GEOPHYSICAL AND REMOTE-SENSINGSURVEYS 109
many archaeological sites coincide with areas of high
sedimentation. This can result in the partial or complete
burial of structures, features and artefacts. While wrecks
with substantial iron content may be found using
magnetic surveying (see below), the only technique
suitable for detecting buried wooden artefacts is sub-
bottom profiling. Furthermore, marine archaeologists
are increasingly interested in the identification of the
environment or landscape in which such artefacts were
deposited, and to understand this it is essential to look
at both the surface and the sub-surface.
Two principal types of systems exist: those that produce
a single-frequency pulse (such as ‘Pingers’ and ‘Boomers’)
and those that produce a swept-frequency pulse (‘Chirp’).
The single-frequency systems suffer from a penetration-
versus-resolution compromise. Put simply, higher fre-
quency sources give better resolution but can penetrate
only a short distance into the sediment; conversely, lower
frequency systems penetrate further but give poorer
resolution. The development of Chirp technology in
the early 1990s attempted to address this conflict by
producing a pulse that can penetrate decametres into the
sea-bed while still retaining decimetre resolution. It
should be noted that the effectiveness of each system is
dependent on the nature of the sediments being imaged,
with coarser sediments (sands and gravels) being harder
to penetrate than fine-grained sediments (silts and clays)
(figure 13.3).
All sub-bottom profilers use a source that generates
sound pulses that travel into the subsurface. These waves
then reflect off boundaries or objects within the sub-
surface and are detected by an acoustic receiver (or hydro-
phone), which is usually mounted close to the source.
Reflections occur where there are differences in density
and/or sound velocity across a boundary. The returning
echoes are then transmitted to a recording device, either
a hard-disk or direct to paper printer (figure 13.4).
Sub-bottom profilers generate a data set that can be
processed to give a cross-section in the direction of
movement of the boat in two-way travel time (the time
taken for the pulse to travel from the source to the
reflector and back to the receiver). With additional
knowledge of the speed of sound through the sediments
present (obtained from in situ measurements of coreFigure 13.3 Chirp (top) and Boomer (bottom) image from the same location within Strangford Lough, Northern Ireland,
showing the detailed layering within the upper, fine-grained sediments in the Chirp profile and the penetration through
stiff glacial sediments into the basal bedrock from the Boomer section. Data acquired using a GeoAcoustics Chirp and
Boomer system. Vertical scale bar represents c.15 m (49 ft). (Courtesy of the National Oceanography Centre/University
of Southampton)