TOBI provides high quality three-dimensional images of the sea floor. It is a deep-towed highly flexible instrument operational in water depths to 6,000 m at 1.5-3 knots. It incorporates a 30 kHz, 6 km swath, 6 m resolution side-scan sonar with a swath bathymetry. It also has a 3 component fluxgate magnetometer and 7.5 kHz sub-bottom profiler (Figure 6).

The tow cable required to maintain a survey altitude of 300-400 m above the seabed is about 1.5 times the water depth. It has proven useful for both exploration and engineering surveys. The Deep-tow platform which houses TOBI also has a 7.5 kHz sub-bottom profiler which penetrates up to 40 m in soft sediment but much less in sand.

This is a deep seafloor mapping system with a number of varyants. SeaMarc II consists of two sets of acoustic arrays mounted in a five meter long neutrally buoyant vehicle that is shallow-towed below the top of the seasonal thermocline approximately 400 meters behind the ship. The acoustic arrays transmit a signal at 11 & 12 kHz. Bathymétrie information is generally reliable out past the first surface-bottom multiple (equivalent to 1.7 times the water depth per side of the tow-fish). Typically a signal pulse produces an average swath width of 10 km (5 km per side) of back-scatter imagery. The system is towed 50-100 m below the sea surface at an average speed of 8 knots. Horizontal resolutions of ca 20-50 m for side-scan and 25-100 m for bathymetry are typical in 500 to 2500 m water depths. Advanced data processing allows for production of high resolution, equal area, essentially geo-referenced datasets.

Figure 7. TOBI image of methane pockmarks at 350 metres in the Barents Sea (B. Murtón, SOC).

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