A sonar device emits one or more pulses of sound waves, which bounce off any objects they bump into and return to their source. A receiver collects the returning energy and records the time it took for the sound waves to make their round trips. The farther away the object is, the longer it takes the sound waves to return. By analyzing the data from the sound waves, computers are able to create an image of the object and determine the size, shape, and even density of underwater objects.

The simplest sonar device, called a fish finder or depth sounder, sends out a single, narrowly focused wave pulse and records the time it takes the wave to return and its strength. This information allows the computer to determine the distance to the target object and the object's relative density. These devices are primarily used to locate fish and to determine sea-floor depth and composition.

In more sophisticated devices, which employ sidescan sonar, the instrumentation is encased in a torpedo-shaped "towfish" that is towed behind and below the boat. These devices send out pulses of sound waves that fan out to the sides and below, so that everything in a 180-degree arc is in their field of view. The amount of wave energy returned depends on the density and texture of an object, and the reflectivity of its surface. The more sound pulses that bounce off an object and the stronger their returned energy, the clearer the sound picture. Typically, rough surfaces return strong echoes while smooth surfaces, unless they are perpendicular to the sound wave, return very little if any energy. Thus, objects such as rocks, metal debris, or large fish show up clearly in the side-scan sonar record, whereas gentle slopes produce only light shadows.

Sidescan sonar devices can create very detailed, three-dimensional pictures of objects in the water through computer analysis of the returning sound waves. These devices are used to map features such as hills and valleys on the ocean floor; to locate submarines, mines, and shipwrecks; and to inspect pipelines, cables, and bridge foundations.