Under our lakeside dock (Lake "de Sede" in France)

Adventure Log
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An example of what the 360 sonar shows us on plastic drums (which support the floating dock) filled with air.
Since these drums are not filled with concrete or other ultrasound-absorbing material, we can properly see the entirety of each drum.

FR VO : Un rapide exemple d’une imagerie sonar contre un quai composé de fûts remplis d’air, la particularité de ces images sonar, c’est que nous pouvons voir la totalité de chaque fût, ils sont traversés par les ultrasons car ils ne sont remplis que d’air (pas de béton, de sable, de matériaux absorbants).

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Hi @FlyingManta, this is really cool - thanks for sharing! :smiley:

I’ve edited your post so that the video is embedded, rather than needing to click the link to open YouTube in a separate tab or window :slight_smile:

If the drums are indeed filled with air, rather than some form of oil or similar, then I find the display quite surprising, since the speed of sound in air is significantly below that of water, so I’d expect the far side of the drums to show up as much further away, rather than nicely following the circular shape. That said, perhaps those are some other kind of reflection, and just end up looking like the other side of the drum because of where they appear…

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Indeed it may seem surprising… I have to come back to dive in this lake this week, I will try again to take some sonar images of these drums but from other angles, to try to understand better.

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That is surprising that the back of the drum is imaged. What’s inside the drum (air? water?), and what is the drum made of?

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I would guess these are blue plastic (ABS? PVC?) and full of air, I’ve seen them used under docks and generally for marine flotation in a lot of applications. Amazing to see!

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Thank you Larry and Anthony …
These are HDPE drums, the type of drums that are used in industries for chemical products or in the food industry.
In my opinion they are filled with air (normally!) but I am starting to doubt… It is not impossible that they are filled, in part, with water?
At the end of the week I will come back to this site and I will post some images for you.
The mystery lurks around the lake :rofl:

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Elliot, Larry, Anthony,
The mystery is solved !!! :smiley:
The drums are not full of air !, in fact they are filled with water to a third of their height (this is used to adjust the flotation for the level of the dock).
The sonar “scanned” this part of the drums which is filled with water…

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Interesting point - each circle does seem symmetrical with a large gap so maybe a just a reflection. Curious effect though and great to keep in mind when interpreting sonar images.

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