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

[NeesiePie]

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.

 

[rob.mwpropane]

I'll preface this by saying I don't have any experience with sonar, using the tech, etc. What I do have is about 8 weeks ago I was asked to go out and help with S&R of the body of a lost fisherman in the Chesapeake Bay. The folks on board had sonar from the Navy as the search area was in front of the Naval Research Laboratory just south of Annapolis, MD.

The sonar readings that they acquired did not penetrate the silt. It showed the bottom silt layer with targets sticking up through it. I assume that the sonar the Navy uses is about as good as it gets.

I hope that helps.. I know it's not a technical answer.

 

[Johnoly]

Depending on the model & type of sonar, Yes it can penetrate both silt & sand to show what's beneath. The oil industry does this every day in geological formations. For boating some models allow you to turn up the "gain" or power. But with the increase in power, you usually also increase "noise" on the screen and if it's so much noise then finding a needle in a haystack is impossible. In addition, sonar operates on frequency and different ones offer different benefits like soundings thru silt or deep or shallow, or currents and even water temperature can effect sonar. As much as people think it's a science,,,,,it's really more of an artform of balance and trade-offs.

 

[Vicko]

Depends on the type of silt, type of sonar (output power, frequency, beam width), type of object and depth in the silt. I know that steel drums and explosive ordinance can be seen even half a meter bellow the silt layer with the correct setup and conditions.
Normal consumer sonars (Garmin, Humingbird) tend to suck at this.

 

[rob.mwpropane]

Depending on the model & type of sonar, Yes it can penetrate both silt & sand to show what's beneath. The oil industry does this every day in geological formations. For boating some models allow you to turn up the "gain" or power. But with the increase in power, you usually also increase "noise" on the screen and if it's so much noise then finding a needle in a haystack is impossible. In addition, sonar operates on frequency and different ones offer different benefits like soundings thru silt or deep or shallow, or currents and even water temperature can effect sonar. As much as people think it's a science,,,,,it's really more of an artform of balance and trade-offs.

That's amazing... would have been nice as I never did feel "bottom" in the muck we were in. It just felt like playing in feathers. What a mess. Still can't get that stuff off my suit.

Do you have an example reading of what that would look like? Looking "through" the silt? This about what we had to work with, but the target was a little more buried.

The picture they received from the Navy was very clear (more so than the one attached). It was clear enough that you could make out a round object and what appeared to be the reflection of a watch which the deceased was supposed to have on that arm, and that was @ 66' deep.

 

[Russjstewart]

I have found the link below useful for understanding what SideScan can and can’t do.

 

[kelemvor]

Cool video. I watched the first 35 minutes, but don't have the remaining 1:45 to finish it out. It was neat to see sidescan sonar in action! Lots of skills involved. Several times I was looking at an image and had no idea there was a person or airplane or truck there until he pointed it out.

 

[BoltSnap]

Several times I was looking at an image and had no idea there was a person or airplane or truck there until he pointed it out.

This is the main problem for me, 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. Very frustrating for me.

 

[DAJ]

Most sonar used in routine recovery dives will show objects that are on the bottom but they won’t generally identify small objects that are buried under heavy silt/mud/sand. The limiting factor for most recovery operations is budget so the type of sonar used and the platform on which it’s employed varies. Sonar is a useful tool when you don’t have a “last seen” point, you need to cover a large search area, search areas that are too deep for regular search techniques, or save resources until you have a possible target. Finding small targets like a knife that is buried is best done by divers on the bottom using a detector or just “brail” diving through the muck while line/grid searching. Most of your penetration sonar centers on finding rather large structures/objects that can return a large enough signature to provide an image.

 

[Akimbo]

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?

I'm not aware of any off the shelf technology that can detect a watermelon under 4' of silt. A gun or hubcap maybe but the range is pretty low. Magnetometer probes would be your best bet for very small areas.

The oil industry does this every day in geological formations.

Offshore seismic surveys do use sound for imaging below the mudline but it is not considered sonar (Sound Navigation and Ranging) in the normal sense. Acoustic waves are generated by massive high pressure air-guns that produce much more powerful signals than sonar, even military ASW (Antisubmarine Warfare) sonar.

They used Dynamite explosive charges in the early days of the industry. Purpose-built seismic survey vessels tow large arrays of sensors and all the data is processed by supercomputers onshore.

Here is a video. Fast-forward to the one-minute mark to skip the dumbed-down hype:

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