Practical search techniques

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The seabed search to locate a specific object is one of the most challenging tasks in diving – and certainly one of the most common in that it has equal application to military, commercial and recreational divers alike. It may be a vital piece of equipment carelessly dropped over the side of a dive boat, a sunken mooring buoy, machinery, small wrecks, or even – in the case of Navy EOD divers – the detection of mines.

Regardless of its purpose, the effort to locate small objects in a vast ocean is a practiced skill. In every instance both the type of object being looked for as well as prevailing environmental conditions will determine how complex the search will be, and what sort of logistics are necessary to a successful outcome.

While modern technology such as ROV’s and side-scan sonars have largely replaced the need to ‘send down a diver’, they are expensive items to operate and maintain, as are hand-held underwater sonars. Neither are they always readily available, nor are they always a practical alternative. In that regard a diver’s ability to conduct a successful manual search still occupies a key role in diving.

Planning
Because there’s no such thing as an easy dive – and because even a quick bounce dive in shallow water to recover a dropped face mask has brought at least one experienced diver to grief – plan the dive in detail. While still on the surface, determine the best type of search based on: the object being looked for? What, if any, special equipment will be required to effectively carry out the search – and subsequent recovery in the case of large items that have to be raised? How best to mark a located object’s position should it prove necessary? and any environmental factors that may impede the search?

Environment
Depending on sea state, adverse currents, underwater visibility, depth and seabed composition, looking for the proverbial needle in a haystack can be a much simpler task.

Even when an accurate surface fix has been taken, currents can carry lighter objects some way from where they were dropped. In deeper waters this may mean that the object is dozens of metres from the search area. Depending on underwater visibility, (particularly in areas close to run off from the land or over sea beds composed of sediment and fine silt that are easily disturbed by a careless fin kick) visual searches may be out of the question, the diver relying, instead, on searching by touch alone.

In the case of heavy objects, they may actually sink into the mud and silt, or be covered and uncovered by tidal action. In the case of smaller objects, they may become lodged out of sight in rock crevices, or be hidden among the fronds of kelp and sea-grass beds.

When initiating a search, environmental factors such as these all have to be taken into consideration in order to determine the most appropriate technique.

SEARCH PATTERNS
There are many search techniques and patterns. The one that’s chosen should be the simplest and easiest method based on the type and size of the object being sought and the environmental conditions.

Visual search
Usually carried out when the underwater visibility exceeds 6-metres, a visual search is best accomplished in open, sandy areas free of mud and weed, and when the diver has a static datum point to refer to, (a shot line or distinct and easily recognisable natural object.)

Employing good buoyancy control, to remain clear of the bottom and a suitable finning motion to prevent stirring up of the seabed, the diver can swim a variety of patterns using both natural and compass navigation techniques.

Towed-diver search
A variation of the visual search and one that lends itself to good visibility when there’s the need to cover a large area of seafloor, a diver is towed behind the vessel while seated on a bar attached to the end of the tow-line. His depth is adjusted by the addition of a 25-kg sinker that can be moved up or down the line.

A light line is attached to the diver and the free end secured to a marker float kept on the vessel. This line is used to signal back and forth between the diver and the boat and also serves to mark the diver’s position should he ‘bail-out’ from the tow.

Also rigged from the boat is a weighted shot line, buoyed at its inboard end, with the sinker trailing above the bottom and within the diver’s field of vision. At a signal from the diver, the shot line is cast free to sink to the bottom and mark a found object.

When towed at a speed of between 2 and 3 knots, the diver can easily control his depth by planning up and down with his body. At higher speeds the diver not only begins to lose control, but will also experience greater difficulty scanning the search corridor.

Circular searches
Used when the position of the object to be located is known with a fair degree of accuracy, the circular search is the simplest and easiest to perform.

With a shot line – either buoyed or lowered straight down from the vessel – in place over the suspected location, and with a distance line secured to its base, the diver can descend down the shot line and hover above the surface. Visually searching the area around the shot, (and carefully feeling beneath it, in the case of a small object.) the diver then takes the distance line and moves, say, 2-metres away from the shot. Keeping the line taut, the diver searches a circle around the shot. On completion, he again moves another few metres away from the shot and still keeping the line taut, completes another circular search, repeating the process until either the object is found, or the diver runs out of distance line. (If on an umbilical hose, each circle is completed in the opposite direction to the one before to prevent fouling of either a life-line or umbilical hose around the shot rope.)

Two-man circular search
A variation of the circular search is to use two divers. While one diver takes the end of the distance line and swims it to its furthest point before commencing a circular swim, the other diver swims up and down the distance line, between the shot and his companion, searching for the object visually or by feel.

If the located object is too heavy to be brought directly to the surface by the diver the distance line can be tied to it, marking its position for later recovery.

Jackstay searches
The term ‘jack-stay’ refers to rope or wire of any size that is laid on the seabed to guide the diver. With the ends securely anchored and buoyed in place and, depending on the line’s length and weight, sinkers placed at periodic intervals to hold it steady, the grid pattern formed by a number of laid jackstays can be used in a variety of ways when carrying out an underwater search. These types of searches are particularly useful in low visibility.

Grid
With one hand holding a line stretched between two jackstays, the diver can move along the line, using his eyes and free hand to conduct the search. At the end of each lap, the diver crosses over the line and swims back in the opposite direction with the same hand retaining hold of the line. If the object is still not found, the diver moves each end of the line further along the jackstay, say 1.5-metres, and the process is repeated.

Where two divers are employed, they start at opposite ends of the line. On completion of each lap, they move the line further along the jackstay by, say 3-metres.

Snagline search
When a large area is to be searched – and particularly where the object is of sufficient size to sit proud of the seabed – parallel jackstays are laid up to 30-metres apart and anchored.

Using a 35-metre long snagline that has been weighted with 30 grammes of lead every 4-metres, two divers descend simultaneously to their respective jackstays and proceed to work their way along them towing the snagline between them.

When the snagline fouls an object, the divers make its ends fast to the jackstay and feel their way along the line to the snag. If it is not the object that they’re searching for, the divers clear the obstacle and continue with the tow.

Although not as thorough as the grid search, the snagline search allows faster coverage of a larger area.

Search Pointers
Because the diver relies on nothing more technical than their hands, eyes, ropes, weights and floats, there’s a general belief that basic search techniques, such as those described, are seemingly easy to perform.

However … handling ropes and lines in a weightless, three-dimensional environment – particularly in zero visibility – does require training and practical experience.

• Practice deployment of SMB’s.

• While gloves are useful in protecting water-softened hands from cuts – and serve to keep them warm in cold waters – they are not practical when conducting a search in zero visibility, where the diver is reliant on touch alone. (In the words of a gnarled old Chief Petty Officer, “You have five eyes on the end of each hand, lad! Bloody well use them!”)

• Any cuts or wounds to the hands sustained during a seabed search should be treated immediately on surfacing.

Above all, however – and no matter how quickly the object is found – never make an underwater search look easy: usually requiring great patience and skill, they very rarely are. And in any event, the longer the time spent underwater searching for, say, a pair of sunglasses dropped from the side of a moored yacht by a non-diver, the greater will be their gratitude.

—ENDS—
The above article was first published in 2004



Categories: Techniques

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1 reply

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