Listening gas exchanges air-water in free waters

ExSONIC: Experimental evaluation of stream atmosphere gas exchange by hydro-acoustics

Camion mesures hydro
David Vilbert and Thierry Labasque (Géosciences Rennes) were in Austria from 14 to 26 July 2019 at the Wassercluster site in Lunz am See (University of Vienna), as part of the Exsonic project led by Marcus Klaus (University of Umea, Sweden) and Jacob Schelker (University of Vienna, Austria). Mission objective: Acoustic and hydrochemical characterization of stream degassing under controlled conditions.
Mission hydro en Autriche - © Thierry Labasque
Streams need to breathe, as do humans, to stay healthy. This breathing, that is to say the gaseous exchange between water and the atmosphere (which is therefore done in both directions), notably allows the dissolution of the oxygen of the air, and thus allows living beings to breathe (fish, algae, molluques, etc.) but also allows the degassing of CO2 (greenhouse gases) which will contribute to global warming. The gas exchange is favoured by turbulence and air bubbles which, in turn, generate the characteristic sound of this exchange. An international team of researchers from the Universities of Umea and Uppsala (Sweden), Padova (Italy), Vienna (Austria), Grenoble and Rennes, have developed a new method that uses sound recordings to estimate the gas exchange coefficient (k) between air and water.

The air-water exchange coefficient (k) is one of the essential components of many studies of biogeochemical and ecological processes in aquatic systems. However, their large spatial-temporal variability is difficult to capture with traditional methods, particularly in turbulent flows. The purpose of the Exsonic project is therefore to study the potential of spectral analysis of sound to derive k in open water (in natural environment), on the basis that turbulence and associated bubbles promote gas exchange, by producing a characteristic sound, i.e. a "sound signature".
To do this, the project partners explore the relationship between k and spectral properties of sound using laboratory experiments and field observations in a wide range of turbulence and bullding conditions. They estimated k using flow chamber measurements, correlating the exchange of several gases of varying solubility (He, Ar, Xe, CO2, CH4) with the sound recorded above and below the water surface, using microphones and hydrophones.

The first results highlight the unique potential of acoustic techniques to predict k, isolate mechanisms and improve spatio-temporal coverage of k estimates in a flow.

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