The aluminium frame of the flying array is a modification of the design detailed in Stevens (2003), scaled up 1.5 times to house the extra bulk of the HD video and Conductivity-Temperature-Depth (CTD) equipment. To make the frame neutrally buoyant, ballast tubes of high-strength plastic were attached to the top of the frame, and calibrated so that the frame was just positively buoyant with all equipment fitted, but without the drag chain. The frame is towed by a floating bridle attached at the two lower front corners and centrally on the upper front panel cross member. It was configured so that the array lifted up slightly as it was towed along. A short length of chain extending from the base of the array allows it to ‘‘fly’’ at a predetermined height above the sea bed; this chain is the only piece of the device making contact with the seabed. When the chain is partly on the bottom the array is neutrally buoyant. Changes in bottom topography result in less or more of the chain off the bottom; the array adjusts its height until equilibrium is again achieved.
The desired height above the bottom can be adjusted by changing the weight of chain used, or the length of a lightweight rope attaching the chain to the array. This rope functions as a weak link, breaking if the chain snags on the sea bed to allow recovery of the array and avoid damage to the umbilical. The weight of chain used is dependent on the environmental conditions, especially current and wave surge. In fair conditions, such as Lyme Bay, an 8 mm stainless steel chain can be used (Length: 3.15 m, Width: 12 mm, Weight 10 kg); in more extreme, high-wave conditions, a shorter thicker chain is more suitable, the compromise being the slightly increased impact that sampling has on the sea bed. This is still, however, minimal compared with traditional equipment.
Learn more about the HD flying camera array (pdf).