Search and recovery in silt?

[Adam]

I am looking for tips for searching for a fairly large object on a silty bottom. I am guessing the silt is 5-8 feet deep.

We are thinking of using a metal detector but we are not sure if it will hit on something that is buried in silt.

Another idea we are thinking of is dragging some sort of snag through the silt in an organized pattern.

The only thing we have to go on are GPS coordinates that we taken just after the loss. So I suspect the search area will be kinda big.

Any ideas?

[jasondbaker]

A metal detector will have limited range in silt depending on the model. For example, if the detector has a range limit of 6ft, you will not be able to locate something buried in 8ft of silt. A boat-towed magnetomometer may have better range.

[DetectorGuy]

A Magnetometer would be best but these are expensive to rent if you can even find one to rent. Side scan sonar will also penetrate loose silt, but I would try a swim over the area to see if you can see a depression in the silt. In 1964 a small plane crashed in the deepest part of Spectacle Lake. We found the area and to this day there is a 3' deep depression in the 8' deep silt where it was removed at 50' deep. The sides of this depression are still vertical in really loose silt. Once you narrow down the area, a 10' long piece of conduit can work for a probe to push down into the silt and 'feel' objects in the silt. I made one of these with 1/8th inch aircraft cable running through it with a large trebble hook on one end and a loop on the other that I can attach a lift bag to. This way I can probe down into the silt, hook the object, and lift bag it with one device and not need to mark it for a later recovery. Obviously some objects won't hook with a trebble hook but you get the idea. Once the object is found you could deploy a dog bone buoy from underwater right over the object to get a better GPS location of the object if a different recovery method is needed. A metal detector could be used to stick down 5' to 8' into the silt but my detector will only read objects 18" from the coil. For body recoveries they use a gang of trebble hooks pulled behind a boat, but a drowning victim is 18 pounds negative if it is a male, and 14 pounds negative if it is a female, so bodies tend to get hung up on the surface of the silt and the hooks are more effective. We did drop a firemans training dummy through the ice into 30' deep water and 8' of silt. The training dummy was 250 pounds on land and 200 pounds negative in the water. It went into the silt like a lawn dart and was hard to locate even directly below the ice hole. Good luck with the search and recovery.
John

[Hydro]

a lot depends on the object and the depth to where the silt is... a boat cover will be easy to find, a steel ice spud will be gone forever in silt 8' deep. if you really want to find it, your best bet is wait till winter and search from a fixed position..... for the right incentive, I could get "a fairly large object on a silty bottom" this winter

[Adam]

Thanks to everyone for the replies. This will help as we continue the search. Interestingly enough the missing stuff is in Spectacle lake. It is a full set of scuba gear. Ooops. At least it will be fun looking for it.

[popolarbear]

drop an anchor tied to a cave reel and do circular pattern. let out afew feet at a time and just keep swimming in circles for ever till you find it. or alternatively drop anchor, swim out to max 100ft or so and do one circle and see what the line snaggs up on.if the silt is deep you are never going to find it. i spent a month on one circle digging in the mud for a large pile of stuff when a boat tipped over for a pal, and found nothing. but again, what kind of gps coordinates you got. the kind that the boat was anchored or drifting and you hit waypoint. or the kind that the owner says i think it was on this side of the lake in front of this beach. its got to be here, then the second person in boat says no it was about 1/4 mile down the shore closer to this and this spot, then the third person says no i for sure got gps mark and that is another 1/4 mile farther down?.. for comparison, i have lost a weight pocket off the side of an anchored boat, and dug for 2omin before i got ran out of time and just went diving instead. also lost a weight pocket off a dock in DEEP mud and just cut my loss and bought a new one... not that i dont mind and enjoy a challenge to dig it out.

[Adam]

The gear has been recovered. I wish I could say that we found it while diving but that's not the case.

About 6 weeks after the gear sank it was found entangled in a dock on the west side of Spectacle Lake. Fortunately my name was on the gear and the person who found it wrote a hand written letter last week with her cell number.

The question remains why did it float back up. Here some of the theories:

I first thought the tank somehow leaked all the air and made the unit more buoyant. This was proven wrong as the tank still had 3000 psi

The lake probably turned over in late September when it was found. Could that have something to do with it. Probably not?

Maybe it didn't sink or came right back up. Maybe. Probably not as I watched it sink out of sight along with all my dreams and ambitions and could not reach it as I was too buoyant in the drysuit.

Lessons learned.

1. Pay attention
2. Everything always works out

[popolarbear]

lots of maybee's

i would add some. some lakes this time of year get strong wind born currents that move garbage around. i like to think of Lake Minnetonka. many areas dont have anything on the bottom even though tons of cigarets and trash get tossed over. theory would be that wind born currents pushed your gear till it hung up on something. ..you got lucky. go buy yourself a single lottery ticket and see where that goes. have to get it before the month ends or your luck might change.

[DetectorGuy]

I'm glad you got it back. My theory is just a guess... I think the mystery of the missing dive gear has to do with Pascal's law and Archemedies Principal. My opinion is that the day you watched it sink to the bottom, your kit would have been slightly positively buoyant if it would have been a low barometric pressure... but if you look back at a weather almanac for that day, I will bet a lunch that the barometric pressure was high that day. This would make a normally positively buoyant kit negative. A day or two later the barometer dropped and your kit became positive, corked to the surface, and the wind took it to the West shore. Couple this together with the lake turning over and it is entirely possible. Here is a link to some information about the Cartesian Diver:

[Terry]

Hello Adam,
I am surprised you got your gear back, w/o diving for it. What if there happened to be some lake water in your BC, likely, and what if over time, some of that bacteria in that lake water in your BC was gas producing, enough that it slowly pulled your gear from the muck, potentially aided by both Popo's and Detectors theory's. The only for sure way to test this is to drain any water from your BC, sterilize the inside of your BC, take her back out to the same spot, and turn her loose. I would use a GPS, and a strong anchor and a submerged float, to be able to recover your gear in the event that this test, is as successful as out current admistrations policy's on a whole bunch of topics. What do you say?