Feature ArticleFreshwater Lens Camera System Surveys Seafloor in High-Turbidity Waters
By Jon Spink
Alpine Ocean Seismic Survey Inc.
Norwood, New Jersey
Gardline Environmental Ltd.
Great Yarmouth, England
The ability to capture clear photographic and video images of the seabed surface is critical for environmental assessment of proposed marine development sites. Underwater camera systems used for environmental work typically have two primary roles. First, they investigate areas other technologies, such as side scan sonar and multibeam echosounder surveys, have identified as potential sensitive habitats. Second, they help characterize the flora, fauna and general habitat across a proposed development site.
The freshwater lens camera system being deployed during an extensive environmental survey project in the southern North Sea.
Standard underwater camera technologies are severely restricted in highly turbid waters, where reduced visibility can render the equipment useless. To overcome this issue and produce clear images in previously unsurveyable environments, Gardline Environmental Ltd. developed a drop-down, freshwater lens (FWL) camera system that can be used in water depths of up to 300 meters. The system has proven particularly valuable to surveys in shallow-water coastal environments, which is where U.K. offshore farms are currently being constructed. The high turbidity common in nearshore waters—created by dynamic tidal regimes, plankton, sediment and organic matter loads from estuaries—makes it difficult to conduct the visual investigations required for many environmental impact assessments.
The FWL system incorporates a Kongsberg OE14-208 camera, which records real-time video and captures digital still images in resolutions from 640-by-480 to 2,592-by-1,944 pixels. The system's integrated design houses a large lens of distilled water in front of the Kongsberg camera, which works to cut down the path length that light must travel through turbid water. Fronting the lens itself is a 15-millimeter clear acrylic base that keeps the water in place and allows light to penetrate naturally. With the FWL positioned in the housing, the distance between the face of the Kongsberg camera and the outside of the acrylic base is 410 millimeters. The FWL system eliminates approximately three-fifths of suspended particulate matter between the camera and the seabed.
In 2008, the system had its first commercial deployment in an extensive, 21-day survey of 160 stations for a proposed wind farm in the southern North Sea. Alpine Ocean Seismic Survey Inc., a Gardline subsidiary, introduced the system to the U.S. in 2011 for use in renewable energy, oil and gas, and other marine projects.
System Design and Challenges
Lighting. To accommodate the added structure of the FWL housing with its large, potentially reflective lens, Gardline had to overcome a number of lighting issues. These included ensuring the system could illuminate the seabed adequately and evenly without making it overly bright. It had to be able to handle differences in the color of light, which changes based on the number and type of light sources (strip lights or spotlights, LED or halogen), the position of light sources on the frame and water turbidity. Another of the challenges was to ensure that light did not reflect on the FWL, as this can obscure images.
To address these challenges, Gardline modified the system to let operators choose the lighting source that will yield the best results based on the level of turbidity. The design accommodates LED lamps, which provide a true reflection of habitat color, and halogen lamps, which provide better illumination in very turbid environments. The system also includes a dedicated flash gun with adjustable intensity for digital-still photography.
Auxiliary Sensors. The system can accommodate a variety of camera options to record images and provide real-time views of the seabed to the surface vessel, ranging from five-megapixel standard-resolution video cameras to high-resolution video cameras that take 14-megapixel digital stills. For enhanced data sets, the design team modified the camera system's in-sea umbilical to free up a line previously dedicated to lighting, enabling researchers to deploy auxiliary sensors such as profiling and imaging sonars, CTDs, fluorometers, altimeters and laser scale bars.
An image of Alcyonium captured by the freshwater lens camera system in reduced visibility water conditions in the southern North Sea. Each of the ruler's bars represents one centimeter.
The umbilical sends all data captured by the camera system and auxiliary sensors to a topside unit on board the vessel. The topside unit splits the signal, sending real-time images to a recording suite via an overlay that integrates water depth, geospatial and job-specific information. The other part of the signal feeds into onboard software that allows operators to control the camera or any auxiliary sensor, adjust the camera's focus, zoom and flash intensity, and take digital images. With the integration of Gardline's environmental event-logging software, the camera can also be triggered based on predetermined time or distance intervals, providing geo-referenced photographs for quantitative assessments.
Image Clarity. Prior to deploying the unit for research operations, scientists placed the camera into the FWL housing and filled the lens with distilled water. Gardline's engineers discovered air bubbles often got trapped on the surface of the camera itself, preventing it from capturing clear images. In early development stages, researchers had to remove the camera and FWL from the housing to eliminate the trapped air, but could not ensure the problem would remain fixed.
To resolve this issue, engineers developed a process that sends water across the lens face to clear it and then removes the trapped air through a bleed valve. This procedure is performed on-deck prior to submersion, and the system is set for the duration of the operation.
System Weight and Seabed Coverage. How high the camera is positioned over the seabed depends on turbidity and visibility levels. In zero-visibility conditions, the system's frame sits on the seabed, with the base of the FWL 20 millimeters above the seabed surface. In greater visibility conditions, the camera is 'flown' above the seabed at the most appropriate height, depending on turbidity. The system has four height increments, each of 100 millimeters. Though flying the camera higher brings a larger section of seabed into the field of view, it also means there is more particulate matter between the camera and the seabed, so researchers must determine the deployment that will deliver the best data based on conditions. To continue this article please click here.
Jon Spink is an environmental project manager for the Gardline Group, based at U.S. subsidiary Alpine Ocean Seismic Survey Inc. He received his B.Sc. in biological sciences and M.Sc. in applied marine sciences from the University of Plymouth, U.K. He focuses on geophysical and environmental surveys for renewable energy projects.
Laurence Read is a technical supervisor for the Gardline Group, based at U.K. subsidiary Gardline Environmental Ltd. He has been in charge of technical support and development since 2007 and specializes in hardware and software engineering for environmental sampling and seabed imaging systems.