SITEMONITORING ANDPROTECTION 165
monitor sites requires multiple site visits over an extended
period.
Video and/or still photography should therefore be
used in all situations where the equipment is available and
the environment, such as underwater visibility, allows. For
still photography, the project design should identify what
needs to be photographed and from what viewpoint.
All photographs should incorporate some form of easily
read and consistent scale and if the photographs are
repeated at a later date, they should be taken from the same
viewpoint. Similarly, the project design should identify what
video footage is required. Great care should be taken to
ensure that the footage shot is steady and slow. If prac-
ticable, video footage should also incorporate scales.
Where conditions allow, the possibility of taking photo-
mosaic and panoramic photographs should be considered.
Unless they are also to be used for measured survey,
photo-mosaics used for monitoring do not usually need
to be precisely scaled and can be prepared using simple
‘swim over’ techniques.
For more detailed information about obtaining, collating
and storing photographic records, see chapter 10.
Taking measurements: Measurements are a valuable
method for identifying and quantifying change, provided
that they are taken accurately and in a way that is repeat-
able. Examples of the way in which measurements can be
used to monitor sites include:
- Measuring changes in the depth of burial for a site.
If a site is buried or partially buried, the depth of
burial can be established by probing at fixed points.
Alternatively, the vertical distance between a fixed
point and the sea-bed surface can be measured.
This fixed point could be a nail attached to an
exposed archaeological feature or a stake or spike
driven securely into the sea-bed. - Using an inclinometer to measure the angle of the
plates or frames of a metal wreck to detect signs of
movement. - Counting the number of species or individual spe-
cimens and measuring the size of marine growth in
sample areas. - Using a depth-gauge to take measurements across
the sea-bed. When compared with previous results,
changes in the shape of the sea-bed can be detected.
If it is intended that repeat measurements should be
taken over multiple visits to a site, then the position of
these ‘measurement points’ should be marked in some way
to ensure that they can be relocated and the measurement
accurately repeated. This is particularly important if
the work is to be carried out by different members of a
group or by a different group altogether.
Sampling: This can take many forms. For example,
sea-bed sediments can be sampled and identified to give
information about the vulnerability of a site to erosion,
or the chemical and biological environment in which
artefacts are buried. Examination of timber samples
can give information about wood-boring organisms and
other biological threats. They can also provide important
information about how long exposed wooden artefacts
are likely to survive. Sampling of marine growth will
allow different species to be identified precisely. This can
help determine which parts of a site are in high- or
low-energy environments, which in turn can highlight
areas of particular vulnerability. Sampling should only
be considered when the information sought cannot be
obtained from other, non-intrusive, methods. The benefit
that is likely to be obtained should always be weighed care-
fully against any damage that may result.Non-destructive testing (NDT): A sophisticated
industry has developed around the regular inspection of
large metal structures in the offshore oil and gas indus-
try. Some of the techniques used in this industry have
been applied or adapted to the monitoring of metal ship-
wrecks and other metal artefacts by archaeologists.
Measurement of corrosion potential (CP) can be used
to provide information about the corrosion history and
vulnerability of metal artefacts. It has been used to study
both large twentieth-century metal wrecks and concreted
iron ordnance. In some cases, the technique can be used
to assist conservators to identify in situconservation
methods, such as the attachment of sacrificial anodes.
Although the application of this technique requires a
certain amount of specialized knowledge, the equipment
is readily available or can be constructed at modest cost
and CP measurement should be within the capability of
most reasonably committed projects.
Similarly, ultrasonic thickness measurement (UTM)
can be used to monitor the thickness, and therefore
corrosion, of metal structures such as iron or steel plates.
Again the equipment is readily available.Geophysical survey: Equipment such as side-scan
sonar and multibeam swath bathymetry can be used to
measure the depth and shape of the sea-bed and to detect
the presence of archaeological material. It can therefore
be used to monitor changes over time (e.g. the movement
of sandbanks over sites and the erosion or deposition of
protective layers of sand and other sediment).
Geophysical survey (see chapter 13) has traditionally
required ‘ground-truthing’ by divers or remotely operated
vehicles to check and interpret the results. However, as both
the quality of data and the interpretative skills of archae-
ologists have improved, the need for complete ground-
truthing is gradually diminishing. This trend is likely to