ADCPs: Action in OOI's Cabled Observatory
By Peter Spain
ADCPs are sonar systems that measure motion underwater. Using sound waves, they work like hand-held radars used by police to catch speeding motorists. To measure motion, ADCPs emit sound bursts along beams angled downward.
Echoes are returned due to scattering off particles. Because zooplankton and suspended sediments are carried by the moving water, echoes scattered off them carry a change in pitch; this is the Doppler Effect. It tells how fast the current is moving and in what direction.
Sound waves propagate through the water column so echoes are returned and processed from many depths. The vertical range of this collection of measurements - called a profile of ocean current velocities - is greater for lower frequency sound waves.
Thanks to a spirited cadre of marine scientists and engineers, high-tech ocean observatories are now operational. These sites provide a continuous presence in the ocean for sustained and interactive observing. Many combine innovative infrastructure with multi-discipline marine sensors.
Installed at various depths, these observatories exist worldwide in diverse marine environments. Their purpose is to measure the oceanic and seabed environments in strategic locations for extended periods. Some supply continuous real-time data via a cable connection to shore.
A prime example is the Cabled Array in the NE Pacific Ocean. This observatory is part of the Ocean Observatories Initiative (OOI), funded by the US National Science Foundation (NSF). Engineered by the Applied Physics Laboratory / University of Washington (APL / UW), the Cabled Array uses dedicated telecoms cables. They provide a high voltage supply and high-speed communication links to nodes as far as 500 km from shore.
Besides its high-tech infrastructure, the Cabled Array holds 150 instruments. Included are nine ADCPs operating at four different frequencies. They equip a range of sites that span different depths, environments, and scientific objectives. These ADCPs are installed in three different ways: Shallow Profiler Moorings (SPM), Benthic Experiment Packages (BEP), and Seafloor Instrument Arrays (SIA).
One focus of the Cabled Array is water-column processes. Topics studied span all facets of ocean science. Some promote cross-discipline cooperation, such as biological-physical coupling, while others entrain citizen science. Sustained observing tackles understanding environmental impact and anticipated climate changes. Long-term observing at high sampling rates will reveal both rapid and slow-changing events. Potentially, this can provide the basis for early warning systems and lessons about adaption.
Currents observed with ADCPs transport important water properties. Examples include heat, momentum, salt, nutrients, plankton, and invertebrate larvae. Large-scale research using the ADCP data will range from the dynamics of eastern boundary currents to episodic events. Cross discipline studies will examine how water currents interact with the environment—from rough topography to ecosystems.
ADCPs — Many Uses
ADCPs analyze returning sound echoes to make four different measurements at once.
- Speed and direction of water currents at many levels through the water depth—a “current profile”
- Spatial distribution of sediments or plankton carried by the water (e.g., a sediment plume)
- ADCP’s speed-over-ground and path of travel (revealed by echoes scattered from the bed)
- Range to boundary. This can be water depth (like an echo sounder) or, when the ADCP’s beams are directed upward, range to surface. The latter provided a new way to measure surface waves.
This collective of data types used individually and together permits a single ADCP to make a diverse range of measurements.
Shallow Profiler Moorings
Besides the underlying power / comms infrastructure, a distinguishing innovation of the Cabled Array is the Shallow Profiler Mooring. Designed and installed by APL / UW, SPMs provide a large, stationary, instrumented platform at 200 m depth. The platform sits at the apex of a unique two-legged mooring. From here, a science pod is winched cyclically through the upper ocean.
The unusual design of the mooring offers the motional stability required for long-term success of the winched method. Robotic vehicles are used to install / recover both the platform and pod. Leaving the two-legged mooring in place mitigates the logistics and cost of servicing the SPM’s instrumentation payload. On a separate mooring line, an instrumented wire crawler measures ocean properties from the seabed to 200 m water depth. In addition, co-located with these moorings, is a Seafloor Instrument Array.
At two deep sites (1A,1C—about 3000 m), a pair of uplooking ADCPs are fitted to the SPM’s large instrument platform. Also onboard are a digital still camera and a multi-discipline suite of probes for measuring water properties and bioacoustics. The ADCP data will inform diverse studies that range from the impacts of climate change to ocean acidification. Others include understanding biogeochemical processes and biological-rich thin layers.
Profiling the Upper Ocean
The ADCPs are a 5-beam 600 kHz WorkHorse and a 150 kHz Quartermaster. The 600 kHz ADCP includes a beam directed vertically that complements the standard Janus configuration.
The fifth beam measures vertical motions directly, ideal for studies of internal waves or the diel migration of zooplankton.
The 150 kHz ADCP remotely profiles water currents from the depth of the platform to the sea surface. Thus it provides time series of the background flow within the 200-m water column sampled by the winched science pod.
Each day, the science pod is winched through 9 cycles from 200 m depth to just beneath the sea surface. The pod carries nine single-point instruments. Due to the winching action, these devices record high-resolution profiles through the upper ocean of physical, chemical, and biological water properties. Interactive command and control from shore is available when interesting features are measured, such as crossing a biologically-rich thin layer. Since late summer 2015, each science pod has made >7,000 cycles.
The SPM’s composite data set includes 18 instruments—platform and pod. They see a wide range of water properties. Plus, their simultaneous measurements have high resolution in time and space. The SPMs and their instrument suites (including the science pod) connect to the network of fiber optic cables. Each SPM has 1 Gbps bandwidth and 3000 watts power. As a result, live data are available on the Internet from sensors on the SPM platforms and their winched science pods.
Benthic Experiment Packages
ADCPs are also aboard a couple of Benthic Experiment Packages (BEP) installed on the Newport Line. It runs from shallow to deep water off Oregon. The deep offshore site (1C) sits at 600 m depth on the continental slope whereas the shallower inshore site (1D) is at 80 m on the continental shelf.
These ADCP data will be used to examine wide-ranging science questions. Examples include flow of currents onto the shelf and hypoxia events. In each case, the ADCP frequency is selected to profile the full water column. Thus a 75 kHz Long Ranger sits at the deep site while a 300 kHz Sentinel is at the inshore location.
Housed in a hazard-resistant frame, the BEP plays a twofold role. It is a mounting for the ADCP and several smaller sensors. It also holds some of the power / comms infrastructure that connect these instruments to the cabled network. The sensors measure chemical signatures in the ocean: acidity (pH), carbon dioxide, salinity and oxygen concentrations. Probes installed nearby address the physics of the bottom boundary layer. As well, a hydrophone sits outside the frame to act as a benthic ear.
Seafloor Instrument Arrays
The cabled Seafloor Instrument Arrays (SIA) permit study of near-bottom and water-column processes. Examples include internal tides and the release of methane from the seafloor into the ocean.
There are three of these arrays (Sites 1A, 1B, 3A). They too carry a suite of instruments. Two SIAs, deployed at 3 km depth (1A, 3A), are fitted with 150 kHz Quartermaster ADCPs. These sites are co-located with SPMs; nearby is a wire-crawler mooring. A third SIA (1B), on the continental slope at 800 m, carries a 75 kHz Long Ranger ADCP.
You can see in the figure that the ADCP sits atop a junction box for the power / comms network. Due to the cabled connection, these ADCP data are available in near real-time and for a long duration—two years so far. This has led to innovative use of the ADCP data.
One example is a bio-geological study at the Southern Hydrate Ridge (1B). Marine geologists at the University of Washington are using the ADCP’s returning acoustic echoes to see bubble plumes of methane.
The plumes seep from gas hydrate deposits in the seabed. Gas hydrates are a solid ice-like form of water that contains methane molecules.
These deposits support biological communities on and within the sediments. The ADCP profiles show bubble plumes through much of the water column. One hypothesis is that these rising plumes of methane might boost biological productivity in the overlying ocean.
OOI’s Cabled Observatory is planned to operate for 25 years. Owing to its unique power/comms infrastructure, this networked array will permit interactive monitoring of diverse ocean sites—from upper ocean to benthic depths. The remote sampling capability of ADCPs deployed throughout the array will extend the reach of observers through the water column. And thanks to the fiber optic connection to the Internet, these data will be available in near real-time for a global community of users.
Peter Spain Ph.D., Teledyne RD Instruments