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The Underwater Sounds of Glaciers
To relate this signal to the ice melt rate requires informa- tion about the density of the bubbles in the ice along with the knowledge of the distribution of gas pressures within the bubbles. A final, critical piece of information required is the fraction of trapped bubbles that are released explosively. This number presumably depends on the ice microcrystalline properties, including its tensile strength and fracture tough- ness, which can vary with temperature and ice history at the terminus. The pressure differential across the bubble cap ice film, which is the difference in pressure between the gas in the bubble and the external pressure, is also important. The external pressure is equal to the hydrostatic pressure of the ocean at the depth of the glacier ice if it is below the sea sur- face and exposed to the ocean.
The fraction of explosive bubble release events decreases at the differential bubble cap pressure decreases, and con- sequently hydrostatic pressure plays an important role in controlling the generation of sound by the glacier terminus. Hydrostatic pressure increases with increasing water depth, which tends to suppress the occurrence of explosive bubble release events and consequently decreases the noisiness of ice melting at greater depths. Measurements of the vertical directionality of the noise radiated by four glaciers in Horn- sund fjord in southwestern Svalbard show that radiation is limited to a layer of ice that extends roughly 20 m below the sea surface. This effect is very important for the estimation of melt rates because the overall level of sound produced is sig- nificantly reduced from what the level would be if the entire melting terminus were generating noise.
Distant Connections
An account of glacier hydroacoustics would not be complete without mention of the “singing icebergs” (Müller et al., 2005). Icebergs can be kilometers or larger in scale, which is large enough to support flow within internal tunnel/crevasse systems and which is thought to create fluid flow-induced vibrations. The signals are in the same spectral band as the harmonic volcano tremor and have similarities in terms of their duration, magnitude, and spectral features.
Iceberg tremor signals observed in the Antarctic have been backtracked to icebergs over distances greater than 800 km. Icebergs of this scale also produce disintegration sounds when they break apart. These are short-duration, broad- band signals in the frequency band of 1-440 Hz, with average sound pressure levels reaching ~220 dB root-mean-square (rms) re 1 μPa at 1 m (Dziak et al., 2013). Talandier et al.
(2002) have reported hydroacoustic signals from large ice- bergs in the Ross Sea, Antarctica, detected by seismic stations in Polynesia, demonstrating that signals from large Antarctic icebergs are detectable at basin-scale ranges.
Challenges and Opportunities
Exploiting the natural sounds of tidewater glaciers to study their dynamics and ice-ocean interactions provides both dif- ficult challenges and exciting opportunities. Notwithstanding the logistical difficulties of collecting a long-term data series of underwater sound in glacial bays, the greatest challenge lies in converting the sounds to quantitative signals, such as the average melt rate of a glacier terminus or the mass of ice lost through calving. The signal, whether from melting, calving, or some other process, is inevitably influenced by propagation through the ocean waveguide, which must be understood and accounted for. If this is possible, the equiva- lent source level then must be inverted for the geophysical process creating it. Natural variability in the sound genera- tion mechanisms, caused by, for example, variation in the shape of an ice block and its angle of entry into the ocean or the microscale tensile strength of melting glacier ice, must be understood. Recent research has made some progress on these issues, but much work remains to be done. If success- ful, the vision of Schultz et al. (2008) for the hydroacoustic monitoring of tidewater glaciers may prove to be a powerful tool for understanding the fate of these critical systems.
Acknowledgments
We acknowledge the contributions of our colleagues Mandar Chitre, Mateusz Moskalik, and Jarosław Tegowski to this article.
References
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Dziak, R. P., Fowler, M. J., Matsumoto, H., Bohnenstiehl, D. R., Park, M., Warren, K., and Lee, W. S. (2013). Life and death sounds of iceberg A53a. Oceanography 26, 10-13. https://doi.org/10.5670/oceanog.2013.20.
Glowacki, O., Deane, G. B., Moskalik, M., Blondel, P., Tegowski, J., and Blaszczyk, M. (2015). Underwater acoustic signatures of glacier calving. Geophysical Research Letters 42, 804-812.
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18 | Acoustics Today | Winter 2019