What is Sidescan Sonar?

Sidescan sonar is a carefully engineered sonar designed to look sideways and at a downward angle from both sides of a towed unit, called a "towfish". The bottom and any objects in the water above the bottom reflect sound waves back to the towed array and from this information an image is produced. Using this type of system, very large areas of the seafloor or lake bed can be mapped quickly and easily, saving time and the expensive labor costs of diver run searches. This method also provides the greatest chance of success in search operations by assuring complete bottom coverage.

Presentation on the sidescan sonar techniques used in imaging the Lake Mead B-29

WHY?

The best way to "see" underwater is with sound. Underwater, light is scattered relatively quickly. Depending upon water conditions, at times even the brightest lights provide a diver (or camera) with only a few feet, or less, of visibility. Imagine trying to see across a room filled with thick heavy smoke. Bright lights become useless. But SOUND would travel well in this environment and travels extremely efficiently through water.

OK, So why Sidescan??

Fishermen have been using fish finding sonars for years with good success. These sonars look down through the water and display objects appearing above the bottom. Depth finders work in much the same fashion. The difference between these types of sonars and sidescan is that fishfinders scan straight down, to a point. So as the boat or platform to which the transducer is mounted moves forward, a number of points are scanned, forming a line. Sidescan, meanwhile, can be thought of as scanning in a line perpendicular to the tow fish. As the fish moves forward, multiple scanned lines form an imaged area with a width up to hundreds of feet on either side of the fish.