Page 14 - WINTER2019

The Underwater Sounds of Glaciers
Making long-term measurements of both calving and melting on the highly resolved timescales necessary for developing predictive models of retreat is an outstanding and difficult problem. Accomplishing this for multiple tidewater glaciers is even more difficult. In response to these monitoring chal- lenges, in 2008, Wolfgang Berger and colleagues organized a workshop in Bremen, Germany, to propose the use of hydroacoustics to study tidewater glaciers, culminating in the publication of a correspondence note (see Schulz et al., 2008). They suggested that “Hydroacoustics could be used for passive listening—for example, to calving, iceberg col- lision, tidal flow, sediment transport and wind action—as well as active echo-sounding (for example Doppler detection of water and ice motions)” (Schulz et al., 2008). At the same time, some of the first measurements to record calving events were being made at Hansbreen Glacier in Svalbard (Tegowski et al., 2011) and the Meares Glacier, Prince William Sound,
AK (Pettit, 2012).
Using Ambient Sound to Study Glaciers
The idea of using ambient sound to study the ocean and the things in it, sometimes called “passive acoustics,” has been around for awhile and has proven effective at providing infor- mation across a diverse range of phenomena including the study of breaking surface waves, monitoring reef ecology, studying marine animals (Mann, 2012), monitoring volca- noes (Matoza and Fee, 2018), and probing the ocean interior structure, to name a few. Active acoustics has a much longer history. Indeed, it is arguably the most important tool ever developed to probe the ocean interior and seafloor. How- ever, the ideas that emerged from Schultz at el. (2008) and the initial measurements made an important contribution in pointing out that these powerful tools could be applied to a pressing and difficult measurement problem in polar regions: the monitoring of tidewater glaciers with hydroacoustics.
Hydroacoustics, more commonly referred to as underwater acoustics in North America, offers some practical advan- tages for monitoring tidewater glaciers over more traditional methods. Active acoustic sensing can provide data about the structure of a glacier terminus that would be virtually impos- sible to acquire otherwise (e.g., Sutherland and Straneo, 2012).
This would include water motions in the glacier bay, which can be complicated by meltwater outflows and direct melting of the terminus interacting with tidally pumped circulation. The concept of passive listening is also attractive because it provides an opportunity to monitor ice-ocean interactions on long timescales with robust and cost-effective technology and
without introducing artificial signals into the ocean. Although it may not be immediately obvious that hydrophones can sur- vive for extended periods in a glacial bay, which is subject to the passage of icebergs that may extend from the sea surface to the seafloor and is often covered with sea ice during the winter months, several groups have now demonstrated that year-long recordings of ambient noise are possible.
The subject of polar underwater acoustics, both active and passive, is a large and important field with a history dating well back into the last century. The breadth and scope of it lie well beyond our reach in this article. However, here we offer some highlights from the new and developing field of tidewater glacier acoustics along with some interesting results from a closely related topic, iceberg acoustics.
The Underwater Soundscape Near a
Glacier Terminus
The bays of tidewater glaciers are one of the noisiest places in the ocean (Pettit et al., 2015). Calving icebergs, wave-iceberg interactions, freshwater outflows and melting glacier ice all contribute to the underwater soundscape (see bit.ly/347NuVF). The variability of sound sources, in both frequency and time, are prominent features of the soundscape.
Figure 3 gives an overview of noise sources in the bay of a tide- water glacier terminus and boundary and waveguide effects influencing sound propagation (note that the spectrogram in Figure 3, inset, is from the video referenced above). The noise
Figure 3. Noise sources and propagation effects shaping the soundscape around the terminus of a tidewater glacier. Inset: spectrogram of sound versus frequency (in kHz on a log scale) and time (total duration of 1 minute) showing a calving event and noise radiated by melting glacier ice in the bay of Hansbreen Glacier.
14 | Acoustics Today | Winter 2019