Topic: what is seen on DI ?

[tournevase]

hello
i ask me questions about what we see on DI
example, on the same record


one target on le left side

[tournevase]

sorry, next

2 targets on the right side



so the DI display seems to mix the 2 side

how can we believe in the DI ?

[tournevase]

and yet a problem 
the 2 targets on the right side

[oschi]

Looks like a Helix 7 G3 or less.
That's blending the left and right SI to a 'fake' DI. So it's always blury.
8" and higher have real DI. (Don't know if the 7" G4 has too)

That's what real DI should look like:

[tournevase]

my equipment is a 10" G4N

i think that on your example the 2 sides are also blended 
on the DI we see the "stakes" of the left side and there are also 2 small white points and a dark dash - belonging to the right side - under the 2nd stake from the right

so DI imaging is a masquerade ? 

[oschi]

No. It's real DI.
Good exmple. There's no blending of right+left SI possible to fake that DI (bottom right). 

Your DI has to be real too.
The DI has a triangular shape. Like SI but much narrower. So it looks a bit to the left and right too. (but you have >20m )
Maybe your image looks that curious because of HumViewer. But I don't understand why your DI shows 20m and more to the side. That's very strange. Maybe you have 2 SI crystals  or it's indeed HumViewer? But I tried the data above with HumViewer and it looks the same.

[oschi]

Ok. Got it messed up with HumViewer, when in QuadraBeam mode ...



Top right is what you get.

And that's the right side of SI.



Right side gets a lot of reflected signal because I'm very close (~2m) to the quay wall. That's why you see the ghost in the SI image.

[MT]

I've read DI will cover up to 2,5 times the underboat's depth (in wide mode), so seeing 15m on the side when there is 5m underboat doesn't seem too disturbing.
On your screenshot the object displays neatly on SI but is fading on DI like it's on the margin of DI coverage.

Now it's also likely that software use SI informations to display the widest part of DI beam, aiming at wider coverage, since in therory one should not be able to achieve such a wide beam using high frequency (the higher the frequency the narrower the beam, that's a physical law).

The way i see it, DI will cover accurately the underboat gap between both SI beams, but there has to be some overlaping (either naturally or by software calculation).

Here is a thread from other forum, where the beam coverage issue is well explained, especially in the 8th message from OThessalonikios. Espacially this sentence is important for a good understanding of how beams work : "In the flashlight analogy, it would be like telling someone "you can't see that object because it's at a place in the light 1/2 the intensity of the center", even though you can CLEARLY see it."

[oschi]

The way i see it, DI will cover accurately the underboat gap between both SI beams

There is no gap between left and right SI beams.

I've read DI will cover up to 2,5 times the underboat's depth (in wide mode), so seeing 15m on the side when there is 5m underboat doesn't seem too disturbing.

Wide/narrow/full mode has nothing to do with DI, only 2D. 

You have to understand the difference between slant range and real distances.
If you have a DI beam at 5m depth and this 2.5 times beam width (1.25 each side), you'll get 6.25m to each side BUT only slant ranges from 5m up to 8m (at a flat bottom).

Common misunderstandings ... - closed -