Sometimes it's easier to try and visualize the water column in the center of the SI image separately from the bottom detail ...
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The bottom detail can best be described by visualizing you are hovering above the boat looking straight down into the water ...
Watch this video showing the displayed SI image morphing into 3D and then back to the displayed SI image to understand how the bottom detail is being presented in the 2 dimensional displayed SI image ...
https://www.youtube.com/watch?v=gsS-Lgg4HD0&feature=youtu.be
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Then once you have a grasp of the bottom detail in your head ...visualize the water column separately ...as a semi circle under the boat that is equal to the water depth ...and any target that resides in that "visualized semi circle" will be displayed in the dark water column of the SI image ...
http://i477.photobucket.com/albums/r...psku8xnkrn.png
Then once you have a grasp of the water column being a semi circle under the boat equal to the water depth ... Use the shadows on the SI image bottom detail to visualize where that target is in the "visualized semi circle" under the boat ...
http://i477.photobucket.com/albums/r...B/896a9686.jpg
Rickie