Synthetic echograms generated from the relative frequency response

Calibrated, digitized data from multi-frequency echo sounders have been used to generate new, synthetic echograms. The relative frequency response measured at four acoustic frequencies (18, 38, 120, and 200 kHz) is the main acoustic feature used to characterize the acoustic targets. Synthetic echograms are used to enhance and colour-code sample volumes with similar acoustic properties. The method is invaluable during scrutinizing, particularly in areas with many types of target. Several broad acoustic categories can be clearly distinguished, and detailed examples based on different fish species and zooplankton are demonstrated. The limitations of the method are described and discussed.