Light is absorbed over very short distances in the water environment. In working underwater, the lack of long range vision is a major limiting factor. In the early days of underwater work, performed manually, limited vision was not as significant because the diver could not move from one place to another very quickly. As robotics and instrumental intervention arrived at the worksite, the need to extend our vision became more vital. This becomes even more important because with our remote presence we can move more quickly from one place to another.

To meet the demands of "seeing" further underwater, engineers have turned away from the visible light spectrum and to another form of transmittable energy underwater: sound. Sound is also attenuated in the dense water environment, but not over as short a distance as light. Although the resolution of acoustic imaging does not approach optics, it does provide a remarkable extension of our vision, as the images of the aircraft and collapsed bridge in the figures on this page show.

Those working underwater, including oceanographers, marine geologists, and ROV Pilots now depend heavily on sound energy to transform the things we cannot see underwater into numbers, graphs, and pictures. The ROV pilot in particular requires that the imaging sonar provide him with accurate and quickly updated images. The instruments that transmit and receive these sound pulses have become sophisticated and more accurate in the past few decades.

Underwater, sound transmission is limited. This is most notable in useable ranges. High-frequency sound energy is greatly reduced by seawater. Low-frequency sound energy is reduced at a much lesser rate. For instance, a sound pulse of 50 Hertz can be transmitted many thousands of kilometers in the ocean, but a pulse of 300 kHz, a common imaging sonar frequency, can be transmitted less than 3,281 ft (1,000 m).

As applied to underwater vehicles, sonar systems in use today include mapping and collision avoidance types. Side scan sonar transducers can be mounted on the sides of a vehicle, such as the one shown to the right, to provide a "map" of the seafloor. An advantage of side looking sonar on an ROV is that a long-range image can be provided out to the side of the vehicle's track. One disadvantage of side scan on a vehicle is that, while vehicles can be flown at low altitude along the seafloor, the side scan requires some amount of altitude in order to gain the necessary range. This problem is not new to the combination of long range acoustic and short-range optical imaging underwater. It is not always possible to fully utilize both simultaneously.

Almost every medium and large vehicle does utilize, however, a forward-looking sonar for navigation, collision avoidance and target delineation. These sonars are most often rotary sonars, commonly known as scanning sonar, such as the MS 900 scanning sonar by Kongsberg Simrad shown to the left. They consist of a transducer head, which rotates and is mounted on an electronics bottle. Common frequencies in these units range from about 300 kHz to 600 kHz and above. Again, the tradeoff between the higher resolution of the high frequency and the longer range of the low frequency comes into play. A vehicle may have more than one rotary scan sonar mounted on it. Two frequencies on two sonar heads working simultaneously, for example, will give a pilot a rapid informational update for targets and terrain on both high resolution and long range.

The fact that towed side scan sonars "fly" high above their targets gives them their ability to observe objects, often through the "shadows" cast by the sonar beam. This is shown graphically in the figure of the ship image to the right.

Today, color monitors and digital processing enhance the sonar operator’s ability to identify targets.