North Pole Mooring Design for 2005 and 2006





Beginning with the April 2001 deployment, the North Pole Environmental Observatory has included an oceanographic mooring anchored to the ocean floor and fixed in position beneath the drifting ice. Instruments mounted on this installation measure properties of the ocean at fixed depths at the North Pole for a year. Each instrument must record internally, as satellite telemetry cannot reach beneath the ice, so the data may only be obtained after recovery. Since spring 2002, operations have included both a mooring recovery and a mooring deployment.

In 2005 and subsequent years, the number of instruments on the mooring has been increased for better resolution and redundancy, and a suite of bio-chemical sensors has been installed by John Christensen of Bigelow Laboratory. Please be aware the diagram to the left illustrates the instrument layout, but not the long distances of mooring line between sensors and reaching the ocean floor. An easier-to-view PDF file of the diagram is available here.

At the North Pole, the ocean depth to the Pole Abyssal Plain is approximately 4200 meters. The top instrument is a Upward Looking Sonar to record ice draft, and must be within 100 meters of the surface. The calculation shows that the mooring has to be over two and a half miles long with little margin for error, so a precise and very deep echo sounding is necessary.

Besides the Upward Looking Sonar fabricated by APL, the instruments include an RDI Workhorse Acoustic Doppler Current Profiler, seven (four before 2004) Aanderaa RCM current meters, and eight (seven before 2004) Seabird conductivity-temperature-depth instruments.

To ensure recovery, also mounted on the mooring are three EG&G 8242 Acoustic Releases now manufactured by Edgetech, two Edgetech XT-7500 Transponders, and a Pieps avalanche beacon to find it when it comes up under the ice. This is an expensive installation that must be recovered to get the data. Successful recoveries the first three years encouraged the addition of more sensors.

Since a year after deployment, there is no certainty of being able to land an airplane at the mooring coordinates, a helicopter is necessary and included in the logistical planning. A mooring is recovered by sending an acoustic message to one of the releases, ordering it to separate the buoyant mooring from the anchor on the seabed. Then when the mooring comes up under the ice, getting the components out of the water is an operation requiring an ROV and/or divers. With a mile and a half of buoyant mooring to float up under the ice, recovering it is expected to be an epic.

Measurements from an ocean mooring are called "Eulerian" because they measure the properties of a moving fluid at fixed points. By contrast, measurements from a drifting buoy are called "Lagrangian" since, in principle, they follow the motion of individual parcels of the fluid.