An operational system for processing and visualizing multi-frequency acoustic data

Calibrated and digitized data from two or more discrete echosounder frequencies can be combined for the purpose of separating and extracting the acoustic scattering from zooplankton and fish in mixed recordings. This method is also useful for quantifying the relative contribution of each frequency to the total acoustic-backscattering when scrutinizing records in large-scale, acoustic surveys. Echosounder hardware requirements are defined which would permit the ideal extraction of such information. These include calibration, transducer specification, pulse resolution and digital representation of the signals. During this initial study a special version of the Simrad EK500 multi-frequency, split-beam echosounder and the Bergen Echo Integrator (BEI) post-processing system were used. The echosounder transmitted pulses simultaneously at four frequencies, 18, 38, 120 and 200 kHz and transferred the received signals to the post-processing system in calibrated, raw, digitized format. Methods are described for echogram manipulation and for the construction of new, synthetic, combined-frequency [c(f)] echograms. Examples of extracted scattering information from mixed layers of fish and small scattering-organisms, such as copepods and euphausiids, are shown, and the potential of the method is discussed.