Sidescan sonar is a method of underwater imaging using narrow beams of acoustic energy (sound) transmitted out to the side of the towfish and across the bottom. Sound is reflected back to the towfish from the bottom and objects present there. Certain frequencies work better than others, high frequencies such as 500kHz to 1MHz give excellent resolutions but the acoustic energy only travels a short distance, therefore the swath or the area of coverage is less (from 50 to 75 meters on each side). Lower frequencies such as 50kHz or 100kHz give lower resolution but the distance that the energy travels is greatly improved and therefore the area of coverage is much greater (250 meters or more on each side).
Side Scan Sonar Towfish
A basic Side Scan Sonar System consists of a topside processing unit, a cable for electronic transmission and towing, and a subsurface unit (a towfish) that transmits and receives acoustic energy for imaging. As stated earlier, most modern units interface to a computer and provide sophisticated software tools for providing the exact GPS position of targets and for calculating the target’s dimensions.
A simple way to understand how to interpret sidescan images follows:
Imagine that the towfish is a extremely strong light source, illuminating to both sides in clear water at night. You are in a helicopter above the towfish looking down. As the towfish moves along and illuminates objects on the bottom of the ocean, you can see the objects, which reflect the light and you will see dark shadows behind each object (from the perspective of the light source).
The diagrams on the following page illustrate this concept.
The towfish generates one pulse of energy at a time and waits for the sound to be reflected back. The imaging range is determined by how long the towfish waits before transmitting the next pulse of acoustic energy. The image is thus built up one line of data at a time. In a standard monochrome image, hard objects reflect more energy causing a light representation on the image; soft objects that do not reflect energy as well show up as shades of grey. The absence of sound, such as the areas behind an object (referred to as shadows), show up as black areas on a sonar image (see example above). Modern sidescan sonars offer a wide range of color options and filters which can be utilized to enhance the images produced.
Sidescan sonar is mainly used for locating ships lost in waters deeper than 100 feet, since the bulk of such a ship will be on the sea floor, not buried under bottom sediment or scattered by storms and wave action. Exceptions occur, however, that make this type of sonar useful for shallow water shipwrecks.
The following images were produced using a color scheme presenting a more natural appearance by having the shadows appear black, and the harder objects reflecting in lighter tones. An experienced sidescan technician is required to interpret the many images produced by sidescan sonar. However, some are readily identifiable. These images show wrecks in varying degrees of deterioration.