difficulties of finding a diver's body

[Ayisha]

One would think that all the metal this guy was likely wearing would make it easier to pick up on sonar. Is that accurate?

A boat captain was tracking us once (unbeknownst to us) with his side scan sonar while we were diving on a wreck in Kingston, Ontario. When we were back on the boat, he showed us our images and we thought it was really neat, so he sent them to us. If you look carefully at the image, you can see us three smaller females together, and a team of 2 taller males. I imagine he might look something like that?

 

[Mark Michaud SELAUSAR]

My dive team guys in a local bayou in scuba. Shallow, so the return is not as good as if they had been deeper but obvious just the same. I had them laying near a sunken bass boat that was near a pier. One shows the shadow of his legs and fins.
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[Jared0425]

There was a case we had with a missing diver that he did not show up on sidescan. We went over the area 4 times and the fourth I finally got a indentifiable hit his tank. Not all divers show up and as I said before, its all about conditions. As mentioned intrepretation is also a huge part in it. A seasoned operator can use past experience to rely on in indentifyimg objects, a lesser trained person is looking for the obvious on what he thinks he will see.

 

[bowlofpetunias]

What a fascinating discussion! IMHO a thread dedicated to the topic of the difficulties of locating/recovering missing/deceased divers would be very worth while for quite a number of reasons. If one of you wants to start such a thread and let me know I can copy or move these very worth while posts to it. That would make the topic much easier to locate if someone wanted to do a search on it.

 

[Mark Michaud SELAUSAR]

There was a case we had with a missing diver that he did not show up on sidescan. We went over the area 4 times and the fourth I finally got a indentifiable hit his tank. Not all divers show up and as I said before, its all about conditions. As mentioned intrepretation is also a huge part in it. A seasoned operator can use past experience to rely on in indentifyimg objects, a lesser trained person is looking for the obvious on what he thinks he will see.

You made a great point here. Interpretation is the main thing. In a certain direction something can look just like it should if you get the right angle to reflect back. Another direction you may get "that doesn't fit" or just a different color....maybe brighter. Too close, as in a vehicle, and you get an undefined bump because there is not enough room for the image to return and take shape. Sonar is a great tool but experience is huge.

I missed a drowning victim 2 years ago. A neighboring sheriff's dept called for assistance. There was no last seen and a few miles of river. A woman was tubing, went for a swim and when everyone made it to the bend she was seen passing....she was gone. The group did not know the area. They thought she had possibly stepped onto the land so they looked for a bit. They could not notify anyone until they got to the end which was a while later. This was a Sunday afternoon in mid summer. I arrived Tuesday morning and the victim should have floated by then but hadn't.

The river is a mess. Firehose current, gravel bottom, lots of debris. Jet drive motors are all you can use. I started sonar from the end of the run and we went upriver. About a mile up, the driver asked what I had just seen. I looked and said, how do you know I saw something? He said my ears wiggled or something. He had been a K-9 handler for a long time. I had, indeed, seen something but it didn't look like anything I had ever seen. It was white and "wavy" but it did not look a hard bottom would, or should, look. The water was 7 1/2' deep here and moving hard. I blew it off as an anomaly. We ended just after sundown with no find. I had a K-9 group called in a few days later. Again we had no last seen, or even a last seen area.

They arrived Thursday due to weather Wednesday. I got a call from the boat driver / deputy. He asked, remember when I asked you what you saw. I said yes. He said "we have her". She was about 100' down river inside of the bend from where I saw something. He then told me she has post-mortem ligature marks on her neck and under her armpits. It hit me then like a brick.

What I saw was her body in the current flapping like a flag. She was held by a trot line and couldn't rise as the line was fully extended. The line finally gave and she ended up where they located her. That is why she could not float. That debrief meant a lot because it accounted for what I had seen and had I probed the water there we would have discovered her. Until then I had no experience to know that. It is what it is. Even though we couldn't keep the boat still, due to current, we could have run a rope across the river and used if to stabilize the boat.

Sonar is one of our best tools but we have to use it often and use it on things we know to be good at interpreting what we see that needs further investigation. Mark

 

[Rich Keller]

He should show. The operators experience and ability to interpret images is a huge benefit to how well a sonar will make a difference. It's not a magic wand, but it is a great tool if the operator has spent time using the unit and keeps a good search grid.

Is there a sonar equivalent to a radar reflector?

 

[Wookie]

Is there a sonar equivalent to a radar reflector?

A sonar returns a hard target. Hard in this case may be an air pocket like lungs, a actual hard thing like a scuba cylinder, but not things with the same density as water. The slower and steadier you can move the sonar, the better the target will present itself. We surveyed pipelines for 115 days on the Spree. We kept 2 engines in forward and one in reverse to go slow enough. For 115 days.

 

[Rich Keller]

I found something. I should have looked before asking the question. They make a small size that can be attached to a diver. Check out their video and they show one at about 1:20 into the video. Home | SonarBell®SonarBell® Might be interesting to see how well it works. Like putting one on each diver you put in the water and seeing how well you could track them during the dive in real time.

 

[Wookie]

I found something. I should have looked before asking the question. They make a small size that can be attached to a diver. Home | SonarBell®SonarBell® Might be interesting to see how well it works. Like putting one on each diver you put in the water and seeing how well you could track them during the dive.

Well, to track someone during a dive, you have to be looking for them. That isn't as easy as it sounds.

For instance, on the Spree, we have a bottom sounder. The bottom sounder has a transducer that looks straight down. Beam angle is 0.5 degrees from the transducer, so I get a very accurate return of what is directly under the boat, but it wouldn't let me see a body unless I were moving, and it were directly under me in 50-100 feet of water. Depth is based on the frequency of the sonar pings, and the number of pings per second.

Now, if you want to see a shape or a body, wreck, ledge, etc. You either need a lot more transducers or you need something called "Backscatter". A multi-beam sonar utilizes multiple beams to create a 3D image of the bottom, where a bottom sounder has one beam, a multibeam may have 50 on a less expensive unit ($50k plus the cost of software) and as many as 1,000 beams on a very expensive unit ($2.5M plus the cost of software. Same software, BTW). Multibeam sonars are pretty much out of the reach of most guys like myself.

Sidescanning sonars work differently, and much of it is PFM and magic smoke, but as I understand it, there 2 lobes of receivers, one on each side of the sidescan (hence the name, get it?) that receive the sonar signal sent out from the nose of the unit. Since different materials reflect back at different speeds, you get a phenomena called backscatter that allows for a 3D image to be formed. Since the receivers are on the sides of the unit, it does not look straight down, as you wouldn't get an effective backscatter. That's why there is a line in the middle of a sidescan image. Sidescan sonars run from $10K for a new JW Fishers to $100K for the latest Edgetech.

The proper tool to track a diver is with some sort of electric pinger and 3D imaging receiver. There is a thread here somewhere about just such a machine to be debuted at DEMA. I did not see it there, although I was interested. I think the cost was around $2500 for a receiver and a few pingers....

 

[Rich Keller]

Now, if you want to see a shape or a body, wreck, ledge, etc. You either need a lot more transducers or you need something called "Backscatter". A multi-beam sonar utilizes multiple beams to create a 3D image of the bottom, where a bottom sounder has one beam, a multibeam may have 50 on a less expensive unit ($50k plus the cost of software) and as many as 1,000 beams on a very expensive unit ($2.5M plus the cost of software. Same software, BTW). Multibeam sonars are pretty much out of the reach of most guys like myself.

This looks like it was geared more for the oil and commercial diving industry where a 50K scanner would not be a big expense. Interesting concept though if it were ever to become economically feasible.