In the field, the comparison between oxygen-rich and oxygen-poor sites requires the description of habitats and communities. Although a myriad of measuring devices exist, none is better than the human eye to gauge the environments and sampling problems faced by scientists. In order to obtain this qualitative information,Examples of typical bottom images a team from the Maurice Lamontagne Institute (MLI) designed a simple, low cost, towed underwater video system that can withstand the conditions found at 350 m below the surface, including rough terrain, total darkness and high pressuree.
A first deployment was done on July 4, 1999 off Les Escoumins by towing the system on the bottom in 312 meters of water. These first results were convincing and these images showed a silt bottom covered with brittle stars, anemones, crabs and fish. The following week saw a series of successful tows that provided good video sequences of the bottom in most areas of the Saguenay fjord between Tadoussac and Ville La Baie. Since these first tryouts, lighting and towing techniques have been improved but the system remains simple and low cost, being built from readily available commercial components that can be easily replaced.
This system has never been seriously damaged and suffered only occasional minor scuffing. It has provided us with several hours of video sequences on snow crab habitat from the Métis and Sept-Iles areas. It has also been used as a model for a more sophisticated instrument now in use with the Canadian Hydrographic Service (CHS) in conjunction with their multi-beam sonar systems.
These 73 second video sequences have been assembled from images taken in 300m of water in the Laurentian Through near Les Escoumins; at various locations in the Saguenay Fjord between 50m and 200m and near Pointe Mitis in the estuary between 100m and 200m.
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Challenges and solutions
Submersible underwater camera systems are not new and are available commercially. Many of these systems can withstand high pressures and provide high quality images. However, their cost is generally high and specialized equipment is often required for deployment. Also, their rapid replacement in case of damage in often not possible.
The success of our system resides in the use of components that are not normally used in scientific equipment and that are easy to find at retail outlets.
All three housings share a common design. They consist of a simple 9 mm thick aluminium cylinder closed at one end by a welded 13 mm plate. The housings have been anodized for protection against corrosion. The other end is closed by a removable 25mm acrylic port. This port compresses an O-ring that provides water tightness. Condensation due to immersion in cold water forms on the metal walls rather than on the front port, leaving it clear at all times. The lighting housings have a removable shorting plug that acts as a switch for the lights. The absence of through holes and movable parts makes for a very robust package. The choice of materials has been found adequate for depths in excess of 300 m and all housings have survived some serious abuse, including encounters with 2-3 meter high boulders.
A domestic camcorder attached insideCamcorder a plastic mounting cage is installed behind the front port. A conversion lens is attached to the camera and provides a wider angle (65 degrees) of view while keeping chromatic and spherical aberrations from the flat port to a minimum. Camera controls are not accessible from outside the housing except for on/off commands using the infrared remote through the port.
The use of high intensity discharge arc lights would have been preferred but cost and complexity of such a system have been reduced by using low voltage halogen lights. Dichroïc reflectors using 50 Watt bulbs and covering 40 degrees have been used. These are powered by 14 Ah 12V lead/acid batteries that provide 50 minutes of run time per immersion. Diffusing filters have been added to the acrylic ports to soften the light and increase the coverage angle.
Again, existing equipment has been chosen over a custom design. The front section of a 3 meter beam trawl (without the net) has been modified to mount the camera and lighting housings. The towing chains have been replaced by nylon cables and lead ballast has been added to the skids to improve stability. This platform is highly stable on different bottom types from soft silt to gravel with the occasional boulder.
- Lighting batteries : verification and installation
- Lubrification of all sealing surfaces
- Camera battery and cassette installation
- Closure and verification of all housings
- Turning on lights and camera
- Immersion while moving (3-5 knots)
- Descent towards bottom with a cable length of 1.5 times the depth
- Towing at 1.5 knots for 20-30 minutes
- Turning lights and camera off
- Visual inspection of all housings
- Opening the camera housing, retrieval of cassette, battery replacement
- Opening lighting housings, replacement of batteries
ProcessingImage processing system
- Quick verification of image quality
- Copy of the analog tape to digital support for archiving
- Rotation of cassettes during the mission (a minimum of four two-hour tapes per mission)
- Editing and digital rendering at the MLI
- Compression and final archival storage on CD-R or DVR-R
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