100 UNDERWATERSEARCH METHODS
extend the length of these with about 2 m (6 ft) of elas-
tic cord (‘shock cord’ or ‘bungee-cord’). The advantage
is that it keeps the line under permanent tension, there-
fore making it less of a problem should a diver acciden-
tally apply pressure to it during the search. It also means
that, provided there is sufficient elasticity, one end of the
jackstay can be leapfrogged at a time with less expendi-
ture of time and effort than if both ends have to be
released before the line can be moved.
The normal width of the corridor between jackstays is
between 2 and 8 m (6 and 26 ft), depending on the num-
ber of divers searching each corridor, visibility and the type
and size of expected targets. Although searches with four
divers can be organized between the jackstays, for inten-
sive work a maximum of two divers for each corridor allows
easier offset measurements to be taken from the jackstays,
resulting in a precise location for each observation.
Experience has demonstrated that a 1 m (3 ft) wide strip
for each diver in a 2 m (6 ft) wide corridor is suitable for
very intensive sea-bed searches (perhaps using a metal-
detector), almost regardless of visibility or nature of the
sea-bed. In areas where the sea-bed is uniformly covered
in fine sand or silt, the width of the search strip can be
greater. It can be as much as 6 m (20 ft) in certain
circumstances, and even wider if the expected target is
relatively large (e.g. a ship-sized ballast mound).Grid searches
If an area needs to be searched thoroughly and features
need to be located with precision, then the grid search has
much to offer. The first step is to lay a series of ground-
lines at 90 degrees to each other at an appropriate spac-
ing to create a grid over the site. The size of the grid will
depend on a variety of factors but is commonly between
2 m and 50 m (6 –165 ft). Ground-lines left on the sea-
bed for any time tend to get damaged or disappear.
However, if the intersecting points or anchorage points
have been adequately fixed, the grid could be recon-
structed in future seasons, even if the actual lines have gone.
Once the grid has been established, the corridor-
search technique can be easily deployed and the location
of points can be readily identified by site coordinates
with, conventionally, a series of numbers on both the
horizontal and vertical axes with the zeros at the bottom
left (south west) corner. Sometimes one of the axes isFigure 12.4 Jackstay (corridor) search. (Drawing by Graham Scott)