Thread: Sonar Quiz #22

Originally Posted by lcookie

I would have thought the red line would have gotten thinner or changed colors as it transitioned from hard to soft bottom. Was this a function of the under lying layer transitioning while to surface layer remained the same?

Most of the Original Pallette snapshots I can find seem to always have the thin red line as the 1st reflection ...

Then a thicker red line (as seen in the mid section below) would indicate larger rocks (or a shale layer as slabbacks referred to) ...

The thick red line in the below pic may even be a rock pile (from the shape of the return in addition to the thicker red line)....



I think cookie may also be correct in that the underlying layers may have something to do with how much of the pulse is reflected ...which would attribute to the multiple echoes we saw in the OP image ...

Rickie

Rickie to the right side of your shot (OP) where the brush and fish are, could this be (for a lack of better terms) sucking up or blocking the beam from reaching the bottom which would account for the weaker returns shown by the other layers in that shot?

That is a possibility slabbacks as anything that reflects some of the sonar is also blocking passes by or through it from reaching anything underneath it as well. However I would expect that most of that is due to a softer bottom in that area. We see where some of that fish/structure area reflected enough sonar to produce a second sonar return, so that spot was harder than what was directly before and after it.

Some things to keep in mind here are not only the color pallet being used but also the sonar frequency, beam width and any menu settings that can affect how the bottom is shown (Sensitivity, any sort of WhiteLine or filtering, TVG or surface clutter settings). Even things like the shape of the bottom terrain can affect how thick the red line is shown. In Rickie’s last pic above; some of the reason for the thicker red line in that area is due to the shape of the bottom. With a slope/drop-off the sonar is receiving a strong sonar return from more than one water depth at the same time and so shows this as a thicker red line. If the same bottom were flat it would show as a thinner red line. Remember, the sonar will try and show all sonar returns from the circular sonar beam and while over a slope it is receiving sonar returns from the shallow, middle and deeper ends at the same time. If they are all of the same density, they would all get shown as a thicker layer or line as the deeper end of the slope is farther away from the transducer – which gets shown as a deeper water depth for us. Of course, a slope or drop-off reflects less sonar directly back to the transducer and so should get displayed as a weaker sonar return color and would be unless the Sensitivity of the unit were turned up too high to show this. Of course when you turn down the Sensitivity in your unit so that it does show this (or even to the point that it could show the difference between bottom types); you may have also set it so low as to not show any fish, which most would never do.

Greg,

Good point on the rise and fall of the bottom causing the thickness to change given the multiple returns from different elevations.

Thanks Greg for the info. What I was taking as a bottom composition change may actually be a result of angle of the return then. Good read, thx again

Has anyone ever seen a multiple return from a thermocline?