Thread: Seeing pvc and plastic structure with sonar/ down imaging

Absolutely ...

Sonar reflects off anything that is of a different density than the water itself ...

The more the difference in density = the more the sonar will reflect ...

So some substances may reflect more sonar (wood, gravel, metal, concrete, etc) ...and show bright hard return echoes ...

Softer substances will also reflect sonar but to a lesser degree (mud, algae, plastic, water logged wood) ...and show Softer return echoes ...

Rickie

Originally Posted by rnvinc

Absolutely ...

Sonar reflects off anything that is of a different density than the water itself ...

The more the difference in density = the more the sonar will reflect ...

So some substances may reflect more sonar (wood, gravel, metal, concrete, etc) ...and show bright hard return echoes ...

Softer substances will also reflect sonar but to a lesser degree (mud, algae, plastic, water logged wood) ...and show Softer return echoes ...

Rickie

I see a lot of things on my down imaging that appear to be something coming up from the bottom in a straight line but the lines are broken so it almost looks like clutter. They come up about 2 foot and look pretty uniform, just wondering if this is some kind of structure


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The pvc structure we placed in C.J.Brown a few years ago ( ODNR approved) was sanded before placing as to get a jump on the algae growth. I have some marked on my gps but I have to really look as about all one can see the bump of the concrete five gal bucket and a little of the pvc going up. I'm able to see the bate / fish around the pvc tree so that's the main giveaway as to it being one of the attractors.

Originally Posted by downtime

The pvc structure we placed in C.J.Brown a few years ago ( ODNR approved) was sanded before placing as to get a jump on the algae growth. I have some marked on my gps but I have to really look as about all one can see the bump of the concrete five gal bucket and a little of the pvc going up. I'm able to see the bate / fish around the pvc tree so that's the main giveaway as to it being one of the attractors.

Gotcha, is that with sonar or down scan?


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IF YOUR UNIT IS SET RIGHT IT WILL SHOW PVC WITH OR WITHOUT ALGAE,I PUT OUT MANY THIS YEAR AND THEN COME BACK THAT DAY TO CHECK ON HOW WELL I COULD SEE IT,YOU HAVE TO REMEMBER IT IS NOT AS BIG AS A TREE(AT LEASE NOT MINE) SO IT IS GOING TO LOOK SMALLER THAN ONE WOULD EXPECT TILL YOU START REALIZING HOW SMALL A OBJECT LOOK ON DI AND EVEN SMALLER ON SI WITH A LONGER DISTANT AND THEN DOUBLE IF USING BOTH SIDES

When a sound wave travels through water alone, nothing happens. If the wave hits something that is a different density than the water it is in, some of the sound is reflected. Air, PVC, fish flesh, wood and even water of a different temperature cause some reflection. The bigger the density difference, or the bigger the object, the greater the reflection.

A plastic bottle full of Coke would be all but invisible to sonar. The same bottle full of air or concrete is a pretty good target.

The density of water is 1. Air is about 1.225. PVC is about 1.4. That means PVC is more visible than air. What makes PVC hard to see is that it is fairly thin since you only count the wall thickness.

Also remember that the PVC pipe is round and it's more stealthy that a flat surface. The sound waves don't all bounce straight back to the transducer but some spread out in other directions. You round wave will pass though one side of the round PVC pipe and the though the water inside the pipe and then hit the back or opposite side (other side of the inside of the PVC pipe). These reflections are going to be coming off the pipe in a fraction of a millisecond different in time. The time it takes for the sound wave to penetrate the pipe on the side closet to the transducer and then a microsecond later they reflect off the back inside of the pvc pipe but they have to travel though the near side again.

They are hard to see if you are set to lower frequency 455 MHz but at 800 Mhz they are easier to see if you set you range shorter and are only looking at one side of the SI screen. That's the only way I could find one of the many PVC 10 ft tall 4" OD crappie trees that I planted in a shallow part of a lake near a concrete launch ramp. I know exactly where they are located as I put them there by having my boat tied to a ramp next to the dock and letting out 75 ft. of rope and letting the wind blow my boat out away from the launch ramp but inline with it. So I could scuba dive down there and find them but it's very hard to see them with the SI or DI on my sonar unit. 898C SI Humminbird Unit is being used. My greatest fear is that the IDNR scuba divers were practicing search and recover in this area and found them while diving off the launch ramp area. If they did they probably removed them or relocated them to another spot that only they know about. They fish too.

The lake I fish is pretty devoid of wood or other structural elements. It had steep drops and some old road beds but they are really in deeper water below the thermocline I the summer months and of not good use. And the bottom is pretty constant except for some grass beds in the shallower water and some big rocks in what use to be a old strip mine.