The arc search, also known as a half-moon or semi-circular search, is a very simple and effective search pattern frequently employed by divers looking for large objects (such as automobiles) or working on non-flat bottoms. In the arc search pattern, the diver works at one end of a taut tether, while a stationary handler holds the other end of the tether. The handler establishes visual or other landmarks on each end of the arc, and the diver moves along the arc, keeping the tether taut so as to maintain the proper distance.
When the diver reaches one end of the arc, the handler reels the tether in by a distance determined by the nature of the search. In general, the larger the object being searched for, the larger a distance the diver will be reeled in. Then the diver proceeds back along the arc at the new, shorter, distance, and the process repeats itself until the entire area of the arc has been covered.
Arc searches are fast, but have a couple of downsides. Although they work well even in scenarios where the bottom changes contour dramatically, this means the diver has to continually equalize their pressure. In addition, if there is a lot of bottom growth (such as weeds), the tether will tend to accumulate that growth quickly and this can put a lot of weight on the line, creating an entanglement hazard for any divers in the area.
Arc searches employing ROVs alone can be very practical. Since the ROV already has a tether, the operator can simply lock the tether length in place in order to perform each leg of the arc search, then reel in tether to set the distance for the next leg. ROVs can also augment a human-performed arc search by following the same pattern and employing sonar in addition to the diver’s visual search.
A circular search pattern can be used by one diver, but can use any number of divers, making it a popular choice when searching very large areas. In a circular search pattern, an anchor marker is dropped at the center of the search. A tether is attached to the anchor, and one or more divers take up stations along the tether at predetermined distances. Each diver marks their starting position so that the team will know when a complete circuit has been made. The divers then swim a complete circle, keeping the line taut. When a complete circuit is made, the divers move to new positions along the line, and the process is repeated. When the line length starts to make the search impractical, a new center point can be chosen and a new search initiated.
The disadvantages of a full circular search are that the search is not very tolerant of bottom variations, since all the divers need to be on the same line. If multiple searches must be done over a wide area, of necessity there will be considerable overlap between the searches, hindering efficiency. In addition, if a large area is being searched, the divers on the outer portion of the search will have to move faster than the divers on the inner portion, which can result in a hasty search that does not thoroughly cover the target area.
ROVs can be used in a circular search pattern to replace or augment human divers, just as on an arc search.
A compass search is a search which relies on the use of underwater compasses rather than search lines for navigation. A compass search may be needed in an area of rough terrain where a line would quickly become entangled, or in visibility conditions so poor that orienting to the line may be difficult or impossible. In a compass search, divers use wrist compasses or inertial navigation devices to maintain proper orientation.
A common type of compass search is the spiral box. In the spiral box search, a diver starts at the estimated position of the target, and then swims in a cardinal direction for a distance about as far as he or she can see under current conditions, counting the number of kicks it takes to cover that distance. The diver then turns 90 degrees, using the compass to set the bearing, either clockwise or anticlockwise depending on the layout of the area, and takes the same number of kicks before turning (in the same direction as before) and then swimming for twice that number of kicks.
For example, if the number of kicks is 10, the diver might kick 10 times, turn clockwise, kick 10 times, turn clockwise, kick 20 times, turn clockwise, kick 20 times, turn clockwise, kick 30 times, and so on. In this way an expanding box is searched, with the compass being used to ensure that the diver is actually covering the correct area. ROVs can make an outstanding contribution to this type of search, since their positioning sensors are more reliable than a diver using a compass; using an ROV to handle the navigation elements of this type of search can free the diver to devote 100% of his or her attention to the actual search.
The jackstay search, also known as a jackleg search or a Z-search, is a very powerful search pattern that is almost guaranteed to find even small target objects, if they are within the search area. It can be used by one or two divers at a time, and it works well in areas where there is either a flat bottom or a consistent slope, such as an embankment. In a jackstay search, two weighted anchors are placed at opposite ends of the search area, with a line taut between them. The diver proceeds along the line, searching along the path. When the diver reaches the terminal anchor, he or she moves the anchor by a short distance, between 60 and 80 percent of the visibility range, in the direction of the search progression. The diver then moves back along the line, covering some new ground and a lot of old ground. When they reach the other terminal, they repeat the process, so that the search line is moved a few feet at a time through the entire search zone. A second diver can follow the first, providing additional coverage.
The disadvantage of the jackstay search is that it is slow and labor-intensive. An ROV can make a major contribution to the efficiency of a jackstay search by filling the role of the backup diver, following the human diver through the pattern and providing both a second set of eyes and a sonar backstop to the visual search.
There are literally dozens of common search patterns, many with specialized applications for certain conditions, and we have only discussed a few of the most commonly used patterns here. However, ROVs can make an excellent addition to almost any search pattern, whether to directly search, to provide navigation and orientation support for human divers, or to conduct preliminary assessments of search areas without risking human divers in dangerous waters.