Blueprint Subsea Starfish 453OEM, Системное руководство

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StarFish 453OEM
System Manual
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4. Understanding Sidescan Imagery
Interpreting side-scan imagery may seem difficult at first, but with practice and some knowledge of how the sonar
works, it doesn’t take very long for an operator to understand what the seafloor is doing below the sonar, and if
there are any targets on it.
Many people try to look at the pictures and understand them as you would a photograph, but this however is not
strictly the case. In the following sections, we will look at several example images, and see how information can
be obtained from them.
4.1. What Is A Side-Scan Sonar?
Sonar ( SO und N avigation A nd R anging) 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
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.
As mentioned previously, your StarFish has two transducers
(transmitter and receiver elements) that are angled 30° down from
the horizontal, and acoustically transmit sound in a “fan beam” of
narrow width (refer to the specification of your StarFish product for
the exact width), but wide vertically with most of the acoustic energy
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.