Towfish images

[Rickard]

Hi,

I haven't seen any snapshot or recording results from towfishes (except from my own old noisy results ) at the forum so I thought it would be interesting to share some. I know how difficult it is to get something you feel worth presenting, maybe that's why there are so few results here.

I'm rather satisfied with my system now so here are some towfish images. The brakethrough came when the honored forum member Kron reported his results privately. He, and I, use 50 m shielded ethernet cable with shielded pairs. The cable is spliced in on the original HB transducer cable. (Don't use HB extension cables, the mistake made by HB to join grounds at the same connector pin will produce severe crosstalk in long extension cables.) The most obvious benefit with this cable is the complete absence of any visibel crosstalk. Noise and interference is very much better than in my earlier cable arrangements (several HB extensions and UTP cables). No cable in the world can do anything about the fundamental drawbacks with a long cable so signal level is noticably weaker but still strong enought to produce meaningful results. I think even a 100 m cable can give acceptable images provided you scan very close the seefloor/target.

First image is from a small (4 m) motorboat. It lies at 35 m and was absolutely invisibel to a hull mounted transducer.

I got an interesting image from an 8 m wreck that is almost completely covered with mud. No currents in this area. Towfish is about 2 m above the bottom. No idea how old it is. In this lake, Mälaren, sedimentation builds a 40 cm layer in 100 years, in the worst case. Depth is 30 m. When scanning close over the wreck, nothing can be seen in any sonar view. No chance to see anything of this from a hull mounted transducer 30 m above.

I hope other people will share their towfish results!

Rickard

[Jolly Roger]

Wow Rickard!
That's awesome! I already wondered if the HBirds will detect small and "absorbed" wrecks like this in 30 + metres with a hullmounted transducer.
Checked the wreck of the "Jura" at 37 metres with good results, but that's a 40 meter ship, so it's hard to be overseen.
Keep them pictures coming!

[Rickard]

Harald,

My focus on small structures at some depth has a simple explanation, it's about 30 m deep everywhere in my waters and there is nothing but small wrecks here! I would love to have something more impressive to scan. Well, yesterday I did make a three hour trip to a site where I saw something that could be worth checking a year ago. And yes, I found a 18 m wreck, probably a bargue, about 30 m down! I scanned it from the surface and from the towfish. I didn,t expect it would rest with the stern deep into the mud and the bow about 7 m above the bottom! I towed the fish over it with only about 2 feet margin at one occasion - oops! I think the boat sank when it was towed from Stockholm to Uppsala 25 years ago. But it's not filed anywhere. I did make some adjustments to the images to get about correct proportions. I scan at one knot or less, that gives too long images in the along track direction on images saved with SiView. I use the sidelobe ghost structures to check the proportions in the images. In most images there are some of these irritating traces of sidelobes but they are actually helpful in aspect adjustments.

I got some disturbances this time, they are caused by the damned engine!

Rickard

First image is the best one obtained from the surface.




The towfish passed very close the nose of the ship but I got a good scan!

[Jolly Roger]

Holy macrowave!
That second picture is a real kicker! That was pretty close indeed.
I found four "wrecks" within the firts two months I had my 981. They were mostly around 20 metres deep. I already wondered if there aren't any wrecks in other depths or if the unit isn't capable to bring them up to the screen, but checking the "Jura" convinced me that it is possible to find wrecks in bigger depths with my 981.

The most interesting wreck is the last one I found in June:

http://forums.sideimagingsoft.com/index ... sg1037#new

The other three were just small sail boats and a steel hull of about 6 m length. That one was also pretty "absorbed" by the ground, but clearly visible on the screen.

Good luck and keep the pictures comming!

[Rickard]

Your find is a real dream find!

But how confusing to learn that you are a suspected looter when you feel you have done something to the benefit of society! I think your experiance from the meeting with the academics tells us all something very important, we should keep a low profile and never reveal positions except to our mothers. Otherwise there is a risk there will come legislations that make sidescans as criminal as metal detectors.

I have also found you can detect rather small structures at large depth, but there are limits of course and you see no detail. I need detail because I gave up diving many years ago because of a bad back so images is the closest I will ever get to the targets.

I took many images from the bargue and here as some more.

This one shows what happens when the fish is towed in front of the ship and below the bow.



This image illustrates the problem with scanning a leaning surface like the deck of this ship. The hull looks twisted as a result of the change in attitude when the fish pass the hull.

[Jolly Roger]

Right Rickard,
I was kinda confused (don't wanna use the "P" word) after the statement of the Doc, but in the end I understand his point. Such sites are very sensitive to any disturbance. For the archaeologists it's paramount to have the site in an as far as possible untouched condition. Otherwise they might get false results when they explore the site.

My feeling is that the gentlemen were quite surprised that there are such units on the market. I mean I was surprised as well, when I found out about the SI technology in January this year. Ten or fiveteen years ago, one has to spent a ransom for a side scan sonar and has to tow a fish behind the boat to get images like this. Nowadays it's still quite some money, but affordable to much more people.
So when I introduced my 981 to them, they propably felt some panic and had a scenario of hundreds of people scanning the lake for fish and structure and finding old wrecks like this in their mind.

Anyways, I have no bad feelings for them and I hope these guys are assured after I promised them to keep my mouth shut about the coordinates of the wreck.

[Rickard]

The find presented above (the ship that stands like a pole in the mud) and the wreck I use in my avatar are now registered in the data base at the Swedish National Heritage Board. They are the authority that deals with archeological and cultural issues in my country. I was a bit nervous when I contacted them, remembering Jolly Roger's experiences, but they welcomed my reports and filed them immediately. They even asked for more since there are no finds filed for the waters I scan. This is surprising indeed since this area is close Stockholm, Sigtuna and Uppsala, places from where our kingdom has been ruled since more than a thousand years. So now anyone can look up my finds on their clickable map and learn something As I understand there is no legislation against searching for historical wrecks here. The strategy seems to be to establish cooperation with divers and wreck searchers to mutual benefit. It's really encouraging to get feedback that tells you have done something with a value to many more people than just yourself. I do this for fun but now it's easier to justify it when I'm confronted with my wallet... and my wife .

Rickard

[Jolly Roger]

Excellent Rickard!

I do this for fun but now it's easier to justify it when I'm confronted with my wallet... and my wife .

I'm with you on that point áll the way .

We'v been out on "my" wreck this week and archaeologists inspected it. So far, they think it's about 150 - 200 years old. Quite an age one would think, but it can't compete with the other wreck they were salvaging during this week:
http://www.suedkurier.de/news/panorama/ ... 67,4020038
(there are some nice pictures in the "bilder galerie")

Working with Dr. Mainberger was quite some fun and really interesting. It's not sure, how they'll procede with my wreck, but it will be left in place and not be salvaged. That would be just too expensive .

Congratulations on your success Rickard!

[nitro78]

First images not easy to do because there is only few minutes to do themwith the tide.


150 m boat cal Léopoldville sink 1944 in north of Cherbourg




french submarine sink during the first day test




down image from military boat sink during the destrution off explosives

and in side