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by Helen Gibbons, George Tate, and Jingping Xu

A deepwater tripod system designed by the U.S. Geological Survey (USGS) will be the first of its kind to be deployed on the bottom of the South China Sea at depths approaching 3,000 meters (10,000 feet). Instruments mounted on the tripod will gather data to help scientists better understand how and where deep-seafloor sediment moves and accumulates. Such knowledge can be applied in deep waters adjacent to the United States as well, to determine where contaminated sediment is likely to accumulate, for example, or to choose favorable sites for undersea cables and other infrastructure.

The free-ascending tripod, or FAT, was designed by George Tate, head of the Marine Facility (MarFac) of the USGS Pacific Coastal and Marine Science Center in Santa Cruz, California. Tate and engineering technician Peter Harkins built the tripod at MarFac, and on February 6, 2013, a team from the science center—Tate, Harkins, Hank Chezar, Joanne Ferreira, Kurt Rosenberger, and Jingping Xu—tested the new system at the Santa Cruz Municipal Wharf. The tripod passed with flying colors, clearing it for use later this year in the project “In-Situ Observation of Bottom Currents and Sediment Transport in the Northeastern South China Sea,” a study co-sponsored by the USGS and Tongji University. The project is funded by the National Natural Science Foundation of China (NSFC). USGS participation is headed by Jingping Xu.

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Project scientists working in the South China Sea will collect current and sediment data in the layer of water just above the seafloor—called the “bottom boundary layer”—at depths of 2,000 meters (6,500 feet) to 3,000 meters (10,000 feet). In the bottom boundary layer, sediment and biological particles on the seafloor interact with the near-bed currents, a process that determines why and how these particles are resuspended into the water column or deposited and accumulated on the seafloor. The scientists aim to better understand (1) bottom-boundary-layer processes and (2) the circulation of near-bed currents in the deep basin of the region. These currents are thought to hold the keys to understanding the source, transport, and deposition of the deep-sea sediment and the evolution of the basin-scale sediment deposits. Using a platform that sits on the bottom, like the new deepwater tripod, is the most efficient way to collect near-seabed data continuously for a long period of time.

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The new tripod is a stainless-steel frame approximately 2 meters (6.5 feet) tall, with a triangular base about 4 meters (13 feet) on a side. It weighs approximately 900 kilograms (nearly 2,000 pounds) in air and has a deployed buoyant weight of about 110 kilograms (240 pounds). The tripod carries a suite of acoustic (sound-based) and optical (light-based) instruments for measuring current velocity, temperature, and sediment concentration in the bottom boundary layer. Many of the instruments make simultaneous measurements at different heights above the seafloor, creating vertical “profiles” of the properties being measured. A high-definition camera system takes photographs of the ocean floor to record variations over time in seafloor morphology (shape) and bioturbation (mixing of seafloor sediment by burrowing organisms).

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What makes the new tripod unusual is the mechanism for raising it from the deep seafloor to the surface so that the data it has collected can be recovered. Similar tripods used to collect bottom-boundary-layer data in relatively shallow water on the continental shelf are fitted with buoys attached to a coiled recovery line. An acoustic (sound) signal sent from a vessel at the sea surface triggers the release of the buoys, which rise to the surface pulling one end of the recovery line with them. Scientists can then motor to the buoys, attach the recovery line to a winch cable, and pull the tripod off the bottom and onto the deck. Such release systems are limited to a maximum depth of approximately 200 meters (650 feet). Because of this limitation, current measurements in the deep ocean are typically collected by instruments attached to a long mooring—a line with a large flotation package at the surface and a heavy anchor at the seabed. The heavy anchor makes unobstructed near-bed flow measurements extremely difficult. Tripods permit more open flow near the seabed, and so the USGS team developed a system for recovering a tripod from the deep seafloor. 

Rather than buoys that pull a recovery line to the surface, the new USGS tripod is fitted with buoyant material that raises the entire tripod to the surface—hence the name free-ascending tripod. The tripod’s recovery buoyancy is provided by hydrodynamic pieces of syntactic foam (a moldable material made of glass microspheres embedded in resin) that surround the instrument bay in the upper section of the tripod. The anchor weight is provided by the three low-profile lead-and-stainless-steel footpads at the ends of the legs. The footpads are attached by high-strength fiber rope to a pair of redundant acoustic releases in the instrument bay. (Ideally, both of these devices will respond to a release command sent from the vessel, but the tripod will be freed for ascent even if only one of them works.) The rope supplies tension to a locking mechanism at the end of each leg that keeps the footpad firmly attached to the leg. When an acoustic command is sent from a vessel at the surface, the tension on the rope slackens, the locking mechanism releases, and the tripod detaches from the footpads, which remain at the seafloor. The syntactic foam provides nearly 300 kilograms (about 650 pounds) of buoyancy, which raises the tripod to the surface for recovery. The release coupling employs an innovative, simplified design developed specifically for the new tripod (see diagram below).

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When deployed, the USGS deepwater tripod will join a dozen or so moorings to form the largest field campaign ever conducted for deep-sea sediment-transport research in the South China Sea. The results are expected to improve understanding of factors that influence sedimentation in the region, including:

• distribution of near-bottom ocean currents in the study area, 
• turbidity currents and the mechanisms that trigger them, 
• processes that determine whether particles in the bottom boundary layer remain suspended in the water or settle on the seafloor, and
• sources and pathways of the bottom sediment that has accumulated in the study area.

Knowledge gained from the research can be applied in practical ways; examples include siting of deep-sea cables and other infrastructure; assessment of hazards related to submarine landslides, including tsunamis; and determination of whether and where polluted sediments are likely to accumulate on the deep seafloor.