| Underwater Search & Recovery Techniques
While these techniques have worked well for me, I am not advocating their use by others. I am not a trained professional nor
a certified instructor.
- do not start where you think the item is, start away and work into the suspected location.
- do not try to "wing it" unless the water is very clear , your information is very good and you don't mind wasting time on a hunch. Sometimes
- be observant ... be aware of depressions (scour marks) tracks, objects that have sharp edges or do not belong.
- Consider density of the item being searched for and the bottom density. Some objects will sit on top of the bottom, others will be fully or partially embedded in the bottom silt or mud.
- if using a line to maintain your lanes, you can use the line to snag the item if it projects above the bottom (and general bottom debris)
- overlap lanes
- search lane width is 60-80% of the visibility range, this allows for lane overlap.
Visibility range is the maximum distance you can search to either side with
absolute certainty of finding the object. With large items this will be determined by water visibility. With small items it will be
a combination of visibility and object size (furthest range you can see a similar sized and colored item). When using a metal detector, the range will be the furthest limit
of the sensor head sweep.
|visibility range and overlap
Used when search lines can not be deployed, a compass search is subject to current and individual diver techniques which will influence the
accuracy of the search pattern : ability to maintain an accurate heading using a navigation board, console compass or wrist compass and maintaining a
consistent kick. Use of a DPV / scooter can increase search range and maintain pattern consistency if proper speed control is maintained.
|Ascending Grid Pattern
Used with a shore based access, useful in a sharply sloped areas to place the deepest part of the dive first, to allow for
outgassing and heaviest air consumption at the start of the search leg, or simply to place the end of the dive closest to egress.
Using either line markers for distance, a tensioned line anchored at the furthermost points with a search line clipped to the tension line
or using compass bearing and kick count.
The first leg of the dive is perpendicular from shore and then parallel from shore at the farthest end of search area. Leg 3 is towards shore for about 80% of the visual search width
Tethered Circle Search
Used to cover an area using a central point to attach the tether. The search grid will be performed by spooling out a specific
amount of line, then swimming in a clockwise or counter-clockwise direction for a 360o sweep. I plant a non-metallic stake or deploy a weight and float at my start point which will also mark my end point when I complete the circle. Upon a full sweep, additional line equal to 60-80% of
the visibility range is paid out, the marker is planted and another 360o sweep is performed.
This technique can be used by one or multiple divers, placing each diver at a point on the line at the proper search lane width. Note: the diver furthest out will of course have to cover more area than the divers closer to the center. You can use this fact to best suit slower/faster divers or to swap out the heaviest work positions
at the completion of each sweep
For good water visibility and a large object, this search technique can be performed mid water to search a wide area quickly even with a cluttered bottom.
For small items or low visibility, the tethered search is best used on a bottom without many obstructions, since you will be closer to the bottom and line snags will impact your progress.
The search can begin at full line extension and work your way in (this is a good choice when the object is expected to protrude from the bottom and will snag the search line) or can begin from the central point. The choice depends upon the object and goals.
This is also my preferred search technique when diving from an unmanned boat, attaching the search line onto the anchor line ensuring I always have a connection to my vessels location. (of course you must make sure the anchor or weight used as the center point is of correct weight and type for the search and will not be moved or released from bottom set. I use a dual anchor setup when diving an unmanned boat). My line is marked at 10 ft intervals with distance markers attached. I have used both fixed line holders (of the type used to tow a dive flag) and spool type reels.
If the object is not discovered, the center point will be moved. Due to the nature of circle searches, there will be either overlap or missed areas
depending upon your next placement and is inefficient for an area that will require many circles to be deployed. note: the overlap image is to convey the overlap and missed area issue and is not to represent the actual optimal pattern. Gray is the unsearched area, white is the overlap.
|tethered circle search
overlap & missed areas
Tethered Partial Circle Search In the case of dropped items where the center point is assumed to be accurate, or searching for an object thrown from shore, this can be a quick search method. If it fails, I would suggest either a tight grid search, or revise your search area parameters.
Same techniques as the Circular Search except it does not allow a full sweep due to structure (pier, shipwreck, dam wall). Each sweep will start against the fixed object, sweep out and back to the fixed object which will be used as search limit indicators, then line will be released for the next sweep.
I prefer a grid based pattern when searching against a large fixed object if it will require a large number of center point relocations due to the overlap inefficiency of the circular pattern.
|tethered circle search
Straight Sweep Search
A line is strung between 2 points (shore to water or 2 points in the water) and placed under tension. Another line to be used by the divers is attached
to the tensioned line with either a large clip or shackle. The diver(s) swim from end to end of the tensioned line. At the end of the run you do a 180o
circle sweep and run a search on the opposite side of the tensioned line.
When the tensioned line needs to be moved often to cover an area
this search has less overlap (or risk of missed areas) than the circle search. For the run parallel to the tensioned line all divers cover the same
distance, at the return sweep the divers furthest from the tensioned line will of course have to cover more distance to cover the circle sweep section.
|straight sweep search
This pattern covers the area with the least amount of unnecessary overlap (other than standard lane to lane safety overlap of 60-80% visibility)
and can be accomplished with divers following tensioned lines on the bottom or compass bearings. For small items it is best to lay lines to ensure
no area is missed. For smaller high value items performing a search of an area twice might be useful. The second sweep is performed over the same area
except you swim 90o from the first pass. (eg: first pass is north and south directions, 2nd pass is east and west) this allows for a different
perspective which sometimes allows a missed item to be seen perhaps due to shape, bottom shadows etc.
When boat support is available, dropping anchored buoys with the end line strung between the anchors is helpful when running a compass course.
If a line is run for reference, it can be run for the center diver with the side diver(s) using a buddy line to the center diver
A boat can drop and move
grid lines when a diver signals at the end of the run by yanking on the anchor marker
If no surface support is available, the divers can move marker buoys
but that requires a lot of shuffling of markers at each run.
Towed Diver Search
A diver can be slowly towed behind a boat. While some people can tow a diver hanging onto a weighted line, I find the use of a dive plane
to make life easier. Basically a sheet of marine plywood or lexan, the diver can tilt the board to dive or ascend, and can fly somewhat to the sides. Some boards
are made simply with hand holds cut into the sheet, the ones I've made have had handles bolted on.
A slow forward speed will be maintained by the
boat operator over a predetermined course. The maximum speed dependant on visibility and the surface area of the divers mask or resistance to the
current on the hose and regulator stressing the divers mouth. A quick release separating the dive plane from the boat can be used (a float on the line will indicate to the
boat captain the diver has separated from the boat (and the change in tow pressure should be noticed). If the board is buoyant it can be spooled up to mark
the divers position so the boat can take surface cover position. The diver should also release a smb to mark their location since the dive plane will not be readily visible. note: simply letting go of the dive plane usually leaves the dive plane bouncing against the bottom in the boards I've built so
the release knot is recommended.
Dive Sleds are more sophisticated than the simple dive plane and can include
a windshield allowing for increased tow speeds, a lay down position with feet supports and left right dive planes.
Patterns from other sources
1-2 divers deploy weighted buoys with a line under tension at the base. The divers proceed from anchor to anchor, at the end point the
anchor buoy is moved the defined search lane (60-80% of visibility range) and the search is performed to the opposite buoy, which when reached, will be moved
the specified distance. This technique will move the search area slowly in a slight diagonal and there will be overlap at each end of the search run since
only one point is moved when returning back along a run. Recommended line length between anchors is 50-75 ft since more length makes it hard to
manage the lines. This pattern makes it easy for shore based personnel to monitor progress. If there is boat support I would prefer for the boat to deploy
the next buoy and anchor set
I have not used this pattern, so it is included here as theory. A line is strung between 2 points within the current. Seems best when used to search
a body of water with land at 2 sides (river or inlet) where hard points can be had on each side of the body of water. Diver(s) swim the line from
anchor point to anchor point. If the bottom is easily stirred, subsequent search patterns should be performed from downstream to upstream. Shore support will
play out or pull in line to change the search area , anchor points can be moved or the diver can use a clip to the line and take up or release more line.
Obviously shore to shore lines are best used with no boat traffic (or they must be weighted to hug the bottom).
Tethered from Shore Search
Another pattern I haven't used. Essentially best used when many divers are searching a body of land bordered by accessible shoreline.
A shore based person holds the end of a line, divers are spaced as visibility range allows perpendicular to shore. The shore person walks the line
along the shore in pace with the divers. If the object of the search is not found within the search range, the further most diver holds position and
the other divers set up station from that point outward. (note: I would have the next to furthest out diver leapfrog the last diver and set the
point to start the next staffing positions). The search is then performed in the opposite direction as the first sweep with the shore based person
Pre-arranged line signals can be used between divers and shore to communicate, and passed along the line.
This pattern is
indicated as being used to search for child drowning victims since they are expected to be close to shore.
Weighted Line Search
Indicated to be used along docks a heavily weighted line is walked along the edge of the dock while the divers attach a clipped line to the
weighted line. This allows divers to receive surface input as to the dock pattern, minimizing wasted searching under the dock cover.
As with any search performed around a crowded environment, the surface support can be helpful to ward off boats which might pose a danger to divers.
next: Mark the location