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Sonar Image of the Adella Shores Shipwreck

05/03/2024 7:17 AM

The sonar image of the shipwreck Adella Shores was confusing.

The image appeared to show a shadow from a low and distant light source. But, the story stated that the image was sonar. The vertical lines that appear in several places in the image also suggest sonar.

After a while I noticed that the shadow has "stepped ghosting" around it. That is a bit like the shadows created from a linear string of LED lights. That is, of course, not what is going on here.

My best guess is that the image is a product of dozens of sonar sweeps being math-a-magically processed into a 3D database. My guess is that the shadow is produced in the processing to make it easier to see vertical items when looking down onto them. The "stepped ghosting" is the result of the imperfections in height measurement being processed out into an artificial shadow. I have no clue as to the vertical white line running the full height of the image (possibly a backscatter reflection from the sonar towing ship?).

Anyone out in CR4 land know about sonar processing? Are my guesses correct? Anything interesting to add?

This image came from https://www.foxnews.com/lifestyle/michigan-researchers-discover-eerie-1909-shipwreck-bottom-lake-superior.

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Guru

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#1

Re: Sonar Image of the Adella Shores Shipwreck

05/03/2024 7:54 AM

It might be Bistatic sonar, where the transmitter and receiver are separated. The advantage is that you get the glancing illumination effect. A similar phenomenon occurs in astronomy, where you can see detail of the moon's surface best near the terminator because of the glancing sunlight.

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#4
In reply to #1

Re: Sonar Image of the Adella Shores Shipwreck

05/03/2024 2:09 PM

"...probably side scan sonar, where the source is not at the same place as the receiver"

https://www.quora.com/On-sonar-images-of-sunken-ships-what-gives-the-appearance-of-a-shadow-on-one-side-or-other

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Guru

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#5
In reply to #1

Re: Sonar Image of the Adella Shores Shipwreck

05/03/2024 8:18 PM

Thanks. This is going to have to rattle around between my ears for a while.

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#2

Re: Sonar Image of the Adella Shores Shipwreck

05/03/2024 9:34 AM

Why does a side-scan sonar system produce images with apparent shadows?

Chat GPT says this

Side-scan sonar systems work by emitting sound waves that travel through water and bounce off objects on the seabed or within the water column. These systems use the time it takes for the sound waves to return to the sensor to determine the distance and location of objects.

When an object is encountered, the sound waves may be partially absorbed or scattered, creating areas of lower intensity or "shadows" behind the object in the resulting image. This effect is similar to shadows in optical imaging, where light is blocked or absorbed by an object, creating darker areas behind it.

Additionally, the angle at which the sound waves hit the seabed or object can affect the intensity of the return signal. Objects that are perpendicular to the sound wave direction may reflect more energy back to the sensor, while objects at an angle may reflect less, creating variations in brightness and shadowing in the image.

Overall, these shadows in side-scan sonar images are a result of the complex interactions between the sound waves and the objects they encounter underwater.

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#3

Re: Sonar Image of the Adella Shores Shipwreck

05/03/2024 12:26 PM

The shadow effect that side scan sonar generates is how a mine-like object is detected when viewing the "waterfall" display from a side-scan sonar as it is towed through the water. That assumes that the field is generally flat without a lot of natural features proud of the bottom.

You can watch this 3-minute video that shows more about side-scan sonar. The AN/AQS-24 uses a synthetic aperture side scan sonar that generates some rather remarkable imaging that starts at about 1:08 into the video.

Another video with some of the imagery you can get with a high-quality sonar system. Check out the boat at 1:07 and the bridge right after that. The system also has a laser line scanner which is explained at about 1:51 that allows optical imaging directly below the tow fish because a side-scan sonar has a blind-spot directly underneath it.

This capability has been in the fleet since the early Eighties. Historically, this mine detection system was towed from an MH-53 helicopter but those airframes are reaching their end of service life and the U.S. Navy is moving towards unmanned surface vessels for this mission. You can watch this aircrew doing its mission. Not for the faint of heart.

https://www.northropgrumman.com/what-we-do/sea/aqs-24b-minehunting-system

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#6
In reply to #3

Re: Sonar Image of the Adella Shores Shipwreck

05/03/2024 8:39 PM

Very impressive. Thank you.

It sure looks like a lot of manual effort is required at the back of the helicopter. I'm surprised it wasn't designed to be easier to deploy.

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#7
In reply to #3

Re: Sonar Image of the Adella Shores Shipwreck

05/04/2024 8:57 AM

Of course, you're right. The ship wreck in the image is far out to the side of the ship with the sonar. The image is generated by the return echo, the farther from the ship, the later the return. The sound produces a shadow behind objects as does any type of illumination.

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