procedures. Offshore, oil and gas companies uses acoustic waves, which are generated and recorded in the ocean, to image the subsurface in order to find hydrocarbon reservoirs. Seismic air guns are the predominant source. A seismic air gun works by discharging highly pressurized air into the water. The air forms a bubble that expands and contracts before dissipating. The oscillations of the bubble generate acoustic waves in the ocean, which couple to the seafloor and are used to image the subsurface properties. There is significant concern about how seismic air guns impact marine life, specifically larger marine mammals like whales that communicate via a frequency range that is similar to that of marine seismic operations. In this project we investigate controls on the air gun signal, also referred to as the source signature, and study how to make seismic air guns more environmentally friendly.
Low frequency sound generation in whales remains an unsolved problem. Here, we propose that whales could produce low frequency sounds through resonance of their lungs, modulated by air flow from the laryngeal sac.
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