Adapting Multi-Frequency Echo-sounders for Operation on Autonomous Vehicles

Low-power, internally-recording multi-frequency echo sounders can acquire continuous profiles of echoes throughout the water column over long periods of time, thus providing a low-cost method to study the behavior and abundance of fish and zooplankton in the ocean. Ocean gliders are growing in importance as components of ocean observing systems, extending measurements of physical oceanography beyond those possible with moorings and expensive oceanographic research vessels. Payloads are typically composed of conductivity, temperature and pressure along with optical measures of chlorophyll reflectance and dissolved organics. The small size and low power consumption of such echo sounders now makes it practical to install them in gliders, providing the means to simultaneously measure biological metrics through the water column over extended areas and linking physical properties and primary productivity to higher trophic levels such as zooplankton and fish. We have recently installed single-beam echo-sounders with up to four acoustic frequencies in gliders. The echo-sounder (ASL Environmental Sciences' AZFP) was designed as an autonomous, moored instrument, and several modifications, mechanical, electronic and software were required to adapt it to installation and operation in gliders. The existing lower-frequency transducers were too large and heavy for the glider; their size was reduced by increasing the beam-width, and the housings were redesigned to reduce weight and improve their profile to reduce drag. The transducer housings are designed to be mounted at an angle on the vehicle body so that they are aimed vertically down during the dive phase. Several modifications were also made to the electronics chassis and shrouds to reduce weight, and the power connection was altered to operate from the vehicle battery rather than its own dedicated supply. Examples of these design alterations will be discussed. The instrument operating software was modified to allow communication and control from the vehicle, although data storage remained in the AZFP itself. In the initial case, a 200 kHz, single-beam echo-sounder was integrated and calibrated in a Slocum Webb electric ocean glider. Calibration and controlled field tests demonstrated reasonable signal-to-noise ratio, allowing for detection of plankton and fishes to greater than 75m range. The noise background was higher than that found when the AZFP is used as an autonomous unit, so improvements to noise isolation in the glider should be able to improve the signal to noise ratio in future. A data analysis workflow was developed that estimates glider position and orientation underwater to correct for depth and range of targets. Trial missions in the eastern Gulf of Mexico traveled over a submerged pipeline and a rocky reef. Acoustic backscatter signals attributed to mid-water plankton layers were co-located with oceanographic features and peaks in chlorophyll. Schools of pelagic and demersal fishes were detected and mapped over charted seafloor features. To date, three multiple frequency versions have been built and integrated into Slocum Webb G2 gliders. Several 3-frequency AZFP (38, 70 and 125 kHz) will be used to assess the spatial and temporal distribution of krill biomass in the Antarctic. Several 4-frequency instruments (125, 200, 455 and 769 kHz) have been built to study whales. A glider with a 3-frequency AZFP (38, 125 and 200 kHz) combination was deployed in Terra Nova Bay (western Ross Sea) and mapped the vertical and horizontal distribution and abundance of zooplankton and pelagic silverfish. Future developments will be aimed at adapting the AZFP to other autonomous vehicles and to reducing the effects of the vehicle's internal noise environment. Ultimately, this glider-based acoustic technology will pave the way for cost-effective, automated examination of food webs and ecosystems in regions throughout the global ocean.