What is a side-wall echo?
The sequence of events occurs when the boat is heading out of Urquhart Bay over water which is getting deeper and deeper. The sonar digital depth reading (20.6M in the picture above) increases, and then, as the visual sonar trace ( the red shape between oldest trace and newest trace ) flattens out when the boat gets over the flat bottom zone, the digital read-out continues to increase. The “real” depth is about 231-233m, but the digital readout goes on, 240, 250, 260 and then suddenly snaps back to 231…this is consistent with the machine receiving ( increasingly weak) echoes from the side wall as it gets left behind the boat, and of course the distance to it increases until the signal it too weak to be read, at which point the “genuine” signal from the bottom beneath the boat once again becomes dominant.
