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Our Work with Direct Velocity and Acoustic Backscatter Measurements

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Our Work with Direct Velocity and Acoustic Backscatter Measurements MTR#8 (18-33):MTR#4 (18-33).qxd 10/7/2009 12:36 PM Page 28 A New World of Velocity Profile Capabilities for Gliders by Eric Siegel, General Manager, Nortek USA Ocean gliders are autonomous underwater vehicles that contour plots of other physical variables is reason enough profile the water column vertically by controlling buoyan- for many researchers. Measuring higher velocity in one cy and transit horizontally by gliding on their wings dur- location compared to another could help explain evidence ing decent and accent. Initially used by academic institu- of upwelling. Observing variance in velocity shear at dif- tions primarily for research interests, gliders are now ferent locations in the horizontal and vertical could pro- becoming more ubiquitous in military, commercial, and vide insight to the formation and dissipation of phyto- operational oceanographic applications. Gliders can be a plankton thin layers. Further, because gliders do not nec- cost effective method of collecting oceanographic data essarily need to surface and communicate their location over a variety of spatial scales from ocean basins, through between dive cycles, some geospatial uncertainty about mesoscale eddies, to microscale turbulence. where the measurements are made is introduced. Current Presently, there are three operational glider manufactur- velocity may help define the glider path between GPS ers: Seaglider from University of Washington/iRobot fixes providing more accurate positioning for other data (Eriksen et al., 2001), Slocum from Webb Research Corp products. (Webb et al., 2001) and Spray from Scripps Institution of So why have direct velocity measurements from gliders Oceanography/BlueFin Robotics Corp (Sherman et al., previously not been included with the standard sampling 2001). The gliders typically descend and ascend through parameters? Making current velocity measurements from the water with about a 25 degree angle from the horizon- gliders is not trivial. Accurately providing an absolute ref- tal. When gliding, their horizontal speed is roughly 25 erence to current velocity data is very difficult due to the cm/s and they move vertically in the water at about 10 glider motion in the horizontal and vertical coordinates. cm/s (Rudnick et al., 2004) In addition, any sensor added to a glider must be small, Gliders can measure a wide range of oceanographic light weight, and require very little electrical power. properties. All gliders measure temperature, salinity, and Ultimately, the power budget forces a compromise pressure. Depending on the applications, other common- between mission length, sampling frequency, number of ly measured variables include optical backscatter, chloro- dives, and types of observational sensors. phyll fluorescence, and dissolved oxygen. Contour plots Since 2005, Nortek has collaborated with leading of these variables, as a function of depth and time (posi- researchers to develop specialized acoustic Doppler cur- tion), can offer views of biophysical phenomena such as rent profilers used to measure current velocity from glid- localized mixing, upwelling, phytoplankton blooms, and ers. The Nortek acoustic Doppler current profilers were thin layers. Vertically averaged ocean currents have been well suited to the glider requirement as they are inherent- inferred by comparing through-water speed and direction ly very small and require less power than most other sim- with distance made good between surface position ilar Doppler equipment (typically less than 1 W when updates. But until recently few direct measurements of pinging). Special mechanical configurations, novel current velocity from a glider have been made. acoustic transducer head designs, and high resolution sig- There are many reasons for making current velocity nal processing methods have been developed based on the measurements on a glider. Simply using profiles of cur- individual researcher’s project goals and requirements. rent velocity structure and shear as a reference to interpret Below, we present case studies of three different applica- 28 MTR October 2009 MTR#8 (18-33):MTR#4 (18-33).qxd 10/7/2009 12:37 PM Page 29 Figure 1 Deployment of a Webb Slocum 200 m glider with self-contained Nortek 1 MHz Aquadopp Profiler. A fleet of similarly-equipped vehicles were used by WHOI researchers to explore relationships between physical and biological fine structure in the ocean. www.seadiscovery.com Marine Technology Reporter 29 MTR#8 (18-33):MTR#4 (18-33).qxd 10/7/2009 12:38 PM Page 30 tions using Nortek acoustic Doppler current profilers on piggy-back to the top of the Slocum glider. After several gliders. successful missions, Dr. Fratantoni collaborated with Nortek to develop a custom transducer head with trans- Zooplankton Biomass Assessment ducers angled to point nominally upwards while the glid- Dr. David Fratantoni and his collaborators at the Woods er is at the 25 degree descent angle. The profiler may run Hole Oceanographic Institute (WHOI) have flown autonomously on internal power and memory, or may be Nortek 1 MHz Aquadopp Profilers on Webb Slocum glid- connected to the Slocum glider to source external power ers since 2005. Initially looking to make only velocity and/or log data. The 1 MHz Aquadopp Profiler can sam- measurements from glider mounted acoustic Doppler cur- ple every 30 seconds continuously for 15 days on small rent profilers, they found the active acoustic instruments internal lithium battery packs. If the profiler is only very useful for mapping and quantifying zooplankton in turned on during the glider descent, the mission can be both the Pacific and Atlantic oceans. The WHOI scien- extended to 30 days. Figure 1 shows the Nortek tists began using an off-the-shelf 1 MHz Aquadopp Aquadopp Profiler with custom transducer head mounted Profiler with a right-angle transducer head mounted on the Webb Slocum glider. Figure 2 Time series of temperature, salinity, and backscatter amplitude collected during May 2005 in the Great South Channel east of Cape Cod, MA. Acoustic backscatter at 1 MHz has been shown to be an effective proxy for the ver- tically-migrating copepod Calanus finmarchicus. 30 MTR October 2009 MTR#8 (18-33):MTR#4 (18-33).qxd 10/7/2009 12:38 PM Page 31 Figure 3 View of 400 kHz Nortek Aquadopp Profiler mounted on the underside of a Webb Slocum glider operated by researchers at Memorial University. Dr. Fratantoni and his team have flown gliders with the for Ocean Gliders focus on new applications using under- Nortek Aquadopp Profilers in the Pacific and Atlantic water gliders to support commercial and research projects. oceans on missions to map and quantify zooplankton in Their initial goal of measuring current profiles from glid- an effort to understand zooplankton thin layers and rela- ers was to provide ocean modelers with velocity data to tionship to whale feeding. Acoustic backscatter data from assimilate into their models and to quantify the accuracy the profiler was corrected for transmission loss and con- of the model predictions. Another aspect of their research verted into volume backscatter. A calibration of the pro- was to use velocity measurements in conjunction with an filer was conducted using a standard target in a tank facil- ice profiling sonar mounted on a glider. This combination ity at WHOI to convert backscatter data from relative of glider sensors provides current velocity profiles in close magnitudes into absolute magnitudes. Acoustic backscat- proximity to icebergs and the means to map the sub- ter at 1 MHz has been shown to be an effective proxy for merged part of the iceberg. Providing modelers with this vertically-migrating copepods. Figure 2 is a time series of combination of glider-based measurements will help gen- temperature, salinity, and backscatter amplitude collected erate better iceberg drift predictions and thus make off- during May 2005 in the Great South Channel east of shore operations in ice infested waters safer and more Cape Cod, MA. The data indicate the abundance of zoo- manageable. plankton is mostly constrained to the surface mixed layer To support their research requirements, the research in the upper ~40 m (Baumgartner and Fratantoni, 2008). team collaborated with Nortek to develop a custom 400 kHz Aquadopp Profiler that is fully integrated in their Offshore Operations Webb Slocum glider (Figure 3). The custom 400 kHz Dr. Ralf Bachmayer and his collaborators at Memorial transducer head mounts directly to the standard 2.5” University and the National Research Council Canada ECO Puck port on the underside of the Slocum glider. A Institute for Ocean Technology and the Canadian Centre fairing is placed around the transducer head to provide www.seadiscovery.com Marine Technology Reporter 31 MTR#8 (18-33):MTR#4 (18-33).qxd 10/7/2009 12:39 PM Page 32 better hydrodynamics and additional buoyancy. The pro- fine structures at the western edge and lower reach of the filer electronics board is short enough to fit in the stan- current. dard electronics bay within the glider. The profiler As a long endurance (up to nine months), battery pow- sources power from the glider and data are logged both to ered, buoyancy driven vehicle, low power, size, and weight the profiler internal memory as well as the glider science are all critical considerations when integrating a new sen- computer. The research team has written the Webb sor onto Seaglider. Developing an instrument to make “proglet” to allow the Slocum computer to control the high resolution current velocity measurements (over the Aquadopp Profiler sampling routine. 1000 m operating range) that meet these size and power The 400 kHz Aquadopp Profiler has a nominal profiling requirements is a significant challenge. range of about 80 m. For this application, the profiler is Seaglider developers at APL-UW collaborated with configured to point downwards and measures the velocity Nortek to develop a custom solution to meet these diffi- in the top 80 m of the water column while the glider is cult requirements.
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