Author Topic: gosht image or not ?  (Read 5570 times)

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Offline abraquelebout

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gosht image or not ?
« on: July 03, 2011, 07:35:45 AM »
hi every towfish men
i make a scan to find an old 1650 war ship
scaning whith towfish, 60 m leng cable, depht 26 m
i take this picture ( i think a navy gun ship) and a gosht image ?
ila_rendered
when i take the scan in drdepht it make this
ila_rendered
it make a picture who can tel me thinking there's two object and not one whith a gosht
i'm waiting for idea of this


Offline Rickard

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Re: gosht image or not ?
« Reply #1 on: July 03, 2011, 12:41:36 PM »
Hi Abra,

I'm afraid I vote for a ghost image. The reason is the perfect symmetry between left and right views with respect to positions along and across track in the DeepView image. The DrDepth image seems to be somewhat misleading since the symmetry disappears.

I think you must blame the long cable for the ghost image. My 50 m cable has fooled me on a couple of occasions in a similar way.

Regards,
Rickard

Offline abraquelebout

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Re: gosht image or not ?
« Reply #2 on: July 03, 2011, 04:48:23 PM »
hi rickard
thank's a lot
if there's a gosht, one of the target is real ?
one gun is real but what ? i think the right one, a dive must answer to this question
i see lot of  gosht picture but they are not beautiful than this !!

Offline Rickard

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Re: gosht image or not ?
« Reply #3 on: July 03, 2011, 05:54:20 PM »
I think it's the right one since it's much clearer. Don't become disappointed if you find two cannons!  :)

I post an image that shows a really strong ghost effect. I used 100 m with the cheapest ethernet cable I could find in one of my first towfish experiments three years ago. The ghost image is caused by crosstalk in the unshielded cable. I was surprised by the image quality and this encouraged me to go on with ethernet cable tests.

Rickard

Crosstalk in 100 m with unshielded ethernet cable.
ila_rendered

Offline Roddy

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Re: gosht image or not ?
« Reply #4 on: July 03, 2011, 10:26:35 PM »
Thank goodness I'an not the only one with ghost !!!!

It would be nice to learn what mil-spec cable you guy's are using?

   Enjoy Roddy >:D
« Last Edit: September 18, 2011, 11:12:05 PM by Roddy »
Scan,Scan and Rescan Roddy

Offline Rickard

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Re: gosht image or not ?
« Reply #5 on: July 04, 2011, 07:16:19 PM »
Roddy,

I use the most shielded stranded patch ethernet cable I could find. It has individually foil shielded twisted pairs and an outer braided shield. There is almost no crosstalk at all in 50 m with this cable.

Regards,
Rickard

Offline Roddy

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Re: gosht image or not ?
« Reply #6 on: July 05, 2011, 12:05:32 AM »
Thanks Richard, There are some new cable out for high compact use I'am going to try. >:D

Thank's Roddy
Scan,Scan and Rescan Roddy

Offline abraquelebout

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Re: gosht image or not ?
« Reply #7 on: August 30, 2011, 07:31:43 AM »
whe dive on it :)
at 25 m depth and 40 m lengh cable towfish whe have take point at 1 m than target, nice
whe dive and see ... a net of rolled up trawler 3.5 m legh and 0,55 cm larg:'(
may be next time...

Offline RGecy

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Re: gosht image or not ?
« Reply #8 on: August 30, 2011, 08:58:26 AM »
I often get ghost images in shallow water at 455khz, especially in areas that have a soft bottom that doesnt reflect well and the hard returns show up on both sides. 

I would asssume it may also have to do with the beam overlap under the transducer.

Robert 
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