The accuracy of length measurements made using imaging SONAR is inversely proportional to the beam width

Multibeam imaging SONARs have been used for a range of measurement
applications, such as measurements of fish lengths. This study aimed to
quantify the accuracy of imaging SONAR systems, that varied in frequency
and beam geometry, to measure the length of synthetic targets positioned
perpendicularly. Blueprint Oculus imaging SONAR systems, with four
different (centre) frequencies (750 kHz, 1.2 MHz, 2.1 MHz, and 3 MHz),
were used to measure the length of three targets of nominal lengths: 10
cm, 20 cm, and at ranges between 1 m and 15.5 m. The effect of beam
geometry on measurement error was then examined using regression analysis.
This study found that there was an overestimation of the actual length of
the target for all measurements that was inversely proportional to the
horizontal beam width. The measurement error can be reduced by normalising
for beam width. However, the variation of measurements (i.e. the
precision), was found to also increase with range, which was attributed to
the increasing beam separation. It is important that the effect of beam
width on the accuracy of target length measurements, by imaging SONARs is
acknowledged in future studies. One approach to mitigating this problem,
is to limit the range at which length measurements are made to a beam
width that produces an estimated level of error that is acceptable for
that study. Keywords: acoustic camera, imaging SONAR, underwater
acoustics, underwater measurements.