Page 15 - WINTER2019
P. 15

of iceberg calving is mostly evident in the sub-500-Hz band and persists for several seconds, whereas the noise of melting glacier ice dominates the noise from around 1 kHz to several tens of kilohertz or higher and is generated without interrup- tion. Other intermittent noise sources include breaking waves on the fjord shoreline, marine mammal vocalizations, rain, wave-iceberg interactions, and the sounds of iceberg disin- tegration. Noise from freshwater outflows from the glacier terminus is thought to generate sound at frequencies below 100 Hz, but not much is known about this source of sound at the present time. Anthropogenic noise from cruise ships, small transport vehicles, and acoustic sensors such as echo sounders and acoustic Doppler profilers can also be present.
The mechanical and acoustical properties of glacier ice play an important role in determining the character of the underwater soundscape in the bay of a glacier terminus. Ice mechanical properties, combined with ocean temperature and other factors such as rain, control how frequently calv- ing events occur, the range of iceberg sizes produced, and the integrity of the ice block as it impacts the sea surface. All these parameters influence the underwater sound of calving.
Remarkably, most glacier ice contains numerous, small bubbles of compressed air (see Figure 4), giving it unique acoustical properties. Trapped at the base of the firm layer in the accumulation zone of the glacier, the bubbles become compacted and pressurized over time by the overburden pressure of accumulating ice above. Gas pressure in glacier bubbles in western Greenland can exceed 2 MPa or 20 atmo- spheres (e.g., a car tire is typically pressurized to around 2 atmospheres; Scholander and Nutt, 1960). Ice-containing bubbles with such high pressures behave in interesting ways. When collected from a terminus bay directly after a calv- ing event, extreme examples of ice containing high-pressure bubbles may fracture explosively during boat transport or fracture into large sections while being cut for processing. The sounds made by the pressurized air bubbles as they escape are coined “Bergy seltzer” (e.g., see, and cubes of glacier ice have been used to both chill and enliven beverages with their pops and cracks.
The subject of air bubbles trapped in glacier ice is com- plicated by many factors including, for example, bubble size and density that depend on the snowfall rate in the glacier accumulation zone; bubbles that can be altered (or removed entirely) if the glacier ice melts and refreezes; and the bubble shape that varies from almost spherical to ellip-
Figure 4. A section of glacier ice showing the inclusion of many small air bubbles.
soidal or even more distorted depending on shear in the ice flow. Notwithstanding the details of bubble production, transport, and heterogeneous distribution, their presence in glacier ice is ubiquitous.
The journey of a bubble trapped in glacier ice may take hun- dreds to thousands of years, but when the terminus bay is finally reached, the bubbles are released into the ocean. The release of bubbles under high pressure by melting ice can be explosive, creating a loud and impulsive burst of sound. The cacophony from millions of bubbles ejected into the ocean every second can be heard up to several kilometers from a glacier terminus (see the band of frequencies labeled “ice melting” in Figure 3, inset). The bubbles also influence the transmission of sound through the glacier ice (e.g., Meyer et al., 2019).
Remote Sensing Using Ambient Sound
Can underwater sound in the bays of tidewater glaciers be used to study glacier-ocean interactions, particularly melt- ing, calving, and outflow? Specifically, can ice mass lost from calving and melting and outflow rate be quantified from measurements of their underwater sound signatures?
A hydrophone is placed on in the water column some distance from the glacier terminus and used to record underwater sound, perhaps over a year-long period. The noise signal contains information about the intensity and statistics of
  Winter 2019 | Acoustics Today | 15

   13   14   15   16   17