Calibrating two scientific echo sounders

The Simrad EK500 has been the state-of-the-art scientific echo sounder for surveying marine fish stocks; the EK60 is its successor. Both echo sounders have been calibrated with the same 38-kHz, 12-deg-beamwidth, split-beam transducer by the standard-target method at the acoustic calibration facility on Iselin Dock at the Woods Hole Oceanographic Institution. The principal measurements were on-axis target strengths and the two-way directivity patterns of the main lobe, measured with a 60-mm-diameter copper sphere. For each echo sounder, the respective split-beam-determined and directly measured angles of the standard target are compared. The directivity pattern as approximated by Simrad firmware is fit to the experimental data, and both the split-beam-determined and newly compensated values of target strength are expressed through histograms. Target strength distributions are compared for two spheres: a 60-mm-diameter aluminum and 38.1-mm-diameter tungsten carbide with 6% cobalt binder spheres.