Inverted echo sounder on a cabled observatory

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Abstract

High-frequency acoustic backscatter measurements have long been used as a method to detect zooplankton populations in the ocean. Ship-borne echo-sounders can map distributions over relatively large areas, but are not practical for following developments over long periods of time. Self-contained echo-sounders, either moored at depth looking upward, or mounted on surface buoys looking downward produce time series of acoustic backscatter which are a means for monitoring long-term behaviour of zooplankton populations. Combining these instruments with a cabled observatory allows this information to be acquired and monitored in near real-time, and thus to contribute to understanding and monitoring the state of one of the key components of coastal marine ecosystems. In this paper, we will discuss what we believe is the first operational example of an inverted echo sounder as part of a cabled seafloor observatory. The first component of the VENUS seafloor observatory, in Saanich Inlet, British Columbia, began operation in February, 2006. The instrument platform is located 3 km from the Institute of Ocean Sciences dock, at 96m depth. A 200 kHz upward looking echo sounder (Acoustic Water Column Profiler™, also known as the ZAP, for Zooplankton Acoustic Profiler) is located on the platform. The instrument collects a profile of acoustic backscatter strength throughout the water column once per second and transmits the data over the cable to the VENUS shore station. Power and control communications are also supplied over the cable. The techniques for command and control of the instrument over the VENUS VPN are described, as are the means for real-time display of the data using TCP/IP protocols. Continuous data retrieval and image posting to the VENUS website are performed automatically using the observatory's DMAS (Data Management and Archiving System). Echograms of acoustic backscattering strength as a function of depth and time are available on the web at hourly intervals, and as daily summaries. These plots, and the data files from which they are constructed, may be used for the detection if zooplankton, fish, near-surface bubbles and internal waves. Their evolution and development can be monitored in real time, and the archived data now provide a year-long record at the Saanich Inlet site. At least two more of these instruments will be deployed at sites in Georgia Strait, when the next phase of the Venus observatory system is installed in October 2007. It is anticipated that the data from these locations will provide a valuable tool to further the understanding of the state of this coastal sea. A new generation of the AWCP is currently under development, which will provide greater A/D resolution and greater data storage for self-contained operation. ©2007 MTS.

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Lemon, D. D., Chave, R. A., Clarke, M. R., Dewey, R. K., & Macoun, P. (2007). Inverted echo sounder on a cabled observatory. In Oceans Conference Record (IEEE). https://doi.org/10.1109/OCEANS.2007.4449192

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