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Bubbling Waters -- What hydroacoustics can tell us about volcanic gas emissions

Ben Roche

Abstract

Since time in memorial humans have reported a correlation between the number of bubbles they see in the waters around a volcano and its relative activity. The underlying theory is incredibly simple; a volcano releases volatile gases which escape into overlying water bodies as bubbles, the more active the volcano the more bubbles. Validating this relationship has however proved an illusive problem, primarily due to difficulties accurately measuring the volume of gas released from subaqueous seeps. Hydroacoustics holds the key to solving this problem. By analysing the spectral acoustic emissions of a subaqueous gas seep (i.e. listing to the sounds of the bubbles), it is possible to determine the size and numbers of bubbles being released each second, thus accurately quantify the volume of gas flow. By correlating these measurements with traditional methods volcanic monitoring, we are able empirically prove (or disprove) a relationship and identify new pre-eruption behaviours. This talk will introduce the topic of bubble acoustics, explain the acoustic inversion process and present several case studies relating volcanic activity to quantified subaqueous gas emissions including Poas (Costa Rica), Panarea (Italy), Dziani Dzaha (Mayotte) and Askja (Iceland) as well as outline future work using new distributed acoustic sensors (DAS).