Question for you guys

[Charles]

Hi,


I could just be way out in left field here but it sounds like you guys might be able to straighten me out. If you get a comercially made side scan sonar, what are the limitations on depth of water it could be used in? I want to try to find a wreck in water that is only 3 to 5 feet deep. It seems that most units seem to be rated to work in a minimal depth that is deeper than that. I read on this forum yesterday where people say you drag the fish near the bottom, hence the need for an extremely durable tow fish. If this is the case:


1) Why would it matter that the water would have to have a minimal depth to work?


2) On many pictures of scans, especially on Sture's site, they appear to be pictures of wrecks that would appear to be pictures looking down from 200feet above the wreck. Why would this be if the fish is pulled near the bottom of the ocean.


I always assumed that SSS wouldn't work for my application since you wouldn't be high enough to get a picture of the complete wreck but it would sound like that isn't the case, however, it seems from what I have read so far that the depth of the water still seems to make a difference. Can someone clear this up for me, I know I'm missing something here. If I could build something that would work in 3-5ft of water, I would be much more motivated in regards to this project.


Thanks,


Charles

[Patrick]

Re: Question for you guys

Charles,


At that depth I would think that you would have a lot of feedback from the transmitter bouncing off the bottom.


One suggestion is to get a fishfinder with a transducer that has a suction cup (portable mounting) and place it on the side facing the direction you want to look. You should also be able to make a mount for a regular transducer to allow it to look sideways. Try mounting it on a stick or small board.


If you do not have one, try to pick up a used one on Ebay. Make sure it has an LCD screen or paper graph.


Patrick


Just my thoughts ...


Patrick


>Hi,


>I could just be way out in left field here but it sounds like you guys might be able to straighten me out. If you get a comercially made side scan sonar, what are the limitations on depth of water it could be used in? I want to try to find a wreck in water that is only 3 to 5 feet deep. It seems that most units seem to be rated to work in a minimal depth that is deeper than that. I read on this forum yesterday where people say you drag the fish near the bottom, hence the need for an extremely durable tow fish. If this is the case:


>1) Why would it matter that the water would have to have a minimal depth to work?


>2) On many pictures of scans, especially on Sture's site, they appear to be pictures of wrecks that would appear to be pictures looking down from 200feet above the wreck. Why would this be if the fish is pulled near the bottom of the ocean.


>I always assumed that SSS wouldn't work for my application since you wouldn't be high enough to get a picture of the complete wreck but it would sound like that isn't the case, however, it seems from what I have read so far that the depth of the water still seems to make a difference. Can someone clear this up for me, I know I'm missing something here. If I could build something that would work in 3-5ft of water, I would be much more motivated in regards to this project.


>Thanks,


>Charles

[Pete]

Re: Question for you guys

There IS a minimum recommended depth for all transducers. The commercial side scan units recommend the fish be towed 100' off the bottom. My bottom scan unit (Marinetec Maximum) will work as shallow as 4' until the screen (video) will "white out".


If you want to build a homemade sidescan unit, why not copy a commercial unit?


Pete

[Pete]

Re: Question for you guys

>Charles,


>At that depth I would think that you would have a lot of feedback from the transmitter bouncing off the bottom.....


Hi guys, Yes, you would get a lot of feedback since there would not be enough time for the sound pulse to go out and come back. Most commercial side scan units have a minimum depth rating which is 100' off the bottom. My bottom scan unit (Marinetek-Maxxim)


will white out (on the video screen) at depths as shallow as 4'.


If you want to build a sidescan unit, why not copy a commercial unit?


Pete

[Patrick Duffy]

Re: Question for you guys

Pete,


>>At that depth I would think that you would have a lot of feedback from the transmitter bouncing off the bottom.....


> Hi guys, Yes, you would get a lot of feedback since there would not be enough time for the sound pulse to go out and come back. Most commercial side scan units have a minimum depth rating which is 100' off the bottom. My bottom scan unit (Marinetek-Maxxim)


I've never seen one in action. However, both Sture and Dan have obviously towed near the bottom or they would not have hit the rocks they mentioned.


>will white out (on the video screen) at depths as shallow as 4'.


> If you want to build a sidescan unit, why not copy a commercial unit?


How do you like your Marinetek? How long have you had it? Could you post photos of it?


Thanks,


Patrick

[Charles]

Re: Question for you guys

Thanks for the reply Pete. I would love to copy a commercial unit however I don't have access to one to duplicate. Any suggestions?


Thanks,


Charles


>>Charles,


>>At that depth I would think that you would have a lot of feedback from the transmitter bouncing off the bottom.....


> Hi guys, Yes, you would get a lot of feedback since there would not be enough time for the sound pulse to go out and come back. Most commercial side scan units have a minimum depth rating which is 100' off the bottom. My bottom scan unit (Marinetek-Maxxim)


>will white out (on the video screen) at depths as shallow as 4'.


> If you want to build a sidescan unit, why not copy a commercial unit?


>Pete

[[email protected]]

Side Scan Performance

From experience:

Side Scan Sonar Units do have a minimum distance to be towed off of the floor. It is dictated usually by the range at which you wish to image. 10% is a good rule. 5% is ok. So for looking out 100 feet, 10 feet off of the bottom is good. 5 feet is ok. It usually isn't for the reason you think. Mostly the towfish needs to be off of the bottom by 10% so you don't plant your fish in the bottom when a bump or small hill or the depth suddenly gets shallower.

Quite a few commercial side scan units can be used in 12' of water or even less. Keep in mind that they will see a surface echoes, however this can be minimized when working in calm water (ie not alot of air bubbles and churning water on the surface). You can get out to 100 feet in 12' of water. That translates to roughly 30 meters. It is relatively easy to do. 5 feet of water should work ok.

Side Scan Sonars have a beam profile that is optimized to have a very wide beam angle vertically and as as small as possible horizontally. This gives you a nice long slice of directly straight out from the transducer from the surface to the sea floor. Usually 40 degrees vertical and less than 1 degree horizontal. Then the transducers are usually pointed down at some angle between 0 and 10 degrees.

In fact scanning in 3 to 5 feet of water would best be done with a side scan / rotating sonar (sector scan) or forward looking sonar and not a down looking sonar. The beam on a down looking sonar would not have very much room to spread and would not cover very much area. So you would have to drive around alot more. A side scan looking out at 10 to 30 meters (~30 to 100 feet) would cover much more ground and for some of the commercial side scan units provide a much more detailed picture. I would suggest using something in the higher frequency range. Definintely something over 300 kHz. Imagenex and Marine Sonic Tech are two companies that come to mind that should (and will work) in those situations.

[UWLocator]

Sidescan in 3 to 5 feet of water

Hi,

Having used Sidescan in water from 2 feet to over 300 feet of water, I can tell you that it can work well in the depths that you suggest. The higher frequency units will work better in shallow water, but all of the units will give acceptable results if properly set up.

The reason for having the transducer higher from the bottom but still near the bottom has to do with getting the best shadows from objects on the bottom. Often it is the shadow that reveals what the item is. The length of the shadow can also be used to calculate the height if the object given the height of the towfish above the bottom and the distance to the object.

In shallow water, I normally run the towfish 10 to 15 inches below the surface with the transducers angled down about 30 degrees. I will adjust both of these to get the best image. You give up distance but gain resolution in the troughs with that arrangement. I use 30 to 60 foot ranges for most of the shallow work.

The best way to do it is to get out there and try. Make adjustments to the towfish depth and the angle of the transducer and see what gives the best image in the water you have to work with.

Rip

[guest76978]

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[tohu]

Why not a laser-scanner?

Uhm, if you need the sensor to be just a few feet away, why don't you look at an optical solution instead of an aucustic?

Unless you've got terribly poor visibility you'd get a far better result with a laser-scanner or a stereoscopic camera...

[Unregistered]

http://www.marinesonic.com/

If you want a unit that will provide hi quality output at very shallow depths, you might take a look at this outfit. They show pictures of objects at just a few feet. Their units are at a much higher frequency than anyone else.

http://www.marinesonic.com/