Listening for Icebergs

We start today with a word you likely haven’t heard before: hydroacoustics. It is exactly what the name implies if you know both the root words; ‘hydro’ relates to water, and ‘acoustic’ relates to sound. Hydroacoustics is thus, as you might have surmised, the study of sounds in water.
The sounds discussed in this post were heard by the International Monitoring System, or IMS. Originally designed to verify nations were abiding by a ban on testing nuclear weapons, the IMS not only hears explosions as the waves move through water, but also picks up waves from underwater volcanoes, earthquakes, mammals that dwell in the ocean, and the topic of today’s post, icebergs.
The A38B iceberg calves in two between April 12 and 15, 2004. Image credit: NASA Earth Observatory.
The A38B iceberg calves in two between April 12 and 15, 2004. Image credit: NASA Earth Observatory.
This brings us to today’s article, “Remote hydroacoustic sensing of large icebergs in the southern Indian Ocean: Implications for iceberg monitoring” written by L. G. EversD. N. GreenN. W. Youngand M. Snellen. The article appeared in 2013 in the American Geophysical Union’s journal Geophysical Research Letters. 
Using the IMS and the waves it receives, which were automatically detected at the International Data Center, and can therefore be traced to their origin. Some signals observed were linked to earthquakes; others implied a moving source. Hello, iceberg.
Signals detected by the IMS suggested an iceberg collision and calving (splitting and shedding a section) during the Southern Hemisphere’s summer (December to March, in this case 2005-2006). Oddly enough, the hydroacoustics said the iceberg was moving to the east, while most icebergs around Antarctica move northwest.
Icebergs can also be seen and tracked by satellites. Using such a method, an iceberg designated C20 was found. Originally C20 moved to the west, but as it drifted northwards, caught the Antarctic Circumpolar Current–which moved it east. The timing and location of the iceberg as seen on satellites and heard by the IMS correlate.
Iceberg B17B, off the coast of Australia. Image taken December 11, 2009. Image credit: NASA Earth Observatory.
Iceberg B17B, off the coast of Australia. Image taken December 11, 2009. Image credit: NASA Earth Observatory.
To verify that other large icebergs could also be heard hydroacoustically, the authors noted the movement of iceberg B17B (pictured immediately above) from November 2009 to January 2010. It was also tracking east, having also been caught up in the Antarctic Circumpolar Current. It was heard for less time than C20, but like C20, satellites see it where and when the hydroacoustic signals suggested.
Hydroacoustics is not a perfect method of tracking large icebergs. They go for periods when they are relatively quiet, for example. However, hydroacoustic signals can be heard when the icebergs are not visible by satellites, i.e. during storms. Storms are more likely to cause the icebergs to calve, therefore producing more signals than normal. As one might expect, an iceberg calving or colliding with others is more hydroacoustically active than one bobbing along with the current.
This article used a simple method of determining the source of the hydroacoustic signals, without taking into account all the variations in the speed of the sound (such as those dependent on the temperature of the water). While it was effective, a more complicated method should give a higher degree of accuracy, and possibly allow signals to be distinguished by their cause. Even as it stands, hydroacoustics can certainly aid in monitoring large icebergs.

 

Bibliography
Evers, L. G., D. N. Green, N. WYoungand M. Snellen (2013), Remote hydroacoustic sensing of large icebergs in the southern Indian Ocean: Implications for iceberg  monitoring,  Geophys.  Res. Lett., 40, 4694–4699, doi:10.1002/grl.50914.
Today’s featured article.
Scott, Michon. Iceberg B17-B Adrift off the Southwestern Coast of Australia. NASA Earth Observatory. December 2009. Web.
An image of the discussed B17B iceberg, the second image in this post. Two other images of the iceberg are also shown.
NASA Earth Observatory. The A38-B Iceberg SplitsNASA Earth Observatory. 24 April 2004. Web.
The source for a series of four photos showing the calving of iceberg A38B, the first image in this post.
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