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Fieldwork

Free-Ascending Tripod Brings Data from the Deep Seafloor of the South China Sea



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A deepwater tripod system designed and built at the U.S. Geological Survey (USGS) Pacific Coastal and Marine Science Center (PCMSC) in Santa Cruz, California, was recovered in late September 2014 after spending 5 months collecting data on the floor of the South China Sea. The Free-Ascending Tripod (FAT) was deployed in April 2014 at 1,900 meters (6,200 feet) water depth—roughly 10 times as deep as most tripods dedicated to measuring currents and sediment movement at the seafloor. Deployment at this unusual depth was made possible by the tripod’s ability to rise by itself to the surface rather than being pulled up by a line. Instruments mounted on the tripod took bottom photographs and measured such variables as water temperature, current velocity, and suspended-sediment concentration. FAT’s deployment was part of a cooperative project with scientists from Tongji University (Shanghai, China) to better understand how and where deep-seafloor sediment moves and accumulates.

The Free-Ascending Tripod (FAT) being deployed in the South China Sea from the vessel Aquilla on April 19, 2014

Above: The Free-Ascending Tripod (FAT) being deployed in the South China Sea from the vessel Aquilla on April 19, 2014. Screenshot from video, below. [larger version]

The deep-water circulation in the South China Sea is believed to control sediment-transport processes and the formation of sedimentary deposits on the slope and in the basin. Researchers have speculated about deep circulation in the South China Sea and modeled it numerically, but until this year no one had measured it directly. To fill this gap, the South China Sea Deep program funded a project (“South China Sea Deep: In-situ observation of bottom flows and sediment dynamics in northeastern South China Sea”) to collect oceanographic and sediment-dynamics data in the water column and in the layer of water just above the seafloor, called the bottom boundary layer. This layer is where seabed sediment and biological particles 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 long-term scientific goals of the South China Sea Deep project are to understand (1) bottom boundary layer processes, and (2) circulation of the near-bed currents in the deep basin that are thought to control the source, movement, and deposition of deep-sea sediment and the evolution of the basin-scale sediment deposits.

Sites in South China Sea where the USGS deepwater tripod and numerous moorings collected data

Above: A, Sites in South China Sea where the USGS deepwater tripod and numerous moorings collected data to shed light on sediment movement and accumulation on the seafloor. Red dots, sites of instrumented platforms. Green triangles, sites of Ocean Drilling Program (ODP) boreholes. Long-dashed yellow line, hypothesized route of deep contour-following current entering from the Pacific Ocean. Short-dashed yellow line, hypothesized alternative route of deep current from the Pacific. B, Seismic-reflection profile along line a–b on map, showing sediment layers in an apparent contourite (deposited by contour currents). Simplified drawings indicate where scientists placed the tripod (TJ-A-2) and several moorings (TJ-A-1, TJ-A-3, TJ-A-4). C, Enlarged map of line c–d, showing sites (red dots) where groups of moorings were placed along the axis of a submarine canyon (Formosa Canyon). [larger version]

In October 2013, the project deployed instruments attached to six deep-water moorings—lines with large flotation packages at the top and heavy anchors at the seabed—at multiple locations around the basin of the northeastern South China Sea (see map above). On April 19, 2014, FAT was deployed near site TJ-A-2. The moorings’ heavy anchors interfere with water and sediment movement near the seafloor, making FAT’s role particularly important: the tripod has a minimal effect on flow near the seabed and so can collect data about natural processes in the bottom boundary layer. The moorings and tripod were recovered in September 2014. Time-series data (measurements taken at regular intervals) from all the instruments will be analyzed by Tongji University scientists and USGS emeritus scientist Jingping Xu.

PCMSC was invited to participate in this project because of its world-renowned research on sediment dynamics and its proven expertise in designing and implementing field experiments for sediment-transport studies, from the continental shelves and slopes to submarine canyons. For example, USGS engineer George Tate, head of PCMSC’s Marine Facility, designed the unique footpad-release mechanism that makes FAT free ascending and thus enables it to be deployed at great depths (see details at “Deep-Sea Instrument Tripod Passes Test in Monterey Bay, California—Next Stop is South China Sea”). Tate and geophysicist Joanne Ferreira traveled to China in April to familiarize collaborators with the equipment and train them in deployment of the system; they returned in September to view preliminary data and pack USGS instruments for shipping back to the United States.

Photograph of the seafloor taken by FAT‘s bottom camera on May 12, 2014, at 1,900 meters (6,200 feet) water depth FAT being pulled onto the deck of the vessel Aquilla on September 26, 2014

Above Left: Photograph of the seafloor taken by FAT‘s bottom camera on May 12, 2014, at 1,900 meters (6,200 feet) water depth in the northern South China Sea. [larger version]

Above Right: FAT being pulled onto the deck of the vessel Aquilla on September 26, 2014. Note that the footpads are gone; they served as weights to hold the tripod on the seafloor and were released to allow the buoyant orange foam to raise the tripod to the surface. Photograph courtesy of Tongji University. [larger version]

Above: The Free-Ascending Tripod (FAT) being deployed in the South China Sea from the vessel Aquilla on April 19, 2014. Video footage courtesy of Tongji University.

The High Energy Benthic Boundary Layer Experiment (HEBBLE) Program investigated sediment transport at the base of the continental rise more than 30 years ago. Since then, only a few studies on deep-water sedimentation and sediment dynamics have been conducted outside of the oil industry, whose data are generally proprietary. The South China Sea Deep project is one of these studies. Data and discoveries from this project will be at the forefront of deepwater marine-geology research for the region and probably for the world. Practical applications include choosing favorable sites for undersea cables and other infrastructure.

Learn more about FAT at “Deep-Sea Instrument Tripod Passes Test in Monterey Bay, California—Next Stop is South China Sea,” and stay tuned for new insights from the South China Sea!


Related Sound Waves Stories
Deep-Sea Instrument Tripod Passes Test in Monterey Bay, California—Next Stop is South China Sea
July / Aug. 2013

Related Websites
South China Sea Deep: In-situ observation of bottom flows and sediment dynamics in northeastern South China Sea
USGS

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in this issue:

Research
Natural Methane Seepage Is Widespread on the U.S. Atlantic Ocean Margin

Spotlight on Sandy
Field Investigations of Hurricane Sandy's Impacts on Fire Island, New York

Seafloor Mapping off the Delmarva Peninsula

#StrongAfterSandy—A Congressional Briefing Hosted by the USGS

This Woman ROCKS!

Fieldwork
Tripod Brings Data from the Deep Seafloor of the South China Sea

Instruments near Martha’s Vineyard Measure Seafloor Bottom Shear Stress

Coastal Streams in Central California Reflect the Region’s Drought

USGS Scientist Participates in National Geographic’s BioBlitz 2014

Outreach
Twenty Years of Ask-A-Geologist

Awards
Advancing Data Sharing Capabilities—2014 DeSouza Award

Staff & Center News
Postdocs Contributing to Climate-Change Studies

Feds Feed Families Food Drive

Publications
Sept. / Oct. Publications

Accessibility FOIA Privacy Policies and Notices

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