Imaging sonar systems, occasionally referred to as sector-scanning sonars, are underwater instruments providing acoustic images of a scene in near-real-time. These images can be updated rapidly to provide a ‘video' display of the scene for use in close-quarters navigation or in placement of engineering equipment. Imaging sonars also have a role as part of the obstacle avoidance systems on ROVs and other underwater vehicles. Visualisation and monitoring of precision dredging operations are possible using these systems, which give the operator confidence in the real-time placement of equipment.

Although it is possible to make the analogy with photography and video, acoustic techniques will operate at distances well beyond the visible range. In many cases, the technology can be coupled with motion sensors and positioning instruments to effectively generate a 3D ‘point cloud' of XYZ positions and signal strength that can be recorded and used to construct a correctly scaled computer model of the scene.

The systems have applications in inspection of harbour walls, bridge piers, pipelines and fixed offshore structures. Many construction projects require the careful placement or matching of structural components underwater and the imaging sonar can be deployed to monitor and record the events. Underwater surveillance in ports and around sensitive installations can be undertaken using imaging sonar systems at fixed locations, and monitored and recorded in a security control centre.

The imaging sonars are deployed on fixed or steerable frameworks from ships, platforms or the quayside, and they are almost essential elements in the suite of sensors mounted on an ROV.

The systems principally rely on active sonar techniques. A high-frequency acoustic pulse is transmitted and the returning echo is received at a number of transducer elements with a fixed geometrical relationship to one another that allows the small components of the echo to be detected and to record direction and strength to be sampled across a scene.

The principal specifications are the maximum range, the area scanned, the image resolution, its update rate and whether a full 3D capability is offered. The instrument housings and connections are usually variable for different applications and water depths.

Here we show one typical example from the principal manufacturers and each offers a range of equipment for specific applications.

The table of specifications does not, of course, match every manufacturer's proprietary technique but, where possible, we have attempted to allow the reader to make an initial comparison of the available systems in this field.