Abstract
- The purpose of the project was to develop a towed video sledge combined with low altitude sidescan system to obtain concurrent visual samples of the seafloor and high resolution acoustic images. It is intended that such a device would bridge the gap between standard sidescan survey and ground truth sampling. The specifications for the device were that the technology would be inexpensive, easy to use, deployable from small vessels and operate in depths of at least 30m.
- A system has been designed meeting these specifications that has been termed a camera and low altitude sidescan (CLASS) device. It consisted of a sledge with sidescan trailing 1.3m above the sea floor from an attachment on the sledge towing cable. Bullet cameras were attached close to the sea floor for viewing in low visibility. A StarFish 450kHz chirp sidescan was connected to the surface using a local area network and the transducer unit was rendered slightly positively buoyant and directionally stable by attaching it to a fish with tail fins and net floats.
- The design went through a series of tests and modifications. The final design of the sledge was small and light-weight and easily constructed from aluminium sheet and threaded steel bars.
- The CLASS device was trialled in areas known to have supported reef created by the Ross worm Sabellaria spinulosa, including the licensed aggregate extraction site Area 107. The device obtained acoustic and video images of the sea floor in all but the strongest tidal currents. However, the device performed best (in terms of directional stability and maintenance of a constant altitude) as a sledge towed on the sea floor in conditions of low currents. With moderate currents the system worked well with the sledge acting as a depressor flown slightly above (but in occasional contact with) the sea floor. The slightly higher sidescan altitude of 3 metres resulted in a marginal reduction in the resolution of small targets.
- The sidescan deployed at low altitude could detect small sea floor features under 10cm high, such as sand ripples and erosion features in sediment consolidated by faunal turf, worm tubes and also sea grass. However, no well defined Ross worm reef was observed and the system has not been tested against the full range of biogenic reef.
- It is considered that the CLASS device would find a use in targeted surveys where it is desirable to describe and map habitats and habitat boundary characteristics in fine detail. Another potential use might be for marine archaeological survey where a potential area of small, dispersed targets (such as wreckage) has been located but needs fine scale investigation.