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Ultradeep-Sea Exploration In the Puerto Rico Trench
Robotic Vehicle System Captures Video of Benthic Life at 8,000 Meters

By Fredrik Søreide

The Puerto Rico Trench robot vehicle system, 11K. (Photo Credit: Promare)
The Puerto Rico Trench is the fourth-deepest ocean trench in the world and the deepest point in the Atlantic Ocean but has remained virtually unexplored. It is located approximately 100 miles north of Puerto Rico and forms the boundary of the Caribbean Sea where the North American tectonic plate is being subducted under the Caribbean plate. The trench can be divided into two parts along 65 to 66 degrees west longitude. The western part includes the deepest sector of the trench and is 10 to 15 kilometers wide and approximately 8,300 meters deep. The bottom is remarkably flat and covered by nonreflective pelagic sediments. The eastern part is slightly shallower (by about 700 meters) and more rugged.

In light of past finds, it is likely that the Puerto Rico Trench has a unique biological life compared to the Pacific Ocean trenches. It holds, for instance, the record for the deepest fish, Abyssobrotula galatheae, which was dredged from the bottom at a depth of more than 8,000 meters in 1970. However, it was dead by the time it reached the surface, and its presence on the bottom has never been confirmed by video data.

The trench was mapped extensively in 2002 and 2003 with a multibeam echosounder by the U.S. Geological Survey during three cruises. In early August 2012, video data and biological samples were retrieved by a team from the nonprofit research group Promare, which dove a prototype robotic underwater vehicle system to the bottom of the trench.

Robotic Vehicle Development
In order to explore the deepest parts of the world's oceans on a small budget, Promare began development of a low-cost, full-ocean-depth robotic vehicle system more than two years ago. The prototype vehicle, 11K, is depth-rated to 11,000 meters and can be operated in AUV or ROV mode. The system measures weighs 60 kilograms.

The basic vehicle consists of a glass sphere from Nautilus Marine Service GmbH (Bremen, Germany) that contains proprietary software and hardware, as well as an assortment of off-the-shelf components, such as a high-definition video camera and lithium-ion battery pack from OceanServer Technology Inc. (Fall River, Massachusetts). In addition, the system includes a high-resolution, full-ocean-depth pressure sensor custom-made by Presens AS (Oslo, Norway), and full-ocean-depth LED lights and a drop-weight system developed by Promare.

In ROV mode, the system incorporates full-ocean-depth thrusters customized by Tecnadyne (San Diego, California) and a fiber-optic connector custom-made by Teledyne Impulse-PDM Ltd. (Alton, England) that connects to a thin single-mode fiber-optic cable to the surface.

The glass sphere is a 1-atmosphere electronics pressure vessel that also provides buoyancy for the vehicle. An external frame made from thermoplastics holds the glass sphere and other external components, such as the thrusters, pressure sensor, lights and drop-weight system.

The drop-weight system was designed with the capacity to drop weights in three stages: one descent weight, which is dropped just above or on the seafloor; several small weights, which can be dropped to obtain neutral buoyancy on the seafloor; and an ascent weight at the end of the dive.

The dimmable LED lights on the vehicle are enclosed in small glass spheres and can operate at 11,000 meters depth. The lights and the rest of the vehicle are powered from the OceanServer lithium-ion battery pack containing approximately 760 watt-hours of energy. The vehicle has a power-control function allowing for long passive periods with minimal battery drain, which is particularly useful during long periods of ascent and descent for abyssal dives. To continue this article please click here.

Fredrik Søreide is the vice president of Promare and is responsible for its new deepwater exploration initiative. He is also a professor of marine engineering at the Norwegian University of Science and Technology. He has been involved in numerous marine scientific and exploration projects around the world.

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