WHAT IS A SIDE SCAN SONAR?

STARFISH WHAT IS A SIDE SCAN SONAR?

Sonar (SOund Navigation And Ranging) and echo-sounding technology dates back to the 1920's, but it was only in the early 1960's that Dr. Harold Edgerton (an electrical engineering professor at the Massachusetts Institute of Technology) started to adapt his techniques on high-speed flash photography to acoustics, having concluded that photography was not best suited to the murky conditions underwater.

By sending 'flashes' of acoustic energy into the water and recording the echoes, Edgerton (who later worked with famous French underwater explorer Jacques Cousteau), developed a towed side-looking sonar that could create a continuous image of the seafloor.

By transmitting a narrow fan-shaped acoustic pulse (ping) perpendicular to its direction of travel, the side-scan sonar sends acoustic pulses outwards. The seabed and other objects reflect some of the sound energy back in the direction of the sonar (known as backscatter), and the travel time of the returned pulse is recorded together with its intensity.

As sounds travels at a known velocity (of approximately 1500 metres per second) through water, we can directly relate the time we received an echo, to the range of the target that reflected it.

This scan-line of information is sent to a topside computer for interpretation and display, and by stitching together data from successive pulses, a long continuous image of the seafloor is created, as the sonar is towed from the survey vessel.

Your StarFish has two transducers (transmitter and receiver elements) mounted in the lower fins.

These transducers are angled 30° down from the horizontal, and acoustically transmit sound in a 'fan beam' 2° wide, by 100° vertically - although most of the acoustic energy is confined to the centre 60° of the beam.

This gives the StarFish the ability to see almost directly below it, to just above the horizontal.

However, despite this field of vision, the StarFish cannot determine where a target lies vertically in its beam (i.e. above or below it), as everything is translated to a planar 2D display.

For example, if there are two targets both 10 metres from the sonar (one horizontally level with it, and one directly below it, and are received on the same channel) they would both appear at the same point on the sonar display, as the display scale is based around time, and both echoes would arrive simultaneously.

With some experience though, image artefacts like acoustic shadows can help the operator make an educated guess to the size of targets and sea-bed features.