One of the most common reasons for divers or ROVs to go into the water is to search for something. The “something” can be enormously variable – an engine that fell off a boat, a weapon, a human body, a sunken ship, an automobile – but regardless of the type of object being searched for, there are some common search patterns that are worth knowing about.
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.
The problem of seeing things underwater is one that has vexed divers and others who work in the water since time immemorial. Ordinary vision works OK, in the daytime or with artificial light, in shallow water, in good conditions – but fails spectacularly in poor light or when water conditions are murky or worse. The modern electronic marvel that let us “see” for thousands or even millions of miles through air and space – radar – is completely useless underwater; water is effectively a brick wall to the short electromagnetic wavelengths that radar employs. Yet it is extremely important that underwater craft be able to sense their surroundings; if your craft is unable to see what is around it, you will pay the price for your lack of vision. Around the beginning of the 20th century, researchers realized that they could use sound waves to “see” underwater.
Unlike radar, sound waves propagate just fine underwater – in fact, they propagate underwater better than they propagate through the air. The first sonar-like devices were used to listen for icebergs; sonar technology advanced enormously during the First World War as a tool to listen for enemy submarines. Sonar still has extensive military uses, but today civilian sonar is the primary area of development.
Although in this post we will talk about “seeing” it should be noted that sonar does not actually produce a direct visual image. Many sonar control suites will translate the sonar data into a visual picture, and high-end sonars actually can produced simulated images that look very much like you are “seeing” what is out there – but the actual data is sound pulses being reflected back. Computers do much of the interpreting of that data stream, making it more accessible to people without advanced training in sonar interpretation.
Most remote-operated vehicles have the ability to carry sonar sets as optional equipment.ROVs use sonar for a variety of purposes, from mapping the bottom of bodies of water to searching for items in the water.
There are three basic types of sonar available for deployment on an ROV:
Scanning sonar operates in a way that will be familiar to anyone who has ever seen a radar dish turning at an airport. The sonar emitter physically rotates, sending out a pulse of sound as it does so, and simultaneously listens for the echoes returning from any objects in the water. The rotational speed of a scanning sonar balances out the time between pulses and the time it takes for a returning pulse to come back to the sonar.
Scanning sonars do not provide a high degree of resolution. They are able to detect large objects (for example, pilings or ships) but lack the discrimination necessary to spot smaller objects, particularly when those objects are on the bottom or next to another object. Because there is only one beam, scanning sonars have no ability to see behind objects; if one object is behind another object in a line to the scanning sonar, the scanning sonar can see only the closer of the two. The strength of the scanning sonar is that it does cover an area of 360 degrees, although only from 10 to 20 degrees above and below the plane of the sonar.
Multibeam sonars (often referred to as “hydrophones”) are significantly more capable. A multibeam array has multiple sonar emitters, which are all digitally rather than manually moved, allowing for a much higher rate of scanning. A combination of hardware and software allows the multibeam sonar to sweep a wider area of the water – about 120 degrees in all directions. Multibeam sonars do not cover a 360 degree arc, however. They make up for this by having a higher degree of resolution than scanning sonars – at a hundred meters, a multibeam sonar can spot a diver in the water. At 10 meters it can distinguish arms and legs. Multibeam sonars are extremely good at looking in one direction at a time.
Side-Scanning sonars have two arrays, each of which has a sonar beam that traverses a 90-degree arc, one horizontally and one vertically. This means that they can see in almost every direction, except for straight up and down. Side-scanning sonars are used to collect data as an ROV moves through the water; unlike scanning and multibeam sonars, they do not provide information in real-time. Instead, they provide a “historical” view of what was there when the ROV was moving past a certain point. The advantage of side-scanning sonars is that they “see” almost everything, providing great resolution and covering an enormous area. Side-scanning sonars are suitable for wide-area searches for stationary objects (such as bodies or objects on the bottom of the body of water), surveying, and environmental monitoring.
The right type of sonar for your ROV will depend on the missions for that ROV. Each sonar type has advantages and disadvantages for various kinds of work. Aquabotix ROVs can be configured with any of these sonar types. As with other accessories, choosing the right tools for the job will go a long way towards ensuring that you get the best possible results.