Going Wireless in the Deep Blue
Deploying instruments to monitor the ocean is one thing. Getting daily reports from them is another.
You’re exploring how currents develop in the ocean, or how salt and spray are exchanged with the atmosphere, or how superheated fluids and lava erupt from the seafloor.
You could make observations from a ship, but expeditions provide only snapshots of the ocean. They don’t allow you to observe changes over months or years, nor can you capture sudden, unpredictable events such as earthquakes or eruptions.
Instruments on satellites offer useful broad and long-term observations of the ocean. But the view is only skin deep, as cameras and lasers can only penetrate the top few meters of water.You could build a cabled observatory, running power and communications cables to an array of instruments on the seafloor and obtaining data in real time. But once built, your observatory is limited to one place on the map, usually close to a coast. And it would be expensive to build.
So how do you get long-term ocean measurements from any spot on the globe, with daily feedback and lower costs? If you are Dan Frye of the WHOI Advanced Engineering Laboratory, you take an old oceanographic concept—the moored buoy—and bring it into the 21st century with wireless technology. He calls it an acoustically linked deepwater observatory.
“The system is like a wireless computer network or hotspot,” said Frye, a senior research specialist in the Applied Ocean Physics and Engineering Department. “It allows users to move in and out in a very flexible and seamless manner, and it eliminates the complexity and expense of cables and connectors used in a conventional wired system.”
High-tech take on an old-school tool
For several years, Frye, Matthew Grund, Lee Freitag, Jonathan Ware and other engineers and buoy specialists at WHOI have been testing the limits of tried and true technology. They have searched for or developed new mooring cables and connectors. They have doubled, tripled, and quadrupled the speed of underwater modems that send data in sound waves. And they have watched and learned from the latest developments in satellite communications.
The result is a wireless buoy observatory, which the team deployed for a trial run from May 2004 to July 2005 with colleagues from the University of Washington and the Scripps Institution of Oceanography. The system was set up in 2,362 meters (7,750 feet) of water along the Nootka Fault, off Vancouver Island, a site rich with fluid seeps, mud volcanoes, hydrothermal venting, and other dynamic geologic processes.
This relatively low-cost observatory allowed researchers to observe daily what was happening on the seafloor, in the water above, and at the surface. Data were sent back to researchers on shore six times a day, with the opportunity to gather information more often when interesting oceanic events occurred. The 12-member research team could talk to the instruments—sending commands to adjust experiments on the fly—while adding others long after the initial deployment.
The keys to the system are satellite transmitters and acoustic modems. For several years, scientists have watched with great interest as commercial companies have established satellite phone and communications networks that enable conversations from anywhere in the world. The Nootka buoy was outfitted with two Iridium satellite transceivers, allowing the research team to send and receive “calls” from their offshore observatory any time of day or night.
Sounds of science
But getting information from a buoy to shore is only part of the challenge. The bigger test is getting signals from the seafloor to the surface. Instead of sending data through cables—which are costly and physically limiting—the Nootka observatory relies on sound waves. Computer codes and data are turned into sound signals—much like those passing through the phone line and modem to your home computer—and transmitted between the seafloor instruments and the acoustic modems on the surface buoy.
Once a novelty, underwater acoustic modems have been made ever faster by WHOI engineers. During the test deployment, roughly half a megabyte of data was sent home per day—a small amount by land-based standards, but a giant leap for remote sensing in the ocean.
With the wireless approach, up to 15 separate instruments can be set up as far as three kilometers (1.8 miles) away from the base of the mooring. Frye and colleagues believe the distance could be stretched even farther by using hydrophones to relay signals along the seafloor from outlying instruments to closer ones that can be heard by the surface buoy.
The distributed design allowed researchers to add a Scripps/University of Washington instrument package to the network several months after the initial installation. With no cables to attach, the research team simply deployed the new instruments on the seafloor and reprogrammed the observatory’s computer systems to listen for another new voice.
The only serious limitation is power. Current battery systems can provide enough juice to get through roughly a year of observations before they need to be replaced.
Because the acoustically linked buoy has a relatively low-cost (two to four times less than buoy systems using cables) and a self-contained infrastructure, researchers believe the system could be deployed and re-deployed almost anywhere in the world—certainly in some places not now reachable with cables.
“The success of the project has been important to the development of ocean observatories because it has demonstrated the feasibility of using acoustic links for long-term, deep-ocean applications,” said Frye. “This system can be installed and maintained using conventional, low-cost logistics as it doesn't require the use of specialized remotely operated vehicles (ROVs) or expensive ships. The sensors can be added or removed without impacting the rest of the system and, since they are independent, they are less likely to experience a system-wide failure.”
The National Science Foundation, the W.M. Keck Foundation, and the Woods Hole Oceanographic Institution provided funding for the Nootka observatory project. Satellite telemetry service was provided through the Oceans.US Iridium project.