As a child, I believed that I could hear the ocean in a seashell. Now when I think about the sounds of the sea, I imagine the roar of waves crashing on the beach. But from the vantage point of a ship with noisy engines, the water seems silent.
In 1490, Leonardo da Vinci observed, “If you cause your ship to stop, and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you.” The observation that sound travels well in water became the basis for the sophisticated sonar systems developed in World War I and II for submarines. The science that formed the basis for those technologies can also be used to explore the ocean.
The ship uses acoustic energy to map the sea floor and sub-seafloor. I measure ocean currents using higher-frequency acoustic pulses that bounce off particles in the water. But I never hear those data as sound; I simply process a digital signal into maps of velocity.
Dr. Erin Pettit, a glaciologist at the University of Alaska, wants to use sound to measure the melt rates of glaciers. The interface between ice, ocean, and bedrock is a site of glacial melting but is difficult to access. It may be possible to use passive acoustics – only listening, not sending out pulses of sound – to quantify the processes in these locations. To test that idea, Erin uses hydrophones to record sounds around glaciers and sea ice.
Erin is currently working on the nearby Flask Glacier. She left the hydrophones with me and Yuribia Muñoz, a student from the University of Houston, so that we could start recording sounds without her. Today was our first opportunity: overcast skies, intermittent snow and a relatively pleasant -2.5˚C (27.5˚F). On foot, we crossed over sea ice, more than a meter thick, floating on top of 500 meters of water. After a core was drilled through the ice to study the algae within it, we lowered two hydrophones into the hole, up to 15 meters down.
What I heard from the hydrophones sounded like a quiet crackling, with some interference from the ship and people walking on the ice above. These data will allow us to relate the sounds of ice to the physical processes that control it and will hopefully show that passive acoustics can be used to quantify rates of melting.