Minimizing errors due to nonlinear effects in fisheries and research echo sounders and sonars, dr.gr.stip.

Under development of a new echo sounder with extremely high accuracy and dynamics, a significant and undesired effect was found in the radiation of acoustic energy from the transducer. At typical drive levels, nonlinear sound was generated in the sea whi ch to a significant degree influences the echo sounder’s primary frequency (level and directivity). Since the primary frequency is used for accurate stock abundance measurements of fish and plankton, it is important to clarify how these nonlinear effects can be avoided, and how the error can be corrected if a too high power level has been used. With typical drive levels (1 kW or less), nonlinear effects may not be so important in stock abundance estimation of fish (using e.g. 38 kHz), but may be more severe for plankton measurements (multiple frequency and broadband methods). However, internati onally there is a tendency towards driving echo sounders at extremely high power, such as up to 10-20 kW. Nonlinear effects will then be dramatic at high frequencies, and may be a serious problem also for lower frequencies (less than 100 kHz), such as in traditional single-frequency methods for fish estimation. A cooperation project between Christian Michelsen Research AS (CMR), the Institute of Marine Research (IMR), the University of Bergen (UoB) and Simrad AS is proposed to address these questions.