Calibrating a 90-kHz multibearn sonar: illustrating protocols

Abstract

Protocols developed for calibrating multibeam sonar have been applied to the calibration of the Simrad SM2000/90-kHz multibeam echo sounder at the WHOI acoustic test facility in a sea well on Iselin Dock. An external transmitter was oriented perpendicularly to the equatorial plane of the receiving array, thus forming a Mills cross. A total of 128 beams, each with nominal beamwidth 1.5times1.5 deg, were formed, collectively spanning an equatorial sector of 100 deg. The stability in performance was evaluated through time series of echoes from an on-axis target These and other measurements, excepting mainly those of the time-varied gain function, were made with a transmit pulse duration of 300 mus and receiver gain function of 20 log r, where r denotes the range. Vertical beam patterns were measured to identify the equatorial plane with high precision. The dependence of the received level on the time-varied-gain function was measured for gain coefficients from 0 to 50, demonstrating linearity over the range from 0 to 30 for the particular target strengths, both about -35 dB, and ranges, 11.7 and 23.2 m, enabling extrapolation for other target strengths and ranges. The directionalities of beams in the equatorial plane and in the 2D central region were both measured. The calibration protocols are sufficient for quantitative applications of the sonar, as in fish-surveying