Search & Recovery question: Can side scan sonar penetrate silt?

[Johnoly]

Do you have an example reading of what that would look like? Looking "through" the silt?

I've seen it on the JW Fisher tow-behinds that had scans at DEMA. They were discussing partially buried artifacts they could identify fully compared to a camera that would miss it. Below is a regular sonar snap of a dive ledge not side scan which is a very different frequency. This is a different frequency and gain adjusted. You can see the thin layer of sand on top, the red rock below the sand and then it's just noise below that. We use this many time to find the difference between a sanded filled in hole versus a good rock hard bottom that lobsters like to hide in. It's not automatic and it's not perfect at all. The bounce back received is just a "good guess" as to whether it's a spot to dive and waste precious bottom time on.

In the end, all the sonars on the market are setup differently. You can't even put 6 different sonars on the same boat because they will likely interfere with each other. So you pick the best aspects that you normally dive and work to get it dialed in the best you can. Even a different transducer will either improve or trash your current setup.

 

[rob.mwpropane]

I've seen it on the JW Fisher tow-behinds that had scans at DEMA. They were discussing partially buried artifacts they could identify fully compared to a camera that would miss it. Below is a regular sonar snap of a dive ledge not side scan which is a very different frequency. This is a different frequency and gain adjusted. You can see the thin layer of sand on top, the red rock below the sand and then it's just noise below that. We use this many time to find the difference between a sanded filled in hole versus a good rock hard bottom that lobsters like to hide in. It's not automatic and it's not perfect at all. The bounce back received is just a "good guess" as to whether it's a spot to dive and waste precious bottom time on.

In the end, all the sonars on the market are setup differently. You can't even put 6 different sonars on the same boat because they will likely interfere with each other. So you pick the best aspects that you normally dive and work to get it dialed in the best you can. Even a different transducer will either improve or trash your current setup.

View attachment 716137

That's pretty interesting. I didn't actually get to see the screen from the captains area... I just saw what they had been given by the Navy. I really should have taken a picture, but I didn't want to come off as morbid.

 

[Gone for diving]

This is a very interesting discussion.
And not to go too off topic,
But has anyone had any experience with setting up a transducer pointing almost 90* forward, to see ahead,more with a non side scan unit..

I assume you're probably going to get some reflection from the surface..

I have a specific application,,, and I want to try it,,, but that requires the display to be underwater sealed... and before I do that,
I thought i would ask...

 

[DAJ]

I realize this is a little far afield of scuba, but hoped one of the S&R folks might know, as I can't seem to find the info anywhere else. If you dropped something overboard (a weapon, a watermelon, a hubcap from a 1957 Chevy) and it settled into 4+ feet of silt, can a SSS penetrate the silt to image it? Or would the silt just reflect back a smooth surface?

Thanks in advance, I need the info for some research and obviously know nothing about sonar.

Here is an example.

Our unit was contacted by the local DA to assist in possibly locating a knife that was used three years prior in a local homicide. The DA recently received reliable information that one of the individuals involved in the killing came forward with details on where and how he disposed of the murder weapon. We reported to a local catch basin/pond and was told that the individual stood roughly at one area and threw the knife out into the water. The water was so full of broken concrete and rebar that I went with a wet suit so I would not trash a dry suit. While the DA had little hope when I started the dive in finding anything, eight diving hours later, I found the knife. Absolute zero visibility, six feet of sludge and sediment on the bottom in about 15 feet of water. I was guiding off of another diver and I just happen to feel the knife bump my arm while crawling through the mud. I grabbed the object and I reached up with my thumb and could feel the tip so I knew I had found the knife. No way sonar would have picked that one out. Sometimes you just need to get in there and dig for what you are looking for This pic was taken by the CSI folks when we transferred custody of the knife.

 

[Jared0425]

Anything deeper than 4 inches and the sidescan's ability to detect objects drop significantly. I've dealt with Klein (all eras) Edgetech, Kraken, Konsberg, and JW fisheries. Missed a car smashed into the bottom by a freighter with 6 inches of dog crap like mud on top. The tires were the only thing sticking up. Unfortunately rubber absorbed most of the return.

 

[NeesiePie]

Thank you everyone, I greatly appreciate your input. The consensus seems to be "Nope"

 

[Russjstewart]

The other variable here is how long it will take for something to disappear into 4 foot of silt.
I once dived in a marina with a very silty bottom looking for a propeller. It turns out the hub was still on the shaft but the blades has sheared off probably when the boat went into gear. I found one of the blades sticking out of the silt.

Based on this I am making a guess that
The gun may go a few inches into the silt, being small and heavy. The hubcap would be sitting on the surface as it is relatively light for its size
The watermelon is going to float, having a large fresh water component

I would be interested to know how far under the silt was the knife that @DAJ found and how long it had been there.

I suspect silt builds up over time to cover objects and bury them deeper in the silt, unless the silt is disturbed.

Incase you can’t tell I am in isolation due to COVID and really really bored.

 

[WeRtheOcean]

not know what I am looking at in the screen or interpreting what the object is, yet people can tell you what type of fish or marine animal it is just by seeing the dots on the screen.

There is a learning curve. The people you refer to have had training in doing that.

 

[Jared0425]

There is a learning curve. The people you refer to have had training in doing that.

And a good place to start is Black Lazer or get yourself a copy of Mark Antertons boom imagery in sound.

 

[BoltSnap]

Black Lazer or get yourself a copy of Mark Antertons boom imagery in sound.

Forgive my ignorance, what are those? I googled them but nothing useful.