Abstract
Multibeam imaging sonars have application to monitoring fish and marine mammal presence and behaviours in the near field of tidal turbine installations, including evaluating avoidance, evasion, and potential blade strikes. SOAR conducted field experiments to help reduce uncertainty in performance of the Tritech Gemini 720is and Teledyne Blueview M900-2250 multibeam imaging sonars for identifying and tracking discrete targets in high-flow environments. This information will help inform the Department of Fisheries and Oceans Canada, tidal energy developers, and other stakeholders in the design and implementation of effective monitoring systems for tidal energy projects in the Bay of Fundy and beyond. These two imaging sonars were the technologies recommended for testing by the subject matter expert for imaging sonars during the first phase (Global Capability Assessment) of the Pathway Program. The Tritech Gemini 720is operates at 720 kHz and has a maximum effective sampling range of approximately 50 m. The Teledyne Blueview M900-2250 has operating frequencies of 900 or 2250 kHz, with a 10 m range for the high frequency transducer head. As per the recommendation from the Global Capability Assessment, this report focuses on the Blueview’s capabilities while operating at 2250 kHz.
Field trials included deployments of an Autonomous Multibeam Imaging Sonar (AMIS) monitoring system in Grand Passage. The depth at the deployment location is approximately 25 m at low water, with flow speeds up to approximately 2.5 m/s. The deployed sonars were oriented with their ensonified areas directed downstream. The instruments’ horizontal fields of view oriented across-channel and vertical fields of view tilted upward from the bed.
Three targets were used during data collection: a 0.45 kg (1 lb.) (9.5 cm long x 3.8 cm max diameter) lead fishing weight, approx. 12 cm diameter basalt rock in a lobster bait bag, and a V-Wing glider (approx. 50 cm diameter) from Dartmouth Ocean Technologies. The targets were suspended beneath research vessel Puffin while drifting through the study area. The Puffin repeatedly travelled to a position upstream from the sonars, then drifted with the tidal flow such that the drift trajectory allowed the targets to pass through the sonars’ ensonified areas. The AMIS system was fully autonomous, so no live view of data collection was available.
The data were manually analyzed to evaluate the performance of the Gemini and Blueview multibeam imaging sonars for detecting and tracking targets in strong tidal flow. The visualization and organization of the data were conducted using the proprietary software packages associated with each sonar: Gemini SeaTec and Teledyne ProViewer. Data from the Gemini were exported into video and organized into training and test data sets, which were shared with 7 sonar observers who conducted the manual analysis to detect, track, and identify the targets. Links to the training and test data sets for are provided below.
Gemini training data: https://vimeo.com/483141927
Gemini test data with 50m range: https://vimeo.com/483142328
Due to the small ensonified area of the Blueview, insufficient sightings of known targets were collected to generate training and test data sets. A manual analysis was conducted by SOAR, with a focus of events of concurrent detection by the Blueview and Gemini including natural targets (primarily fish) and occasionally the artificial targets used in our methodology. A link to a video file with 21 comparative cases is provided below.
Concurrent Blueview and Gemini: https://vimeo.com/487808248
The Tritech Gemini 720is received high scores from the observers in the ability to identify the presence of, visually detect, and track targets in videos displaying sonogram data output. The observers correctly identified the presence of a target in 99% of cases, and gave average scores greater than 4 out of 5 describing their visual detection and tracking ability. Targets were correctly identified roughly 50% of the time. No significant relationship between flow speed and ability to detect and track the targets was observed.
The Teledyne Blueview M900-2250 MKI is an impressive technology that offered the ability to resolve finer scale features of the targets and their movements in some cases. However, persistent high-noise bands resulting from a known hardware issue and an apparent transducer alignment issue represented substantial impediments to reliable target detection and tracking. We conclude that data from the Blueview did not add substantial value or insight to the target analysis when used in conjunction with the Gemini. This should not rule out potential use of other MHz frequency multibeam sonars for monitoring the 10 m range in a combined sonar approach, including MKII of the Blueview.
SOAR recommends use of the Tritech Gemini 720is for application to monitoring interactions between marine animals and tidal turbines. The Gemini demonstrated a high level of utility for detecting and tracking targets from vessel and bottom mounted orientations in tidal flows up to approximately 2.5 m/s in Grand Passage. It is likely that this technology will contribute significantly to effective monitoring and advancing knowledge of importance to regulators and other stakeholders. Tidal flows are faster at the FORCE site in the Minas Passage, with flow speeds exceeding 2.5 m/s 30 to 40% of the time.
With respect to deploying multibeam sonars from the surface (i.e., vessel) or seabed, the sonars performed well from both positions, despite increased levels of air entrainment in the vessel mount case. The selection of deployment position for monitoring tidal turbines is likely to be defined by the nature of the tidal device (floating or seabed mounted) and the questions to be addressed by the monitoring.
The project addressed the objective of assessing the performance of bottom deployed multibeam imaging sonars for target detections, including the extent of signal interference from waves/turbulence, and entrained air.
Further testing of bottom mounted multibeam sonars would be useful in four focus areas, including:
- fish and other marine animals in locations and seasons (times) with high levels of animal abundance and variety,
- evaluating most effective sonar orientations for monitoring the near field of tidal turbines,
- flow speeds that exceed 3 m/s, and
- increasing efficiency in data assessment, possibly including reliable automation.
This work should build upon success in Grand Passage to conduct next steps in stronger flows present in Petit Passage and Minas Passage. The report titled “Field Assessment of Multi-beam Sonar Performance in Surface Mount Deployments” (Trowse et al. 2020) provides similar analysis for the case of surface mounted Gemini 720is and Blueview M900-2250.