Deploying Instruments on the Seafloor in the Deep Ocean
Oceanic plates are born at mid-ocean ridges, where hot mantle rocks are brought very close to the surface, partially melt, and then cool and crystallize. The newly formed rocks move outwards from the mid-ocean ridge, making way for the next batch of hot rock rising from below. Inch by inch, over millions of years, oceanic plates progress through a life cycle of birth at the mid-ocean ridge, cooling and aging in the open ocean basins, and destruction at a subduction zone, where they dive back into the mantle.
Because rocks contract inward as they cool, oceanic plates deepen considerably with age: from approximately 2500 meters depth at mid-ocean ridges to as much as 8000 meters depth in subduction-zone trenches. The NoMelt study region has matured to a middle-aged 70 million years (a plate age roughly equivalent to 40 human years), and sits at a seafloor depth of just over 5000 meters. That’s 3 miles of seawater, with the temperature at the bottom just above freezing – a very inhospitable environment to deploy our seafloor equipment.
Four days after departing Honolulu, we began deploying ocean-bottom seismometers (OBS) and seafloor MT instruments, over a grid spanning 360 miles by 250 miles. The instruments come in four flavors, designed for different types of measurements, but they have several components in common. First, they all deploy via “free fall” – they are hoisted over the side of the ship using a crane, and dropped into the water. They weigh several hundred pounds each and sink to the bottom within a few hours. Each contains a sensor such as a seismometer or a magnetometer, a low-power computer to record the data, and acoustic transceivers capable of receiving and replying to simple commands, such as “turn on” or “reply to this ping.” All are stocked with a battery supply capable of running the instrument for the duration of the experiment – up to a year for some instruments. All of these electronic components are housed in precisely engineered aluminum tubes and glass and titanium spheres designed to withstand the crushing pressures at 6000 meters below the sea surface.
Our deployment strategy poses some risks. We cannot ensure they land nicely in good spots on the seafloor. The combination of pressure and corrosion continuously wears on the instrument over a year-long deployment, and it can be difficult to withstand. If a problem occurs, then the instrument and any data it contains may be lost. And problems do occur – glass spheres implode, aluminum cases corrode and leak, instruments can float prematurely to the surface because they accidentally release from their anchors.
Tiny “upgrades” in instrument design can prove catastrophic. In one legendary case, a new disk drive was just heavy enough to make the anchorless instruments neutrally buoyant; instead of floating to the surface at the end of the experiment, they hovered 10 meters above the seafloor, never to be seen again. But there is no affordable alternative for deploying equipment on the seafloor in the open ocean, and over the last 15 years, the seafloor geophysics community (see www.obsip.org) has learned many lessons for minimizing the risk.
Working around the clock for four days, our team of technicians (from Scripps Institute of Oceanography and Woods Hole Oceanographic Institution), students, and PI’s deployed 61 OBS and nine MT instruments. Our time is tight, so we dropped them over the side and moved quickly to the next site, never knowing whether they reach a safe resting place on the bottom.
In a little over a week, we will return to recover 34 of the OBS (short-period instruments designed specifically to record the airgun shots from the Langseth) and two of the MT instruments. Only at that point will we truly learn if the deployment has been successful. We will not know the fate of the remaining 27 OBS and seven MT for another year.