Earth Observatory Blog
Clever Contraptions – All Digital, One Analog
Although a number of seafloor samples will be cored towards the end of our four-week expedition, the main data-acquisition instruments aboard the research vessel Marion Dufresne are an echosounder and a sub-bottom profiler, both mounted on a gondola attached to the ship’s hull. These two instruments will produce the bathymetry that is so essential to the mission of the MIRAGE.
To understand how these two instruments work, I spoke with Ms Elodie Duyck and Mr Bertrand Beunaiche, both of whom are studying hydrography and oceanography at Ensta Bretagne in Brest, France. In a way, they suggested, hydrography is bathymetry, since it teaches students to use equipment such as echosounders to map the seafloor, as well as to make sense of the data these machines produce.
For the MIRAGE, we will use the Marion Dufresne’s Kongsberg EM 122, which is a 12 kilohertz (kHz) multibeam echosounder, designed to perform in as much as 11 kilometres (km) of water. That’s well beyond the 3-km depth we will mostly encounter in the Wharton Basin.
One of the most important characteristics of the EM 122 is its frequency, a low 12 kHz. When acquiring data in deep water, there are lots of opportunities for attenuation (signal loss) and absorption (in which the signal is absorbed by the mass of the water). Low-frequency acoustic waves are not 100-percent immune from either condition, but they are far less susceptible to them, which is why low-frequency echosounders are widely used in deep-water research. For similar reasons, the frequency of the ship’s sub-bottom profiler is even lower.
Of course, there’s much more to acquiring bathymetry than flipping a switch and sitting back while the data roll in. One must understand the characteristics of the water the sound waves will travel through during their 3-km journey from surface to seafloor and back again. Key variables include the water’s salinity, temperature, and pressure, which increase with depth. Together, these variables are expressed as celerity, which is essentially the speed of sound in water.
To determine celerity, one fires a small probe off the aft deck. The components of this device are actually quite simple — upon being fired, a weight containing a temperature sensor pulls a slender thread of copper about 1,200 metres until the copper reaches the end of its spool and snaps free. In a matter of minutes, information about temperature and salinity is relayed back to the ship to determine the water’s celerity — computer models are then used to extrapolate the celerity beyond the length of the copper wire, completing the picture needed for accurate bathymetry.
In addition to the echosounder and sub-bottom profiler, a scalar magnetometer is also used. It is towed behind the ship rather, than attached to its hull, because the ship’s metal construction and large complement of electronic hardware would interfere with its readings. The magnetometer helps scientists deduce magnetic anomalies related to seafloor spreading. As one scientist told me, the magnetometer will not provide new data — we already know that the Earth’s poles have flipped their polarity more than 40 times over the last 80-million-years — but when the data we collect is combined with existing data, it will create a clearer picture of seafloor spreading.
Finally, there is the log book, a curiously analog item aboard a ship that is so digital and buzzing with electronics. In the log book, basic information such as time, date, latitude, longitude, speed, and course are recorded. This happens every 15 minutes, 24 hours a day, necessitating rotating teams of scientists and students to make sure the log book is always up to date.
When I asked Ms Duyck if this wasn’t a redundant activity since all of this data was being captured electronically anyway, she likened the log book to a map that can help a scientist find her way through enormous data sets. And besides, she said, computers can fail.
To continue to follow the progress of MIRAGE, please check the EOS blog throughout the month of July, and spread the word using #MIRAGEcruise.
All photographs are taken by Ben Marks, unless otherwise stated.