Re: [PSUBS-MAILIST] sonar question

[LynnDarnel@aol.com]

Dave:

Side scan sonar is usually a horizontally aimed phased-array sonar system.  I have not seen the latest technology, but I have worked with side scan sonar in the past.

The unit itself is a towed, torpedo-shaped, device with a row of transducers on each side.  It transmits sound pulses out and records echo intensity.  It does not observe things in 3-D, but effects like sonic shadows behind tall objects allow a trained operator to determine that something is sticking out of the bottom and approximately how tall it is.  The shape of things is not always clear in 2-D unless you get the right view.   Example: An upright sailboat seen from the stern might look like a block with a stick in the top.  A perpendicular view from either side, however, would present a clear view of the hull profile from which you can calculate length (knowing travel speed) and height (from the cast shadow).  

Operation of side scan is a lot like looking out a distorted vertical slot on each side of a submarine.  What you see instantaneously doesn't mean much, but if you record what you see and show it all at once you get a reasonable horizontal view.

The effective width of the sound beam, horizontally, is very narrow like viewing through a vertical slit. The vertical spread of the beam is essentially from top to bottom.  The vertical pattern is such that it does reflect off the bottom giving a good indication of altitude of the side scan.  The sound also reflects off the surface, in shallow water, presenting clutter in the images.

I hope that helps.

Lynn Darnell
 




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