Underwater searches are procedures to find a known or suspected target object or objects in a specified search area under water. They may be carried out underwater by divers, manned submersibles, remotely operated underwater vehicles, or autonomous underwater vehicles, or from the surface by other agents, including surface vessels, aircraft and cadaver dogs.
A search method attempts to provide full coverage of the search area, and to do this a search pattern is usually applied, which is a systematic procedure for covering the search area. This is greatly influenced by the width of the sweep or sensor swath, which largely depends on the method used to detect the target. For divers in conditions of zero visibility, this is as far as the diver can feel with their hands while proceeding along the pattern, while for towed pinger locators it may be more than a kilometre to each side. When visibility is better, it depends on the distance at which the target can be seen from the pattern, or detected by sonar, optical sensors or magnetic field anomalies. In all cases, the search pattern should completely cover the search area without excessive redundancy or missed areas. Overlap is needed to compensate for inaccuracy and sensor error, and may be necessary to avoid gaps in some patterns.