MERMAIDs reveal secrets from below the ocean floor

Seismologists use waves generated by earthquakes to scan the interior of our planet, much like doctors image their patients using medical tomography. Earth imaging has helped us track down the deep origins of volcanic islands such as Hawaii, and identify the source zones of deep earthquakes.

"Imagine a radiologist forced to work with a CAT scanner that is missing two-thirds of its necessary sensors," said Frederik Simons, a professor of geosciences at Princeton. "Two-thirds is the fraction of the Earth that is covered by oceans and therefore lacking seismic recording stations. Such is the situation faced by seismologists attempting to sharpen their images of the inside of our planet."

Some 15 years ago, when he was a postdoctoral researcher, Simons partnered with Guust Nolet, now the George J. Magee Professor of Geoscience and Geological Engineering, Emeritus, and they resolved to remediate this situation by building an undersea robot equipped with a hydrophone — an underwater microphone that can pick up the sounds of distant earthquakes whose waves deliver acoustic energy into the oceans through the ocean floor.

A MERMAID recently launched near Tahiti is sending messages to the satellite before diving a mile underwater to begin monitoring for earthquake signals.

Drifting a mile below the surface, MERMAIDs cover a large area. The red circles show where a MERMAID picked up a seismic signal.

If mantle rock is hot, it will slow down seismic waves. This plot shows (in percent) how much slower the wave travels in a cross-section cutting along the longitude 91 degrees west. The plot extends all the way to the core of the Earth (2,890 km, or about 1,800 miles), and ranges from 20 degrees south latitude to 20degrees north. The reddish colors show where the waves slow down. Galápagos is near the equator, from which a broad plume-like structure near 1 degree north latitude descends to a depth of 1,900 km (about 1,200 miles).

A photo taken while developing the MERMAIDs shows one rising to the surface after it has recorded an earthquake wave. Once at the surface, it sends a seismogram via satellite to the scientists. Simons and Nolet tested early MERMAID prototypes with research partners at the Scripps Institution of Oceanography in La Jolla, California. The prototypes successfully recorded earthquake waves, but their idea continued to meet with skepticism because of the high noise level from ocean waves. But when Nolet transferred to emeritus status at Princeton in 2008 and moved to Geoázur at the University of Nice in France, funding from the European Research Council enabled him and his lead engineer Yann Hello to fully develop the MERMAIDs, which have since become commercially available.

These two Son-O-Mermaid instruments, a later generation of the seismic sensors, were brought to Bermuda by Simons and his team to be deployed in partnership with the Bermuda Institute for Ocean Sciences. In this photo, the instruments are secured on deck before deployment in the water. Unlike traditional ocean-bottom seismometers, which are placed in stationary locations and must be retrieved to obtain their data, MERMAID and Son-O-Mermaid drift with ocean currents and regularly report data back to scientists using wireless technology. Several can be deployed for the same cost as one ocean-bottom seismometer.

Joel Simon, a Princeton graduate student in geosciences whose research focuses on analysis of the Son-O-Mermaid data, adjusts cables and prepares the instrument for a test run. Simon went to Bermuda a week ahead of deployment with the engineers to unpack the pieces of the instrument from the shipping container and assemble them.

A Son-O-Mermaid instrument is attached to a crane and lowered into the ocean. The researchers will next reel out 1,000 meters (more than 3,200 feet) of cable that connect the surface buoy to hydrophones, microphones that record sound in water, which "listen" for earthquakes in the ocean. When an earthquake is detected, the device sends a seismograph by email to the scientists.

Harold "Bud" Vincent, a research professor at the University of Rhode Island and main collaborator in designing and building Son-O-Mermaid, prepares part of the instrument before deployment. After the first prototype was hit by Hurricane Sandy off the coast of the Bahamas in 2012, Simons and Vincent have worked to fine-tune Son-O-Mermaid over the last three years, adjusting part of the design to make it more robust.

A Son-O-Mermaid instrument in the water. The research team deployed and retrieved the buoys several times to test that everything was working and to identify remaining kinks. Now the task ahead is to construct a robust new generation of Son-O-Mermaid instruments to add to the growing number of earthquake recorders in the oceans. With each new instrument deployed, Simons and colleagues will help fill in the picture of our planet's interior.

This week, Nolet, Simons and an international team of researchers published the first scientific results from the revolutionary seismic floats, dubbed MERMAIDs — Mobile Earthquake Recording in Marine Areas by Independent Divers. The researchers, from institutions in the United States, France, Ecuador and China, found that the volcanoes on Galápagos are fed by a source 1,200 miles (1,900 km) deep, via a narrow conduit that is bringing hot rock to the surface. Such "mantle plumes" were first proposed in 1971 by one of the fathers of plate tectonics, Princeton geophysicist W. Jason Morgan, but they have resisted attempts at detailed seismic imaging because they are found in the oceans, rarely near any seismic stations.

MERMAIDs drift passively, normally at a depth of 1,500 meters — about a mile below the sea surface — moving 2-3 miles per day. When one detects a possible incoming earthquake, it rises to the surface, usually within 95 minutes, to determine its position with GPS and transmit the seismic data.

By letting their nine robots float freely for two years, the scientists created an artificial network of oceanic seismometers that could fill in one of the blank areas on the global geologic map, where otherwise no seismic information is available.

The unexpectedly high temperature that their model shows in the Galápagos mantle plume "hints at the important role that plumes play in the mechanism that allows the Earth to keep itself warm," said Nolet.

"Since the 19th century, when Lord Kelvin predicted that Earth should cool to be a dead planet within a hundred million years, geophysicists have struggled with the mystery that the Earth has kept a fairly constant temperature over more than 4.5 billion years," Nolet explained. "It could have done so only if some of the original heat from its accretion, and that created since by radioactive minerals, could stay locked inside the lower mantle. But most models of the Earth predict that the mantle should be convecting vigorously and releasing this heat much more quickly. These results of the Galápagos experiment point to an alternative explanation: the lower mantle may well resist convection, and instead only bring heat to the surface in the form of mantle plumes such as the ones creating Galápagos and Hawaii."

To further answer questions on the heat budget of the Earth and the role that mantle plumes play in it, Simons and Nolet have teamed up with seismologists from the Southern University of Science and Technology (SUSTech) in Shenzhen, China, and from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Together, and with vessels provided by the French research fleet, they are in the process of launching some 50 MERMAIDs in the South Pacific to study the mantle plume region under the island of Tahiti.

"Stay tuned! There are many more discoveries to come," said Yongshun (John) Chen, a 1989 Princeton graduate alumnus who is head of the Department of Ocean Science and Engineering at SUSTech, which is leading the next phase of what they and their international team have called EarthScope-Oceans.

"Imaging the Galápagos mantle plume with an unconventional application of floating seismometers," by Guust Nolet, Yann Hello, Suzan van der Lee, Sébastien Bonnieux, Mario C. Ruiz, Nelson A. Pazmino, Anne Deschamps, Marc M. Regnier, Yvonne Font, Yongshun J. Chen and Frederik J. Simons Scientific Reports, 2019, doi: 10.1038/s41598-018-36835-w. The MERMAIDs and their development were financed by ERC Advanced Grant 226837 "Globalseis". The University of Nice (now: Université de la Côte d’Azur) and the Observatoire de la Côte d’Azur contributed additional funding and INOCAR provided the vessel that launched the MERMAIDs.