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Cabled Observatories: An Ideal Platform for Acoustic Devices

Richard Dewey
Associate Director of Research,
VENUS Coastal Observatory, Ocean Networks Canada

Recent advancements in cabled ocean observatories have resulted in a resurgence of acoustic technology applications for deep-sea exploration. Cabled observatories offer two features that align well with the needs of modern ocean acoustic devices: continuous power and high-bandwidth communications.

Since the early 1950s, the U.S. Navy has used cabled SOSUS (Sound Surveillance System) arrays to monitor ambient sounds in the Atlantic and Pacific oceans. The Martha's Vineyard Coastal Observatory, operated by the Woods Hole Oceanographic Institution, has demonstrated modern oceanographic acoustic instruments as a natural fit on cabled systems. Both active and passive acoustic devices represent core instrumentation on Ocean Network Canada's Victoria Experimental Network Under the Sea (VENUS) and Northeast Pacific Time-Series Undersea Networked Experiments (NEPTUNE) observatories and on plan'ned U.S. Ocean Observatories Initiative installations. These instruments include active inverted echosounders, acoustic Doppler current profilers, acoustic Doppler velocimeters, scanning sonar imaging and passive hydrophones.

Active acoustic systems transmit and receive specialized signals to probe environmental conditions. Monostatic systems use arrays of single transducers for the dual purpose of transmitting and receiving acoustic pulses, either narrowband or broadband. Bistatic systems include separate transducers for the transmission and reception of sound energy. Frequencies range from relatively low (38 kilohertz) to high (6 megahertz), supporting far- and near-field characterization of marine conditions, respectively. Broadband (1 hertz to 100 kilohertz) passive hydrophones listen for ocean sounds, including geophysical processes, marine mammal vocalizations and anthropogenic noise. Acoustic devices have used internal processing to reduce internal data storage, but with fiber-optic cabled observatories, many acousticians now demand raw data at 10 to 100 megabits per second.

In this new generation of science-driven cabled observatories, not only can existing technologies provide continuous power to operate all active systems and the data bandwidth to record all raw signals without needing to operate in burst or data limited/averaging modes, but the interactive capabilities can also support adaptive configuration to optimize performance. Colocated systems can be individually triggered or run in master-slave modes to avoid interference, and configuration parameters can be adjusted in real time to optimize performance under a wide range of marine conditions or to sample for particular features. Using specific digital network protocols, cabled networks also allow for accurate timing across both short and regional scales, so signals from distributed sensors can be used to form coherent directional arrays. Power and bandwidth improvements also allow more complex technologies, such as sonar imaging video systems, to be deployed on cabled observatories. This year, VENUS will add a high-frequency DIDSON Aris sonar imaging device from Sound Metrics Corp. (Lake Forest Park, Washington).

Data from observatory sensors flow in real time back to onshore stations, where they are time-stamped and recorded on arrays of buffering hard disks. Within seconds, data are pushed to centralized archives for permanent storage, backup and distribution over the Internet. Ocean Network Canada's data management and archive system manages data from several hundred sensor types, with acoustic devices representing 90 percent of the typical 42 gigabytes of data logged daily.

Active and passive acoustic devices are, and will be, core instruments on cabled ocean observatories as they provide unique capabilities for probing various ocean conditions and benefit from continuous power and nearly unlimited bandwidth. A few challenges remain, however, as visualization tools for exploring long time series from high-volume acoustic devices are limited. Although interoperable data file formats exist, they were not designed for the terabytes of streaming data from observatories. Our industry can overcome these challenges with collaboration and standardization but such complex initiatives can be difficult to rush.

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