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Old 08-30-2009, 09:41 PM   #1
Senior Member
Join Date: Jun 2005
Location: Exeter NH
Posts: 527
Default Side-scan-sonar and air bubbles

Before I post-perhaps Don could create a section for "Diving Winnipesaukee"? Lots going on as it's a great place to dive, but the "Boating" section is the only place I think I should post this, my history-related posts aside.

People have asked about how well air bubbles show up on side-scan; One area of mis-conception is that air bubbles are not a good reflector of side-scan-sonar waves, not so. I caught these recently of a dive buddy dropping to 85-100 feet or so to check something out. I scanned him and caught these images. For the curious, yes it was dark and viz with powerful halogen lights was 8 feet.

- Image 1 (if they post in order): Depth, 89.5 feet; Each singular horizontal line represents an exhalation by the diver as he moved parallel to the direction my boat was moving; Although not readily apparent, he was descending at the time, that's why his bubbles appear to the right of the long white vertical line (the bottom), which puts him in the water column on his way down to the bottom; The diver is nowhere to be seen, due to his rubber-ized drysuit, although a wetsuit will also make him nearly if not totally, invisible; Bottom is mud;

- Image 2: Long white trail 1/3 of the way down from the top of the image is his bubble-trail, starting to mix together as the bubbles ascend, hence the long white "tail", left to right from the bottom; Above that near the top you will see my flag in the middle of the water column (actually on the surface), and float line extending to the bottom at 95.8 feet; Look carefully and you will see evenly spaced dots-those are the 1" diameter stainless steel split-rings I have on the down-line from the flag every 5 feet or so that allow me to take slack out of the line so the surface marker doesn't drift more than necessary; Hard to believe the rings even show up;

- Image 3: Left to right you'll see sharp, elongated bubble-trails over the bottom of the lake that start to blend together as they get larger, eventually blending into one combined trail after passing by the bottom (the long vertical, top-to-bottom white line);

Objects with air in them-depending on how deep into the object the air is are-will show up on side-scan; Some fiberglass boats with alot of air-type insulation will produce similar results.

To those who asked, I hope this has answered your questions.
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