The newest and most dangerous island in the world is about to get a robotic sentinel. Since bursting to life 1,000 kilometers south of Tokyo in 2013, a massive marine volcano called Nishinoshima has erupted dozens of times, spewing red-hot lava that engulfed a neighboring island. As the volcano has grown, so has the risk it represents to 2,500 people living on the nearby Japanese archipelago of Ogasawara. Should Nishinoshima’s rocky slopes collapse during an eruption, they could trigger a deadly tsunami that would reach the Ogasawara islands within 20 minutes.
Now scientists are planning to give the islanders a robotic protector: a $180,000 autonomous Wave Glider drone that harvests wave and solar energy to move and power itself for months at a time. The bot is kind of a high-tech surfboard connected to an array of underwater wings that convert the up-and-down bobbing of waves into forward motion, moving the machine as fast as three knots.
In successful tests last year the Wave Glider provided real-time surveillance of the volcano. It serves as a crucial link, relaying messages from an innovative tsunami detector thousands of meters below the waves to satellites overhead that broadcast a warning to people. The robot should start full-time patrols off Nishinoshima this May, providing crucial early warning of tsunamis, 24/7.
The need for a tsunami detection system became clear in 2011, after the Thoku earthquake spawned a giantwave that wrecked the nuclear power plant at Fukushima and killed thousands of people along Japan’s northeastern coast. Like most tsunamis, this one followed a massive temblor at sea, but others are caused when offshore volcanoes erupt or collapse.
Geologist Hiroko Sugioka, then a chief scientist at the Japan Agency for Marine–Earth Science Technology (JAMSTEC), realized that a mobile system stationed near fault lines or smoldering volcanoes could be simpler and cheaper than a permanent observatory. “A buoy costs 10 times as much as a Wave Glider,” Sugioka says. “Moreover, it needs an anchor and thousands of meters of strong wire, requiring a large ship and days to deploy. A Wave Glider can be launched in 10 minutes from a small boat, and go back to a port for maintenance by itself.”
The key part of JAMSTEC’s system is a Vector TsunaMeter (VTM). This device, which rests on the seabed 4,000 meters below the surface, combines a pressure gauge to detect changes in the water level with an electromagnetometer. Salty ocean water is a good electrical conductor. As a huge mass of displaced seawater—the start of a tsunami--moves through the Earth’s magnetic field, electric fields are generated, which in turn induce secondary magnetic fields. The VTM correlates disturbances in these fields with pressure changes to detect how big a tsunami is and where it is headed. It can operate on battery power for a year.
Sugioka programmed a Wave Glider to keep station directly above the VTM. If the tsunameter detects a tsunami, it uses an acoustic communication device to send an alert to the Wave Glider, which then beams the warning to a satellite. The entire process from a tsunami passing over the VTM to Sugioka receiving an alert takes just three to four minutes. “Considering that it took 30 minutes for the Thoku tsunami to attack the coast after the arrival of the seismic waves, we think that’s good enough,” Sugioka says.
The robot’s journey to Nishinoshima was stormy, with an early version getting lost at sea for weeks before being netted by bemused Japanese fisherman hunting for swordfish. That experiment failed because the robot kept turning in too tight a circle, twisting the cable running between the surface surfboard and the underwater wings several meters below, reducing its speed and mobility. The drone was then at the mercy of currents that swept it into the fishermen’s nets.
Sugioka revamped the system. The robot is now almost impossible to sink, can cope with typhoon-force winds and even avoid approaching ships. If it does get into trouble, it will send an e-mail message home. In 2014 it had its first success, detecting a micro-tsunami less than one centimeter high associated with a magnitude 8.2 earthquake in far-off Chile.
In October 2016 Sugioka, now with Kobe University, took the combined system to Nishinoshima. The Wave Glider has its own time-lapse camera, microphone and hydrophone to monitor the island. It circled Nishinoshima for a day, maintaining contact with the VTM the whole time.
Sugioka thinks the Wave Glider is now ready to graduate. In May 2017 she plans to deploy the tsunami monitoring system to operate autonomously at Nishinoshima for three months or longer. If it works well, Japan, which is home to about 10 percent of world’s active volcanoes, might roll out a network of surfing, tsunami-spotting drones. “There are many isolated island and submarine volcanoes in Japan, where our system could be a powerful tool for remote monitoring,” she says. When Japan’s next big tsunami inevitably rolls in, it could find people—and robots—ready to meet it.