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Author Topic:   SONAR Echo Interpretation
jimh posted 09-06-2013 07:14 AM ET (US)   Profile for jimh   Send Email to jimh  
In the SONAR echoes shown below, are the echo returns near the lake bottom from fish near the bottom or from structure on the bottom?


HDS-8 SONAR, 200-kHz P66 Transducer

Jefecinco posted 09-06-2013 09:28 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
I'm unsure. The returns look to me like classic arch shaped fish returns. If you are more or less stationary and the returns change position they are not structure.

Butch

Tom W Clark posted 09-06-2013 10:21 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Those returns do not look like classic arch shaped fish returns.
jimh posted 09-06-2013 10:42 AM ET (US)     Profile for jimh  Send Email to jimh     
Since changing the transducer to the AIRMAR P66 I have not seen one, classic, arched, fish return.
tmann45 posted 09-06-2013 11:37 AM ET (US)     Profile for tmann45  Send Email to tmann45     
Since some of those returns appear to be tilted to the right (/), they look like the beginning of fish arches, even the small ones in the middle depths. This indicates the your transducer is tilted forward some, instead of being perfectly vertical.

It also appears that you might improve your picture with some fine tuning of your settings. I can't offer any suggestions there since my sonar looks similar but not as good.

I think I had better fish returns with the older software versions. I've been going back and forth between 3.5, 4.0 and 4.1 trying to get better arches.

Since most of the bottom returns are not attached to the bottom, I would say that is not bottom structure, only the bump on the very left is bottom.

Buckda posted 09-06-2013 01:03 PM ET (US)     Profile for Buckda  Send Email to Buckda     
Lake Trout...sitting just off the bottom.
Buckda posted 09-06-2013 01:03 PM ET (US)     Profile for Buckda  Send Email to Buckda     
Lake Trout...sitting just off the bottom.
kwik_wurk posted 09-06-2013 01:12 PM ET (US)     Profile for kwik_wurk  Send Email to kwik_wurk     
Well if they are not fish, what are they, jelly fish...I think not. --- Yes you are getting half arches most of us agree, reduce the gain more and you'll only get the top of the arch. The angle of the transducer can be the issue. (transom or thru-hull?) I have had two fish finder units side by side present different returns, but each had some sort of object on the screen that was discernible from the ground or other structure. -- Honestly I would love it if OEM wrote a bit more on what sample return images are supposed to look like and how to adjust accordingly. (other than marketing literature with perfect arches) -- I remember by older Furuno radar had a very detailed section on how to interpret and adjust setting for things like rain, excessive motion, double echos, etc.
K Albus posted 09-06-2013 01:14 PM ET (US)     Profile for K Albus  Send Email to K Albus     
It's only 25 feet deep there. We'll have to take a mask and some fins next time, and take a closer look. My opinion is that most of the marks near the bottom are sunken logs and tree branches.
jimh posted 09-06-2013 02:10 PM ET (US)     Profile for jimh  Send Email to jimh     
Regarding the angle of the transducer:

The AIRMAR P66 does not allow for much adjustment of the transducer angle. There is only a single wedge block supplied. If the transom angle is between a certain range, the wedge is not used. If the transom angle is in another range, the wedge is used. The wedge can be used two ways, with either the skinny end up or down. My installation is not using the wedge block. The transducer face does seem to be aimed slightly forward from vertical or straight down.

According to

http://www.actionfishingtrips.com/sonar.htm

some speed is needed to get good fish arches. As you can see, when I took these images there was not much speed on the boat; the forward speed was only 1.7-MPH.

The above reference also mentions to "make sure your transducer is pointing straight down." As I mentioned, the P66 has a slight forward throw in my installation.

They also mention to "be sure there is no oil or wax residue on the face, which could cause small air bubbles to stick to the face and interfere with the sound waves." I have not done anything to the transducer face. I don't think there is any oil or wax on it, unless it picked up something on the roadway.

Lowrance also talks about transducer angle in

http://www.lowrance.com/Root/Lowrance-Documents/US/ X-4_OM-IM_EN_988-1014-001_w.pdf

See page 12 in the above.

jimh posted 09-06-2013 02:53 PM ET (US)     Profile for jimh  Send Email to jimh     
The AIRMAR installation instructions include a diagram, Figure 10, showing use of the shim or wedge. See

http://airmartechnology.com/uploads/installguide/17-247-03.pdf

I am not using the wedge, as the instructions indicate for transom angles of 12 to 18-degrees.

tmann45 posted 09-06-2013 03:54 PM ET (US)     Profile for tmann45  Send Email to tmann45     
quote:
According to

http://www.actionfishingtrips.com/sonar.htm

some speed is needed to get good fish arches. As you can see, when I took these images there was not much speed on the boat; the forward speed was only 1.7-MPH.


Nice general information, I wish they would have been more specific with the time in the cone to give a good arch.

I would think your speed was sufficient to show a good arch, but maybe not. The calculated time transiting the diameter of your cone is about 2 seconds. Aside: the cone diameters for 200 kHz shown on the Airmar website need a 12-degree cone angle to calculate correctly instead of the 11-degree specified. The difference in cone diameter transit time is 1.9 versus 2.1 seconds for the 11 versus 12-degree cone angles at your 1.7 mph speed and 25 ft. depth.

What is your ping speed set on?

silentpardner posted 09-08-2013 04:04 PM ET (US)     Profile for silentpardner  Send Email to silentpardner     
[Had no opinion on the echoes but began a patronizing sidebar, which has been deleted--jimh].
jimh posted 09-08-2013 06:44 PM ET (US)     Profile for jimh  Send Email to jimh     
Re the set-up of the HDS-8, the HDS has been set up for the particular transducer in use, an AIRMAR P66. Prior to navigating down this narrow channel, the SONAR was in the "Deep Water" mode, as most of the depths of Lake Superior are greater than 200-feet around Isle Royale. As we were coming down this two mile passage in the cove, I switched the mode to something like "Shallow Water" or "Fishing Mode" in order to get a better presentation of the echoes. (I think people who are not familiar with Lake Superior might not realize that it has depths to 800-feet not far offshore, so most of the time the SONAR mode was not set for finding fish in a 40-foot depth. Perhaps people who are accustomed to having to go 50-miles offshore to get to 800-foot depths might be confused by the rapid change in depths around Isle Royale.)

As I am not really concerned about angling for fish, I don't spend much time--really no time--obsessing about what settings to adjust. I usually set the NOISE REDUCTION to something like MEDIUM and the SURFACE CLARITY to MEDIUM, and I might adjust the AUTO GAIN up or town a few percent. I have used the HDS-8 enough now in the two years that I have owned it that I feel fairly comfortable with getting good echo returns from it. If someone thinks they are a real expert, maybe they could publish a good article on how to make expert adjustments to the HDS. This would be much more beneficial to the boating community in general than to just jump into this thread to suggest that the root of the problem is that I am an ingenue with SONAR device.

I don't think the "rain" in the SONAR echoes is due to electrical interference. I have seen actual electrical interference in the display and it looks different.

Let's review the discussion so far. The question at hand is the interpretation of the 2013 SONAR echoes, and whether the echoes are fish or structure. So far the responses have been as follows:

--the echoes look sorta like fish arches, but can't be sure if they're fish or structure (Jefecinco)

--the echoes do not look like fish arches (Tom Clark)

--echoes are mostly fish; possible problem with transducer angle (tmann34)

--echoes are big lake trout (Buckda)

--probably fish (kwik_wurk)

--echoes are structure like logs and debris (K Albus)

Here is a screen shot I took in 2010 of the SONAR on my HDS-8 in about this same area. It shows the bottom but without the 3X zoom. The echoes are more suggestive of the classic fish arch. The transducer was the Lowrance Skimmer transducer that is bundled with the device:


HDS-8 with Skimmer Transducer, 83-kHz setting, from 2010

The 2010 SONAR echoes of McCargoe shows that at some time in the past, using a different transducer, the Lowrance Skimmer model, I was able to get classic fish arch echoes with my HDS-8 in this general location.

The most credible information presented so far seems to suggest that the angle of the P66 transducer points a bit too forward.

There may be other differences between the Skimmer and P66. For example, the Skimmer is described as being a very wide beam device. I think it is said to be something like a 60-degree cone. I do not think the P66 at 200-kHz is such a wide cone device. This could affect the appearance of the echoes produced by the devices.

It might be interesting to see if anyone has a screen shot of an HDS using a P66 in about 40-feet of water at 200-kHz where there are fish echoes that resemble the classic fish arch shape. If anyone has such a screen shot, please join the discussion. (You can email the screen shot to me and I will make it part of this discussion.)

jimh posted 09-09-2013 11:59 AM ET (US)     Profile for jimh  Send Email to jimh     
Also, I realized that I must have had the HDS in 2010, so that makes 2013 my third year of using it. I regret misleading people by previously mentioning I had been using it for two years. It is true that I have been using it for two years, but people may have misinterpreted that to mean I had not used it longer than two years. I apparently have been using it for at least three years.
tmann45 posted 09-09-2013 04:00 PM ET (US)     Profile for tmann45  Send Email to tmann45     
If you do the math, you will see that fish arch is directly dependent on cone angle. A very narrow cone angle will only give you a dot or blob, a wide angle will give a nice big arch.

Jimh said:

quote:
I do not think the P66 at 200-kHz is such a wide cone device.

tmann45 said earlier:
quote:
Aside: the cone diameters for 200 kHz shown on the Airmar website need a 12-degree cone angle to calculate correctly instead of the 11-degree specified.

Also, to get good arches the fish must transit the diameter of the cone, if it transits a secant the result will not be as good.

jimh posted 09-12-2013 01:37 AM ET (US)     Profile for jimh  Send Email to jimh     
tmann is precisely onto the real problem with the P66 and fish arches. The P66 directivity is given in this excellent data sheet from AIRMAR:

http://airmartechnology.com/uploads/CeramicDesignation/50_200A.pdf

The data sheet specifies that the 200-kHz element (which is the one in use in my SONAR echoes that is the topic of this discussion) has a very narrow beam. The -3dB beam width is only 11-degrees. The -6dB beam width is only 16-degrees, and the -10dB beam width is 21-degrees. This is a much narrower beam than the Lowrance skimmer transducer, which seems to have a -3dB beam of about 60-degrees.

With such a narrow beam at 200-kHz, the SONAR echoes from an AIRMAR P66 is not likely to produce fish arches like those seen on the much wider-beam Skimmer.

jimh posted 09-12-2013 11:53 AM ET (US)     Profile for jimh  Send Email to jimh     
There are several newer SONAR units now on the market using much higher frequencies, in the range of 400 to 800-kHz, and the beam width of those transducer is probably much smaller. When you seen SONAR echoes from those devices, there are no "classic fish arches", but, instead, fish appear as small dots in the water column. These higher-frequency SONAR's produce a much more detailed view of the bottom structure, often showing details like branches of sunken trees or other objects on the bottom. But they do not produce fish arches.
Peter posted 09-16-2013 07:57 AM ET (US)     Profile for Peter  Send Email to Peter     
My Garmin 740s is connected to an Airmar P66 and is driven at 50 and 200 khz. In the 200 khz image, I cannot recall ever seeing any "classic" fish arches. However, the "bedsheet ghost" like blobs appearing in the 200 khz image sometimes appear as classic fish arches in the wider 50 khz image.

I note that the arches appearing in the second image in this thread are at 83 khz.

jimh posted 09-17-2013 01:30 AM ET (US)     Profile for jimh  Send Email to jimh     
Peter--Thanks for the first-hand information that when using the AIRMAR P66 you have not seen any classic fish arches. That is my experience, too.

The notion of seeing classic fish arches was introduced into this discussion by Tom Clark. I don't know for certain if Tom has first-hand experience with the AIRMAR P66 transducer, and if he has seen classic fish arches produced by it on the 200-kHz setting.

By the way, I like your term bed sheet ghost blobs--that is a good description of the echoes seen in my screen capture in the initial article. Do you think the echoes are from fish or from bottom structure?

Peter posted 09-17-2013 07:55 AM ET (US)     Profile for Peter  Send Email to Peter     
I think they are fish based on looking at the 83 kHz image. I've sent you a screen capture from the Garmin 740s with both 50 and 200 kHz images side by side.
jimh posted 09-17-2013 12:13 PM ET (US)     Profile for jimh  Send Email to jimh     
Here is a screen capture of a Garmin 740s using an AIRMAR P66 that shows the 200-kHz and 50-kHz SONAR echoes side-by-side.

It is quite interesting that in the 200-kHz presentation the echoes look like the little goblins and in the 50-kHz presentation the same targets make classic fish arch echoes. Thanks so much for sending this screen shot. It really shows how the difference in transducer frequency and beam width affect the echo returns.

tmann45 posted 09-17-2013 06:03 PM ET (US)     Profile for tmann45  Send Email to tmann45     
Peter, really nice clean interference free sonar display, that is as good or better than Garmin's website "screen shots".
jimh posted 09-18-2013 08:09 PM ET (US)     Profile for jimh  Send Email to jimh     
Peter's screen capture of the Garmin 740s shows an interesting mode: the sounder is able to use two frequencies at the same time and display different representations of the same water column. I believe that this is accomplished by alternating between the 50-kHz ping and the 200-kHz ping.

As far as I can tell, the Lowrance HDS-8 does not have this option. When you select 50-kHz the sounder pings at that frequency, and when you select 200-kHz it pings at that frequency. You cannot see a split screen display of the two different frequencies and the echoes the received from the water column simultaneously.

jimh posted 09-19-2013 12:28 PM ET (US)     Profile for jimh  Send Email to jimh     
To get back to the initial question of interpretation of the echoes, I am now quite confident that the echoes are from fish near the bottom. I should also mention that this is based on having seen a smooth bottom contour for most of the 1.5-miles of travel in this particular portion of the lake.
saumon posted 09-19-2013 02:09 PM ET (US)     Profile for saumon  Send Email to saumon     
quote:
As far as I can tell, the Lowrance HDS-8 does not have this option. When you select 50-kHz the sounder pings at that frequency, and when you select 200-kHz it pings at that frequency. You cannot see a split screen display of the two different frequencies and the echoes the received from the water column simultaneously.

I was surprised to read this, as even an older Lowrance LMS-522c that I had can display both frequencies at the same time.

In the Lowrance HDS 8 manual ( http://www.lowrance.com/Root/Lowrance-Documents/US/ HDS-8-10_OM_EN_988-10042-001_w.pdf ) on page 30, it's written "You can view both available frequencies
at the same time by setting up a sonar split screen."
.

jimh posted 09-19-2013 02:54 PM ET (US)     Profile for jimh  Send Email to jimh     
I know it says that in the manual, but in the next section, immediately following, it explains how to set up a split screen. In that section there is no explanation of how to set up a split screen with two frequencies shown at once. I will have to experiment with my HDS-8 to see if this is possible. But my reading of the manual seems to say you cannot do it.
jimh posted 09-19-2013 03:05 PM ET (US)     Profile for jimh  Send Email to jimh     
After a few minutes of experimenting, I found the solution. Yes, you can display two different frequencies of SONAR echoes at once on a Lowrance HDS-8, but it certainly is not very well explained in the operating manual. You don't use the Split Screen menu option in the SONAR menu, as is implied. In order to get two different SONAR frequencies shown at once you must use the PAGE carousal and select the option of SONAR + SONAR. This gives you two SONAR pages on screen at once, and you can independently adjust them to the different frequencies.

I don't recall doing this before, so the next time I am out with the boat in some deep water I will set up from the PAGES carousal a SONAR + SONAR display.

This is something different than the Split Screen option provided in a single SONAR display; that option lets you see the full water column on one side of the screen and a zoomed in version, a bottom locked version, or a flasher presentation on the other side.

jimh posted 09-22-2013 09:54 AM ET (US)     Profile for jimh  Send Email to jimh     
Now for a second question:

In the SONAR echoes shown in the initial article, are those smaller blue blobs scattered in the water column also echoes from small fish?

saumon posted 09-22-2013 10:55 AM ET (US)     Profile for saumon  Send Email to saumon     
Other than an underwater camera, I haven't heard of a way to distinguish between small scattered fishes and debris/leaves.
Hoosier posted 09-22-2013 11:45 AM ET (US)     Profile for Hoosier  Send Email to Hoosier     
The Loons were feeding on something, small fish is a good bet.

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