Thread: Sonar Quiz #25

I think it is possible that the fish could have been in the outer fringes of the DI fan beam and be somewhat missed by the 2d cone ...

However...this example would tend to follow the explaination from Airmar ... (the notable aspect being the sharp rise in the bottom) ...

From Airmar ...
http://www.airmartechnology.com/uplo...Operations.pdf

What is target masking?

There are areas within the transducer's range which seem to be invisible to the echosounder. This is know as target masking. It can happen if the lake or sea bottom drops off suddenly or contains a large rock. The sound waves will bounce off all of the sea bottom within the sound beam and return as strong echoes. The echoes from the highest point, the rock or dropoff, return to the transducer first, falsely indicating the apparent depth of the bottom. Small fish below the highest point will produce relatively small echoes which will return after the larger ones. Therefore, fish can be swimming around the sides of a large rock or a dropoff, be in the sound field, and yet remain invisible to the echosounder.



Rickie

I wonder if part of the problem is the 2d td installation? Maybe kicked up in the back?

Originally Posted by kosmo

So if there were a flat bottom, same fish, but no quick rise of the bottom .would those fish show on the 2d? Have you ever heard anyone say that 2d won't lie? If it's down there,2d will show it? Does 2d have a blind spot in some cases?

Kos I believe 2d does have blind spots, on the edge of said cone, the bottom of the cone is not flat it is shaped just like an ice cream cone upside down with a round scoop of ice cream in it , so now the cone has spots that don't reach nor read bottom, 2d sonar gives bottom readings from the first contact made with the bottom

I'm not much good at explaining things, I hope I got this correct

Originally Posted by wilbur

Im gonna say they would show. The 2d shows 11ft. but the fish are about 17 ft. when the 2d beam hits the bottom it levels out it doesn't follow the slope.

I.m thinking that not only those fish would not show, but even, let's say a diver that was six feet tall standing on the bottom and close to that sharp break, would not show on the 2d, but would be displayed on the di.what do yeall think?

Kos you are correct, my thinking is that if the diver is at a level below where the 2d cone makes first contact with the bottom he will not show up on 2d but he would on DI

here's a crude picture I did in paint real quick to kind of show my thoughts

I hope I'm correct if not I was told wrong or miss understood by my late uncle that was a sub man in the cold war era

Originally Posted by kosmo

I.m thinking that not only those fish would not show, but even, let's say a diver that was six feet tall standing on the bottom and close to that sharp break, would not show on the 2d, but would be displayed on the di.what do yeall think?

+1

Originally Posted by short grub

here's a crude picture I did in paint real quick to kind of show my thoughts

I hope I'm correct if not I was told wrong or miss understood by my late uncle that was a sub man in the cold war era

so if this is the case, when traveling perpendicular to a sharp dropoff and you see fish returns, can you assume the fish is on the leading edge of the cone since the rest is dead zone?