The Design and Evaluation of a Phase-Steered Parametric Sonar System Intended for Sediment Characterisation

The ultimate aim of the European Commission's MAST-II REBECCA project** was to study the viability of characterising sub-bottom sediment structures entirely by acoustic means. Part of the system developed to attempt to realise this aim was a parametric sonar system. The mode of operation was to insonify the sea bed at different incidence angles, using a variety of transmitted acoustic pulses, and subsequently to examine reflected and forward-scattered signals. The paper describes firstly the transmitting system designed and developed at Loughborough University. It then presents the results of static calibration tests in Loch Duich, Scotland and further results obtained during sea trials on two. French research vessels off the coast of Brittany, when the parametric sonar array was integrated into a tow-fish with an attached hydrophone streamer. The sonar transmits a phase-steerable primary beam centred at 75 kHz and generates in the water useful secondary signals at typically 3 kHz to 7 kHz, with a narrow sidelobe-free beam (2˙5° × 2˙5°), at Source Levels up to 196 dB re 1 μPa at 1 m at 5 kHz.