A scanning sonar is used to build up an image of the bottom. In many cases the sonar image of a target will closely resemble an optical image of the same object. In other cases, the sonar image may be difficult to interpret. A sonar image will always have less resolution than an optical image, due to the nature of the ultrasonic signals used to generate it.

Some objects, such as smooth plane surfaces, may be difficult to see with sonar.