To understand Earth’s interior, geophysicists rely on tomography. Using seismic-wave velocity data collected from networks of stations around the world, researchers produce and analyze three-dimensional images of the rock through which those waves propagated. The images provide insight into processes that include the convection of slowly deforming rock in the mantle, the movement of tectonic plates, and earthquakes. Nearly all the seismic networks, however, are located on land, which precludes seismologists from getting a complete picture of the planet’s internal churning. For example, it’s hard to measure the dynamic heat transport of hot rock up through the mantle by plumes, because nearly all of them are in the ocean.
Adding instrumentation to the 71% of Earth’s surface covered by oceans would dramatically improve the data density and provide seismologists with close-up, heretofore unseen detail of mantle plumes, fault lines, and deep-sea earthquakes. But deploying instruments in the oceans has challenged seismologists for decades due to technological and financial hurdles.
Today there are a handful of methods that geophysicists can use to probe Earth’s interior from marine environments. For several years, scientists have been collecting seismic data from in situ instruments on the ocean floor. Meanwhile, researchers advocating two other methods are making progress toward pilot or larger-scale studies. Rather than compete, those research groups may benefit most by using the different techniques cooperatively.
Ocean-bottom seismometers
In the 1930s researchers began developing seismometers that could be anchored to the ocean floor and withstand the tremendous pressures and other harsh environmental conditions. Since then, the number and quality of the sensors have increased. Still, their deployment is relatively limited: Whereas about 1900 seismometers have collected data in the ocean over the past 20 years, the nongovernmental International Seismological Centre lists about 11 500 active sensors on land globally.
The engineering of a modern-day ocean-bottom seismometer (OBS) is amazing, says Jeff McGuire of the US Geological Survey. “They’re basically like robots that don’t talk to home for a year,” he says. Scientists aboard research vessels bring OBSs to locations of seismological interest and essentially drop them overboard. Because an OBS sits on the ocean floor, it doesn’t pick up much noise from the overlying water column, and it is quite sensitive to the compressional P waves, the shear S waves, and the surface waves that every earthquake produces.
The biggest challenge to deploying OBS technology across the ocean is expense. McGuire estimates that a high-quality OBS costs about $100 000 to build, whereas a land-based seismometer is about one-fifth of that price. And those figures don’t include deployment costs, which add up because scientists need to retrieve the sensors every 18 months or so to upload data, replace batteries, and make repairs. “A lot of the expense is sending the humans out to sea once a year,” says McGuire.
Even with the downsides, researchers continue to use OBSs. Last August, NSF chose the Woods Hole Oceanographic Institution (WHOI) in Massachusetts to operate the new Ocean Bottom Seismograph Instrument Center, which will collect oceanographic seismic data for the science community. According to WHOI senior researcher John Collins, the center currently uses a network of 134 seismometers spanning the Atlantic and Pacific Oceans. Various research groups have claimed time on those instruments through 2022.
Cable hookups
About a million kilometers of cable laid by the telecommunications industry crisscross the ocean floor, shuttling internet traffic and other information from one continent to another in near-real time. That infrastructure also has the potential to help seismologists. In 2012 an international team led by Bruce Howe of the University of Hawaii at Manoa developed the Scientific Monitoring and Reliable Telecommunications (SMART) research program under the auspices of the United Nations. Over the past several years, Howe’s team and other scientists have shown that it’s feasible to attach seismic sensors to fiber-optic cable with a negligible effect on the cable’s reliability.
Seismometers known as SMART sensors would collect seismic data using a small fraction of the cable’s power and bandwidth. According to the program’s most recent workshop report, power and communication lines would connect packages of sensors to a cable about every 65 km at the repeater interfaces, where optical amplifiers boost the signal. The sensors would measure seismic-wave velocity, pressure, and water temperature.
The cables of the telecom industry can be less expensive than OBSs and can provide real-time data. Though there hasn’t been any large-scale implementation yet, at least one project has succeeded: A coaxial cable stretching from Hawaii to California that was retired and donated by AT&T was used from 1999 to 2003 by the Hawaiian-2 observatory to collect real-time seismological data. All the data are archived and freely available via the Incorporated Research Institutions for Seismology. About 150 earthquakes were recorded during the Hawaiian-2 observatory’s first year of operation.
Persuading other cable owners to share some bandwidth has been a challenge. Researchers with the SMART program are corresponding with cable owners and raising funds to develop a pilot experiment.
Floating seismometers
The shortcomings of OBSs and transoceanic cables have spurred other geophysicists to pursue a new strategy. It’s a floating network of seismometers known as MERMAID, or Mobile Earthquake Recording in Marine Areas by Independent Divers. The instruments are mobile, are about a third as expensive as OBSs, and can be deployed in remote locations without the need for a return trip to collect data. They drift and float in the water column listening for earthquakes. Each time they sense one, they record the data, move to the surface, and transmit the information to a satellite.
“We call it a near-real-time system,” says principal investigator Frederik Simons of Princeton University. “Now we can ask questions about specific places . . . at specific depths.” The MERMAID project began in 2002, and earlier this year it had its first major research success by imaging the Galápagos mantle plume. The program “has been going on an absolute shoestring budget,” says Simons, which is why it took a long time to get results. This map shows the location in the Pacific of the 25 currently deployed MERMAIDs.
The portability does come with a price: poorer-quality data. Floating in the water column, a MERMAID must contend with omnidirectional pressure waves stemming from marine wildlife, ships, and other sources. “The ocean-bottom seismometers are the Rolls-Royce devices, and MERMAIDs are the scooters weaving in and out of traffic,” says Simons. Filtering and other quality-control checks can improve the signal-to-noise ratio. There’s another limitation: Floating seismometers provide very little data from S waves, which propagate only through solids.
Simons and collaborators from institutions in China, France, Japan, and South Korea founded the EarthScope Oceans program in 2016 to coordinate efforts to build a global network of floating seismometers. Like those involved with the SMART program for seismic cable, they’re raising funds so that they can acquire and deploy more MERMAIDs. The latest generation of instruments is designed by Guust Nolet and Yann Hello of the Géoazur Laboratory in France and manufactured by the French engineering company OSEAN.
The collaboration plans to more than double the Pacific array in August, when a pair of teams led by Chen Yongshun from the Southern University of Science and Technology in Shenzhen, China, and Masayuki Obayashi of the Japan Agency for Marine-Earth Science and Technology are scheduled to deploy 35 additional MERMAIDs.
All three methods to gather seismic data from the oceans have risks, so the best data collection programs may be those that incorporate information from all of them, says Simons. If an OBS or cable network collects a faint, faraway signal, for example, a MERMAID located closer to the source could collect higher-intensity seismic data. “It’s important to jointly observe the Earth’s interior from the ocean,” he says. “We need to spread ourselves out.”
