Towed Ocean Bottom Instrument TOBI: A New Deep-Towed Platform for Side-Scan Sonar and Other Geophysical Surveys

Abstract

Abstract. TOBI is a new deep-towed geophysical surveying vehicle (figure 1) developed at the Institute of Oceanographic Sciences, Deacon Laboratory, U.K. Its standard instruments include: 31 kHz side-scan sonar (6 km swathe); a 7kHz high-resolution sub-bottom profiler (>50 metres penetration); a narrow beam, 1-20kHz scatterometer; and a high quality triaxial fluxgate magnetometer. It can operate in water depths between 200 and 6000 metres, and is towed from a steel armoured coaxial cable. Since its completion in 1989, TOBI has completed almost 3000km of survey lines in water depths from 1500-6000 metres. These surveys were conducted from both British and American ships, in winds up to gale-force! TOBI images revealed targets ranging from subtle sedimentary textures to ship wrecks in 5km water depth. Introduction. Since 1965, when Scripps Institution of Oceanography built the world's first deeptowed system, a variety of these devices have been developed across the world 1. The basic requirement common to all of them was to be able to generate relatively high resolution images of the seafloor in a swath between 500 and 6000 metres wide. The reason for this is because there is a large leap in scale between surface-towed side-scan sonar systems and seafloor photography. This can prevent the engineer and scientist from either interpreting an image or mapping a sufficient area of seafloor. It was also recognised that having got an instrument to the deep ocean floor it would be cost effective to make more than one measurement. The variation in swath width of deeptowed side-scan sonar systems is roughly dependant upon the frequency of the sound used. So the SAR system developed in France achieves a swath of 1.2 km at a frequency of 180 kHz, whereas the Towed Ocean Bottom Instrument (TOBI) described here achieves 6 km at 31 kHz. When these systems were developed they were usually compared only by resolution and swath width. Recent results using a variety of side-scan sonar systems operating from 6.5 kHz to 31 kHz indicate that at these relatively low frequencies, acoustic energy penetrates sedimented seafloors z. So the images they produce are not of the sedimentiwater interface but a sum of the scattering strength of the upper few metres or centimetres of sediment. Therefore deep-towed systems must be assessed not only according to their resolution and range, but also by their ability to penetrate the sediment surface. Acoustic penetration increases as frequency decreases, so TOBI (which uses a low frequency compared to other deep-towed systems) achieves the greatest penetration This property is of great importance where the target of interest to the engineer and scientist (e.g. manganese nodules or archeological remains) may be buried by a thin veneer of sediment. Recent results indicate that TOBI side-scan sonar may penetrate more than 25 cm into the sediment z. This effectively removes the masking effect of holocene sediments which are draped over many deep ocean seafloors.