High-resolution calibration of a multibeam echo sounder

Calibration can greatly increase the utility of collecting seafloor backscattering strength with multibeam echo sounders (MBES). A calibration procedure to determine high resolution, three dimensional transmit and receive beam patterns of a Reson SeaBat 7125 MBES was performed using the fresh water calibration tank at the University of New Hampshire. The measurements employed an omni-directional Reson TC4034 projector/hydrophone as a reference transducer at an acoustic distance of 13 m. The multibeam sonar system was configured for 256 beams equi-angle mode and operating frequency of 396 kHz. The transmit beam pattern was computed for across-track angular range between +/-90° in 0.1° increments and along-track angular range between +/-2.55° in 0.09° increments. Receive directional characteristics were measured for the across-track range of +/-120° in 0.1° increments with along-track angular range of +/-1.23° in 0.18° increments. With these measurements, a three-dimensional plot of the transmit beam pattern was computed for the entire range of angles, although for the receive beam pattern it was only determined for the 0° along-track angle. The reduced angular coverage of the receive beam pattern was due to discrepancies in the off-axis results that are thought to be related to receiver saturation. Measurements for the transmit beam pattern show a -3 dB beamwidth of 0.99° for across-track angle of 0° with side lobes below -17 dB. The results show an unexplained curvature in the transmit beam pattern that would have gone un-noticed during a two dimensional beam pattern measurement. An interference effect causing ripples and attenuation of the transmit beam pattern was also observed. This effect is thought to be caused by a second 200 kHz projector mounted parallel to the 396 kHz projector. This interference occurs at an across-track angular region between +30° and +75° with gradual decrease in amplitude of about 3 dB. The receive beam pattern measurements show a -3 dB beamwidth of approximately 0.62° for the most inner beams (beams 128 and 129) and of approximately 1.40° for the most outer beams (beams 1 and 256) at along-track angular position of 0° with side-lobes below -26 dB for the most inner beams and below -17 dB for the most outer beams.