TOBI on R/V Marion Dufresne

TOBI (Towed Ocean Bottom Instrument) is based at the National Oceanography Centre in Southampton. TOBI provides high-resolution deep-sea surveys, using co-registered side-scan and swath bathymetry technology, to provide high quality three-dimensional images of the sea floor. Marine scientists use it worldwide on behalf of government departments and agencies, research institutes and industry. TOBI operates to depths of 6,000 metres and incorporates a 30 kHz, 6 km swath, high resolution side-scan sonar with swath bathymetry capability, and carries a variety of other scientific instruments; Sub-bottom chirp (6-10 kHz) profiler (up to 70 m penetration of sediment layers and sub-metre resolution), CTD (Conductivity, Temperature, Depth), 3-axis fluxgate magnetometer, LSS turbidity sensor, custom self logging devices (biological and chemical sensors)- application specific.

The TOBI vehicle weighs around 2.5 tonnes in air but is neutrally buoyant in water owing to its complement of syntactic foam. It has a total payload capacity of 400 kg and an open frame construction that simplifies the addition of extra instrument packages. Power, data and control signals are multiplexed on to a single coaxial cable contained within the steel torque balanced towing wire of up to 10 km in length (tow cable length to operating water depth is around 1.6:1 ratio).

TOBI is "flown" by its technical staff (left to right; Duncan, Dave & Andy) on a 4 hour on, 8 hour off watch keeper rota 24/7 during the time it is in the water. Duties include monitoring and controlling optimum altitude (350 - 600 metres depending on roughness of seabed terrain) and ship's speed (1.5 - 3.0 knots depending on tow cable length in the water). The only two controls for TOBI's flying altitude are tow cable length and ship's speed.

Data from the two side-scan channels (port & starboard) are corrected for geometric distortion before being displayed on both a high-resolution monitor (2nd left) and thermal paper record (below display). Sub-bottom profiler data are displayed directly to an on-line monitor (1st left) as a measure of vehicle altitude (used for flying), and to reveal sub-bottom reflections. Data from other sensors are displayed in digital form on 2 monitors (1st & 2nd right) to aid flying the vehicle and to observe instrumentation performance.

All data from the vehicle are stored on magneto-optical disks which are archived (burned) to compact disks / DVDs. These can be replayed to produce a corrected and geographically indexed mosaic of the surveyed area. Further image processing can be carried out on more powerful workstations.

A typical end product of image mosaicing is shown below (volcanic debris flow off the island of Stromboli, TOBI on R/V Urania 1998). Each TOBI image swath is 6 km wide and here we can see 2&1/2 swaths, with overlap for continuity, merged with a contour map of the island of Stromboli. The high intensity parts of the image show debris flows of up to 12 km.