SEARCH PATTERNS
There are a variety of search techniques that are applicable to the underwater environment. While in theory, some patterns based on land or air search patterns should work. . . . in practice they prove to be inadequate underwater due to such factors as limited visibility, chaotic bottom contour, and submerged hazards. Experience has shown that to be efficient underwater a search pattern must be simple. The simpler - the better.
All search techniques rely on one common element: the adoption and execution of a defined search pattern. The pattern must commence at a known point, cover a known area, and terminate at a known end point. A search is conducted by employing search sweeps that slightly overlap. The last known position of the object of the search is used as a starting point.
Different patterns lend themselves better to a particular type of search than others, depending on the type of bottom, depth, visibility, number of divers, size of the missing object, and how the base of operation is set up. Ten search techniques (patterns) are listed below. There may be others that are more practical to use under ice, in rivers, or in dams where the bottom is filled with trees or other growth; however, an imaginative combination of two or three of these patterns will prove effective in almost any instance.
A. THE CIRCULAR SEARCH PATTERN
B. THE SEMI-CIRCULAR SEARCH PATTERN
C. THE STRAIGHT SWEEP SEARCH PATTERN
D. THE GRID OR CHECKERBOARD SEARCH PATTERN
E. THE STRAIGHT SWEEP IN CURRENT
F. THE STRAIGHT SWEEP ALONG A SHORE LINE
G. SEARCH WITH A WEIGHTED LINE
H. THE PLANING BOARD IN SEARCH
I. TOWING DIVERS BEHIND A BOAT
K. THE JACK-STAY OR "Z" PATTERN
It is critical, regardless of the type of search pattern selected that each diver participating in the search be able to guarantee one of two things:
1. The object is in his assigned search area (assuming only one object is being searched for, only one diver will be able to say this).
2. The object being searched for is positively not in his assigned search area.
Divers participating in a search must keep in mind that the search can be greatly complicated by stirring up the bottom. Everything possible should be done to keep from disturbing the bottom and stirring up silt, sand, etc. This is normally done by maintaining slightly positive buoyancy, a head down feet up orientation and arching your back as you kick.
Divers should refrain from: wading about in water adjacent to the search area; treading water in a vertical position when near the bottom; touching the bottom with hands, fins, or body anymore than is absolutely necessary; entering and exiting the water in the vicinity of the immediate search area; or deploying weights and anchors by dropping them from the surface.
The circular search pattern can involve any number of divers, yet it is effective for single diver. The first item of importance in using this pattern is to make sure that the anchor used as the base of theascending/descending line is of the correct shape and weight to remain fixed during the search. Next, be sure to mark the starting point of the circle so that you will know when you have completed 360 degrees.A line is secured to the anchor and one or more divers are spaced along a bottom (search) line according to the visibility of the water. When a 360 degree sweep has been completed, the diver furthest from the anchor remains in position, otherwise the size of the first circle will not be known and a small object might be missed as a result of the larger second sweep. The number of circles made from one reference point is dictated only by the length of the line. It is the responsibility of the outside diver on the line to keep the line taught, consequently his/her area of search is less than the others. Thus, you have another reason for this diver not moving at the end of the completion sweep.
If the object is not found from the first point of reference, the location of the first anchor and the distance of the first search is marked with surface buoys. The anchor can then be moved to a new location and the search repeated.
We utilize a flat 25-pound weight (brake drum), attach a 2- to 3-foot length of chain to it with a swivel at both ends. The chain will extend above silt and give the diver something to snap his or her search line into, the swivel allows it to rotate to prevent fouling. The ascending/descending line is then attached to the top swivel and goes to the float (mini-mother) at the surface and excess line (scope) is allowed to suspend itself with a 5-pound counter weight below the surface. The mast on the mini-mother is used for ranging and positioning from the beach. Two shives at least 12 inches apart (or a spreader system) are used to keep the ascending/descending line from fouling with the counter weight. In zero visibility searches the diver uses the ringed line and a double snap to snap into the chain. The search line is used with better visibility - in either case a perimeter marker is utilized to mark starting and ending points and expansions are done by coming inside the already searched area and letting out appropriate line or rings and then moving out to the new perimeter with marker and repeating the pattern.
THE SEMI-CIRCULAR SEARCH PATTERN
The semi-circular search pattern is most effectively used from a pier. The guide is tied to a piling near the bottom and the same technique is followed as is used in the circular search pattern. From a pier, the base makes an excellent anchor. The pilings themselves indicate starting and ending points . From shore a mini-mother positioned in the middle of your search area and utilizing the banks as turning points works well and avoids confusion.
THE STRAIGHT SWEEP SEARCH PATTERN
A straight sweep from a single guide line is generally used when there are many divers participating and there is a fair amount of visibility.
The anchor should be placed sufficiently beyond a point where the lost object is thought to be. The guide line is made taut from the anchor to a point on the shore, and a second line along which the divers swim, runs at a right angle to the guide line. This line is then tied to the guide line with a loop.
The divers swim out to the anchor. Then the diver closest to the anchor holds his/her position while the others swim a 180 degree arc so that they are headed back toward shore. They determine that they are in line by prearranged signals that can be given down the rope which they are holding. When they are in a straight line they swim back to the shore. In this manner ten divers (assuming 10 foot visibility) can, in one sweep, cover a two hundred foot area in one sweep.
In practice this is a very difficult feat. One problem is that the anchor deployed in the water must be very heavy as this deployment is important to keep the line taut. A second problem is that it is extremely difficult to keep very many people abreast in the water without utilizing something rigid like a pole. A rope doesn't get it. (We don't swim at the same rate, nor equalize easily all the time, and there are too many things to go wrong).
THE GRID OR CHECKERBOARD SEARCH PATTERN
The grid or checkerboard search pattern requires four anchors and four floats. Guide lines beneath the surface may be used. The divers may swim the guide lines or, if none, may use a compass to swim back and forth within the area marked off by the buoys.
In this pattern, as in others, it is possible to control the divers by a signal from the surface. If there are no obstructions on the bottom the divers may use a line between them to maintain their position as the pattern is swum. When they reach one edge of the pattern a man on the surface with a line to one of the divers signals him of this fact. A jerk on the line alerts the other divers who then make another sweep according to their prearranged instructions.
If bottom obstructions or kelp prevents the use of a line between divers, signals from the man on the surface can be sent to the divers by pounding on a hollow pipe which is partially submerged in the water. The divers in this situation swim the pattern by compass. This is an especially useful technique when searching in deep water.
If the search is expected to be fairly extensive, enough pattern is laid out to last the length of one tank of air.
In locations where there is a current in one direction, the straight sweep is used. The anchor is secured to a fixed location, and rope is played out from shore a given distance each time the divers (one or two only in this pattern) swim out to the anchor and back. Consider starting the search in a current downstream in order to avoid divers stirring up the search area thereby lessening visibility.
THE STRAIGHT SWEEP ALONG A SHORE LINE
An effective method for searching a quiet body of water, such as a lake or pond, is to have divers spaced along a guide line as visibility will permit. The guide line is then held by someone on shore who walks along with the swimmers as they search. If the object is not found, the last diver holds position while the others take a position further out on the guide line, and another sweep is made.
This type search is effective when searching for small children who may have drowned, as they will generally be close to the shore.
When a search is required around a pier that is oddly shaped or one that has pilings to provide boat slips, divers may swim from a guide line which is held by a person on the pier.
Using the heavily weighted line to the surface as a reference point, divers may take positions along a bottom line. It is important that they swim slowly so they do not get ahead of the guide line and swim under the pier.
The heavy weights will keep the line taut while the person walks around the edge of the pier.
If a large area is to be searched and the object being sought is a large one, it is possible to use a planing board (sea sled) behind a boat. The planing board enables the diver to stay near the bottom, but the diver must always be conscious of the fact rapid ascents might result in serious problems.
When using this technique a marking device should be carried by the diver. When the object of the search is sighted, the diver merely drops from the board and swims to the object. A small line is then tied to the object and the marker is sent to the surface. The marker assures finding the object again and should keep the diver from being struck by the boat, which should stay a safe distance from the marker until the diver surfaces.
In the ocean where there is unlimited visibility, or in rivers where it is possible to see the bottom, divers may be pulled behind a boat without the aid of a planing board. The number of divers towed depends on the current of the water and speed of the boat. The best method is to have a rope for each diver being pulled.
The same marking technique is used in this search as was used in the one before on the planing boards.
At times, conditions are such that it is not feasible to use search lines. When such is the case a search can be conducted using an underwater compass. Visibility permitting, the use of the underwater vehicle (e.g., DPV, Scooter, etc.) provides an efficient method of searching an area by use of the compass. The underwater vehicle not only allows the diver to search a given area faster, but also greatly increases bottom time by reducing expended effort and thus air consumption.
The jack-stay pattern, sometimes referred to as a "Z" pattern can be utilized when searching a given area involving a steep embankment. The advantage of this search method in these circumstances is that a diver (or divers) can conduct the search from deeper depths to shallower depths. (The use of the circular pattern requires a diver to constantly change depths when conducted on an embankment.) The disadvantage of this search method is that it is slow to employ and manpower intensive.
The jack-stay rig consists of two down lines with small floats (area marker floats). Attached to each down line is a 15-20 pound anchor. A length of line 50-75 foot long is attached at each end to the two anchors. Longer lines tend to be hard to control and do not work as well. An advantage of the down lines and surface floats is that surface personnel can readily track the progress of the search.
One diver or two divers maximum, swims along the line from anchor to anchor. Upon reaching the opposite anchor, the diver pulls the line taught and moves the anchor in a predetermined direction a distance which is dependent upon the visibility and the size of the object of the search. The diver then reverses direction somewhat overlapping his previous pattern and repeats the sequence.
A diver (or divers) terminating a search simply ascends on one of the down lines. Should it be necessary to continue this pattern, another diver can descend the same down line and pick up where the first diver left off.
For more search patterns, see Basic Search and Recovery from the National Association of Resuce Divers.
Photos courtesy of Charlie Curtis