Thread: What am I looking at?

Originally Posted by crappiepappy

While that may be true ... I'd think a vertical stream of bubbles would return a ping signal as a solid "object", just like the tree. And if the bubbles showed up at an angle, why wouldn't a tree show up "angled" (according to Rickies hypothesis) ??

Heck, for all we know, it could be a 12ft long Y shaped water logged tree branch ... that hasn't quite soaked up enough water to drop all the way to the bottom. (well, it could be )

That still doesn't explain the gargantuan "arch" in the image, or why the fish ID symbols are right along the angled sonar return. (and we already know that fish ID symbols are not always indicative of actual fish, since submerged trash/leaves/moss/etc can give false signals to a 2d sonar unit).

.... cp

Pappy, you are over thinking this one my man. That bubble would have to be traveling at lightning speed to give a solid, vertical return. Say you wanted to capture pictures of a car driving by on a country road, with a huge tree in the background. As the car comes speeding by, you quickly snap say, 20 shots and inadvertantly lower the camera from top to bottom as it goes by. When you flip through the pictures, you will see that the car slowly moves at an angle from one side to the other, but the tree is still vertical because it is stationary. Same concept.

Originally Posted by Cmj

Pappy, you are over thinking this one my man. That bubble would have to be traveling at lightning speed to give a solid, vertical return. Say you wanted to capture pictures of a car driving by on a country road, with a huge tree in the background. As the car comes speeding by, you quickly snap say, 20 shots and inadvertantly lower the camera from top to bottom as it goes by. When you flip through the pictures, you will see that the car slowly moves at an angle from one side to the other, but the tree is still vertical because it is stationary. Same concept.

Yes .... but it's not ONE bubble that's rising from the bottom of the lake, it's a STREAM of bubbles; each one of which would give a pixel or two of sonar return, the instant they entered the sonar cone's perimeter. That 2d unit can't possibly distinguish a rising stream of bubbles from a tree trunk, so if the return signal is generated off a stream of bubbles the same way it's generated off a tree ... then regardless of whether the boat or bubbles ARE moving, it would only seem logical that the vertical stream of bubbles would display on the screen as a VERTICAL object, not a angled object.

Even Rickie's explanation doesn't add up, unless the cone angle was very wide, and the boat speed was just slow enough to stack the image history at that angle. But, even then, why would it not also "angle" anything else that came into the cone while the boat speed was the same. It just doesn't add up, IMHO. And it still doesn't explain why the supposed "bubble trail" is split into two separate lines.

Just so you all know .... I'm not trying to be difficult, or argumentative, and I'm not dissing the unit (or even 2d sonar). I'm just not convinced that a stream of air bubbles would be shown on the screen any differently than any other vertical object. That's not logical, in my mind.

But ... like my buddy Carl says : you may never know unless someone puts a underwater camera down there & sees it first hand, when an image like that appears on the screen.

Now, if someone sees a bunch of bubbles on the surface, and runs their 2d unit over that spot & gets a similar sonar return image ... then I might concede the point. But, I'd still not understand why the image is split into two columns ... why the big honkin arch is present ... and why the fish ID icons are right in the middle of all this "chaos"

... cp

I think a better way to imagine the bubbles rising from the lake bottom would be more of a "glob" of bubbles instead of a "stream" of bubbles...

This scenario would allow the echo to be displayed as a few pixels (similar to a fish echo) ...then the next ping would capture the "glob" a little higher in the water column (as the screen scrolls left)....and repeat over and over..



More pix here...(TMike was indicating how to catch fall bass but he posted several good shots of the lake turnover and how the bubbles appear on the 2d sonar....)

Turnover Fishing... (Pic heavy)

I think pappy's term of "chaos" in the OP pic is very true..



*We have "globs" of bubbles rising to the surface...
*We have a "huge" fish arch...
*We have several smaller fish that the unit painted with Fish ID symbols...

The "huge" fish arch must have been so big that the unit's Fish ID algorithm didn't think it was a fish ...so it just plotted the arch... instead of a Fish ID symbol...(I'm guessing on this part)...

Rickie

I'm wondering if that big arch is not a tightly (being attacked) schooled bait fish (it's around 5' long) and the fish icons are actually game fish. That would be the normal position of feeding game fish in relation to bait fish. If this is the case then the two streaks could be game fish busting through the bait fish.

In Drifyt4s image the two diagonal lines come from the same place, or very near. In tmikes images the diagonal lines seem to come from different locations on the bottom.

I don't know, interesting shot though.

Wow! Sorry I missed the discussion yesterday folks, however; I still think it is air bubbles. Not a stream of them (which would show more as a tree trunk cp) but just one or two or two smaller groups. Air bubble rise at a constant rate and this is shown by the very linear shape of the angled sonar returns. Now I don’t know that all sizes of air bubbles rise at the same rate, so that could partially account for the two different angled sonar returns but also, though they appear to have originated from the same location; all we really know for sure is that both air bubbles were at the same distance from the transducer when they entered the sonar beam. It is obvious that this unit is using a very wide sonar beam, or that it has some very wide side lobes. The air bubbles could have started at different spots on the bottom of the lake and travelled upwards through different parts of the sonar beam and give the same result. If you look at the colors shown for both returns you see that they returned less sonar energy to begin with, than more, then less again, but one of them returned more sonar energy for a longer period of time versus the other. Perhaps it was more centered in the sonar beam as the boat passed over it and the other more off to one side? Those fish symbols could be from the air bubbles as they tend to follow the same pathway that the air bubbles took. That large arch is definitely from something that reflected a large amount of sonar and not what I would expect an air bubble to look like nor even a school of fish (it should have been filled in if it were multiple fish).

Originally Posted by Redge

HB Greg, why do bubbles show up like that? Is it the gas? It's water, why does it show a return? Sorry don't mean to hijack the thread but it does pertain to the thread.

Redge,
To answer your question: basically sonar reflects off of a change in density. There is a large change in the density of air versus water. So that is why air is such a good sonar reflector. I’ve read where some experts state that 50% of a fish’s sonar return is from the air in its swim bladder. Not just that but a free rising air bubble will try and maintain a circular shape which means there will always be a surface of the air bubble that is perpendicular to the transducer. Part of what controls the amount of sonar that gets reflected back to the transducer is the shape of the object. Think F117 Nighthawk here because its shape helped reflect radar away from the radar station that sent it. So a small bubble of air could end up getting displayed as a large sonar return.

You know, Rickie .... all this thread & the discussion about what the 2d image might be has done, is to convince me that my depth finder is better off without 2d sonar

When I look at the split screen picture that you posted from T Mike ... my attention is more on the DI image, where the "bubbles" & "chaos" are absent, and potential "fish" are present.

Fished with a buddy, this past weekend, that has a 1198 ... and now I understand what all the hubbub was about, concerning the DI from SI image discussions. I had the whole screen on DI, and still couldn't determine much about what was in the water column ... but, did find the SI portion of that image to be interesting.
I do remember seeing these angled lines on the 2d screen section, and there were "bubbles" rising in the areas we fished .... so I concede that point. There were also many schools of Shad, some deep & some at the surface, so I originally thought those angled streaks to be the Shad schools coming up or going down. I'll know better, next time I fish with him !!

Thanks all, for the lesson in sonar image interpretation. It's been a fascinating & humbling experience.

Thanks HBGreg that makes perfect sense on the bubble return. For the life of me I couldn't come up with the logic!!