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. This is greatly influenced by the width of the sweep 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 his hands while proceeding along the pattern. When visibility is better, it depends on the distance at which the target can be seen from the pattern, or detected by sonar 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.
Diver searches are underwater searches carried out by divers. There are a number of techniques in general use by Commercial, Scientific, Public service, Military, and Recreational divers. Some of these are suitable for Scuba, and some for surface supplied diving. The choice of search technique will depend on logistical factors, terrain, protocol and diver skills.
As a general principle, a search method attempts to provide 100% coverage of the search area. this is greatly influenced by the width of the sweep. In conditions of zero visibility this is as far as the diver can feel with his hands while proceeding along the pattern. When visibility is better, it depends on the distance at which the target can be seen from the pattern. In all cases then, the pattern should be accurate and completely cover the search area without excessive redundancy or missed areas. Overlap is needed to compensate for inaccuracy, and may be necessary to avoid gaps in some patterns.
The general procedure is to start from a fixed central point, and to search the circumference of a circle where the radius is defined by a search line anchored at the central point. The radius of the circle is dependent on visibility, and is increased after each circle has been completed, by an amount which allows the diver to either see or feel an overlap between the current arc and the previous arc.
One end of the distance line is carried by the diver and the other is attached to the datum position by any appropriate method. E.g. clipped to the base of a shot line, pegged into the bottom, tied to a fixed object on the bottom or held by another diver. The diver may tow a surface marker buoy if conditions allow. The diver unreels a section of distance line appropriate to the visibility and mark his start position by a peg, loose marker, compass heading, or a pre-laid marker line extending outwards from the datum position. Then, keeping the line taut, the diver swims in a circle with the line as radius, searching visually or by feel until back at the start position. He then unreels another section of line of the same length and repeats the procedure until he finds the object, runs into obstacles or runs out of line, air or time.
The amount of distance line increment for each sweep should allow some overlap of sweeps to avoid the risk of missing the target between sweeps. If a buddy is involved the most efficient place is alongside the controlling diver on the line, and the extension of distance line for each sweep can be roughly doubled. Depending on the circumstances, control of the pattern may be from the surface, from a diver at the central point, or by the diver at the end of the search line, who would in that case control the search line reel.
Divers should be well trained in general diving skills before attempting this type of search. The search diver is responsible for maintaining sufficient tension on the search line so the signals can be transmitted and received. If a surface marker is used, slack in the line should be kept to a minimum to avoid entanglement. This is easiest if a reel is used to control the line, or alternatively the line should be buoyant, to keep it as far from the divers as possible, but buoyant lines will still tend to wrap around the shotline in the centre if there is enough slack.