Abstract

Abstract: A 12kHz side-scanning sonar for fisheries applications has been developed. The sonar consists of a 2.8°by 120°b eam-width, 40-element array which may either be towed or mountedon the sea floor. In the towed configuration salmon were detected at ranges up to 7km. The sonar was also deployed in a shallow (1O-14m deep) channel in Denmark on a tripod that mechanically rotated the transducer heading through a 50°sector. Herring schools were detected at ranges up to 1200m. RODUCTION A significant challenge in fisheries acoustics is the acquisition of data covering a sufficient area or volume so that accurate estimates can be made about the size of particular fish stocks. The use of traditional vertical echo-sounders and echo-integration techniques are well-established, however the accmacy of these measurements is compromised by the limited sampling volume and consequent need for statistical compensation. For this reason the potential of horizontally oriented sonars at intermediate to long range has been investigated over the past 30 years (1-4). Here we describe initial experiences gained with a sonar designed for horizontal operation in relatively shallow waters at ranges up to 7 km. k order to achieve a reasonable operating range while at the same time keeping the transducer within manageable dimensions, a 12kHz operating frequency was chosen. Cost considerations firther restricted the design to a fsxed 2.5m aperture array in which the 40 elements were wired in parallel, providing a nominal beam-width (to -3 dB) of 2.8°in azimuth and 120°in elevation. The sonar was driven with a modified EDOWestern model 248 Sonar Transceiver delivering approximately 2kW (electrical). The transmit pulse was a linear FM sweep sptig 11.2 to 12.8kHz with duration varying from 50 to 200ms. Extra capacitance was added to the transmitter power supply to maintain the &lving voltage during these longer pulses. Rearward acoustic radiation was reduced by 20dB (oneway) with either an epoxy foam or corprene backing material. A time-varying gain pre-amplifier was added to the array to boost the return signal up the 200m cable. The transceiver output was connected to a PentiumPC system for conversion, processing, display, and data storage. The coherently sampled echoes (up to 10s afier transmit) were correlated with the chirp pulse template, providing a processing gain of approx~ately 25dB. The 12kHz sidescan array was used in both towed and bottom-mounted cotilgurations @igure 1). During towing operations the array was mounted on the starboard side of a fibreglass tow-body. This was deployed behind and beneath the ship at a depth of nominally 40m while towing at speeds of 3 to 4 knots. An orientation package ww included inside the tow-body to provide measurements of depth, tempemture, salinity, heading and tilt. The trrmsceiver was located on the vessel. The vessel location and speed were derived from differential GPS. When the sonar was deployed on the seabed, it was mounted on a mechanically driven device that changed the azimuthal orientation under remote command. The motor was set to sweep in 2°steps over a 50" sector, requiring 6s between transmissions for data acquisition and rotating the array. 1663