GEOPHYSICAL AND REMOTE-SENSINGSURVEYS 107
(sea-bed discrimination) systems. Most are based on
single-beam ‘profile’ technology and so give an indication
of the material type at individual points. These systems
look at either the shape of the sea-bed return directly or
in conjunction with a multiple reflection of the sea-bed
to determine both the ‘hardness’ and the ‘roughness’ of
the bed. Thorough ground-truthing of these parameters
is then used to determine the grain size of the sea-bed.
The available literature on bottom-classification systems
suggests they are capable of delimiting broad sediment
types (bedrock, gravel, sand and mud) and indeed they
have been used for some archaeological surveys (plate
13.10). For these systems to become really useful tools
for imaging archaeological materials, research needs to
be undertaken into both their sensitivity to the rapid
changes of material type common on archaeological sites
and the actual acoustic properties of typical archaeolo-
gical materials. Archaeologists are actively researching
the potential for extracting such information from
multibeam swath bathymetry data, but the extraction of
non-normal incidence backscatter to material type is a
major challenge. It should also be noted that those sys-
tems based on single-beam acoustic sources suffer the same
horizontal resolution problems described above for the
single-beam echo-sounders.
Sidescan Sonar
Sidescan sonar is a method of underwater imaging using
a wide-angle pulse similar to those of the multibeam
swath bathymetry systems. Rather than calculating depth
information from the returning echo, the sidescan sonar
system displays the intensity of the sound scattered back
to the tow-fish from the sea-floor sediments and objects
exposed on the sea-bed. Sidescan data can be processed
to provide undistorted images of the sea-floor in real time.
The area of the sea-floor covered in a single pass is con-
trolled by the surveyor altering the altitude of the tow-
fish above the sea-bed. This can be done by adjusting either
the speed of the vessel or the length of the tow-cable.
Traditionally, a swath width equalling ten times the water
depth was recommended but, while this may be adequate
for basic sea-bed surveys, it does not always provide
the optimum geometry for archaeological information.
For detailed surveys, the angle at which the acoustic
energy reaches the target is very important and running
the same survey lines at very low and very high passes
can often reveal very different, but complementary,
information.
Sidescan sonar systems are available in a variety of
types, depth ratings and operating frequencies. Recent
developments that are now commercially available include
‘Chirp’ systems (so called because of the noise emitted),
which utilize a sweep across a range of frequencies, and
synthetic aperture systems, which provide excellent defini-
tion at longer ranges. Standard systems usually connect
directly to a laptop computer to display the data. Such
systems employ one of two industry-standard frequencies
for imaging: 100 kHz and 500 kHz, although they may
vary considerably from these, depending on the model and
the manufacturer. In general terms, a 100 kHz operat-
ing frequency is chosen for regional surveys with swath
widths in excess of 100 m (325 ft) per side. 500 kHz sources
are generally used where a higher resolution is required,
such as for shipwreck or waterside structure surveys
(figure 13.1). Very high-frequency systems of up to 2.5 MHz
provide even better definition but their effective range
is limited, sometimes to less than 10 m (33 ft), and so
are only useful on small, well-defined sites with excellent
tow-fish positional control.
Material properties (primarily roughness characteristics)
of the area being surveyed determine the strength of the
echo (backscatter) from the sea-bed. Rock, gravel, woodFigure 13.1 500 kHz sidescan sonar image of a v-shaped
fish-trap from the River Barrow, County Wexford, Ireland.
Data acquired using an EdgeTech Model 272-TD acquisition
system. (Courtesy of the Centre for Maritime Archaeology,
University of Ulster, Coleraine)