ACOBAR - ACoustic Technology for OBserving the interior of the ARctic Ocean
The main objective of ACOBAR is to develop a system for environmental monitoring of the interior of the Arctic Ocean by assimilation of data obtained with acoustical methods including tomography, data transmission from underwater platforms, communication and navigation of floats and gliders under the ice-cover.
The large gap in data from the Arctic Ocean introduce severe uncertainties in detection of temperature and salinity changes, understanding of ocean-ice interactions and in modelling of processes and climate. During the previous projects (FP6 ASOF-N, FP7 DAMOCLES IP) and International Polar Year 2007-2009, several new instruments and platforms for ocean observations were deployed and the collection of data was significantly enhanced. ACOBAR continues this work by contributing to the Arctic Ocean observing system from 2009 to 2013 by use of acoustic tomography, in combination with data collection by floats, gliders and standard oceanographic moored instrumentation. These components are envisaged to be implemented as part of a future Arctic Ocean observing system designed under iAOOS and under progress in the framework of Arctic ROOS as a contribution to EuroGOOS.
Under ACOBAR two complementary ocean observing systems were implemented in Fram Strait: acoustic tomography array, providing spatially averaged temperature and currents with a high temporal resolution along several sections, and the oceanographic moored array combined with autonomous gliders to collect ocean data with high spatial and temporal resolution along a single section across the whole strait. The acoustic tomography array, operated by the Nansen Environmental and Remote Sensing Center (NERSC) consists of three tomographic sweeping frequency sound sources, distributed in as a triangle in the eastern, northern and western Fram Strait and one tomographic receiver in the central, deepest part of the strait. The tomography array measures acoustic travel times along six different paths, since each sound source mooring is also equipped with a receiver. Tomographic sources provide also RAFOS transmissions for navigation of gliders and floats.
Since 1997 the Alfred Wegener Institute (AWI) has maintained the array of oceanographic moorings across Fram Strait with the main aim to measure the oceanic fluxes between the North Atlantic and Arctic Ocean. The array consists of 16 deep ocean moorings (with the westernmost four operated by Norwegian Polar Institute), covering the entire width of the strait from the shelf west of Svalbard to the East Greenland shelf and equipped with point and profiling current meters, and temperature and salinity sensors located at five depth levels. Since 2009 these observations have been continued under the ACOBAR project and the existing array was augmented with a wide range of new technologies for acoustic communication and navigation. To increase a spatial resolution of the array, the high resolution CTD measurements have been provided by autonomous gliders, covering the section parallel to the mooring line.
Towards the operational data transfer
Developing operational capability for the near-real time (NRT) data transfer from the moored array is focused on two main aims: to test the long-range acoustic data transfer between moorings and to develop the communication to shore using a moored surface unit capable of satellite data transmission. To achieve the first goal, in 2009 three low-frequency long-range acoustic modems, interfaced with current meters, were deployed for a one-year long field test in the eastern Fram Strait. Recorded data were transferred in a relay mode (sequential data transfer from the farthest mooring towards the central communication unit). Since acoustic data transmission over the typical range between moorings of O(30 km) proved to be unreliable, the distance between long-range modems was reduced by adding a relay-link mooring with an additional modem between the instrumented moorings. The results of the first field test revealed significant problems related to the high level of ambient noise and low signal-to-noise ratio. A performance of the long-range modems and quality of the acoustic data transfer is currently under evaluation after a recovery in summer 2010 and a set of improved modems will be deployed in summer 2012.
Fig. 3: The Fram Strain oceanographic array with the components of data transfer system
For NRT data transfer to shore, a communication mooring was designed, consisting of an acoustic modem receiving data from other moorings, a central unit, and an underwater winch equipped with a profiling CTD at the top, capable of Iridium data transfer. During summers 2010 and 2011 the underwater winch with CTD profiler was deployed for the first short-term field tests, without being acoustically linked to the moorings. These field tests of the winch-profiler system revealed that under the strong currents in the eastern Fram Strait, most of the time, drag on the profiler body prevented it from surfacing. In addition, drifting sea ice poses a risk for the profiler when at the surface. An improved system is currently under development and will be further tested in 2012.
To complement oceanographic measurements at the moored array with spatially high-resolution CTD sections, autonomous, buoyancy-driven gliders have been deployed since 2008 in close collaboration with Dr. Craig Lee from the Applied Physics Laboratory, University of Washington (APL-UW). The Fram Strait observatory employs Seagliders equipped with temperature, conductivity, pressure, fluorescence, optical backscatter and dissolved oxygen sensors. Gliders profile the upper 1000 m with a horizontal resolution of approx. 7–9 km during deep dives. During each surfacing, data are transmitted to shore. Under the ACOBAR project six glider missions with a total duration of 416 days were performed during summer and autumn in the eastern and central Fram Strait, and about 2000 vertical profiles have been measured.
For acoustic positioning and navigation of gliders profiling under the sea ice, an array of three to five RAFOS 260-Hz sound sources have been deployed since 2010 in the northern and western Fram Strait. The tomographic sources also provide RAFOS signals between tomographic transmissions. In collaboration with the Applied Physics Laboratory, University of Washington (APL-UW), Seagliders were equipped with RAFOS receivers and dedicated software for acoustic navigation. This system was developed at APL-UW for under-ice glider missions in Davis Strait and adopted for glider operations in Fram Strait. When RAFOS transmissions were available, the gliders calculated navigational solutions for testing purposes, remaining mostly in open water. The maximum range of RAFOS signals received by the Seagliders in Fram Strait varied between 50 and 300 km, depending on the type of sound source (two RAFOS systems were used, as well as tomographic sources), sea ice extent between source and glider, and the type of RAFOS receiver carried by the glider. During the glider mission in 2011, 72% of acoustically obtained positions had an error of less than 10 km, while nearly half of the positions were accurate to 5 km. Several under ice profiles were collected by the glider in summer 2011 and a longer under ice mission is planned for summer 2012.