Page 16 - WINTER2019

The Underwater Sounds of Glaciers
underwater noise sources, which include the splashing sounds of calving events, bubbles bursting out of the glacier terminus as it melts, and low-frequency sounds generated by ice fracture, movement, and submarine freshwater out- flow. But the signal also contains other sources such as the sounds of melting and disintegrating icebergs along with other potential sources such as ships, marine mammals, breaking waves, and rain. Moreover, the sound is influenced by propagation effects in the ocean waveguide and reflection from the terminus. The ocean waveguide contains the usual complications that arise when considering the propagation of sound through the ocean, which include scattering and coherent reflection from the sea surface and seafloor and scattering and refraction from the thermohaline structure in the ocean interior (see the article by Dall’Osto in this issue of Acoustics Today).
Recent progress has been made toward using the sounds of both calving and ice melting to quantify ice mass loss and melt rates from a glacier terminus. Here we discuss each of these topics in greater detail.
The Sounds of Iceberg Calving
Anyone who has observed an iceberg calving event can attest to its drama; booms and cracks accompany an ice fracture event followed by a splashing ocean entrance and the forma- tion of a mini tsunami. The impact of many tons of ice on the sea surface also creates underwater noise. The two first stud- ies of the underwater sound signature of calving events were conducted independently in Svalbard (see bit.ly/347NuVF; Tegowski et al., 2011) and Alaska (Pettit, 2012). The under- water sound of calving from above the waterline (subaerial calving) is most pronounced at frequencies below 1,000 Hz (see Figure 3, inset). Calving noise typically persists for sev- eral seconds and is energetic and clearly discernable above other, more persistent sources. There are distinct phases of a calving event that generate sound: (1) infrasound rumble at the onset followed by (2) ice fracturing and cracking, (3) block-water impact, (4) iceberg oscillations, and (5) surface wave action.
Of these various processes, water entry is the most energetic and spectacular. Figure 5 illustrates three phases of sound
Figure 5. The sounds produced by a block of ice falling into a pool of water. A-D: distinct, sound-producing phases of block impact: ice-water impact (A), cavity formation (B), and cavity pinch-off (C and D). E: these phases are annotated in a time-series plot of acoustic pressure. E: red letters, letter designation in A-D. The most energetic phase of sound production occurs with cavity pinch-off, photographed both above the water surface (C) and below the water surface (D).
16 | Acoustics Today | Winter 2019