A team of scientists have developed a device to measure earthquakes in the oceans. This team comprises of Princeton researchers Frederik Simons, Associate professor of Geosciences, and Guust Nolet, the George J. Magee Professor of Geoscience and Geological Engineering, Emeritus.
In the process to test and improve the device, Frederik Simons, an associate professor of geosciences at Princeton University, set off on a ship to the coast of Bermuda, fastened a rope around a six-foot-tall white cylinder affixed with solar panels and with various wires suspended in a metal frame. Once everyone got into position, a crane slowly raised the device and prepared to lower it into the ocean. The researchers, after working three years on design modifications and fine-tuning, are preparing to test the apparatus for the first time.
The instrument is called ‘Son-O-Mermaid’. It will detect and record waves, but not the kind that are rolling under the research vessel making Simons seasick. Son-O-Mermaid measures sound waves created by the quaking of the earth far beneath the ocean surface.
Geologists can use earthquake data to survey the interior structure of the planet. With two-thirds of the Earth covered by water, MERMAID and Son-O-Mermaid are among the few instruments able to record earthquakes under the ocean.
The origin of research
The research began more than two decades ago with an idea provided by collaborator Guust Nolet, who was then a professor at Princeton and is now the George J. Magee Professor of Geoscience and Geological Engineering, Emeritus. He is also a professor of geophysics, emeritus, at the University of Nice-Sophia Antipolis in France.
While on sabbatical at the Scripps Institution of Oceanography in San Diego, California, Nolet met a colleague who was measuring sound waves in the ocean. Among the whale songs and passing freighters, Nolet detected the signal of an earthquake that came all the way from Alaska. He realized that it might be possible to use sound waves—which are the audible equivalent of seismic waves that travel through the Earth—to build a better picture of the planet’s structure in regions that are inaccessible underneath the oceans.
Geoscientists can use seismic waves, the elastic vibrations emitted by earthquakes, to map the structure of Earth’s interior. These waves travel faster when passing through colder, denser regions deep inside the Earth, such as subduction zones where tectonic plates collide and one slides under the other, and slower in hotter regions such as mantle plumes, which are upwellings of hot rock. By careful analysis of how the waves travel and change, geophysicists can create 3D maps of inside of the Earth’s crust and mantle. On smaller scales, similar principles are used to map prospects for oil, gas and other mineral deposits.
Although many seismic stations dot the continents, few have been installed in the oceans. Nolet explains that if you think of the Earth as a patient in the hospital, the present situation is like trying to do a CAT scan with two-thirds of the sensors broken. Nolet realized that hydrophones, which are microphones that record sound in water, could be deployed on mobile devices floating in the oceans. But at the time, the technology did not exist to make the project workable, and Nolet put the idea aside.
After a long gap of 10 years, Nolet brought the project to Simons, who was then a postdoctoral researcher in the Department of Geosciences at Princeton. Simons recalled, ‘Guust sent me a brown envelope marked ‘confidential”. ‘Inside was a picture of a seismogram—the record of a seismic wave—and a proposal to build a dedicated instrument that could record earthquakes in the ocean.’
Simons and Nolet envisioned deploying tens or hundreds of these instruments throughout the world’s oceans. They built the first version of the device with colleagues at Scripps in 2008 and named it MERMAID, short for Mobile Earthquake Recording in Marine Areas by Independent Divers.
Working of the earthquake detection in the ocean
Once in the water, MERMAID sinks to about a mile deep, drifts along deep ocean currents, and “listens” to acoustic signals. Nolet explained that when there is a ship passing or a whale singing it will analyze the signal and decide whether the sound is due to an earthquake or not. When it decides that it is an earthquake, MERMAID comes to the surface, takes a GPS measurement of its location, and sends a seismogram by email.
A step ahead
Before MERMAID, undersea earthquake data could be obtained only by traditional ocean-bottom seismometers, which are placed in stationary locations and must be retrieved to obtain their data. Simons explained that sometimes when one gets those instruments back, something might have gone wrong and months of waiting would have been in vain.
The high cost of manufacturing and ship time required for ocean-bottom instruments also greatly restrict their use.
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