A low-cost calibration facility for high-frequency acoustic backscatter instruments

Full-system calibration of acoustic instruments used to measure high-frequency backscatter from zooplankton and small fish in the ocean is required if the data are to be used to estimate biomass and populations from signals at different frequencies. Here we describe a test tank facility for calibrating high-frequency, narrow-band sonars operating at one or more frequencies from 125 kHz to 775 kHz and with beam-widths between 1.8 and 11 degrees. The all steel tank is cylindrical with a diameter of 2.43 metres and a length of 6.10 metres and holds 28.5 cubic metres of water. The absorbing material on the end wall is a coarse artificial turf which is easily available at minimal cost and reduces high-frequency reverberations in the tank to negligible levels after 6 reflections. Pulsed operation at pinging rates up to 10 Hz is therefore possible without interference from reverberations. Echoes from the target are recorded over a series of pings; the difference between the target strength computed from the mean of those echoes and the known target strength is a measure of the accuracy of the nominal instrument response calculated from the manufacturer's transducer characteristics and bench measurements of the system electronics, and therefore allows a calibration correction to be made. An acoustic propagation model of the tank has also been developed to characterize the response of instruments placed in it and to identify optimum target placements.