Measuring the melt rate of glacier ice with underwater noise

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The development of a novel measurement of glacier ice melt rate is proposed using the underwater noise generated by bubbles bursting from the ice as it melts. The motivation for this work lies in the critical role melting plays in ice-ocean interactions in polar regions, and consequent sea level rise, combined with the great difficulty currently encountered when making direct or indirect measurements of ice melting. Glacier ice contains bubbles of air, trapped by falling snow and compressed over time by increasing pressure due to the accumulation of overlying ice. The explosive release of gas trapped within the bubbles as ice melts at the front of the glacier radiates an intense pulse of sound into the water, making glacial bays and fjords some of the noisiest places on the planet. Because bubble release rate increases with increasing ice melt rate, it should be possible to use bubble-generated noise to quantify melt rate. This project will contribute to STEM workforce development by providing support for the training of a post-doctoral associate. It will continue existing international collaborations with colleagues in Poland and Singapore. Outreach will occur through contributions to an innovative, planned exhibit at the Birch Aquarium ? Expedition Earth! Finally, a successful project will be an initial step in the development of a potentially transformative remote sensing technique for observing glacier-ocean interaction.This project will utilize a two-pronged effort to initiate development of an acoustic methodology for observing the melt rate of marine-terminating glaciers: direct measurement of the vertical distribution of noise sources on a terminus with a hydrophone array and recording the melting noise from large numbers of blocks of ice whose physical properties have been measured. An approach for the analytical analysis of the field measurements and initial development of an inversion technique is based on earlier work modeling noise radiated from bubbles in breaking waves. Data will be collected in front of a small group of glaciers terminating into a single fjord to address the following questions that presently limit the usefulness of acoustic data:1. What is the density, mean internal pressure and distributions of radii of bubbles in randomly selected samples of glacier ice, and how do these distributions correlate with ice density, which is a proxy for ice depth?2. How does the bubble source spectrum vary with hydrostatic pressure, bubble radius, internal gas pressure, and water temperature?3. What is the variability in the distributions of bubble properties between glaciers, and what are the minimal set of ice property measurements required to generalize the method from one to many glaciers? That is, what variables must be measured on a glacier-by-glacier basis to make the technique work across many glaciers?