Sonar question to think about...kindof long

[Dank]

My Garmin fishfinder manual mentions a 'dead' spot which occurs when a fish or structure is below the highest point of 'ground' within the sonar 'cone'. Since the first strong reading is interpreted as the bottom, it can't see anything below that point. No big deal on a hump, because eventually a fish or log will be above the interpreted 'bottom'as the boat moves. But what about a 4-foot wide channel in 20' of water?. A typical cone would be about 6' wide at the bottom. Based on my interpretation, you'd never be able to see fish or structure in that channel unless it came up higher than the sides of the channel. For that matter, you wouldn't even be able to see the channel itself.<br />Has anyone else out there considered this phenonenon, or am I just missing the point?

 

[mattttt25]

Re: Sonar question to think about...kindof long

wow, don't we all have too much time on our hands during the off season. you make a good point. i would guess (without doing the geometry) that if your sonar cone is at 6' wide at 20' depth, and you are in a 4' wide channel with 20' depth, your sounder may only show 16-18' depth and miss fish below that point. not a real big deal. that's assuming the "cone" of the sonar extends evenly as it travles down. i should really know more than this, i learned a lot about sonar propogation in my surface warfare classes (navy), but seemed to have forgot it all. go figure-

 

[Gadgetman]

Re: Sonar question to think about...kindof long

Dank<br />Good question,I've never looked at it that way.If your theory is correct,you should be able to read two different depths at the same time over that channel,buy using two graphs with different cone angles(12 and 21 degree for example).This would only work if the cone angles don't cross,or if there is a big enough spread in the frequencies not to interfere with each other.