The Sounding of the Ocean

Whether literally or metaphorically, you’ve no doubt heard the phrase “sounding the depths” before. “Sounding” actually comes from the Old English word sund, which means “sea”, and for many millennia, humans used archaic technologies to measure the depths of the ocean. Ironically enough, we now use sound.

History of Sounding. Way back when in 1800 B.C., the Egyptians used sounding poles and lead lines to measure the depths of ye old sea. A pole, when inserted into the ocean and hit bottom, would reveal the depth of the sea floor. Pretty straightforward method, but understandably, highly unrealistic for depths greater than 30-40 feet (aka the vast majority of the ocean).

Ancienty Egyptians using sounding poles to measure the depths. Image from http://www.history.noaa.gov/

Ancienty Egyptians using sounding poles to measure the depths. Image from http://www.history.noaa.gov/

Bathymetry, the study of ocean depths, could be nominated as arguably the slowest evolving scientific field of all time. Let’s leave 1800 B.C. and fast forward to 1838. Charles Wilkes is leading the United States Exploration Expedition and in a radical departure from his Egyptian predecessors 3500 years ago, uses copper wire (!) instead of hemp line to sound the depths. Unfortunately, the copper wire proves too pliant and Charlie becomes too impatient to explore other options. Granted, I can’t blame him because who could really tolerate letting out a line for a whole day only to get a single reading? But still. This is the same guy who decided to haul a pendulum up to measure gravity at the peak of Mauna Loa in Hawaii, employing hundreds of natives to a) drag all of his crap up the mountain and b) blaze a new path even though a well-worn trail already existed. What a kook!

Anyway, the resolute Sir James Clark Ross picked up the slack a year later and got real about the whole ocean-depth sounding idea. After failing repeatedly to obtain readings, Ross pushed onward and refused to be defeated: he ordered a 4-mile long hemp sounding line to be constructed on board his vessel. In 1840, he successfully recorded the first abyssal sounding: 2,425 fathoms (2.75 miles).

By the mid-1850s, the beginning of the sounding technology revolution had begun in response to the desire to lay a Transatlantic cable. Lord Kelvin developed a sounding machine that was modified, over the next fifty years, into several variations.kelvin machine

Using Sound to Sound the Depths. Aristotle, Leonardo da Vinci, and Francis Bacon had all, in some form or another, contributed to the idea that sound could be used to measure the depths of the ocean. But it wasn’t until 1807 that the idea was revisited, however briefly, by a French scientist. Forgoing the opportunity to embellish his Wikipedia page, Dominique Francois Jean Arago floated the idea, but did not follow through on any conceivable method or means to accomplish this objective.

 

fessenden oscillator

Fessenden and his oscillator. Image from http://www.dosits.org/

Alas, matey. We finally arrive at the last century when sounding technology really went wild. In response to the sinking of RMS Titanic in 1912, Reginald Fessenden invented an oscillator that could emit sounds underwater and receive their echoes. During an experiment which confirmed the oscillators ability to detect icebergs, Fessenden curiously noted an echo that was returned only two seconds after the outgoing pulse; that echo was the bottom of the ocean.

The discovery that the oscillator could work both horizontally and vertically set off a sounding technology revolution and a flurry of ever-improving transducers. Sonic sounding devices became the new norm and for the next fifty years, the meteoric-rise in technological innovation in the field of sounding began to help elucidate the sea floor topography. And so, the relatively nascent theories of seafloor spreading and plate tectonics were born!

Modern SONAR (SOund NAvigation and Ranging). The impetus for developing advanced sonar devices was, like all good innovations in ocean technology, defense-oriented. In the 1970’s, the Navy pushed development in order to create seafloor maps for submarine navigation. By 1977, the technology was commercially used. Over the past forty-years, improvements in multi-beam sonar devices have primarily focused on increasing the resolution of the maps (via higher frequency pulses and more beams). Today, you can take a tour through a 3-D map generated by some of these enormous transducers (and sophisticated data processing software). As compared to the leadline, SONAR is the crown jewel of oceanography. 

To give you an idea, the lead-line method represents one sounding data point. With the multi-beam sonar device used to map Roatán, each ping or sound emitted from the transducer could return up to 864 pings or echoes!

To give you an idea, the leadline method returns one sounding per survey. With the multi-beam sonar device used to map Roatán, each ping or sound emitted from the transducer returns up to 864 soundings! Image from http://celebrating200years.noaa.gov/

The ship that surveyed Roatán, the RV Falkor, used a Konsberg multibeam echosounder that was mounted to the bottom of the ship’s hull (similar to what’s pictured below). After more than a year of data processing, here is a higher resolution map that has been created, showcasing Roatán’s steep topography and an awesomely deep gully toward the west end of the island. For more maps and info, check out this poster available on the Schmidt Ocean Institute’s website.

Konsberg transducer. Image from http://www.km.kongsberg.com/

Konsberg transducer. Image from http://www.km.kongsberg.com/

 

3-D view of the west end of the island.

3-D view of the west end of the island. Map created by Matt Rittinghouse

Sources:

http://www.oceanexplorationtrust.org/#!remotely-operated-vehicles/c1lm4

http://www.schmidtocean.org/story/show/1770

http://www.history.noaa.gov/stories_tales/poletobeam.html

http://www.merriam-webster.com/dictionary/sound

Carson, Rachel (1989) The Sea Around Us. Oxford University Press, USA.

Wikipedia entries for Charles Wilkes and Sir James Clark Ross

http://www.dosits.org/people/history/early1900/

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~ by twonakedapes on June 1, 2014.

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