Feature ArticlesArvor-C: A Coastal Autonomous Profiling Float
By Xavier Andr'
Serge Le Reste
Profiling Floats Project Manager
A growing number of users, from private individuals to professionals, need forecasts or real-time information about the coastal seas. They require information on hydrodynamic circulation, seawater temperature, sea state, biogeochemical state and primary production in order to study ocean evolution and changes and to monitor sea quality.
The Operational Coastal Ocean-ography (OCO) program, and particularly the Previmer project, conducts forecasts in the coastal zone. It has developed numerical models to make such predictions.
These models require a large volume of real-time, in-situ information from the shelf zone over many months or even years. These data are used to validate such models and, in the near future, as input to be assimilated within these models.
OCO also involves observations in real time to study physical and biogeochemical processes in water masses. These observations require good quality time series of real-time, in-situ data.
Currently, instruments deployed in coastal zones provide data that are either limited in terms of deployment duration, spatial distribution or water-column sampling.
A major breakthrough in coastal observation has been made by Ifremer, the French Research Institute for Exploitation of the Sea, to make up for these drawbacks. The Arvor-C, an autonomous profiling float, was designed to make repetitive in-situ measurements along profiles from the seafloor to the surface, providing complete three-dimensional, high-rate data.
This article includes detailed information about the Arvor-C architecture, its standard operation and its fields of application.
Challenges to In-Situ Monitoring
There are many autonomous instruments capable of delivering real-time, in-situ data. However, their use is typically restricted to the shore, shallow waters of only a few meters of depth or to the sea bottom.
Automated 3D measurements are currently carried out by equipment requiring complex mechanical setups and costly maintenance work (measuring along moored cables or using winches, for example).
Gliders are state-of-the-art 3D ocean monitoring instruments, but they require particular attention and skill, especially when navigating close to shore. Moreover, their autonomy is typically limited to approximately three months, which is restrictive for long-term observations.
Ifremer developed the Arvor-C because all of these instruments fail to meet the new requirements of OCO: to provide in-situ data over long periods through the entire water column, with high rate and resolution.
The Arvor-C is an autonomous vertical profiling float that weighs fewer than 20 kilograms, is 2.1 meters high and has a diameter of 11 centimeters.
The float behaves like a virtual mooring for short to long-term observations. It can take measurements at the same location for each profile thanks to its optimized time of ascent and descent through the water column, short time of transmission at the surface, and anti-drift capability when grounded on the seabed.
The Arvor-C provides a standard set of measurements (pressure, temperature and conductivity) as well as technical information. Multidisciplinary sensors can be integrated on this vertical vehicle, which is designed as an open platform. Additional sensors are currently being fitted to measure dissolved oxygen, turbidity and fluorescence.
In standard mode, the Arvor-C operates autonomously. One of the major features is its Iridium' (Bethesda, Maryland) satellite bidirectional link, which offers a fast uplink to transfer data when surfacing after each profile and a downlink remote control to reconfigure the mission parameters during operation. For example, users can increase the number of profiles per day and the sensor sampling frequencies when a bloom is detected.
Arvor-Cs can be deployed in a network for 3D monitoring of coastal seas.
The Arvor-C is based on the Provor and Arvor deepsea profiling floats, which are able to make profiles from 2,000 meters' depth to the surface. They have proven their reliability at sea over the last 10 years, especially in the Argo program, a global array of 3,000 floats that studies global ocean changes.
On the upper endcap, the float houses a Sea-Bird Electronics Inc. (Bellevue, Washington) pumped conductivity, temperature and depth (CTD) sensor. It has a biband Iridium™ and global positioning system (GPS) antenna for data transmission, remote control and positioning and a Bluetooth antenna for configuration and testing.
An external bladder is fitted on the bottom endcap to adjust the buoyancy when descending and ascending along the water column, as well as anti-drift claws to prevent drifting when grounded on the seafloor.
The Arvor-C is a coastal profiling float, designed to withstand pressures as far down as 450 meters. It can perform as many as 320 profiles when cycling at 200 meters' depth, and the profile repetition rate can be configured from one profile every hour.
Its ascending speed reaches 15 to 20 centimeters per second. For instance, a two-second sampling period provides one measurement approximately every 35 centimeters. Data are then averaged into one-meter-high slices to reduce transmission duration.
Setup. Before its deployment, the Arvor-C is configured in the laboratory by users (scientists or technical assistants). The main parameters to be set up are the profile repetition rate and the sensor sampling frequencies.
Deployment. To deploy, the user removes a magnet to power the float. It sends its GPS location and technical information to the data center, retrieves remote commands (if any) and then starts its profiles.
Descent. Starting from the sea surface, the Arvor-C quickly reduces its buoyancy by pumping oil from its external ballast bladder to its internal tank until its density becomes less than the surrounding seawater.
Anchoring. During the descent, pressure is monitored. When the float reaches the seafloor, it anchors in the sediment.
Seabed Stationing. The Arvor-C is grounded on the seafloor until the next programmed time of ascent.
Ascent. The Arvor-C starts pumping oil from its internal tank to its external ballast bladder, increasing its buoyancy. Data acquisition starts on the seafloor and continues all the way up to the sea surface.
Remote Control. Each time the float reaches the surface, it retrieves commands sent by the user via satellite and adjusts its operation according to the new requested configuration.
Data Transmission. Measurements and technical information are sent to France's Coriolis data center via satellite to be processed and made available to scientists.
Recovery. In standard operation, the Arvor-C cycles through these phases to complete its profiles. When recovery is desired, the user sends a command to the float so that it stays at the surface. It will then regularly send its GPS position until recovery.
Risks. Risks during deployment include the destruction of the instrument due to trawling or difficulties in starting the ascent when anchored in clay soils, which can lead to a delay in surfacing.
Results at Sea
ASPEX Scientific Cruise. One Arvor-C was deployed for technical tests during the Aquitaine/Armorican Shelves and Slopes Physics Experiment (ASPEX) physical oceanography cruise, led by Louis Mari' of Ifremer's Laboratoire de Physique des Oc'ans (LPO) in July 2009.
The aim of this cruise was to study the physical processes governing the hydrological structure and subtidal circulation on the Armorican and Aquitaine shelves and slopes over two years.
The cruise focused on the vertical and temporal structures of the circulation around the 'cold pool' as well as the slope currents and cross-slope exchanges.
Eight drifting buoys, 10 ocean-bottom frames and two tethered moorings were deployed during this cruise, making it a major setup for the study of ocean circulation on the Bay of Biscay continental shelf.
These instruments provide measurements on subsurface currents in real time or record bottom hydrology and velocity profiles.
Meanwhile, eight sections totaling 800 nautical miles were completed with a towed undulating vehicle over 10 days, collecting CTD measurements from 100 meters' depth to the surface. Vessel-mounted acoustic Doppler current profilers collected current measurements along the way.
However, all of these measurements are snapshots, taken either over a limited period or spread over a whole year, but limited in terms of distribution along the water column.
In this way, current patterns can be observed, but a full understanding of the mechanisms driving them also requires observations with a high spatial and temporal resolution in the whole water column.
The Arvor-C was deployed on July 15, and will be recovered this spring or summer.
It provides complete CTD profiles at regularly timed intervals (from one to three days, depending on the season) from the seabed to the surface. Data are transmitted in real time to the Coriolis database so that no data are lost in the event of instrument loss.
In the first five months of deployment, the Arvor-C drifted fewer than 200 meters per day north of its deployment position, despite rough wave and tidal conditions. At the same time, a free-drifting profiling float drifted more than 4.3 kilometers per day.
Ifremer has developed a profiling float for coastal applications: the Arvor-C. It can be deployed in any shallow seas where hydrological and biogeochemical monitoring is required and makes repetitive in-situ profiles from the seafloor to the surface, providing information at a high rate.
It was successfully deployed during the ASPEX cruise in the Bay of Biscay, and it has provided CTD profiles since July 2009. Other deployments are planned this year in the Gulf of Lion.
The Arvor-C promises to become a part of the monitoring network along the French coasts, which is comprised of other systems such as those developed by Ifremer (Marel and Recopesca, for example).
The Arvor-C is produced by one of Ifremer's industrial partners, NKE (Hennebont, France), which already manufactures the Provor and Arvor profiling floats.
The authors would like to thank the staff at Ifremer that contributed to the development of the Arvor-C: Jean-Fran'ois Masset, Jacky Dupont, Laurent Bignon, Philippe Bouquet and Yannick Aoustin.
They also wish to thank the Previmer team (led by Fabrice Lecornu), the OCO team (led by Jacques Legrand) and all of the staff members who participated in the design and qualification of the Arvor-C.
The authors would also like to offer thanks to Ifremer's industrial partner NKE, particularly Patrice Brault and J'r'me Sagot; Louis Mari' of Ifremer's LPO for his valuable contribution to this paper and the Arvor-C project; as well as Steven Herbette of LPO, Pascal Lazure of Ifremer's D'partement Dynamiques de l'Environnement C'tier and Gilles Reverdin of CNRS/LOCEAN (Paris, France) for their interest in the project.
For a full list of references or additional information, please contact Xavier Andr' at firstname.lastname@example.org.
Xavier Andr' is head of the Arvor-C project at Ifremer. He is an electronics and software engineer and has been involved in the development of many multidisciplinary underwater systems such as military sonar, underwater communication and positioning systems, and scientific instrumentation.
Serge Le Reste is an electronics engineer and the project manager of float instrumentation at the Department of Technology in Ifremer. His interests include coastal and deepsea developments in operational oceanography and scientific applications.
Jean-Fran'ois Rolin is a mechanical engineer and a graduate of the Institut Sup'rieur de M'canique in Paris, France. He has designed several subsea landers, benthic stations and geotechnical equipment for deepsea and coastal monitoring. As head of the Instrumentation System Technology Department at Ifremer, he was involved in the Arvor-C specification.