Moderated Discussion Areas
ContinuousWave: Small Boat Electrical
|Author||Topic: SONAR Echograms|
posted 09-06-2011 11:58 PM ET (US)
Here are two SONAR echograms I recorded on my Lowrance HDS-8. They're moderately interesting.
This echogram shows two cables we passed over. The cables are used for the IRONTON ferry. We passed astern of the ferry and about 100-feet off. The cables are only about 12 to 14 feet below the transducer, or about 14 to 16 below the water. The cables were about 30-feet apart. They're steel wire rope cable about an inch in diameter. They show up rather clearly.
This echogram shows the wreck of the KEUKA in Lake Charlevoix. It is not far from the city boat ramp in the Northwest part of the lake. The wreck is not in its charted position, but it's close. The KEUKA was 172-feet long. It is sitting upright on the bottom. The HDS-8 gives a nice image. It looks like there are some decent sized fish swimming atop it. It is in 45-feet of water and the upper part is about 25-feet below the surface. If the lake water is not churned up and you have a calm day, it is said to be visible from the surface. It was quite visible on SONAR.
posted 09-07-2011 07:40 AM ET (US)
I think Jimh posts these sonar images just to tease the fishermen amongst us. Maybe if I had an HDS-8, I would see and find more fish?
posted 09-07-2011 07:54 AM ET (US)
The HDS-8 seems to be a good SONAR. I think there is an old saying that "90-percent of the fish are in 10-percent of the lake," and the HDS-8 certainly helps you find that 10-percent of the lake.
posted 09-07-2011 10:59 AM ET (US)
A bit of history on the Keuka wreck, and an old photo of the ship, can be found about halfway down the following page: http://www.lakecharlevoixassociation.org/historyphotos2.html
Videos of scuba divers on the wreck are also available on YouTube, like the following not-particularly-exciting video: http://youtu.be/D2ycXJbEmSo
posted 09-07-2011 11:02 AM ET (US)
I find it interesting that the echogram of the wreck very distinctly shows the lake bottom, including the area directly below the wreck. Can we take that to mean that the wood of the wreck is somewhat transparent to the sonar waves?
posted 09-07-2011 11:22 AM ET (US)
The echogram shows both the wreck and the bottom due to the width of the SONAR signal beam. I think the transducer is rated for a beam width of 30-degrees; let's just use that figure for this discussion. At a depth of 50-feet, for a nice round number, the SONAR will be detecting a volume of a cone whose radius and height are
radius = sin15 x 50-feet = 12.94-feet
The bottom area in the cone is a circle with a diameter of about 25.8-feet. In the echogram I am showing, my transducer must have been located above the wreck in such a way that it received echoes from both the wreck and from the bottom within that 25.8-foot diameter circle.
Of course, when I mention a circle diameter, this is just a way of describing the general area that will be most likely to return echoes. The sensitivity of the transducer is most uniform in that 30-degree cone area. The sensitivity begins to rapidly fall off at angles that exceed the 30-degree, but you can still get echoes. (You can see that in the other echogram of the cables. The cables show up even when the boat is well beyond them, and the cables are only in the fringe of the SONAR beam.)
The presentation of the echoes uses a coloration of the echo signal in relation to its strength. The stronger echoes get a brighter color, the reds, oranges, and yellow. The coloration is arbitrary, and there are many color palettes to chose from. This particular palette, number 13 I think, is a good one for my application because it tends to color the bottom brown, which helps in the visualization and interpretation of the echogram.
The sound did not pass through the wreck and reflect from the bottom. The bottom echo was from a portion of the sound beam than missed the wreck and bounced back from the bottom.
I have some other views of the wreck which don't show the bottom in them, probably because I must have been centered on the wreck. I will post one later that shows this.
posted 09-07-2011 12:12 PM ET (US)
There is a new type of SONAR which uses much higher frequency and much narrower beams. This is often called down-scan imaging or DI to distinguish it from the traditional SONAR. The transducers are operating at 800-kHz and have very narrow beam width. This lets them produce a higher resolution image of whatever it is they pass over.
posted 09-07-2011 12:40 PM ET (US)
A tip of the hat to Kevin for the link to the many interesting historic photographs of Charlevoix. Thanks for that pointer.
posted 09-07-2011 05:51 PM ET (US)
Does the HDS-8 have "print screen" functionality? How do you get these images?
posted 09-07-2011 08:26 PM ET (US)
Dave--The HDS-series devices provide a screen capture. The screen image is saved as a 800 x 600 PNG image to the device's internal hard drive. From the internal hard drive the image file can be easily moved to a removable SecureData (SD) memory card using a FILES utility. Once on the SD memory card, the card can be removed from the HDS and moved to my laptop computer, which has a SD memory card reader accessory. The image file can easily be mounted on most computer file systems that can read FAT32 formatted SD cards. From the SD memory card the file can be transferred to a computer workstation file system. I changed these screen captures to JPEG files to reduce their size and quality somewhat for better use with a web server and browser.
posted 09-07-2011 08:34 PM ET (US)
To me, the image below the wreck looks like one of those beasts from the Alien movie series....
posted 09-07-2011 08:35 PM ET (US)
I gotta stop drinking single malt. Does it every time...
posted 09-07-2011 10:02 PM ET (US)
Don--It is odd how the SONAR paints some strange pictures on its screen.
I think those fish are Walleye. Based on the dive video, not the SONAR.
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