Underwater searches
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.
Diver searches
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.
Search patterns controlled by ropes and lines
Circular search
An underwater circular search is a procedure conducted by a diver moving around a fixed reference point, usually by swimming, at a series of distances.The circular search is simple and requires little equipment. It is useful where the position of the objects of the search is known with reasonable accuracy.
Procedure
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 to show their position to the support team if conditions allow. The diver unreels a section of distance line appropriate to the visibility and marks the 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 proceeds in a circle with the line as radius, searching visually or by feel until back at the start position. The diver then unreels another section of line of the same length and repeats the procedure until they find the object, run into obstacles or run 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.
Variations on the circular search
In some cases a second diver can establish themself at a fixed point on the bottom and act as both the central point and line tender. The diver and line tender communicate with each other using line pull signals. When the diver has completed a full revolution of the search, the tender signals the diver and advances another section of line so the search can be expanded further from the central point. Another variation uses more than one diver along the search line. The divers are evenly spaced at a distance depending on visibility, and the increase in radius allows overlap of search area only for the innermost diver on the line. This variation becomes more difficult to coordinate with a larger number of divers, particularly in poor visibility.A major variation on the circular search is the pendulum search, also known as the arc or fishtail search. in which the diver stops and changes direction at the end of each arc. This is used when there is insufficient space to complete a circle, as when controlled from the shore, when the search area is limited to a sector to one side of the control point, or there is a major obstruction limiting the extent of the searchable sector. Divers on surface supply may change direction at the end of each arc even when using a full 360° pattern to avoid twisting the umbilical. The pendulum search can also be done with more than one diver on the search line, but this requires considerable skill and co-ordination, particularly in low visibility.
Another variation is used where the target is large enough to snag the search line, In this case the diver may go out to the full radius of the search area and make a single sweep, hoping to snag the target with the line. If on his return to the start line or bearing, he finds he is closer to the centre point, he will swim back along the line in the expectation of having snagged something. With some luck it will be the target of the search.
If the target is not found by the time the search pattern has reached maximum convenient radius, the centre point may be shifted and another search started. This can be repeated as often as necessary, but the positions of the centre points must be chosen to allow the full search area to be covered. This implies quite a lot of overlap, and the pattern is not efficient. The most efficient pattern uses an equilateral triangular grid, but this may have to be modified to suit the site.
The circular search is very popular as it does not require complicated setup and can be done by most divers without a great deal of special training. It is effective where the position of the target is known with reasonable accuracy, where the bottom terrain does not have major snags, and where the depth variation during each arc is acceptable.
Safety
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.Jackstay searches
An underwater jackstay search is a procedure conducted by divers swimming along a search line, the jackstay. There are various techniques for performing a jackstay search.Procedure
The procedure for a search using two fixed jackstays and a movable search line is described:The distance between the fixed jackstays will depend on circumstances, but should not be so long that reliable overlapping of sweeps is prevented. This will depend on the bottom terrain. Two divers are generally used on this search system. Two heavy jackstays are laid parallel to each other across the bottom of the search area. A lighter movable jackstay is used to connect the fixed jackstays at one end of the search area. This line is kept reasonably taut, but must not pull the fixed jackstays together.
The divers start at opposite ends of the movable jackstay and swim along it, each diver holding the line with his left hand and searching the bottom visually or by feel on his side of the line until he passes the other diver and reaches the other fixed jackstay, at which point he will signal to the other diver that he has reached this point by a pull signal on the movable jackstay.
When both divers are at the fixed jackstays they will shift the movable jackstay along the fixed jackstays by an agreed distance depending on conditions. The distance should be large enough to reduce excessive overlap, but small enough that there is no risk of missing the target between traverses. This usually means that the distance is between the reach of the divers searching by feel in low visibility, and the distance they can see to the sides plus width of the target in good visibility. Care must be taken to always shift the movable jackstay in the same direction. This can be easily confused in low visibility, so a compass can be used to prevent this problem.
The divers then repeat this process until they find the object or run out of fixed jackstay, time or air. When a diver finds the object he should signal this to the other diver by rope pulls. The second diver can join him to confirm the finding and mark it or continue the search. If the movable jackstay snags it should be freed by the divers as the pass the snag. The sweep may have to be repeated after freeing a snag. The method of attaching the movable jackstay should be easily adjustable, but reliable.
If a series of sweeps does not find the object, one of the fixed jackstays may be lifted and re-laid on the opposite side of the remaining one, and the process repeated until the target is found or the entire search area has been searched.