Acoustic Doppler current profilers (ADCPs) have revolutionized our ability to record ocean current movements. But they can only measure what they can “see”, and that is largely determined by what they are attached to, which may be a surface buoy or a frame on the seabed.
That’s perfect for many uses, but if you are seeking to find out about the movements of currents across a large swathe of ocean, then moving ADCPs on frames from location to location is a time-consuming business and doesn’t provide a complete picture of the currents in the whole area. Mounting an ADCP on a moving surface vessel is a good option for many, but some users are put off by the work and cost involved with hiring and crewing a vessel.
Accurate analysis of currents across tens of kilometers of ocean is now possible due to Alseamar’s successful integration of the Nortek ADCP into its SeaExplorer glider. Photo: Alseamar.Underwater gliders and ADCPs – an efficient and cost-effective way to profile a large volume of water
However, rapid advances in underwater glider technology have provided an exciting new alternative. Slimline battery-powered gliders are capable of cruising through the oceans for months at a time, carrying a multitude of instruments measuring many facets of the underwater world close up.
Typically, gliders move in a sawtooth, or wave-like, movement between the top of the water column and depths of around 1000 m. So, if you can get accurate measurements from an ADCP fixed to a glider, you have an efficient and cost-effective way to profile a huge volume of water in a relatively short period of time.
That possibility prompted researchers at French marine tech company Alseamar to investigate how an ADCP could be made to work with their SeaExplorer glider. The company produces high-tech marine and submarine equipment, such as gliders (or UUVs – underwater unmanned vehicles) with very long mission endurance.
Their investigation of ADCPs was accelerated when Alseamar researchers joined the CITEPH innovation program in 2015. The research was done in collaboration with French energy firm Total and two French academic research laboratories, with Nortek eventually providing the ADCP as the project continued. Total describes the glider program as a success, saying that the innovation that came as a result of it was “a world first in the deep offshore, [making] our exploration more efficient while cutting costs and HSE risks”.
Today, the SeaExplorer – equipped with a specially adapted 1 MHz OEM (original equipment manufacturer) version of a Nortek ADCP and a multitude of other instruments – is being used by Alseamar and its customers around the world. It is proving to be a versatile autonomous vehicle, well equipped for application in scientific research, military deployments and commercial subsea operations such as oil and gas exploration.
The results benefit all of us. The detailed analysis of current movements made possible by the Nortek ADCP, combined with data from the glider’s other instruments, gives users access to highly accurate data on how our oceans work. That enhances our understanding of human impact on the marine environment – enabling us, for example, to monitor the effects of climate change on the ocean, and understand the interaction between structures such as oil and gas rigs, offshore wind turbines or coastal infrastructure with the surrounding subsea world.
Video: See how Ecopetrol used a fleet of SeaExplorer underwater gliders in the Colombian Caribbean to acquire in-situ data at sea and support its offshore activities. (Video courtesy of Ecopetrol.)
“Glider missions contribute to the international efforts of the Global Ocean Observing System (GOOS),” says Orens de Fommervault, oceanographer at Alseamar.
The UNESCO-led GOOS program coordinates observations in the ocean globally, focusing on three critical themes: climate, operational services and marine ecosystem health.
“It brings together marine scientists and engineers operating gliders around the world to observe the long-term physical, biogeochemical and biological ocean processes and phenomena that are relevant for societal application,” he explains.
“The most widespread applications are boundary currents, tropical and extratropical storms and water-masses transformations. In these three key areas of focus, measurements of currents in the ocean (with ADCPs) are of particular interest,” de Fommervault adds.
An ideal current profiler for underwater gliders
Nortek’s 1 MHz current profiler was an attractive option for Alseamar due to a multitude of reasons.
“The Nortek ADCP design is really ideal for a profiling platform like ours. You could almost say it was dedicated to our application,” says Orens de Fommervault, who is the oceanographer at Alseamar leading the ADCP glider integration project.
This project truly breaks new ground in the oceanographic research industry. Some select academic institutions are also able to estimate water currents from a glider, but the real step forward is that this is now available as a service from a private company. Alseamar is currently the only known private company able to estimate water currents from a glider and to provide this service to other companies, organizations and researchers.
“Glider-mounted ADCPs offer the ability to collect high-resolution, dense-data water velocity measurements at an unprecedented spatio-temporal scale resolution without the constant use and expense of support vessels,” de Fommervault points out.
The Nortek ADCP’s angled transducers provide current profiles across a wide area of the water column. Photo: Alseamar.The geometry of the ADCP’s four transducers, which are angled away from each other, means that the instrument can measure more of the water column on its angled trajectory than if the transducers were all oriented in the same direction.
This also means they can carry out current profiling measurements when the glider is rising back to the surface, as well as when it is diving – effectively doubling the amount of information that can be gathered compared to other ADCPs that can only take accurate measurements when diving (see Figure 1).
Figure 1: A Nortek ADCP on a glider provides unprecedented opportunities for measuring full ocean current profiles, whether the vehicle is ascending or descending. A small physical size coupled with low power consumption and advanced broadband signal processing allows this ADCP to be used in a wide range of applications.The relatively small size and low power consumption of Nortek’s ADCP are also plus points when you are seeking to pack as much instrumentation on to a glider as possible and get the most out of its on-board battery.
Integrating the ADCP and accurately interpreting oceanographic data from the moving glider
But selecting the right ADCP was only the start of the task for the Alseamar team. Processing data gathered from a glider moving at an angle through the water column to give accurate current profiles is far from easy.
“We worked for almost a year on integrating the ADCP on the glider. We wanted to give the end user an opportunity to tune the many adjustable parameters to meet their scientific objectives,” says Orens de Fommervault.
First, the ADCP needed to be fitted onto the SeaExplorer, a glider packed with instruments, where space was at a premium.
Alseamar is now working to fine-tune data transmission and interpretation from the glider to make more detailed oceanographic current profiling data from the Nortek ADCP available in real time. Photo: Alseamar.“We had to reinforce the structure of the glider to fit the ADCP, but in terms of streamlining, it fitted very well and didn’t cause any problems for the hydrodynamics. Other instruments were more challenging in that regard,” he says.
Then the team came up with a configuration for the ADCP that represented a good compromise between the current profiler’s sampling rate and the velocity and angle of the glider’s trajectory, so that it provided good coverage (or sampling) over the entire water column.
The next step was to develop an algorithm that could accurately interpret data harvested from a moving glider. “A lot of effort has been put into adapting and improving existing data-processing techniques to the specificities of the SeaExplorer. Several missions have been performed to adjust calculation and make the algorithm as robust as possible,” says de Fommervault.
The results have been impressive, with the processed data from the ADCP on the glider comparing well with that from fixed-point-mounted and vessel-mounted current profilers.
In a separate project, the team was also able to use the ADCP’s ability to estimate the glider’s motion in relation to the surrounding water (under certain assumptions) to fine-tune the hydrodynamic model, which describes how the glider moves through and interacts with the water.
Responsive feedback and collaboration key for glider development success
The company collaborated closely with Nortek in the early days of the project.
“When we wanted to improve our capabilities with more accurate estimations of the backscatter index, vertical velocity and so on, we had a lot of interaction with Nortek to get their expertise. We were very happy with the collaboration. I got fast responses with exactly the answers I needed,” says Orens de Fommervault.
Cristobal Molina, the Sales Engineer at Nortek who provided support in the early days of the project, says working with Alseamar was a different proposition from the norm.
“Glider makers usually find post-processing results from the instrument a challenge, but Alseamar brought in Orens to do it, and he has done a very good job. We provided technical support, but they have been pretty self-sufficient,” he says.
“We supply the ADCPs, but they can handle all the post-processing independently,” says Elsa Defachelle, a Technical Sales Engineer at Nortek.
“When anyone makes enquiries about gliders, I always suggest they go to Alseamar, because it’s a company that does things properly,” Molina adds.
De Fommervault says Alseamar’s relationship with Nortek continues to be fruitful.
“We use the ADCP ourselves, but we are also very happy to sell it to our customers. We provide a bridge between Nortek and end users, and we offer post-processing for the Nortek ADCP for our glider customers,” he says.
ADCP-equipped glider proves accuracy in a number of different environments
The ADCP-equipped glider has proved its accuracy in a number of different environments.
Figures 2 and 3 show how current profiles obtained by the ADCP on the SeaExplorer glider across 60 km in the Mediterranean Sea off southern France correlate with temperature, salinity and chlorophyll measurements taken by the glider at the same time.
Figure 2. Current profiles acquired by a SeaExplorer glider equipped with a Nortek ADCP in the northwest Mediterranean, crossing a well-known geostrophic current. The left figures represent depth-resolved water currents acquired along the glider transect (the gray line on the right figure). The color indicates the velocity of the currents. In the left figures, the current is decomposed (i.e. the velocity measurement is split) into its eastward (top) component (red = positive = eastward, blue = negative = westward) and northward (bottom) component (red = north, blue = south). Y-axes are depth and X-axes the distance to the coastline. The map to the right represents a snapshot of the surface current obtained from a regional CMEMS model. Data shows the ADCP-equipped glider is perfectly capable of capturing interesting and known features of the northwestern Mediterranean Sea. In particular the Northern Current, a permanent flow that results from the large-scale thermohaline circulation of the Mediterranean Sea, was sampled. In the transects presented here (to the left), the flow had a westward direction and reached a maximum velocity of 60 cm/s, which agrees with model simulations. The Northern Current was located at around 30 km distance from the coast and had a maximum vertical extent of 300 m depth on the north edge. At 40 km, the current dropped below 10 cm/s. Source: Alseamar.
Figure 3: Hydrological and biological data relating to the same mission across the geostrophic current in the northwest Mediterranean. The data is the same as previously but with the other parameters shown. The black lines in the four data plots are isopycnal levels (density) and the white lines indicate the mixed layer depth. Source: Alseamar.Whereas Figures 2 and 3 show current profiles obtained by the ADCP on the SeaExplorer glider across 60 km in the northwest Mediterranean, Figures 4 and 5 show data obtained following a deployment of a SeaExplorer glider equipped with an ADCP in a relatively shallow coastal area where variability within a short timescale is prevalent.
Figure 4 shows how the ADCP was recently able to measure tidal currents in considerable detail through the depth profile in the Atlantic off northwest France.
Meanwhile, Figure 5 illustrates how water turbidity measurements taken using the ADCP during the same mission match up very closely to those obtained conventionally using optical instruments. This was a major success, demonstrating the high quality of both Nortek’s ADCP technology and Alseamar’s data-processing skills (see below for more on turbidity).
Figure 4. Tidal current in the Atlantic off Brittany, northwest France, over a 12-hour period, measured by the Alseamar SeaExplorer glider equipped with the Nortek ADCP glider current profiler. Source: This data was acquired in the framework of the MELANGE project (ASTRID MATURATION) in collaboration with the SHOM and the CNRS (Perpignan University).
Figure 5. Backscatter index estimated by the signal amplitude of the ADCP on the glider compared to optical measurements of turbidity (FLBBCD). Both data sets were taken during the same mission as the tidal data in Figure 4. Source: This data was acquired in the framework of the MELANGE project (ASTRID MATURATION) in collaboration with the SHOM and the CNRS (Perpignan University).Improving real-time data interpretation from ADCP glider missions still further
But configuring the ADCP-equipped underwater glider to gather vast amounts of usable oceanographic data is not the end of the story. While it’s straightforward to store the data from all the glider’s instruments on board and then download it when the glider returns to base at the end of its mission, accessing it during the mission is not.
“One big remaining challenge is to get access to as much of that data as possible in real time – something that is vital for our customers in the oil and gas industry, for example,” Orens de Fommervault says.
The glider can transmit data each time it reaches the surface, but it’s not practical for it to stay there long enough to offload all the data from its dive, given the low transfer rates possible in remote locations using satellite communications.
The Nortek ADCP’s small form and low profile enabled Alseamar to fit it to the SeaExplorer with little impact on the glider’s hydrodynamic properties. Photo: Alseamar.“The glider would have to spend most of its time at the surface to send all the data and would end up doing very little actual profiling,” he says.
So, the challenge has been to devise algorithms that could provide high-quality results while using as little data as possible. Getting the balance right is vital – if you transmit too little data, you risk having insufficient information to provide a complete profile across the glider’s trajectory.
Alseamar has made considerable progress with this already, and is currently working to improve real-time data interpretation still further, de Fommervault says.
Video: See how the SeaExplorer underwater glider was operated at the ODYSSEA Project’s observatories based in Al Hoceima in Morocco and in the Thracian Sea in Greece.
Measuring water turbidity with ADCPs – a new frontier for the energy industry
Another current area of focus for Alseamar with the ADCP is developing more accurate profiling of water turbidity – a function of particular interest for oil and gas companies seeking to identify promising subsea areas to explore for hydrocarbons by identifying bubbles of gas seeping into the water from the seabed.
The glider is already proving popular among Alseamar’s customers. Such additional functionality will broaden its appeal still further.
Video: The SeaExplorer is a powerful autonomous sensing platform dedicated to collecting water-column data profiles with very large spatio-temporal coverage (from regional to local scale). Driven by changes in buoyancy, the vehicle silently glides without wings, facilitating launch and recovery operations, avoiding wing breaks and limiting risks of entanglements.
