98 UNDERWATERSEARCH METHODS
Many dive-computers have audible alarms to alert the diver
in the event of a rapid ascent. In conditions of poor
visibility, there should also be some capability for detect-
ing potentially dangerous obstructions in the path of
the towed diver. One solution is for the towboat to be
equipped with forward-scanning sonar.
Swimline (freeline) searches
Practice, patience and perseverance are prerequisites for
this method but it has been used with success on many
archaeological projects in the past. The system relies
on a string of divers, often between two and six, spaced
along a tape-measure or graduated line at intervals
equivalent to less than the limit of clear visibility, so that
complete coverage is possible (figure 12.2). The search
is usually undertaken with a ground-line to guide a con-
troller. To enable positions of objects to be fixed, it is
useful also to graduate the ground-line in convenient
increments. This technique is in fact a large-scale offset
survey (see chapter 14). These ground-lines can be as
long as necessary (up to a kilometre has been known)
and should be reasonably straight. To achieve this, they
are probably best laid from a moving boat, keeping the
ground-line under tension while it is being deployed.
It can be difficult to lay long ground-lines precisely,
but with practice it is possible. As long as the line is
straight, the positions of the two ends can be fixed (see
chapter 11) so that there will be a record of where the search
took place.
The main difficulty with this technique is maintaining
the line of divers at right angles to the ground-line,
which inevitably becomes more difficult in poor visibil-
ity. Another difficulty encountered with this method is
maintaining effective communication between the divers
to allow the line to advance smoothly. This problem
increases proportionally to the number of divers on the
line, and it is also intensified by poor visibility. Usually,
if a diver needs to stop to make an observation, or is ready
to move off again, this is indicated to the others by a code
of signals along the line. With practice, it is possible to
communicate quite complicated messages along the con-
necting line using a set of bells (short tugs) and pulls (long
tugs). Standard rope signals can be found in many div-
ing manuals and additional ones appropriate to the
individual operation can be devised.
Another difficulty is the variation in dive duration
between individuals at the same depth and in the same
environment caused by dissimilar equipment and breath-
ing rates. Matching the contents of the divers’ breathing-
gas supply will help. It is considered best practice to
terminate the search when one of the divers needs to
surface for whatever reason, unless a buddy diver can
accompany the diver to the shore or safety-boat.Figure 12.2 Swimline (freeline) search. (Based on original artwork by Ben Ferrari)