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Tools for Biological Assessment Using Split Beam Hydroacoustics
Split Beam Echosounders for Defense, Observation and Fisheries Projects

Eric Munday

Tim Acker

James Dawson

Imaging and split beam sonar system: study area diagram, Hood Canal, Puget Sound, Washington.
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/Installation. 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 fixed-station 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. To continue this article please click here.

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.

James 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.

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