Tools for Biological Assessment Using Split Beam Hydroacoustics

Split Beam Echosounders for Defense, Observation and Fisheries Projects

Split beam echosounders are a standard instrument of scientific fisheries acoustics. To augment the information gained from mobile hydroacoustic surveys, researchers have developed innovative methods, devices and platforms to deploy split beam echosounders in new environments. Here, we describe three recent inventions that facilitate the use of split beam echosounders in nonconventional applications: a hybrid split beam and imaging sonar system for defense-related applications, an autonomous, submersible echosounder for long-term seafloor observation, and a towed echosounder engineered for use with a Liquid Robotics (Sunnyvale, California) Wave Glider.

Hybrid Split Beam, Imaging Sonar System

Split beam echosounders detect, locate and measure targets at relatively long ranges (+500 meters) and are largely unaffected by turbidity or light. Imaging sonars provide high-resolution data for visual verification of targets, but are more range-limited and affected by water conditions than split beam. BioSonics Inc. (Seattle, Washington) and Teledyne BlueView Inc. (Seattle) developed a prototype system integrating split beam and imaging sonar to combine long-range detection with visual ground truthing capabilities.

Active sonar systems for security applications are designed to detect and classify specific targets. Performance can be diminished due to nuisance alarms caused by nonthreatening targets (debris, aquatic vegetation and marine mammals) that reflect acoustic energy in a similar way when compared to an intruder. An experiment was conducted in the fall of 2011 to identify the acoustic signatures that could cause nuisance alarms. Information gained will aid in the development of data filtering and target classification algorithms to minimize the nuisance alarm rate (NAR) for active sonar systems used in security applications.

Experimental Design/lnstallation. A BlueView P900-130-D multibeam sonar and a BioSonics DT-X split beam echosounder were deployed as an integrated system that operated continuously for three weeks. The two systems were aimed at an overlapping 7-square-meter study region that varied in depth from 1 to 4 meters.

Each sonar head was mounted on a dual-axis rotator, precisely installed to insure coverage of the vertical extent of the study region during all tidal cycles. Alignment of the sonar beams was verified using a point-source reflector carried by tethered scuba divers.

A bin detection range of 60 to 68 meters was programmed into the BioSonics sonar to insure that target detections inside the study region were the only ones to trigger an alert.

Control Unit Description. Both sonar systems were controlled by a BioSonics Automated Monitoring System Control Unit (AMSCU), a rack-mount electronics package consisting of a heavy-duty computer, uninterruptible power supply (UPS), broadband firewall/router and a remote power management system (RPMS) all contained in an environmental controlled enclosure (Purcell RAC 48). BioSonics System Watchdog software monitored the system status and sent daily reports via cellular internet that included a record of power outages, sonar aim, operational status and all alert activities. The cellular communication system also allowed for remote control of each sonar system.

Data Collection and Analysis. All sonar data were time stamped by the system clock and logged to RAID storage in daily directory file folders. The BioSonics system is automated to alert when a target of sufficient signal strength enters the detection area and persists for a specified amount of time. The BioSonics AMSCU is programmed to trigger an alert when echoes exceed an intensity threshold for a consecutive number of pings at a particular range. Upon alert, the system changes its indicator status from "Green" to "Red" and the information (time, 3D location, speed and direction of travel) about the echoes that generated the alert are recorded in a track list. The software automatically copies a section of data extending from 10 seconds before the alert to 10 seconds after. The time stamp from this section of raw data is used to make a video clip from the corresponding imaging sonar data. These video clips provide a means to verify the cause of the alerts.

Autonomous Programmable Split Beam Echosounder

Variability in fish and other biological organism distribution and abundance has been documented as a function of short-term behavioral variation. This variation may be due to predation avoidance, migration between resting and feeding areas, feeding, territorial or spawning activities, or response to environmental changes. These highly variable biological activities are important to understand for siting and operation of tidal energy generation facilities.

A better understanding of biological distribution and behavior can be obtained through measurements from fixed locations over a period of weeks or months. This type of fixedstation hydroacoustic monitoring typically involves the collection and analysis of water column backscatter data utilizing an upward or side-looking echosounder recording data to a shore-based control station.

Data transmission and configuration of conventional split beam echosounders is accomplished via a continuous communication link (typically Ethernet) with a computer running specialized data acquisition software. Installation of a communication tether cable to the surface can be impractical or impossible in deep-water, remote and offshore projects. Therefore, fixed-station deployments of split beam echosounders in these conditions have been quite rare or previously not possible. Recent invention of a programmable, autonomous, split beam system (BioSonics DT-X SUB) allows for untethered deployments and long-term data collection in remote or deep-water environments. This submersible echosounder system is capable of collecting and recording split beam data over an extended period of time with no surface tether required.

The University of Washington and the Northwest National Marine Renewable Energy Center (NNMREC) working with BioSonics, successfully deployed a DT-X SUB echosounder in Admiralty Inlet, Puget Sound, for five weeks beginning June 2011. The study location is a proposed Snohomish Public Utility District tidal energy demonstration project site. Information gained is useful to project developers in assessing the relative suitability of locations for longterm hydrokinetic facility deployments.

Deployment Methods. The DT-X SUB was deployed as part of an array of battery-powered monitoring sensors affixed to a seafloor observatory platform. The system was autonomous and required no surface tether. Echosounder configuration and duty cycle were programmed at the surface. The echosounder operated at a 10 percent duty cycle, alternating between active pinging and sleep modes to maximize temporal coverage and battery life. An integrated orientation sensor recorded the aiming angle of the transducer for accurate water column volume and fish target angle calculations. The system was retrieved using acoustic releases, and the data files were processed for fish abundance, distribution and behavior information.

The DT-X SUB system components were as follows: PREVCO (Fountain Hills, Arizona) A811SSTB pressure housing containing a DT-X split beam echosounder; a 200-kilohertz split beam transducer with internal orientation sensor; electronics and specialized software for automated operations, data handling/storage, and shutdown/restart capabilities; four 80-ampere-hour, 24-volt, deep-cycle batteries, manufactured by DeepSea Power & Light (San Diego, California); and an Oceanscience (Carlsbad, California) Sea Spider tripod mount.

Results From DT-X SUB Deployment. Processed data verified the expected density distributions across time and space. Overall, NNMREC concluded that it is possible to adapt existing acoustic instrumentation for autonomous deployments in high-flow environments. However, it was also noted that while data storage capacity is not an issue, limited power availability restricts sampling density, therefore it is important to ensure that power supplies meet sample design requirements. Researchers also concluded that more predeployment testing to optimize echosounder data collection parameter settings would have improved data quality.

Wave Glider Towed Split Beam Fisheries Echosounder

Rising costs of ship time have reduced ship-borne fisheries survey efforts and compromised their value to fisheries managers. Autonomous vehicles now carry split beam sonar systems to collect data at a reduced cost and in environments not before possible. A hybrid machine, conceived by Dr. Charles Greene of Cornell University, was codeveloped by Liquid Robotics and BioSonics. The device combines a Liquid Robotics Wave Glider with a BioSonics DT-X SUB echosounder housed in a missile-shaped towed vehicle attached to the submerged glider unit with a compliant tow cable. This "Fisheries Glider" requires fewer man hours than typical shipboard acoustic surveys, uses no fuel, and thereby operates at a reduced cost.

System Description. The Wave Glider is a vehicle comprised of a submerged glider tethered to a surface float. The vehicle is propelled by the conversion of wave energy into forward thrust. The Wave Glider maintains an average forward speed of about 1 meter per second (1.5 knots) in seas with 0.5-to-1-meter wave height. The system uses solar panels and 650 watt-hours of battery capacity to power onboard sensors and payloads. The Wave Glider can withstand openocean waves and strong winds and has proven capable of transoceanic missions.

The DT-X SUB is a programmable split beam echosounder in a submersible housing engineered for deployments where a cable to the surface is not practical. The DT-X SUB can be configured with a range of transducer frequencies for detection of objects ranging in size from zooplankton to marine mammals. The echosounder can provide information including: bathymetry, substrate classification, fish sizing and abundance, and the distribution and location of aquatic plants and or ocean mixing layers. Data are logged internally and downloaded upon recovery of the vehicle. Real-time data summaries are transmitted via satellite connection during deployment. The echosounder can be operated continuously for approximately 10 hours between battery recharging from the Wave Glider solar panels.

Researchers use Liquid Robotics's Wave Glider Management System (WGMS) to monitor and control the vehicle via the Iridium satellite network from any Internet connection. WGMS provides the ability to turn payloads on and off, navigate to desired locations, and monitor vehicle status.

Test Description. Liquid Robotics conducted a series of tests in Kealakekua Bay, Hawaii, in March 2011 to evaluate the ability of the system to detect and observe the migration of the mesopelagic boundary community. The system tested included a Wave Glider and towed echosounder with 70- and 200-kilohertz transducers. The Wave Glider was equipped with a custom umbilical cable and payload box to provide power and communications capabilities to the echosounder towed body. The Wave Glider was programmed to collect data in an operational test area of 1 nautical mile by 100 meters. The speed of the Wave Glider pulling the tow body during this test was approximately 1.2 knots in a moderate sea state. The operations lasted two nights to test and validate the system's ability to collect stable, high-quality acoustic data.

Results. The system proved to be an effective platform for collecting acoustic data and studying patterns in the distribution and migration of marine organisms. Biological backscatter was clearly detected by the echosounder at ranges in excess of 100 meters and observed to ascend vertically over time. Significant biological backscatter was visible along the ocean bottom at a depth of 70 to 90 meters, as well as a likely aggregation of fish at approximately 30 to 50 meters.

The compliant cable provided mechanical isolation from the surging motion of the Wave Glider propulsion system and allowed the towed body to maintain a stable horizontal orientation suitable for the collection of highquality hydroacoustic data. The towed body contained an orientation sensor logging positional data approximately nine times per second. The pitch of the tow body typically stayed within a range of +/- 7 degrees and roll within +/- 2 degrees. The Wave Glider effectively propelled the echosounder towed body while maintaining an averaged 90 percent of the velocity of a nearby, un-encumbered Wave Glider.

The testing demonstrated that acoustic data collected from the "Fisheries Glider" are suitable for quantitative analysis. Overall, the system proved suitable and highly efficient for observing aquatic organisms in the water column over extended periods of time.

References

For a list of references, contact Eric Munday at [email protected].

Eric Munday, owner of SUBSEA 20/20, Inc., has worked in the hydroacoustics field since 2008, providing sales training in sonar operation, survey methods and data processing while directing BioSonics's global sales and marketing efforts. He received a bachelor's in fisheries science from the University of Washington, with certifications in salmonid research and environmental technology.

Tim Acker is president and CEO at BioSonics. He earned his geosciences degree from Pennsylvania State University and studied electrical engineering at the University of Washington. He has launched several government-funded product development programs to further grow the company in the areas of aquaculture and homeland security.

lames Dawson has worked in the development of mobile survey and fixed-location hydroacoustic techniques since 1981. He has successfully developed software for detection and tracking of underwater acoustic targets, and has extensive experience deploying sonar in challenging environments on hundreds of hydroacoustic projects in rivers, lakes, reservoirs and marine environments.