© 2005 International Council for the Exploration of the Sea
Comparing two 38-kHz scientific echosounders
a Northeast Fisheries Science Center (NOAA/NMFS) 166 Water Street, Woods Hole, MA 02543, USA
b Woods Hole Oceanographic Institution Woods Hole, MA 02543, USA
c Northwest Fisheries Science Center (NOAA/NMFS) 2725 Montlake Blvd. E., Seattle, WA 98112, USA
*Correspondence to J. M. Jech: tel: +1 508 495 2353; fax: +1 508 495 2258. e-mail: michael.jech{at}noaa.gov.
The EK500 has been the state-of-the-art scientific echosounder for surveying marine fish stocks for over a decade; the EK60 is its successor. Ensuring comparability in performance is vital during the transition from the EK500 to the EK60. To quantify the respective performances, each echosounder was calibrated in tandem by the standard-target method using the same 38-kHz, 12° beam width, split-beam transducer, with alternating pinging by means of an external triggering-and-switching system. The principal measurements comprised split-beam-determined angle and target strength, on-axis sensitivity, and directionality in the plane normal to the acoustic axis, as measured with a 60-mm-diameter copper sphere. Ambient noise, including volumetric reverberation, was also measured. Principal comparisons included those of the time-series and histograms of split-beam-determined target strength; respective alongship and athwartship angles as determined by the split-beam system; and as expected, difference in the split-beam-determined and experimental target-strength values in the plane normal to the acoustic axis. The mean absolute difference in off-axis angle values was also compared. While the performance of the two echosounders is generally similar, systematic differences exist. For the particular calibration measurements, the time variability in measurements of on-axis target strength was of the order of 1 dB for the EK500 and 2 dB for the EK60. The target-strength distribution for measurements made with the EK500 was normal, with standard deviation 0.2–0.3 dB, whereas for the EK60, the target-strength distribution was distinctly skewed and the standard deviation varied over 0.3–0.5 dB. Differences were found between the split-beam and physical-angle measurements. They were noticeably larger in the case of the EK60. Differences in performance between the two echosounders suggest refinements to the new system that will help realize its full potential in scientific work.
Keywords: calibration, echosounder, fisheries acoustics
Received 14 September 2004; accepted 7 February 2005.
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