Side Scan vs Magnotometer

[Gareth]

Hi All,

Gareth from Dive Frontier Australia.

Just a quick question for you guys. Side Scan Sonar vs Magnotometer for wreck hunting.

1. Which do you feel is better and why.
2. Costs
3. Ease of use.

From the research i have been doing, it appears magnotometers are the weapon of choice in most wreck hunting scenario's.

Thanks.

[Richard Lundgren]

Hi Gareth

Good to have you here! Gareth is as indicated a member of the GUE affiliated Dive Frontier group, much similar to Ocean Discovery. They operate from Sydney but have ambitions in the whole Oceania region.

Regarding sidescan sonars vs. magnetometers

Sidescan sonars have the clear advantage since they produce pictures of the sea bottom. Loads of information can be drawn from the picture produced and it is easy to identify type and general age and also plan dives from these. The swat, the over all width, search are more limited compared to a good magnetometer but still covers up to 600m of the sea bed. The only negative about Sidescan sonars is the price.


Picture: Typical Sidescan sonar image (*image changed by admin)

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Magnetometers will give echoes of magnetic hits on the sea bottom. Fancy topside workstations give a good overview of these hits and show their relative position and some also what kind metal being processed. The search can be bigger than compared to a Sidescan system. Magnetometers tend to be far cheaper than Sidescan sonar system hence more widely used by wreck hunters. On the negative side they don’t produce images and some areas, like the Baltic, is far from ideal since major part of the sea bed consist of magnetic minerals like iron etc.

I’m sure Ingemar, our expert hunter, will fill in as soon as he wakes up. He isn’t an early bird

Take care,
Richard Lundgren

[Tulle]

And a another disadvantage in the swedish part of the Baltic, it's illegal!
(like all metalldetectors, you must have a permit, in sweden)

[Ingemar Lundgren]

As Tulle said Mags are illegal in Sweden, therefore our experience with them are limited. I have used them abroad though and they certainly have advantages for some kind of targets. Working with Kevin Gurr on the Santa Margarita, the sister ship of the Atocha treasure Galleon we used mags to locate stray artifacts such as gold chains and silver coins. We also did a 24h on shift search with side scan sonar for 4 days looking for one of its undiscovered sister ships. In these waters there would be nothing left of the hull, what we where hoping for was a mound of ballast stones.


For targets that are heavily damaged and deteriorated i think a mag is the way to go. Depending on the bottom characteristics side scans are more or less useful. Finding a broken up wooden ship on a rocky bottom is very hard. The reason being that the bottom return from the rocky bottom is stronger then the return from the wood. You would only see the shadows of the wreck and if it is broken up and lay flat against the sea bed there is not that much shadow. The shadows would also risk being hidden or blended in with shadows from rocks etc. Looking for something with a side scan sonar is very much like holding a flash light near the floor in a dark room. What you are looking for is the shadows. The closer to the floor you hold the light the longer the shadows from the objects appear. Therefore it is an advantage to run the fish as close to the sea bed as you can.

With a mag it doesn't matter how the bottom looks. It will pick up magnetic returns anyway so in some searches it has clear advantages. It will pick up everything though and a pile of scrap metal could have you fooled. Also some sea beds contain a lot of iron such is the case in the Baltic sea in some area making the use of a Mag somewhat limited. Does a wooden wreck have a magnetic return? Yes it often does although far from as strong as the return from a modern iron ship. The ballast stones used back then often contain high amounts of iron.

The advantages of a side scan sonar is clearly that you have a means of identifying and measuring the target. For more intact wrecks and 90% of search scenarios it is the way to go in my opinion.

Link regarding Atocha http://1622atocha.com/
Some more info on side scans http://www.ocean-discovery.org/sidescan.htm

[Martin Quensel]

Sture Hultquist, Peter lögdberg and some others, me included used both Magnetometer and Sidescan a couple of years ago searching for some wrecks. We had some trouble with the magnetometer, and neither one of us had experience from using one before. It just gave readings, and it was hard to actually do something with these.. We had a display that showed some numers that kept changing all the time. From the computer we got a log and from that it would be possible to draw a graph of the magnetometer readings..

We used it at the same time as we ran the Side Scan, and i must say i was not really impressed by the magnetometer.

Sure it works, but for it to make sense you have to have some nice software that also marks the readings on a map with the help of a GPS. If you are running the magnetometer from a large steel ship then you have to have a considerable lengthy cable.

On the other hand you probably go a few knots faster with the boat using a mag and thus covering a larger area.. But im not sure.

[Matthew White]

I`m not sure that there are to many undescoverd wreck`s on the Eastern coastline over here!
i picked up a theory once,that talking to the local fisheries or fisherman(person) and finding there areas of no go can often turn up the remains of a wreck(or a lot of deep reef diving...)
sometimes they don`t even know why the area is not fished only that the history of the site has been ruled out.
Dose anyone speak "wharfy".http://www.ocean-discovery.org/forum/images/icons/icon6.gif

[Matthew White]

Disregard text previous,i`ve just read thread;Graf Zepplin:o

[Ingemar Lundgren]

Talking to local fishermen is by far the best way to get leads on new finds. Often you can get very accurate positions to. Are they using bottom trawls in your area? Here they use a pair of trawl doors about 30m apart with a big net in between. They drag this along the bottom and sometimes they get stuck in wrecks. Expensive these trawls so they keep a log on "trawl snag" positions to avoid them in the future.
Other fishermen using other methods actively seek out wrecks to lay their nets on. Wrecks attract a lot of fish.

There might be a lot more wrecks then you think so it is well worth a try to interview the locals.

[Ingemar Lundgren]

Martin, could you tell us some more about Sture´s side scans? I have seen some pretty amazing images from his sonars. What KHZ is he running them at to get that level of detail? Is he still producing/selling side scans?

One of Sture´s side scan images.
http://www.abc.se/~pa/uwa/paula.htm

http://www.abc.se/%7Epa/mar/img/sture/3paula3-x.jpg

Take a look at this one! http://www.abc.se/~pa/mar/img/sture/fritzen-x.jpg

[Matthew White]

Actually that`s what got my attention with the fishie`s point in all this.
there is a wreck that is popular and close to shore that has huge cargo netting laid heavily across the bulk in 45m,has a density of aquatic inhabitance.
Mainly shrimp and carvel trawlers that use the same netting setup you mentioned,wide(boom)gates swing out to lower the drag net to depth`s that calculate the fish finders,Don`t quote that though!

The side scan data look`s impressive,very vivid shadow.

[Martin Quensel]

Martin, could you tell us some more about Sture´s side scans? I have seen some pretty amazing images from his sonars. What KHZ is he running them at to get that level of detail? Is he still producing/selling side scans?

One of Sture´s side scan images.
http://www.abc.se/~pa/uwa/paula.htm (http://www.abc.se/%7Epa/uwa/paula.htm)

http://www.abc.se/%7Epa/mar/img/sture/3paula3-x.jpg

Take a look at this one! http://www.abc.se/~pa/mar/img/sture/fritzen-x.jpg (http://www.abc.se/%7Epa/mar/img/sture/fritzen-x.jpg)

I have not talked SideScan with Sture for 2 years. We have been out searching for wrecks a couple of times, and mostly talked about a Unix based software for the SideScan.

There was talk about mass producing these sonars, and a small project was planned where the sonar body would be CNC machined instead of mounted in a tube as the usual design. I saw some CAD drawings of this design and it looked nice (but have no experience to judge designs, it looked nice in my eyes). The project planned to build 10 of these, just to see if it would be profitable, and to see if the cost could be kept under 10 000 for each unit.
But i was never really involved in this. Sture and i only talked about the software part of the project, since that is what i am good at, and that was 2 (maybee even 3) years ago, and i have only spoken to him once since then (last week actually) So i dont know if the plans still exist. He has however produced a number of sidescans, and i dont think he minds producing more. What seems to be the really great thing in his sonars is the analog to digital converter, that he built, and that actually does some signalprocessing before the digital signal reaches the software.

The images are quite stunning, but to get these images a wreck somtimes has been passed over 10 times, just to try to get a good angle.

I can check what kind of frequencys they operate between (you can adjust it if i recall correctly), i never cared about that, i just cared about the digital image processing ;-)

[Briton diver]

I am using Aquascan Mag and Hultqvist sonar in France and overseas since 2000.(Sture's sonar is a very nice machine)
When you search wrecks, especially old, you need the best because olds ones are difficult to find. Mag is easy and useful for metallic wrecks, but give false signals if the zone contains geological magnetics rocks. To prevent this it necessary to see the sinusoidal signal generated by the Mag on a wreck. It is the best way to make te difference between wreck and magnetic rocks.
In volcanic countries you must use a gradiometer (expensive)
This year I want to make tests to find the best solution using two coupled Mags.
Sonar is more easier to use, but need pratice to memoryse all the differents pictures you can get

[Ingemar Lundgren]

It would be exciting to hear some more of the wrecks you have discovered in France. Do you have some side scan images that you could post on the forum?

[Tulle]

Martin, could you tell us some more about Sture´s side scans? I have seen some pretty amazing images from his sonars. What KHZ is he running them at to get that level of detail? Is he still producing/selling side scans?


When Sture did these images of Paula and Fritzen, he had good help from L-G Utberg, owner of the company "Promare". And L-G told me that they "washed" the pictures digitaly to get the great result. Don't know if I understand it correct, but they took away some frequency's that desturbed the result... maybe you guys know what he mean?! :o

[Briton diver]

Hello
Have a look on http://sonar.se/sonar.html. Go to "a flat towfish design.." and "back to side scan". You can see my face and the very nice 440 kHz probe builded for me by Sture. You see also my latest hit in 2005 on Corsica Island, a 500 years old wreck, 50 m deep, an anchor and a cannon mouth. This hit was made with my second probe 200 kHz. This year I am coming back in Corsica to begin a new search and make new pictures with 440 kHz. I try to send you enclosed my best actual picture.

best regards

andre/Volumes/DD 002/A-PROJETS ARCH 2005/PROJET SEAS 2005/RETOUR DE CORSE/RAPPORT DRASSM ST FLORENT 2005/IMAGES EPAVES RPRT 2005/F 71005/F 154337.bmp

[Ingemar Lundgren]

Sounds like you got some awesome wreck hunting gear! I particulary like this picture of the old wooden wreck. Shows the level of detail. How about doing a joint effort to locate Svärdet next summer? We could run 2 boats each with side scan.

BTW to insert a picture from another website you can drag and drop.
http://sonar.se/4CROWN.JPG

[Martin Quensel]

When Sture did these images of Paula and Fritzen, he had good help from L-G Utberg, owner of the company "Promare". And L-G told me that they "washed" the pictures digitaly to get the great result. Don't know if I understand it correct, but they took away some frequency's that desturbed the result... maybe you guys know what he mean?! :o

Yes. I know what you mean.
The washing i have heard of is contrasts etc that has been manipulated a bit in photoshop. Some pictures has been reversed when it comes to colours (black -> white, white->black) go get a nicer looking result.

[Ingemar Lundgren]

Sure they have been manipulated but you can´t really add detail to a picture. You can fix contrast etc but if the picture have low resolution you can´t do much about it. It is evident regardless of post processing that Stures side scans have outstanding resolution.

The famous Fritzen picture was done by a 1000khz tow fish i believe. That frequency have very limited range and would not be ideal for actual "wreck hunts".

[Gareth]

Guys,

Thanks for all your replies, it appears the the side scan is the way to go.

FYI: We will be diving some new marks on the 20th of this month, we will update you with the results.

[Richard Lundgren]

Good luck! Give your mates my best regards and see ya soon!

[Gareth]

Hi All,

First round searches and dives did not yield any new wrecks unfortunately (what we expected).

Having said that the team enjoyed a 75m scooter dive in 3m vizibility. I was unfortunate enough :D to miss that dive as i tore my neck seal the previous day.

Not to be put down the team was back in the water on Sunday, scootering up a storm on one of the local favourites.


Guys,

Thanks for all your replies, it appears the the side scan is the way to go.

FYI: We will be diving some new marks on the 20th of this month, we will update you with the results.

[Gareth]

Hi Ingemar/Richard,

What sort of Sidescan is Ocean Discovery running? Looks like something our team may need to consider, initial searches for the Nemesis have yielded some, yet rather vague results. Understand if you dont want to discuss online, shoot me an email if you get a chance.

Cheers Boys.

[Ingemar Lundgren]

We are using an Imagenex model 858 330khz side scan sonar. Not to be confused with the new cheaper "sport scan".


Link to Imagenex http://www.imagenex.com/

BTW We would like to upgrade the software. Anyone know of any good third party software that could be used?