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VIDEOGRAB

The Video Grab is an electro-hydraulically actuated clamshell grab sampler. The design satisfies a number of objectives: (1) ability to visually select the sample area; (2) minimise the disturbance to the sample area; (3) ability to close or open the bucket remotely; and, (4) verify that the bucket is closed prior to recovery. Video cameras with remote control zoom and macro capability, combined with 1000 watts of quartz halogen lights, allow the operator to survey the study site in preparation for sampling. When a suitable area is identified, the Grab is lowered to the seafloor, with the bucket poised 20 cm above the bottom. By keeping the umbilical cable slack, the Grab is decoupled from the motion of the ship, affording a detailed view of the bottom using the high resolution colour video camera looking through the open bucket. On command, the bucket is closed, taking a 0.5 square meter sample of the upper 10 to 25 cm. of the ocean bottom with minimum disturbance.

VIDEOGRAB

The Video Grab weighs approximately 1 tonne in water, which determines the maximum force available to drive the bucket into the benthic layer. At full penetration, the volume of sediment sampled is 0.1 cubic meters. Closure of the bucket simultaneously closes a retractable lid which reduces wash-out during recovery of the grab. The system is designed to operate in depths up to 500 metres.

The entire system includes the customised winch, 30-conductor electro-mechanical cable, and large diameter sheave block. From the console in the ship's laboratory, the operator can raise, lower, or land the Grab while controlling the video cameras, the video recording equipment, underwater lights, and bucket actuator. The video images are coded with time and differential global positioning (DGPS) and recorded on SuperVHS tape and displayed on the vessels bridge.

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