Optimal acoustic beam pattern corrections for split beam transducers

Precise in situ target strength estimates of fish can only be made when the effect of the transducer directivity is totally removed from the recorded target amplitudes. By guiding a standard calibration target through the acoustic beam while simultaneously recording the amplitudes and angular positions of the target, a precise reconstruction of product of transducer transmit and receiving directivity can be made. From several thousand point measurements taken in a cross section through the acoustic beam, the recorded data have been fitted to a generalized three dimensional model by the use of non-linear estimation. The suggested model functions yield very low residual beam correction error with 95% of the data within ± 0.5 dB. within the half power points of the beam, the residual beam correction error is close to the observed ping to ping system stability of ± 0.1 dB. The precision and repeatability of the method is demonstrated through data from four split beam transducers mounted on four different research vessels. These are three ES-400 transducers and one EK-500 transducer, all working on 38 kHz