Sonar interpretation?

[Larry3215]

Another hint that the bottom might not be flat is how thick the bottom is drawn on the screen.

That isnt 100% certain because there is more than one thing that determines how thick and dark the bottom layer is drawn.

Most sonar manuals say that a "hard bottom" gives a stronger darker bottom layer - and thats true...but...
Things like Gain settings - more gain = thicker and darker.
How smooth the bottom is - a smooth flat bottom gives a stronger return even if it is sand or mud.
Then a sloped or un-even bottom also makes for a thicker bottom layer.

Lets go back to my earlier screen shot. This is at Toliva Shoal just off the south end of Fox Island jigging for lings last year. The bottom rises up pretty steeply and the top of the shoal is pretty uneven. The bottom is drawn in very thickly on that shot for that reason, plus its very rocky there and I had the Gain turned up.

There are some other things to note in that picture. Note the difference between the digital depth and where its drawn on the right side.

On the left side its hard to see, but there are some fish arches that are partly to mostly buried in the bottom return where it is rising steeply. Those fish were actually well above the bottom or they would not have shown up at all. See the first drawing I did with the sloped bank. Thats another indication of a sloped bottom.

Inside the black circle are some fish arches hugging the bottom. They are uneven because of the wave action on the boat.

The whitish lines are our jigs moving in the water. The darker ones were more in the cone and the lighter ones were on the edges. Lines that lean to the right were where the jig was being reeled in. Ones leaning to the left were going down.

[Larry3215]

Of course, the best way to tell what the bottom is doing is to have a good chart with lots of contour lines. If the lines are close together, then the depth is changing rapidly. If they are far apart, then its relatively flat.

Here is the Toliva Shoal area on my chart. The red line is more or less our path on that drift. We started out in deeper water south of the buoy. You can see that the contour lines are close together which means it was a steep slope. Then we got to the top of the shoal and it flattened out and the lines got further apart. Finally we stared down the other side, but its not as steep as shown by the lines being further apart.

[Sideburns]

Well I guess Im busted for the fish id too. I now understand that my downrigger ball interference is the extra long release clip and ball mounted attractors!

The wider cone width not being able to detect holes/depressions in the bottom now makes alot of sense, finally understand the reasoning for the fish arch, and in general I'm realizing what I saw in various past circumstances.... Thanks for your well thought out/presented explanations!

But what about these? I see these stacked images occasionally. Usually in my unfortunately narrow 20* 2d view. Even the 800khz down imaging screen saw them this time. a lot of the time they are full arches and look like christmas trees. Stacked fish? Or a scenario like your spread out fish pictures? Can 2d sonar even really detect stacked objects?

Having trolled previously forested reservoirs before, I was immediately panicking and pulling up all my gear thinking I was about snag a tree.

[Larry3215]

Thats an interesting pattern. My first guess is some sort of transient electrical interference. Bilge pump or live well kicking on, or some other electrical device switching on, trolling motor, or??

The things that make me think its electrical are the fact they are so perfectly aligned vertically and they all look to be almost identical in intensity plus they all change depth in exactly the same amount at the same rate and keep the same spacing. I'm having a hard time imagining that many fish of just the right sizes, spreading out in just the right places within the beam to create such a perfectly aligned pattern, all moving in perfect lock step.

At the same time I cant think of anything you could have driven over in the water that would make that pattern either. It would have to be something that was on an angle that perfectly matched the outside angle of your sonar beams cone angle. Thats the only way they could all enter and leave the cone at the same time. Something like a buoy anchor line could be like that, BUT then it would have to have a series of things spaced along it that created each individual return. All of those items would have to be different sizes for them to all show up looking the same size when they were at different depths.

It could also be a brief pulse of interference from the sonar or some other item on another near by boat.

Interesting screen shot!

[Amx]

And if the bottom sediment is soft enough you'll get a 'false' reading from the hard bottom below the sediment.

[Sideburns]

Not much as far as electrical on my boat... bilge is manual, stereo disconnected, lighting was all off. This boat has seen those types of images on other lakes.

This was sammamish last month, and I think the last time I saw similar shapes was on Lake Washington, SE of Mercer. ......Ohhh.... The Minn Kota I-Pilot Link trolling motor is on the same aux electrical system, I'm sure it has contactors, etc.. for the drive and steering motors. It was running every time ive seen those marks.

[Sideburns]

Do you use any kind of voltage isolator/ power conditioner for your electronics? I was thinking I should have installed something like that, but was told that I didn't need it.

[Larry3215]

Trolling motor interference is probably the most common type there is. Im lucky and have no issues, but I have my trolling motor completely isolated from all other boat electrical systems on its own battery system. I also do not have a transducer mounted on the trolling motor and all cables are well separated.

Minnkota has a web page dedicated to this issue and how to fix it. It covers all the bases pretty well. They also sell kits for DIY installation and sell ferrites and filters. For some trolling motor models where they have acknowledged there is a problem (internal grounding issues) they even give away the parts for free. You have to call and talk to them though.

http://www.humminbird.com/Category/Supp ... erference/" onclick="window.open(this.href);return false;

[Sideburns]

I do have their ferrite power supply cable, only does so much though. when the humminbird was originally connected to the main battery temporarily, the main engine's charging system was super noisy! The main and auxiliary systems are now completely isolated, but the minn kota is on the aux with the sonar. Since they are networked together, I was a little concerned about having them wired to different voltage sources. Being a mechanic, I'm a ground Nazi. The Minn Kota and all other devices are redundantly grounded to the neg battery terminal with large wire!

I think this small interference issue is livable though... if I ever think I see bothersome electrical interference in the future, Its pretty easy to shut down the Minn Kota to verify.

A little off topic, but have you ever seen what magnetic induction does to an electronic navigational or auto pilot compass?
Run any high current wiring near any autopilot compass sensor, and you'll be turning circles!

[Larry3215]

Yeah, on some boats it can be very difficult to find a good spot for an electronic compass. Even laying a screw driver on the dash can cause issues!.

One problem with trolling motor interference is that is so variable. Two people can have the same motor and the same sonar setup and yet have very different types and amounts of interference. On top of that, especially with common grounds, what fixes it on one boat may actually make it worse on another one. Sometimes it helps to bond all the grounds together and other times it helps to disconnect them.

You've got it much better than a lot of guys do.

[Larry3215]

I think we have the basics covered pretty well on 2D, but I want to go back to cone angles briefly.

I mentioned before that what was going on in some of my drawings wasnt quite accurate. In fact, talking about cone angles at all is really a gross over simplification of a very complex situation. I would not even bring this up but its important to know that the examples I gave above about how fish interact with the "cone" and how they display on the screen isnt always the case. Those are not "rules" you can count on in any way.

Remember in the Lowrance tutorial I linked to earlier, they hinted about this when they mentioned that the specified "cone angle" was not the same as the "fish detection" angle. A transducer that lists a 20 deg cone angle could have a fish detection cone angle of 60 degrees or more - but - that angle will vary greatly depending on the depth and the gain settings.

Here are a few pictures showing what the true "cone" is like on any transducer. The cone isnt really shaped anything at all like my drawings or the ones you see in any fish finder manual or advertisements. Those simplified, perfectly geometric, triangular cones help to get the basic idea across, but they are complete and utter lies.

Its even worse because there is no industry standard on how to measure cone angles. Airmar uses -3DB (1/2 signal strength) and Humminbird uses -10DB (1/10th signal strength) and and I have no idea what some of the other brands use because they keep those specs close.

The point I want to make is that you cant really tell anything about where a fish is based on the 2D images on the screen. Some of those examples I gave above would look very different if the fish were a little deeper or shallower because the fish would be entering the cone at very different times and distances.

Also, the listed cone angles are pretty much useless when trying to compare different transducers - especially from different mfg's.

[Larry3215]

Before starting on downview and sideview I want to talk about a relatively new technology in sport sonar - CHIRP - and talk a little about frequencies.

CHIRP stands for Compressed High-Intensity Radiated Pulse. I wont go into the technical mumbo jumbo except for one thing. Regular sonar pulses are one frequency but CHIRP sends out pulses or pings that sweep over a range of frequencies.

Frequency affects three things on the sonar returns - cone angle, detail level and depth penetration. Higher frequencies tend to give better target separation and more detail because of the shorter wavelengths. They do not penetrate as far though the water so there is less range. Lower frequencies give less detail but penetrate further and have more range. For any given transducer, the higher frequencies will have narrower cone angles and the lower frequencies will have wider cone angles.

So if you have a transducer that does 2D on 50 and 200 khz, the 50 khz frequency will have a wider cone and penetrate deeper but give less detail. The 200 khz frequency will have a narrower cone and not penetrate as deep but will give more detail.

In that example above with a fish hiding in a pothole, its possible that the 50 khz setting will not show the pot hole at all, but the 200 khz setting might show it just fine - and show the fish - because the bottom coverage is small enough to pick it out. The 50 khz might also show several fish close together as a single large return where the 200 khz might show them as individual fish arches.

Where CHIRP comes in is that it sweeps across a range of frequencies from low to hi. This does a few things that are all good. It gives the benefits of hi and low frequencies at the same time. You get better penetration and more detail and better target separation at the same time. The other big benefit is you get a much cleaner screen because the software is able to reject noise much better. That in turn allows you to run the sonar at much higher gain levels without a lot of clutter and noise on the screen.

Bottom line - CHIRP is good to have. I highly recommend getting a CHIRP capable sonar next time you are looking to up-grade. You will pay a little extra, but it is well worth it.

[TrackerPro16]

Huh. For a minute there I thought I was looking at circumcised vs. uncircumcised pictures...
Wound up reading a detailed article (more like a technical manual) about military sonar tonight at work kind of by accident. It was of more interest because of this thread. Neat stuff.
Keep up the articles! If I don't blow a head gasket I might actually learn something!

[Larry3215]

hehehe You're going to get one of us in trouble ;)

By the way, everyone please feel free to jump in with questions, comments, disagreements, additions, corrections or what ever. Im fine with any of that.

I'll be starting on side and down view next.

[Amx]

I'm thinking that circumcision isn't a problem, it's the object that is circumcised that isn't allowed to be mentioned by name.

[zen leecher aka Bill W]

What would 2D sonar show this as?

In my crude drawings above, the right side is supposed to be the screen on your plotter. In both cases, the bottom would just be a straight line across the screen assuming the boat was either not moving, or was following a constant depth.

In this next screen shot, there is no obvious way to tell if I have passed over a pot hole like in the second drawing. It just wont show up unless its pretty large in relation to the beam width on the bottom.

There is however, a strong hint that I am driving over a sloped bottom or an area where the bottom is not flat within the cone. Look at the digital depth reading in the top left corner. It says the depth is 202.2 ft, but if you look at the black arrow on the right side, the bottom is being drawn on the screen at about 185 ft. Thats a good indication that the bottom is not flat. You will see that discrepancy between the digital depth reading and where the bottom is drawn in many of the screen shots Im going to post later.

03MAY16_1615_00.jpg

Im pretty sure the reason for that is because the sonar decides what to call the "depth" by averaging out the different returns that it decides are from the "bottom" and not suspended targets.

Larry, I meant "down imaging", not 2D. I called it the wrong name. I wondered if DI would show the contour dip with the fish sitting in it. I'd imagine that image would be accurate and not hide the fish.

[Sideburns]

well it seams like I can find ravines or old creek bottoms with the di,when I cant with the 2d, because of its thin, but wide slice of coverage. (one small crappie pond in particular) But probably only if Im traveling perpendicular to the creek? I'm curious about the "hole" scenario too.

[Larry3215]

You are more likely to see things like that hole on downview, but it all depends on the size of the hole vrs the beam widths on downview. Its a little more complex than it might seem. And yes, you will see a creek bed more easily if you hit it at 90 degrees, but again - size matters

Im about to get into that in more detail......

[zen leecher aka Bill W]

You are more likely to see things like that hole on downview, but it all depends on the size of the hole vrs the beam widths on downview. Its a little more complex than it might seem. And yes, you will see a creek bed more easily if you hit it at 90 degrees, but again - size matters

Im about to get into that in more detail......

Old style fishers usually have a paper map in the boat so they have an idea what the bottom looks like (for bigger lakes)

[Larry3215]

Yes, paper and sonar charts can help a lot if they are detailed enough.