1 Korneliussen, R. J., and Heggelund, Y. Processing 4-D acoustic data from the Worlds most advanced multi-beam fisheries sonar. Proceedings of the 30 th Scandinavian Symposium on Physical Acoustics, Geilo 28 - 31 January 2007. PROCESSING 4-D ACOUSTIC DATA FROM THE WORLDS MOST ADVANCED MULTI-BEAM FISHERIES SONAR Korneliussen, Rolf. J. 1 and Heggelund, Yngve. 2 1 Institute of Marine Research 2 Christian Michelsen Research Abstract Acoustic methods are widely used for estimating fish abundance. Institute of Marine Research (IMR) uses data from echo sounder to calculate abundance of many fish species. Vertical echo sounders does not cover the region close to the sea-surface, and is therefore not suited to investigations at those times the fish swim close to the surface. The move towards investigation of complete ecosystems force simultaneous investigations of many species, i.e. also when fish are too close to the sea-surface to be investigated with echo sounders. IMR specified requirements to a quantitative multi-beam SONAR to be used in combination with the echo sounder as recent development of technology has made it possible to make both such a SONAR and a system to post-process its data. Currently the most advanced multi-beam SONAR of the world can deliver calibrated output to be used for abundance estimation of fish. Potentials of such SONARs are discussed, and simple applications are demonstrated. Correspondence to: Korneliussen, R. J., Institute of Marine Research, P.O. Box 1870 Nordnes, NO-5817 Bergen, NORWAY, email: rolf@imr.no Introduction Acoustic methods are widely used for estimating fish abundance. Echo integration at one frequency, supported by biological sampling, is the general method used (MacLennan, 1990). Scrutiny of acoustic data is generally done by analysing and correcting echograms in digital format using a dedicated post-processing system, e.g. the Large Scale Survey System, LSSS (Korneliussen et al., 2006). Vertical echo sounders do not cover the region close to the sea- surface, and is therefore not suited to investigations at those times the fish swim close to the surface. Currently, surveys targeting abundance estimation of single species is done at those times of year when that species is optimal distributed for abundance estimation, which inherently means that the species is not close to the surface. However, investigations of complete ecosystems are advantageous to understand the interaction between different species, both fish and plankton. This inherently forces simultaneous investigation of species, i.e. investigation of some species when they are not optimal for abundance estimation based on echo sounder data. IMR specified requirements to a quantitative multi-beam SONAR to be used in combination with the echo sounder as recent development of technology has made it possible to make such a SONAR and a system to post-process its data. One of the requirements to the SONAR was to cover the entire school in one ping. The acoustic needs for abundance estimation of species of