Down Scan Mille Lacs Mud Flat

I am working on a good description of how StructureScan works; details on the cone shape and cone angles for side scan and down scan. But for this page we will keep it simple.

Down scan is similar to side scan in that it looks down and to the sides and not to the front or back. Traditional sonar has a cone shaped area under the transducer so it ‘sees” front, back and to the sides. Down scan also uses a higher frequency (455/800) than traditional sonar (200).

These pictures are of a mud flat in Mille Lacs Lake, Minnesota. The mud flats are mid lake humps composed of hard mud and some have rocks. Top and bottom of the drop off are softer than the sides of the drop off. The edges of the drop off are deeply rutted.

Example 1 is traditional sonar (200 kHz transducer Broadband) showing small fish and large fish. The small fish are difficult to separate from the bottom. I dropped my Aqua-Vu and the large fish were walleye and the small fish were perch. I did not see any other species. The bottom appears to be rolling humps but I am actually driving the boat along the drop off.

Example 2 is down scan (800 kHz transducer) recorded at the same time. The small fish are much easier to see and the ruts are more obvious. Fish do not display as arches since the down scan doesn’t “see” them in front and back. The tails of the fish arch are the fish on the edge of the cone as you drive to and away from them.

Example 3 is StructureScan (800 kHz transducer) which is side scan that I recorded at the same time as the traditional and down scan sonar. The ruts are everywhere and shown in detail. The perch and walleye I identified are probably the same ones as seen on the down scan the traditional sonar.