Simple calibration technique for the split-beam echo-sounder

Abstract

The split-beam echo-sounder is used to measure the target strength of fish. Calibration of this instrument requires sensitivity measurements to be made throughout the cross-section of the acoustic beam. It is necessary to measure the sensitivity variation as well as the mean value. The indicated target strength distribution is broadened if the sensitivity changes significantly with the target direction. The theory of this effect is developed. A calibration technique is described which requires few measurements to achieve good accuracy. For the ES400 echo-sounder, measurements at 30 points in the beam determine the mean sensitivity to ± 0.5 dB or better. The sensitivity measurements of a particular transducer and echo-sounder varied over a 3 dB range. Reasons for this unexpected variation are discussed.