UNDERWATERSEARCH METHODS 99
Plotting the position of observations during the search
can be achieved in the same way as described when using
the offset survey method. All that is required is to record
the distance along the ground-line and a distance out
at right-angles to the ground-line. To achieve a detailed
survey two divers can move slowly forward using a
short rigid rule or tape, at right-angles to the graduated
ground-line (figure 12.3). Alternatively, markers can be
dropped at points of interest and the position fixed later.
If the number of markers needed by each diver is exces-
sive, an alternative search method, such as the jackstay
system (see below) may be more suitable.
Swimline searches are often more effective in clear,
shallow water, but this technique can be deployed suc-
cessfully in deeper water if consideration is given to using
a secondary pair, or team, of divers to record the points
of interest located by the search team. With discipline, good
communication and rehearsed procedures this techni-
que can be very flexible, allowing the search-line to be
stopped periodically to record the nature of the sea-bed,
or count the number of surface-indicators (potsherds, for
example) to compile a distribution-map.
Jackstay (corridor) searches
This is a useful system if total coverage of an area of sea-
bed is required during visual or metal-detector searches,
but it requires more pre-search organization than the
two methods outlined above. The minimum require-
ment is usually two long ground-lines laid parallel at a
convenient distance apart, often 10 m, 30 m or 50 m, and
a further line, the jackstay, laid at 90 degrees between the
two ground-lines. It is more efficient to use two jackstays
to define a corridor because an area with defined edges
can be more effectively searched. Once the area between
the jackstays has been searched, another corridor is laid,
usually by leapfrogging one line over the other (figure 12.4).
It is important to take into account the potential safety
implications of divers being separated during this part of
the operation.
Fixing ground-lines to the sea-bed can be achieved
with methods similar to those suggested for survey
points (see chapter 14). In fact the ends of ground-lines
are likely to be survey points. One of the problems with
ground-lines is keeping them straight on the sea-bed. In
shallows, water-movement may mean the line has to be
weighted or pinned to the sea-bed to prevent unaccept-
able lateral displacement. Even in deeper water, ground-
lines may have to be placed along the line of maximum
prevailing current to help prevent sideways movement; and
even then, fixing to the sea-bed is likely to be required if
the lines are going to be used as part of a site coordinate
system for locating observations.
The jackstays themselves are less permanent features and
may only be in position for as little as 5 or 10 minutes,
depending on the size and intensity of search. The
method by which they are anchored must depend on the
nature of the sea-bed, and that might be totally different
from one end of the line to the other. Like many survey
points, line fixing will probably rely on either the weight
of an object, such as a 25 or 50 kg (55 or 110 lbs) metal
block, or on a pin or other fastening forced into bedrock,
an immovable boulder, or sediment. Even heavy weights
can be pulled across the sea-bed with surprising ease
(except when you want to move them yourself, of
course), so it is common for a weight to be pinned to the
sea-bed for additional security.
If the ground-lines and the jackstays are graduated, one
of the ground-lines should be considered as a zero axis.
The zeros of the jackstays can then be positioned on that
line. It is difficult to keep the distance between ground-
lines constant and therefore the other end of the gradu-
ated jackstay will not always coincide with the second
ground-line at the required distance. Rather than spend
unnecessary time making everything perfect, and without
sacrificing offset-survey precision, simply concentrate on
the line of the jackstay passing across the appropriate
graduation of the ground-line. To do this, it helps if the
jackstays are over-length. While ground-lines are usually
made from rope or line (leaded line is useful), the jack-
stays are often tape-measures or, better still, thin plastic
measuring lines that are available in 50 m (165 ft)
lengths. In many circumstances it can be advantageous toFigure 12.3 The offset method can be used to record the
position of features in relation to jackstays and ground-lines.
In this example a hand-held metal-detector is being used
to locate metallic features. (Drawing by Graham Scott)